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B.Sc. in Counselling Psychology
Complementary Course
II Semester
(2011 Admission onwards)
Calicut University P.O. Malappuram, Kerala, India 673 635
School of Distance Education
B.Sc in Counselling Psychology
II Semester
Complementary Course
Prepared and
scrutinised by :
Physiological Psychology
Prof. (Dr.) C. Jayan
Department of Psychology
University of Calicut
Computer Section, SDE
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Nervous System
Sensory Processes
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Module 1
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that
coordinate the actions of an animal and transmit signals between different parts of its body. In most
animals the nervous system consists of two parts, central and peripheral. The central nervous system
of vertebrates (such as humans) contains the brain, spinal cord, and retina. The peripheral nervous
system consists of sensory neurons, clusters of neurons called ganglia, and nerves connecting them to
each other and to the central nervous system. These regions are all interconnected by means of
complex neural pathways. The enteric nervous system, a subsystem of the peripheral nervous system,
has the capacity, even when severed from the rest of the nervous system through its primary
connection by the vagus nerve, to function independently in controlling the gastrointestinal system.
Neurons send signals to other cells as electrochemical waves travelling along thin fibres called axons,
which cause chemicals called neurotransmitters to be released at junctions called synapses. A cell that
receives a synaptic signal may be excited, inhibited, or otherwise modulated. Sensory neurons are
activated by physical stimuli impinging on them, and send signals that inform the central nervous
system of the state of the body and the external environment. Motor neurons, situated either in the
central nervous system or in peripheral ganglia, connect the nervous system to muscles or other
effector organs. Central neurons, which in vertebrates greatly outnumber the other types, make all of
their input and output connections with other neurons. The interactions of all these types of neurons
form neural circuits that generate an organism's perception of the world and determine its behavior.
Along with neurons, the nervous system contains other specialized cells called glial cells (or simply
glia), which provide structural and metabolic support.
Nervous systems are found in most multicellular animals, but vary greatly in complexity. Sponges
have no nervous system, although they have homologs of many genes that play crucial roles in
nervous system function, and are capable of several whole-body responses, including a primitive form
of locomotion. Placozoans and mesozoans—other simple animals that are not classified as part of the
subkingdom Eumetazoa—also have no nervous system. In Radiata (radially symmetric animals such
as jellyfish) the nervous system consists of a simple nerve net. Bilateria, which include the great
majority of vertebrates and invertebrates, all have a nervous system containing a brain, one central
cord (or two running in parallel), and peripheral nerves. The size of the bilaterian nervous system
ranges from a few hundred cells in the simplest worms, to on the order of 100 billion cells in humans.
Neuroscience is the study of the nervous system.
The nervous system derives its name from nerves, which are cylindrical bundles of tissue that emanate
from the brain and central cord, and branch repeatedly to innervate every part of the body. Nerves are
large enough to have been recognized by the ancient Egyptians, Greeks, and Romans, but their internal
structure was not understood until it became possible to examine them using a microscope. A
microscopic examination shows that nerves consist primarily of the axons of neurons, along with a
variety of membranes that wrap around them and segregate them into fascicles. The neurons that give
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rise to nerves do not lie within them—their cell bodies reside within the brain, central cord, or
peripheral ganglia.
All animals more advanced than sponges have a nervous system. However, even sponges, unicellular
animals, and non-animals such as slime molds have cell-to-cell signalling mechanisms that are
precursors to those of neurons. In radially symmetric animals such as the jellyfish and hydra, the
nervous system consists of a diffuse network of isolated cells. In bilaterian animals, which make up
the great majority of existing species, the nervous system has a common structure that originated early
in the Cambrian period, over 500 million years ago.
The nervous system is primarily made up of two categories of cells: neurons and glial cells.
The nervous system is defined by the presence of a special type of cell—the neuron (sometimes called
"neurone" or "nerve cell").Neurons can be distinguished from other cells in a number of ways, but
their most fundamental property is that they communicate with other cells via synapses, which are
membrane-to-membrane junctions containing molecular machinery that allows rapid transmission of
signals, either electrical or chemical. Many types of neuron possess an axon, a protoplasmic protrusion
that can extend to distant parts of the body and make thousands of synaptic contacts. Axons frequently
travel through the body in bundles called nerves.
Even in the nervous system of a single species such as humans, hundreds of different types of neurons
exist, with a wide variety of morphologies and functions. These include sensory neurons that
transmute physical stimuli such as light and sound into neural signals, and motor neurons that
transmute neural signals into activation of muscles or glands; however in many species the great
majority of neurons receive all of their input from other neurons and send their output to other
Glial cells
Glial cells are non-neuronal cells that provide support and nutrition, maintain homeostasis, form
myelin, and participate in signal transmission in the nervous system. In the human brain, it is
estimated that the total number of glia roughly equals the number of neurons, although the proportions
vary in different brain areas. Among the most important functions of glial cells are to support neurons
and hold them in place; to supply nutrients to neurons; to insulate neurons electrically; to destroy
pathogens and remove dead neurons; and to provide guidance cues directing the axons of neurons to
their targets. A very important type of glial cell (oligodendrocytes in the central nervous system, and
Schwann cells in the peripheral nervous system) generates layers of a fatty substance called myelin
that wraps around axons and provides electrical insulation which allows them to transmit action
potentials much more rapidly and efficiently.
Anatomy in vertebrates
The nervous system of vertebrate animals (including humans) is divided into the central nervous
system (CNS) and peripheral nervous system (PNS).
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The central nervous system (CNS) is the largest part, and includes the brain and spinal cord.[11] The
spinal cavity contains the spinal cord, while the head contains the brain. The CNS is enclosed and
protected by meninges, a three-layered system of membranes, including a tough, leathery outer layer
called the dura mater. The brain is also protected by the skull, and the spinal cord by the vertebrae.
The peripheral nervous system (PNS) is a collective term for the nervous system structures that do not
lie within the CNS. The large majority of the axon bundles called nerves are considered to belong to
the PNS, even when the cell bodies of the neurons to which they belong reside within the brain or
spinal cord. The PNS is divided into somatic and visceral parts. The somatic part consists of the nerves
that innervate the skin, joints, and muscles. The cell bodies of somatic sensory neurons lie in dorsal
root ganglia of the spinal cord. The visceral part, also known as the autonomic nervous system,
contains neurons that innervate the internal organs, blood vessels, and glands. The autonomic nervous
system itself consists of two parts: the sympathetic nervous system and the parasympathetic nervous
system. Some authors also include sensory neurons whose cell bodies lie in the periphery (for senses
such as hearing) as part of the PNS; others, however, omit them.
The vertebrate nervous system can also be divided into areas called grey matter ("gray matter" in
American spelling) and white matter. Grey matter (which is only grey in preserved tissue, and is better
described as pink or light brown in living tissue) contains a high proportion of cell bodies of neurons.
White matter is composed mainly of myelinated axons, and takes its color from the myelin. White
matter includes all of the peripheral nerves, and much of the interior of the brain and spinal cord. Grey
matter is found in clusters of neurons in the brain and spinal cord, and in cortical layers that line their
surfaces. There is an anatomical convention that a cluster of neurons in the brain or spinal cord is
called a nucleus, whereas a cluster of neurons in the periphery is called a ganglion. There are,
however, a few exceptions to this rule, notably including the part of the forebrain called the basal
At the most basic level, the function of the nervous system is to send signals from one cell to others, or
from one part of the body to others. There are multiple ways that a cell can send signals to other cells.
One is by releasing chemicals called hormones into the internal circulation, so that they can diffuse to
distant sites. In contrast to this "broadcast" mode of signaling, the nervous system provides "point-topoint" signals—neurons project their axons to specific target areas and make synaptic connections
with specific target cells. Thus, neural signaling is capable of a much higher level of specificity than
hormonal signaling. It is also much faster: the fastest nerve signals travel at speeds that exceed 100
meters per second.
At a more integrative level, the primary function of the nervous system is to control the body. It does
this by extracting information from the environment using sensory receptors, sending signals that
encode this information into the central nervous system, processing the information to determine an
appropriate response, and sending output signals to muscles or glands to activate the response. The
evolution of a complex nervous system has made it possible for various animal species to have
advanced perception abilities such as vision, complex social interactions, rapid coordination of organ
systems, and integrated processing of concurrent signals. In humans, the sophistication of the nervous
system makes it possible to have language, abstract representation of concepts, transmission of culture,
and many other features of human society that would not exist without the human brain.
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Neurons and synapses
Major elements in synaptic transmission. An electrochemical wave called an action potential travels
along the axon of a neuron. When the wave reaches a synapse, it provokes release of a puff of
neurotransmitter molecules, which bind to chemical receptor molecules located in the membrane of
the target cell.
Most neurons send signals via their axons, although some types are capable of dendrite-to-dendrite
communication. (In fact, the types of neurons called amacrine cells have no axons, and communicate
only via their dendrites.) Neural signals propagate along an axon in the form of electrochemical waves
called action potentials, which produce cell-to-cell signals at points where axon terminals make
synaptic contact with other cells.
Synapses may be electrical or chemical. Electrical synapses make direct electrical connections
between neurons, but chemical synapses are much more common, and much more diverse in function.
At a chemical synapse, the cell that sends signals is called presynaptic, and the cell that receives
signals is called postsynaptic. Both the presynaptic and postsynaptic areas are full of molecular
machinery that carries out the signalling process. The presynaptic area contains large numbers of tiny
spherical vessels called synaptic vesicles, packed with neurotransmitter chemicals. When the
presynaptic terminal is electrically stimulated, an array of molecules embedded in the membrane are
activated, and cause the contents of the vesicles to be released into the narrow space between the
presynaptic and postsynaptic membranes, called the synaptic cleft. The neurotransmitter then binds to
receptors embedded in the postsynaptic membrane, causing them to enter an activated state.
Depending on the type of receptor, the resulting effect on the postsynaptic cell may be excitatory,
inhibitory, or modulatory in more complex ways. For example, release of the neurotransmitter
acetylcholine at a synaptic contact between a motor neuron and a muscle cell induces rapid contraction
of the muscle cell. The entire synaptic transmission process takes only a fraction of a millisecond,
although the effects on the postsynaptic cell may last much longer (even indefinitely, in cases where
the synaptic signal leads to the formation of a memory trace).
There are literally hundreds of different types of synapses. In fact, there are over a hundred known
neurotransmitters, and many of them have multiple types of receptor. Many synapses use more than
one neurotransmitter—a common arrangement is for a synapse to use one fast-acting small-molecule
neurotransmitter such as glutamate or GABA, along with one or more peptide neurotransmitters that
play slower-acting modulatory roles. Molecular neuroscientists generally divide receptors into two
broad groups: chemically gated ion channels and second messenger systems. When a chemically gated
ion channel is activated, it forms a passage that allow specific types of ion to flow across the
membrane. Depending on the type of ion, the effect on the target cell may be excitatory or inhibitory.
When a second messenger system is activated, it starts a cascade of molecular interactions inside the
target cell, which may ultimately produce a wide variety of complex effects, such as increasing or
decreasing the sensitivity of the cell to stimuli, or even altering gene transcription.
According to a rule called Dale's principle, which has only a few known exceptions, a neuron releases
the same neurotransmitters at all of its synapses. This does not mean, though, that a neuron exerts the
same effect on all of its targets, because the effect of a synapse depends not on the neurotransmitter,
but on the receptors that it activates. Because different targets can (and frequently do) use different
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types of receptors, it is possible for a neuron to have excitatory effects on one set of target cells,
inhibitory effects on others, and complex modulatory effects on others still. Nevertheless, it happens
that the two most widely used neurotransmitters, glutamate and GABA, each have largely consistent
effects. Glutamate has several widely occurring types of receptors, but all of them are excitatory or
modulatory. Similarly, GABA has several widely occurring receptor types, but all of them are
inhibitory. Because of this consistency, glutamatergic cells are frequently referred to as "excitatory
neurons", and GABAergic cells as "inhibitory neurons". Strictly speaking this is an abuse of
terminology—it is the receptors that are excitatory and inhibitory, not the neurons—but it is
commonly seen even in scholarly publications.
One very important subset of synapses are capable of forming memory traces by means of long-lasting
activity-dependent changes in synaptic strength. The best-known form of neural memory is a process
called long-term potentiation (abbreviated LTP), which operates at synapses that use the
neurotransmitter glutamate acting on a special type of receptor known as the NMDA receptor. The
NMDA receptor has an "associative" property: if the two cells involved in the synapse are both
activated at approximately the same time, a channel opens that permits calcium to flow into the target
cell. The calcium entry initiates a second messenger cascade that ultimately leads to an increase in the
number of glutamate receptors in the target cell, thereby increasing the effective strength of the
synapse. This change in strength can last for weeks or longer. Since the discovery of LTP in 1973,
many other types of synaptic memory traces have been found, involving increases or decreases in
synaptic strength that are induced by varying conditions, and last for variable periods of time. Reward
learning, for example, depends on a variant form of LTP that is conditioned on an extra input coming
from a reward-signalling pathway that uses dopamine as neurotransmitter. All these forms of synaptic
modifiability, taken collectively, give rise to neural plasticity, that is, to a capability for the nervous
system to adapt itself to variations in the environment.
Neural circuits and systems
The basic neuronal function of sending signals to other cells includes a capability for neurons to
exchange signals with each other. Networks formed by interconnected groups of neurons are capable
of a wide variety of functions, including feature detection, pattern generation, and timing. In fact, it is
difficult to assign limits to the types of information processing that can be carried out by neural
networks: Warren McCulloch and Walter Pitts showed in 1943 that even networks formed from a
greatly simplified mathematical abstraction of a neuron are capable of universal computation. Given
that individual neurons can generate complex temporal patterns of activity all by themselves, the range
of capabilities possible for even small groups of interconnected neurons are beyond current
Historically, for many years the predominant view of the function of the nervous system was as a
stimulus-response associator. In this conception, neural processing begins with stimuli that activate
sensory neurons, producing signals that propagate through chains of connections in the spinal cord and
brain, giving rise eventually to activation of motor neurons and thereby to muscle contraction, i.e., to
overt responses. Descartes believed that all of the behaviors of animals, and most of the behaviors of
humans, could be explained in terms of stimulus-response circuits, although he also believed that
higher cognitive functions such as language were not capable of being explained mechanistically.
Charles Sherrington, in his influential 1906 book The Integrative Action of the Nervous System,
developed the concept of stimulus-response mechanisms in much more detail, and Behaviorism, the
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school of thought that dominated Psychology through the middle of the 20th century, attempted to
explain every aspect of human behavior in stimulus-response terms.
However, experimental studies of electrophysiology, beginning in the early 20th century and reaching
high productivity by the 1940s, showed that the nervous system contains many mechanisms for
generating patterns of activity intrinsically, without requiring an external stimulus. Neurons were
found to be capable of producing regular sequences of action potentials, or sequences of bursts, even
in complete isolation. When intrinsically active neurons are connected to each other in complex
circuits, the possibilities for generating intricate temporal patterns become far more extensive. A
modern conception views the function of the nervous system partly in terms of stimulus-response
chains, and partly in terms of intrinsically generated activity patterns—both types of activity interact
with each other to generate the full repertoire of behavior.
Reflexes and other stimulus-response circuits
The simplest type of neural circuit is a reflex arc, which begins with a sensory input and ends with a
motor output, passing through a sequence of neurons in between. For example, consider the
"withdrawal reflex" causing the hand to jerk back after a hot stove is touched. The circuit begins with
sensory receptors in the skin that are activated by harmful levels of heat: a special type of molecular
structure embedded in the membrane causes heat to generate an electrical field across the membrane.
If the electrical potential change is large enough, it evokes an action potential, which is transmitted
along the axon of the receptor cell, into the spinal cord. There the axon makes excitatory synaptic
contacts with other cells, some of which project to the same region of the spinal cord, others projecting
into the brain. One target is a set of spinal interneurons that project to motor neurons controlling the
arm muscles. The interneurons excite the motor neurons, and if the excitation is strong enough, some
of the motor neurons generate action potentials, which travel down their axons to the point where they
make excitatory synaptic contacts with muscle cells. The excitatory signals induce contraction of the
muscle cells, which causes the joint angles in the arm to change, pulling the arm away.
In reality, this straightfoward schema is subject to numerous complications. Although for the simplest
reflexes there are short neural paths from sensory neuron to motor neuron, there are also other nearby
neurons that participate in the circuit and modulate the response. Furthermore, there are projections
from the brain to the spinal cord that are capable of enhancing or inhibiting the reflex.
Although the simplest reflexes may be mediated by circuits lying entirely within the spinal cord, more
complex responses rely on signal processing in the brain. Consider, for example, what happens when
an object in the periphery of the visual field moves, and a person looks toward it. The initial sensory
response, in the retina of the eye, and the final motor response, in the oculomotor nuclei of the brain
stem, are not all that different from those in a simple reflex, but the intermediate stages are completely
different. Instead of a one or two step chain of processing, the visual signals pass through perhaps a
dozen stages of integration, involving the thalamus, cerebral cortex, basal ganglia, superior colliculus,
cerebellum, and several brainstem nuclei. These areas perform signal-processing functions that include
feature detection, perceptual analysis, memory recall, decision-making, and motor planning.
Feature detection is the ability to extract biologically relevant information from combinations of
sensory signals. In the visual system, for example, sensory receptors in the retina of the eye are only
individually capable of detecting "points of light" in the outside world. Second-level visual neurons
receive input from groups of primary receptors, higher-level neurons receive input from groups of
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second-level neurons, and so on, forming a hierarchy of processing stages. At each stage, important
information is extracted from the signal ensemble and unimportant information is discarded. By the
end of the process, input signals representing "points of light" have been transformed into a neural
representation of objects in the surrounding world and their properties. The most sophisticated sensory
processing occurs inside the brain, but complex feature extraction also takes place in the spinal cord
and in peripheral sensory organs such as the retina.
Intrinsic pattern generation
Although stimulus-response mechanisms are the easiest to understand, the nervous system is also
capable of controlling the body in ways that do not require an external stimulus, by means of internally
generated rhythms of activity. Because of the variety of voltage-sensitive ion channels that can be
embedded in the membrane of a neuron, many types of neurons are capable, even in isolation, of
generating rhythmic sequences of action potentials, or rhymthic alternations between high-rate
bursting and quiessence. When neurons that are intrinsically rhythmic are connected to each other by
excitatory or inhibitory synapses, the resulting networks are capable of a wide variety of dynamical
behaviors, including attractor dynamics, periodicity, and even chaos. A network of neurons that uses
its internal structure to generate temporally structured output, without requiring a corresponding
temporally structured stimulus, is called a central pattern generator.
Internal pattern generation operates on a wide range of time scales, from milliseconds to hours or
longer. One of the most important types of temporal pattern is circadian rhythmicity—that is,
rhythmicity with a period of approximately 24 hours. All animals that have been studied show
circadian fluctuations in neural activity, which control circadian alternations in behavior such as the
sleep-wake cycle. Experimental studies dating from the 1990s have shown that circadian rhythms are
generated by a "genetic clock" consisting of a special set of genes whose expression level rises and
falls over the course of the day. Animals as diverse as insects and vertebrates share a similar genetic
clock system. The circadian clock is influenced by light but continues to operate even when light
levels are held constant and no other external time-of-day cues are available. The clock genes are
expressed in many parts of the nervous system as well as many peripheral organs, but in mammals all
of these "tissue clocks" are kept in synchrony by signals that emanate from a master timekeeper in a
tiny part of the brain called the suprachiasmatic nucleus.
In vertebrates, landmarks of embryonic neural development include the birth and differentiation of
neurons from stem cell precursors, the migration of immature neurons from their birthplaces in the
embryo to their final positions, outgrowth of axons from neurons and guidance of the motile growth
cone through the embryo towards postsynaptic partners, the generation of synapses between these
axons and their postsynaptic partners, and finally the lifelong changes in synapses which are thought
to underlie learning and memory.
All bilaterian animals at an early stage of development form a gastrula, which is polarized, with one
end called the animal pole and the other the vegetal pole. The gastrula has the shape of a disk with
three layers of cells, an inner layer called the endoderm, which gives rise to the lining of most internal
organs, a middle layer called the mesoderm, which gives rise to the bones and muscles, and an outer
layer called the ectoderm, which gives rise to the skin and nervous system.
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Human embryo, showing neural groove
Four stages in the development of the neural tube in the
human embryo
In vertebrates, the first sign of the nervous system is the appearance of a thin strip of cells along the
center of the back, called the neural plate. The inner portion of the neural plate (along the midline) is
destined to become the central nervous system (CNS), the outer portion the peripheral nervous system
(PNS). As development proceeds, a fold called the neural groove appears along the midline. This fold
deepens, and then closes up at the top. At this point the future CNS appears as a cylindrical structure
called the neural tube, whereas the future PNS appears as two strips of tissue called the neural crest,
running lengthwise above the neural tube. The sequence of stages from neural plate to neural tube and
neural crest is known as neurulation.
In the early 20th century, a set of famous experiments by Hans Spemann and Hilde Mangold showed
that the formation of nervous tissue is "induced" by the underlying mesoderm. For decades, though,
the nature of the induction process defeated every attempt to figure it out, until finally it was resolved
by genetic approaches in the 1990s. Induction of neural tissue requires inhibition of the gene for a socalled bone morphogenetic protein, or BMP. Specifically the protein BMP4 appears to be involved.
Two proteins called Noggin and Chordin, both secreted by the mesoderm, are capable of inhibiting
BMP4 and thereby inducing ectoderm to turn into neural tissue. It appears that a similar molecular
mechanism is involved for widely disparate types of animals, including arthropods as well as
vertebrates. In some animals, however, another type of molecule called Fibroblast Growth Factor or
FGF may also play an important role in induction.
Induction of neural tissues causes formation of neural precursor cells, called neuroblasts. In
drosophila, neuroblasts divide asymmetically, so that one product is a "ganglion mother cell" (GMC),
and the other is a neuroblast. A GMC divides once, to give rise to either a pair of neurons or a pair of
glial cells. In all, a neuroblast is capable of generating an indefinite number of neurons or glia.
As shown in a 2008 study, one factor common to all bilateral organisms (including humans) is a
family of secreted signaling molecules called neurotrophins which regulate the growth and survival of
neurons. Zhu et al. identified DNT1, the first neurotrophin found in flies. DNT1 shares structural
similarity with all known neurotrophins and is a key factor in the fate of neurons in Drosophila.
Because neurotrophins have now been identified in both vertebrate and invertebrates, this evidence
suggests that neurotrophins were present in an ancestor common to bilateral organisms and may
represent a common mechanism for nervous system formation.
The nervous system is susceptible to malfunction in a wide variety of ways, as a result of genetic
defects, physical damage due to trauma or poison, infection, or simply aging. The medical specialty of
neurology studies the causes of nervous system malfunction, and looks for interventions that can
alleviate it.
The central nervous system is protected by major physical and chemical barriers. Physically, the brain
and spinal cord are surrounded by tough meningeal membranes, and enclosed in the bones of the skull
and spinal vertebrae, which combine to form a strong physical shield. Chemically, the brain and spinal
cord are isolated by the so-called blood-brain barrier, which prevents most types of chemicals from
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moving from the bloodstream into the interior of the CNS. These protections make the CNS less
susceptible in many ways than the PNS; the flip side, however, is that damage to the CNS tends to
have more serious consequences.
Although peripheral nerves tend to lie deep under the skin except in a few places such as the elbow
joint, they are still relatively exposed to physical damage, which can cause pain, loss of sensation, or
loss of muscle control. Damage to nerves can also be caused by swelling or bruises at places where a
nerve passes through a tight bony channel, as happens in carpal tunnel syndrome. If a peripheral nerve
is completely transected, it will often regenerate, but for long nerves this process may take months to
complete. In addition to physical damage, peripheral neuropathy may be caused by many other
medical problems, including genetic conditions, metabolic conditions such as diabetes, inflammatory
conditions such as Guillain-Barré syndrome, vitamin deficiency, infectious diseases such as leprosy or
shingles, or poisoning by toxins such as heavy metals. Many cases have no cause that can be
identified, and are referred to as idiopathic. It is also possible for peripheral nerves to lose function
temporarily, resulting in numbness as stiffness—common causes include mechanical pressure, a drop
in temperature, or chemical interactions with local anesthetic drugs such as lidocaine.
Physical damage to the spinal cord may result in loss of sensation or movement. If an injury to the
spine produces nothing worse than swelling, the symptoms may be transient, but if nerve fibers in the
spine are actually destroyed, the loss of function is usually permanent. Experimental studies have
shown that spinal nerve fibers attempt to regrow in the same way as peripheral nerve fibers, but in the
spinal cord, tissue destruction usually produces scar tissue that cannot be penetrated by the regrowing
Autonomic nervous system
The autonomic nervous system (ANS or visceral nervous system) is the part of the peripheral
nervous system that acts as a control system functioning largely below the level of consciousness, and
controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation,
perspiration, diameter of the pupils, micturition (urination), and sexual arousal. Whereas most of its
actions are involuntary, some, such as breathing, work in tandem with the conscious mind.
It is classically divided into two subsystems: the parasympathetic nervous system and sympathetic
nervous system. Relatively recently, a third subsystem of neurons that have been named 'nonadrenergic and non-cholinergic' neurons (because they use nitric oxide as a neurotransmitter) have
been described and found to be integral in autonomic function, particularly in the gut and the lungs.
With regard to function, the ANS is usually divided into sensory (afferent) and motor (efferent)
subsystems. Within these systems, however, there are inhibitory and excitatory synapses between
The enteric nervous system is sometimes considered part of the autonomic nervous system, and
sometimes considered an independent system.
The sympathetic nervous trunk consists of sympathetic ganglia running directly adjacent to the spinal
column. The adrenal medulla can be considered a sympathetic ganglion; although separate from the
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main trunk, the sympathetic fibers run through the sympathetic trunk before synapsing in the adrenal
medulla. The parasympathetic division consists of a sacral and cranial part. In the cranium the PSN
originate from cranial nerves CN III (oculomotor nerve), CN VII (facial nerve), CN IX
(glossopharyngeal nerve) and CN X (vagus nerve). In the sacral region of the body the PSN is derived
from spinal nerves S2, S3 and S4, commonly referred to as the pelvic splanchnics. The reflex arcs of
the ANS comprise a sensory (afferent) arm, and a motor (efferent or effector) arm. Only the latter is
shown in the illustration.
Sensory neurons
The sensory arm is made of “primary visceral sensory neurons” found in the peripheral nervous
system (PNS), in “cranial sensory ganglia”: the geniculate, petrosal and nodose ganglia, appended
respectively to cranial nerves VII, IX and X. These sensory neurons monitor the levels of carbon
dioxide, oxygen and sugar in the blood, arterial pressure and the chemical composition of the stomach
and gut content. (They also convey the sense of taste, a conscious perception). Blood oxygen and
carbon dioxide are in fact directly sensed by the carotid body, a small collection of chemosensors at
the bifurcation of the carotid artery, innervated by the petrosal (IXth) ganglion. Primary sensory
neurons project (synapse) onto “second order” or relay visceral sensory neurons located in the medulla
oblongata, forming the nucleus of the solitary tract (nTS), that integrates all visceral information. The
nTS also receives input from a nearby chemosensory center, the area postrema, that detects toxins in
the blood and the cerebrospinal fluid and is essential for chemically induced vomiting or conditional
taste aversion (the memory that ensures that an animal which has been poisoned by a food never
touches it again). All these visceral sensory informations constantly and unconsciously modulate the
activity of the motor neurons of the ANS
Motor neurons
Motor neurons of the ANS are also located in ganglia of the PNS, called “autonomic ganglia”. They
belong to three categories with different effects on their target organs (see below “Function”):
sympathetic, parasympathetic and enteric.
Sympathetic ganglia are located in two sympathetic chains close to the spinal cord: the prevertebral
and pre-aortic chains. Parasympathetic ganglia, in contrast, are located in close proximity to the target
organ: the submandibular ganglion close to salivary glands, paracardiac ganglia close to the heart etc...
Enteric ganglia, which as their name implies innervate the digestive tube, are located inside its walls
and collectively contain as many neurons as the entire spinal cord, including local sensory neurons,
motor neurons and interneurons. It is the only truly autonomous part of the ANS and the digestive tube
can function surprisingly well even in isolation. For that reason the enteric nervous system has been
called “the second brain”.
The activity of autonomic ganglionic neurons is modulated by “preganglionic neurons” (also called
improperly but classically "visceral motoneurons") located in the central nervous system.
Preganglionic sympathetic neurons are in the spinal cord, at thoraco-lumbar levels. Preganglionic
parasympathetic neurons are in the medulla oblongata (forming visceral motor nuclei: the dorsal motor
nucleus of the vagus nerve (dmnX), the nucleus ambiguus, and salivatory nuclei) and in the sacral
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spinal cord. Enteric neurons are also modulated by input from the CNS, from preganglionic neurons
located, like parasympathetic ones, in the medulla oblongata (in the dmnX).
The feedback from the sensory to the motor arm of visceral reflex pathways is provided by direct or
indirect connections between the nucleus of the solitary tract and visceral motoneurons.
Sympathetic and parasympathetic divisions typically function in opposition to each other. But this
opposition is better termed complementary in nature rather than antagonistic. For an analogy, one may
think of the sympathetic division as the accelerator and the parasympathetic division as the brake. The
sympathetic division typically functions in actions requiring quick responses. The parasympathetic
division functions with actions that do not require immediate reaction. Consider sympathetic as "fight
or flight" and parasympathetic as "rest and digest".
However, many instances of sympathetic and parasympathetic activity cannot be ascribed to "fight" or
"rest" situations. For example, standing up from a reclining or sitting position would entail an
unsustainable drop in blood pressure if not for a compensatory increase in the arterial sympathetic
tonus. Another example is the constant, second to second modulation of heart rate by sympathetic and
parasympathetic influences, as a function of the respiratory cycles. More generally, these two systems
should be seen as permanently modulating vital functions, in usually antagonistic fashion, to achieve
homeostasis. Some typical actions of the sympathetic and parasympathetic systems are listed below.
Sympathetic nervous system
Promotes a "fight or flight" response, corresponds with arousal and energy generation, and inhibits
Diverts blood flow away from the gastro-intestinal (GI) tract and skin via vasoconstriction.
Blood flow to skeletal muscles and the lungs is not only maintained, but enhanced (by as much
as 1200% in the case of skeletal muscles).
Dilates bronchioles of the lung, which allows for greater alveolar oxygen exchange.
Increases heart rate and the contractility of cardiac cells (myocytes), thereby providing a
mechanism for the enhanced blood flow to skeletal muscles.
Dilates pupils and relaxes the ciliary muscle to the lens, allowing more light to enter the eye
and far vision.
Provides vasodilation for the coronary vessels of the heart.
Constricts all the intestinal sphincters and the urinary sphincter.
Inhibits peristalsis.
Stimulates orgasm.
Parasympathetic nervous system
Promotes a "rest and digest" response, promotes calming of the nerves return to regular function, and
enhances digestion.
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1. Dilates blood vessels leading to the GI tract, increasing blood flow. This is important following
the consumption of food, due to the greater metabolic demands placed on the body by the gut.
2. The parasympathetic nervous system can also constrict the bronchiolar diameter when the need
for oxygen has diminished.
3. Dedicated cardiac branches of the Vagus and thoracic Spinal Accessory nerves impart
Parasympathetic control of the Heart or Myocardium.
4. During accommodation, the parasympathetic nervous system causes constriction of the pupil
and contraction of the ciliary muscle to the lens, allowing for closer vision.
5. The parasympathetic nervous system stimulates salivary gland secretion, and accelerates
peristalsis, so, in keeping with the rest and digest functions, appropriate PNS activity mediates
digestion of food and indirectly, the absorption of nutrients.
6. Is also involved in erection of genitals, via the pelvic splanchnic nerves 2–4.
7. Stimulates sexual arousal.
Neurotransmitters and pharmacology
At the effector organs, sympathetic ganglionic neurons release noradrenaline (norepinephrine), along
with other cotransmitters such as ATP, to act on adrenergic receptors, with the exception of the sweat
glands and the adrenal medulla:
Acetylcholine is the preganglionic neurotransmitter for both divisions of the ANS, as well as
the postganglionic neurotransmitter of parasympathetic neurons. Nerves that release
acetylcholine are said to be cholinergic. In the parasympathetic system, ganglionic neurons use
acetylcholine as a neurotransmitter, to stimulate muscarinic receptors.
At the adrenal cortex, there is no postsynaptic neuron. Instead the presynaptic neuron releases
acetylcholine to act on nicotinic receptors.
Stimulation of the adrenal medulla releases adrenaline (epinephrine) into the bloodstream
which will act on adrenoceptors, producing a widespread increase in sympathetic activity.
Emergency Theory or Fight-or-flight response
The "fight-or-flight response", also called the emergency theory. Its a set of physiological changes,
such as increases in heart rate, arterial blood pressure, and blood glucose, initiated by the sympathetic
nervous system to mobilize body systems in response to stress.
Physiology of the stress response
Normally, when a person is in a serene, unstimulated state, the "firing" of neurons in the locus
coeruleus is minimal. A novel stimulus (which could include a perception of danger or an
environmental stressor such as elevated sound levels or over-illumination), once perceived, is relayed
from the sensory cortex of the brain through the hypothalamus to the brainstem.
That route of signaling increases the rate of noradrenergic activity in the locus coeruleus, and the
person becomes alert and attentive to the environment. Similarly, an abundance of catecholamines at
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neuroreceptor sites facilitates reliance on spontaneous or intuitive behaviors often related to combat or
If a stimulus is perceived as a threat, a more intense and prolonged discharge of the locus ceruleus
activates the sympathetic division of the autonomic nervous system. This activation is associated with
specific physiological actions in the system, both directly and indirectly through the release of
epinephrine (adrenaline) and to a lesser extent norepinephrine from the medulla of the adrenal glands.
The release is triggered by acetylcholine released from preganglionic sympathetic nerves. The other
major factor in the acute stress response is the hypothalamic-pituitary-adrenal axis.
These catecholamine hormones facilitate immediate physical reactions associated with a preparation
for violent muscular action. (Gleitman, et al., 2008) These include the following:
Acceleration of heart and lung action
Paling or flushing, or alternating between both
Inhibition of stomach and upper-intestinal action (digestion slows down or stops)
General effect on the sphincters of the body
Constriction of blood vessels in many parts of the body
Liberation of nutrients (particularly fat and glucose) for muscular action
Dilation of blood vessels for muscles
Inhibition of the lacrimal gland (responsible for tear production) and salivation
Dilation of pupil (mydriasis)
Relaxation of bladder
Inhibition of erection
Auditory exclusion (loss of hearing)
Tunnel vision (loss of peripheral vision)
Acceleration of instantaneous reflexes
Behavioral manifestations of fight-or-flight
In prehistoric times when the fight or flight response evolved, fight was manifested in aggressive,
combative behavior and flight was manifested by fleeing potentially threatening situations, such as
being confronted by a predator. In current times, these responses persist, but fight and flight responses
have assumed a wider range of behaviors. For example, the fight response may be manifested in
angry, argumentative behavior, and the flight response may be manifested through social withdrawal,
substance abuse, and even television viewing.
Males and females tend to deal with stressful situations differently. Males are more likely to respond
to an emergency situation with aggression (fight), while females are more likely to flee (flight), turn to
others for help, or attempt to defuse the situation – 'tend and befriend'. During stressful times, a mother
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is especially likely to show protective responses toward her offspring and affiliate with others for
shared social responses to threat.
Negative effects of the stress response in humans
The stress response halts or slows down various processes such as sexual responses and digestive
systems to focus on the stressor situation and typically causes negative effects like constipation,
anorexia, erectile dysfunction, difficulty urinating, and difficulty maintaining sexual arousal. These are
functions which are controlled by the parasympathetic nervous system and therefore suppressed by
sympathetic arousal
Prolonged stress responses may result in chronic suppression of the immune system, leaving the body
open to infections, however there is a short boost of the immune system shortly after the fight or flight
response has been activated. This may be due to an ancient need to fight the infections in a wound that
one may have received during interaction with a predator.
Stress responses are sometimes a result of mental disorders such as post-traumatic stress disorder, in
which the individual shows a stress response when remembering a past trauma, and panic disorder, in
which the stress response is activated by the catastrophic misinterpretations of bodily sensations.
Peripheral nervous system by effects
The peripheral nervous system is functionally as well as structurally divided into the somatic nervous
system and autonomic nervous system. The somatic nervous system is responsible for coordinating the
body movements, and also for receiving external stimuli. It is the system that regulates activities that
are under conscious control. The autonomic nervous system is then split into the sympathetic division,
parasympathetic division, and enteric division. The sympathetic nervous system responds to impending
danger, and is responsible for the increase of one's heartbeat and blood pressure, among other
physiological changes, along with the sense of excitement one feels due to the increase of adrenaline
in the system. The parasympathetic nervous system, on the other hand, is evident when a person is
resting and feels relaxed, and is responsible for such things as the constriction of the pupil, the slowing
of the heart, the dilation of the blood vessels, and the stimulation of the digestive and genitourinary
systems. The role of the enteric nervous system is to manage every aspect of digestion, from the
esophagus to the stomach, small intestine and colon.
A polygraph (popularly referred to as a lie detector) is an instrument that measures and records
several physiological indices such as blood pressure, pulse, respiration, breathing rhythms/ratios, and
skin conductivity while the subject is asked and answers a series of questions, in the belief that
deceptive answers will produce physiological responses that can be differentiated from those
associated with non-deceptive answers.
Polygraphy is widely rejected as pseudoscience by the scientific community. Nonetheless, polygraphs
are in some countries used as an interrogation tool with criminal suspects or candidates for sensitive
public or private sector employment. US federal government agencies such as the FBI and the CIA
and many police departments such as the LAPD use polygraph examinations to interrogate suspects
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and screen new employees. Within the US federal government, a polygraph examination is also
referred to as a psychophysiological detection of deception (PDD) examination.
The idea that lying produces physical side-effects has long been claimed. In West Africa persons
suspected of a crime were made to pass a bird's egg to one another. If a person broke the egg, then he
or she was considered guilty, based on the idea that their nervousness was to blame. In ancient China
the suspect held a handful of rice in his or her mouth during a prosecutor's speech. Because salivation
was believed to cease at times of emotional anxiety, the person was considered guilty if by the end of
that speech the rice was dry.
Early devices for lie detection include an 1885 invention of Cesare Lombroso used to measure
changes in blood pressure for police cases, a 1904 device by Vittorio Benussi used to measure
breathing, and an abandoned project by American William Marston which used blood pressure and
galvanic skin response to examine German prisoners of war (POWs).
Sir James Mackenzie of Scone, Scotland invented an early lie detector or polygraph in the 1900s.
MacKenzie's polygraph "could be used to monitor the cardiovascular responses of his patients by
taking their pulse and blood pressure. He had developed an early version of his device in the 1890s,
but had Sebastian Shaw, a Lancashire watchmaker, improve it further. "This instrument used a
clockwork mechanism for the paper-rolling and time-marker movements and it produced ink
recordings of physiological functions that were easier to acquire and to interpret. Interestingly, it has
been written that the modern polygraph is really a modification of Dr. Mackenzie's clinical ink
A device recording both blood pressure and galvanic skin response was invented in 1911 by Dr.
Reginald A. Larson of the University of California and first applied in law enforcement work by the
Berkeley Police Department under its nationally renowned police chief August Vollmer. Further work
on this device was done by Leonarde Keeler.
Several devices similar to Keeler's polygraph version included the Berkeley Psychograph, a blood
pressure-pulse-respiration recorder developed by C. D. Lee in 1936and the Darrow Behavior Research
Photopolygraph, which was developed and intended solely for behavior research experiments.
Marston wrote a second paper on the concept in 1915, when finishing his undergraduate studies. He
entered Harvard Law School and graduated in 1918, re-publishing his earlier work in 1917. According
to their son, Marston's wife, Elizabeth Holloway Marston, was also involved in the development of the
systolic blood pressure test: "According to Marston’s son, it was his mother Elizabeth, Marston’s wife,
who suggested to him that 'When she got mad or excited, her blood pressure seemed to climb' (Lamb,
2001). Although Elizabeth is not listed as Marston’s collaborator in his early work, Lamb, Matte
(1996), and others refer directly and indirectly to Elizabeth’s work on her husband’s deception
research. She also appears in a picture taken in his polygraph laboratory in the 1920s (reproduced in
Marston, 1938)." The comic book character, Wonder Woman, by William Marston (and influenced by
Elizabeth Marstoncarries a magic lasso which was modelled upon the pneumograph (breathing
monitor) test.
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Marston was the self-proclaimed “father of the polygraph” despite his predecessor's contributions.
Marston remained the device's primary advocate, lobbying for its use in the courts. In 1938 he
published a book, The Lie Detector Test, wherein he documented the theory and use of the device. In
1938 he appeared in advertising by the Gillette company claiming that the polygraph showed Gillette
razors were better than the competition.
A device which recorded muscular activity accompanying changes in blood pressure was developed in
1945 by John E. Reid, who claimed that greater accuracy could be obtained by making these
recordings simultaneously with standard blood pressure-pulse-respiration recordings.
Testing procedure
Today, polygraph examiners use two types of instrumentation: analog and computerized. In the United
States, most examiners now use computerized instrumentation.
A typical polygraph test starts with a pre-test interview to gain some preliminary information which
will later be used for "control questions", or CQ. Then the tester will explain how the polygraph is
supposed to work, emphasizing that it can detect lies and that it is important to answer truthfully. Then
a "stim test" is often conducted: the subject is asked to deliberately lie and then the tester reports that
he was able to detect this lie. Then the actual test starts. Some of the questions asked are "irrelevant"
or IR ("Is your name Chris Drozdz?"), others are "probable-lie" control questions that most people will
lie about ("Have you ever stolen money?") and the remainder are the "relevant questions", or RQ, that
the tester is really interested in. The different types of questions alternate. The test is passed if the
physiological responses during the probable-lie control questions (CQ) are larger than those during the
relevant questions (RQ). If this is not the case, the tester attempts to elicit admissions during a posttest interview, for example, "Your situation will only get worse if we don't clear this up".
Criticisms have been given regarding the validity of the administration of the Comparative Questions
test (CQT). The CQT may be vulnerable to being conducted in an interrogation-like fashion. This kind
of interrogation style would elicit a nervous response from innocent and guilty suspects alike. There
are several other ways of administrating the questions.
An alternative is the Guilty Knowledge test (GKT), or the Concealed Information Test (CIT). The
administration of this test is given to prevent potential errors that may arise from the questioning style.
The test is usually conducted by a tester with no knowledge of the crime or circumstances in question.
The administrator tests the participant on their knowledge of the crime that would not be known to an
innocent person. For example: "Was the crime committed with a .45 or a 9 mm?" The questions are in
multiple choice and the participant is rated on how they react to the correct answer. If they react
strongly to the guilty information, then proponents of the test believe that it is likely that they know
facts relevant to the case. This administration is considered more valid by supporters of the test
because it contains many safeguards to avoid the risk of the administrator influencing the results.
Polygraphy has little credibility among scientists. Despite claims of 90-95% validity by polygraph
advocates, and 95-100% by businesses providing polygraph services, critics maintain that rather than a
"test", the method amounts to an inherently unstandardizable interrogation technique whose accuracy
cannot be established. A 1997 survey of 421 psychologists estimated the test's average accuracy at
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about 61%, a little better than chance. Critics also argue that even given high estimates of the
polygraph's accuracy a significant number of subjects (e.g. 10% given a 90% accuracy) will appear to
be lying, and would unfairly suffer the consequences of "failing" the polygraph. In the 1998 Supreme
Court case, United States v. Scheffer, the majority stated that "There is simply no consensus that
polygraph evidence is reliable" and "Unlike other expert witnesses who testify about factual matters
outside the jurors' knowledge, such as the analysis of fingerprints, ballistics, or DNA found at a crime
scene, a polygraph expert can supply the jury only with another opinion..." Also, in 2005 the 11th
Circuit Court of Appeals stated that “polygraphy did not enjoy general acceptance from the scientific
community”. Charles Honts, a psychology professor at Boise State University, states that polygraph
interrogations give a high rate of false positives on innocent people. In 2001 William G. Iacono,
Distinguished McKnight University Professor of Psychology and Neuroscience and Director, Clinical
Science and Psychopathology Research Training Program at the University of Minnesota, published a
paper titled “Forensic “Lie Detection": Procedures Without Scientific Basis” in the peer reviewed
Journal of Forensic Psychology Practice. He concluded that
Although the CQT [Control Question Test] may be useful as an investigative aid and tool to induce
confessions, it does not pass muster as a scientifically credible test. CQT theory is based on naive,
implausible assumptions indicating (a) that it is biased against innocent individuals and (b) that it can
be beaten simply by artificially augmenting responses to control questions. Although it is not possible
to adequately assess the error rate of the CQT, both of these conclusions are supported by published
research findings in the best social science journals (Honts et al., 1994; Horvath, 1977; Kleinmuntz &
Szucko, 1984; Patrick & Iacono, 1991). Although defense attorneys often attempt to have the results
of friendly CQTs admitted as evidence in court, there is no evidence supporting their validity and
ample reason to doubt it. Members of scientific organizations who have the requisite background to
evaluate the CQT are overwhelmingly skeptical of the claims made by polygraph proponents.
Summarizing the consensus in psychological research, professor David W. Martin, PhD, from North
Carolina State University, states that people have tried to use the polygraph for measuring human
emotions, but there is simply no royal road to (measuring) human emotions. Therefore, since one
cannot reliably measure human emotions (especially when one has an interest in hiding his/her
emotions), the idea of valid detection of truth or falsehood through measuring respiratory rate, blood
volume, pulse rate and galvanic skin response is a mere pretense. Since psychologists cannot ascertain
what emotions one has, polygraph professionals are not able to do that either.
Polygraphy has also been faulted for failing to trap known spies such as double-agent Aldrich Ames,
who passed two polygraph tests while spying for the Soviet Union. Other spies who passed the
polygraph include Karl Koecher, Ana Belen Montes, and Leandro Aragoncillo. However, CIA spy
Harold James Nicholson failed his polygraph examinations, which aroused suspicions that led to his
eventual arrest. Polygraph examination and background checks failed to detect Nada Nadim Prouty,
who was not a spy but was convicted for improperly obtaining US citizenship and using it to obtain a
restricted position at the FBI.
The polygraph also failed to catch Gary Ridgway, the "Green River Killer". Ridgway passed a
polygraph in 1984 and confessed almost 20 years later when confronted with DNA evidence.
Conversely, innocent people have been known to fail polygraph tests. In Wichita, Kansas in 1986,
after failing two polygraph tests (one police administered, the other given by an expert that he had
hired), Bill Wegerle had to live under a cloud of suspicion of murdering his wife Vicki Wegerle, even
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though he was neither arrested nor convicted of her death. In March 2004, a letter was sent to The
Wichita Eagle reporter Hurst Laviana that contained Vicki's drivers license and what first appeared to
be crime scene photographs of her body. The photos had actually been taken by her true murderer,
BTK, the serial killer that had plagued the people of Wichita since 1974 and had recently resurfaced in
February 2004 after an apparent 25 year period of dormancy (he had actually killed three women
between 1985 and 1991, including Wegerle). That effectively cleared Bill Wegerle of the murder of
his wife. In 2005 conclusive DNA evidence including DNA retrieved from under the fingernails of
Vicki Wegerle, demonstrated that the BTK Killer was Dennis Rader
Prolonged polygraph examinations are sometimes used as a tool by which confessions are extracted
from a defendant, as in the case of Richard Miller, who was persuaded to confess largely by polygraph
results combined with appeals from a religious leader.
Law enforcement agencies and intelligence agencies in the United States are by far the biggest users of
polygraph technology. In the United States alone all federal law enforcement agencies either employ
their own polygraph examiners or use the services of examiners employed in other agencies.
This is despite persistent claims of unreliability. For example in 1978 Richard Helms, the 8th Director
of Central Intelligence, stated that:
"We discovered there were some Eastern Europeans who could defeat the polygraph at any time.
Americans are not very good at it, because we are raised to tell the truth and when we lie it is easy to
tell are lying. But we find a lot of Europeans and Asiatics can handle that polygraph without a blip,
and you know they are lying and you have evidence
Several countermeasures designed to pass polygraph tests have been described. Asked how he passed
the polygraph test, Ames explained that he sought advice from his Soviet handler and received the
simple instruction to: "Get a good night's sleep, and rest, and go into the test rested and relaxed. Be
nice to the polygraph examiner, develop a rapport, and be cooperative and try to maintain your calm."
Other suggestions for countermeasures include for the subject to mentally record the control and
relevant questions as the examiner reviews them prior to commencing the interrogation. Once the
interrogation begins, the subject is then supposed to carefully control their breathing during the
relevant questions, and to try to artificially increase their heart rate during the control questions, such
as by thinking of something scary or exciting or by pricking themselves with a pointed object
concealed somewhere on their body. In this way the results will not show a significant reaction to any
of the relevant questions.
2003 National Academy of Sciences report
The accuracy of the polygraph has been contested almost since the introduction of the device. In 2003,
the National Academy of Sciences (NAS) issued a report entitled "The Polygraph and Lie Detection".
The NAS found that the majority of polygraph research was "Unreliable, Unscientific and Biased",
concluding that 57 of the approximately 80 research studies that the APA relies on to come to their
conclusions were significantly flawed. These studies concluded that a polygraph test regarding a
specific incident can discern the truth at "a level greater than chance, yet short of perfection" though
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NAS did restrict almost all of its conclusions to polygraph usage for "security screening" purposes. It
did not focus on forensic applications, polygraph testing commonly plays a role in helping to resolve
criminal investigations. The report also concluded that this level of accuracy was overstated and the
levels of accuracy shown in these studies "are almost certainly higher than actual polygraph accuracy
of specific-incident testing in the field."
When polygraphs are used as a screening tool (in national security matters and for law enforcement
agencies for example) the level of accuracy drops to such a level that "Its accuracy in distinguishing
actual or potential security violators from innocent test takers is insufficient to justify reliance on its
use in employee security screening in federal agencies." In fact, the NAS extrapolated that if the test
were sensitive enough to detect 80% of spies (a level of accuracy which it did not assume), this would
hardly be sufficient anyway. Let us take for example a hypothetical polygraph screening of a body of
10,000 employees among which are 10 spies. With an 80% success rate, the polygraph test would
show that 8 spies and 1,992 non-spies fail the test. Thus, roughly 99.6 percent of positives (those
failing the test) would be false positives. The NAS concluded that the polygraph "...may have some
utility" but that there is "little basis for the expectation that a polygraph test could have extremely high
The NAS conclusions paralleled those of the earlier United States Congress Office of Technology
Assessment report "Scientific Validity of Polygraph Testing: A Research Review and Evaluation”.
Admissibility of polygraphs in court
Recently an Indian court adopted the brain electrical oscillations signature test as evidence to convict a
woman, who was accused of murdering her fiance. It is the first time that the result of polygraph was
used as evidence in court. On May 5, 2010, The Supreme Court of India declared use of narcoanalysis,
brain mapping and polygraph tests on suspects as illegal and as against constitution. Article 20(3) of
the Indian Constitution-"No person accused of any offence shall be compelled to be a witness against
himself.". But with the consent of the suspect it may be done
Autonomic balance
Autonomic balance is your own body’s internal balance system void of conscious control. Much like
the other systems of your body, including your heart, lung, kidney, and digestive tract, your balance is
under the control of the autonomic nervous system.
This specific autonomic balance system is similar to two GPS systems lying side by side within your
skull; each expressing to your brain where you are in space. These specialized organs are called
semicircular canals and are self regulating and function optimally when your temporal bones are
perfectly positioned. Misalignment of these bones can produce extreme balance disorders, like vertigo,
Menieres disease, etc.
The brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some
primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain,
while sponges lack any nervous system at all. In vertebrates, the brain is located in the head, protected
by the skull and close to the primary sensory apparatus of vision, hearing, balance, taste, and smell.
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Brains can be extremely complex. The cerebral cortex of the human brain contains roughly 15–33
billion neurons, perhaps more, depending on gender and age, linked with up to 10,000 synaptic
connections each. Each cubic millimeter of cerebral cortex contains roughly one billion synapses.
These neurons communicate with one another by means of long protoplasmic fibers called axons,
which carry trains of signal pulses called action potentials to distant parts of the brain or body and
target them to specific recipient cells.
The brain controls the other organ systems of the body, either by activating muscles or by causing
secretion of chemicals such as hormones. This centralized control allows rapid and coordinated
responses to changes in the environment. Some basic types of responsiveness are possible without a
brain: even single-celled organisms may be capable of extracting information from the environment
and acting in response to it. Sponges, which lack a central nervous system, are capable of coordinated
body contractions and even locomotion. In vertebrates, the spinal cord by itself contains neural
circuitry capable of generating reflex responses as well as simple motor patterns such as swimming or
walking. However, sophisticated control of behavior on the basis of complex sensory input requires
the information-integrating capabilities of a centralized brain.
Despite rapid scientific progress, much about how brains work remains a mystery. The operations of
individual neurons and synapses are now understood in considerable detail, but the way they cooperate
in ensembles of thousands or millions has been very difficult to decipher. Methods of observation such
as EEG recording and functional brain imaging tell us that brain operations are highly organized,
while single unit recording can resolve the activity of single neurons, but how individual cells give rise
to complex operations is unknown.
Vertebrate brain regions
Neuroanatomists usually consider the brain to consist of six main regions: the telencephalon (cerebral
hemispheres), diencephalon (thalamus and hypothalamus), mesencephalon (midbrain), cerebellum,
pons, and medulla oblongata. Each of these areas in turn has a complex internal structure. Some areas,
such as the cortex and cerebellum, consist of layers, folded or convoluted to fit within the available
space. Other areas consist of clusters of many small nuclei. If fine distinctions are made on the basis of
neural structure, chemistry, and connectivity, thousands of distinguishable areas can be identified
within the vertebrate brain.
Some branches of vertebrate evolution have led to substantial changes in brain shape, especially in the
forebrain. The brain of a shark shows the basic components in a straightforward way, but in teleost
fishes (the great majority of modern species), the forebrain has become "everted", like a sock turned
inside out. In birds, also, there are major changes in shape. One of the main structures in the avian
forebrain, the dorsal ventricular ridge, was long thought to correspond to the basal ganglia of
mammals, but is now thought to be more closely related to the neocortex.
Several brain areas have maintained their identities across the whole range of vertebrates, from
hagfishes to humans. Here is a list of some of the most important areas, along with a very brief
description of their functions as currently understood (but note that the functions of most of them are
still disputed to some degree):
The medulla, along with the spinal cord, contains many small nuclei involved in a wide variety
of sensory and motor functions.
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The hypothalamus is a small region at the base of the forebrain, whose complexity and
importance belies its size. It is composed of numerous small nuclei, each with distinct
connections and distinct neurochemistry. The hypothalamus is the central control station for
sleep/wake cycles, control of eating and drinking, control of hormone release, and many other
critical biological functions.
Like the hypothalamus, the thalamus is a collection of nuclei with diverse functions. Some of
them are involved in relaying information to and from the cerebral hemispheres. Others are
involved in motivation. The subthalamic area (zona incerta) seems to contain action-generating
systems for several types of "consummatory" behaviors, including eating, drinking, defecation,
and copulation.
The cerebellum modulates the outputs of other brain systems to make them more precise.
Removal of the cerebellum does not prevent an animal from doing anything in particular, but it
makes actions hesitant and clumsy. This precision is not built-in, but learned by trial and error.
Learning how to ride a bicycle is an example of a type of neural plasticity that may take place
largely within the cerebellum.
The tectum, often called "optic tectum", allows actions to be directed toward points in space. In
mammals it is called the "superior colliculus", and its best studied function is to direct eye
movements. It also directs reaching movements, though. It gets strong visual inputs, but also
inputs from other senses that are useful in directing actions, such as auditory input in owls,
input from the thermosensitive pit organs in snakes, etc. In some fishes, such as lampreys, it is
the largest part of the brain.
The pallium is a layer of gray matter that lies on the surface of the forebrain. In reptiles and
mammals it is called cortex instead. The pallium is involved in multiple functions, including
olfaction and spatial memory. In mammals, where it comes to dominate the brain, it subsumes
functions from many subcortical areas.
The hippocampus, strictly speaking, is found only in mammals. However, the area it derives
from, the medial pallium, has counterparts in all vertebrates. There is evidence that this part of
the brain is involved in spatial memory and navigation in fishes, birds, reptiles, and mammals.
The basal ganglia are a group of interconnected structures in the forebrain, of which our
understanding has increased enormously over the last few years. The primary function of the
basal ganglia seems to be action selection. They send inhibitory signals to all parts of the brain
that can generate actions, and in the right circumstances can release the inhbition, so that the
action-generating systems are able to execute their actions. Rewards and punishments exert
their most important neural effects within the basal ganglia.
The olfactory bulb is a special structure that processes olfactory sensory signals, and sends its
output to the olfactory part of the pallium. It is a major brain component in many vertebrates,
but much reduced in primates.
The cerebral cortex is the part of the brain that most strongly distinguishes mammals from other
vertebrates, primates from other mammals, and humans from other primates. The hindbrain and
midbrain of mammals are generally similar to those of other vertebrates, but dramatic differences
appear in the forebrain, which is not only greatly enlarged, but also altered in structure. In nonmammalian vertebrates, the surface of the cerebrum is lined with a comparatively simple layered
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structure called the pallium. In mammals, the pallium evolves into a complex 6-layered structure
called neocortex or isocortex. In primates, the neocortex is greatly enlarged, especially the part called
the frontal lobes. In humans, this enlargement of the frontal lobes is taken to an extreme, and other
parts of the cortex also become quite large and complex. Also the hippocampus of mammals has a
distinctive structure.
Unfortunately, the evolutionary history of these mammalian features, especially the 6-layered cortex,
is difficult to trace. This is largely because of a missing link problem. The ancestors of mammals,
called synapsids, split off from the ancestors of modern reptiles and birds about 350 million years ago.
However, the most recent branching that has left living results within the mammals was the split
between monotremes (the platypus and echidna), marsupials (opossum, kangaroo, etc.) and placentals
(most living mammals), which took place about 120 million years ago. The brains of monotremes and
marsupials are distinctive from those of placentals in some ways, but they have fully mammalian
cortical and hippocampal structures. Thus, these structures must have evolved between 350 and 120
million years ago, a period that has left no evidence except fossils, which do not preserve tissue as soft
as brain.
Primates, including humans
The primate brain contains the same structures as the brains of other mammals, but is considerably
larger in proportion to body size. Most of the enlargement comes from a massive expansion of the
cortex, focusing especially on the parts subserving vision and forethought. The visual processing
network of primates is very complex, including at least 30 distinguishable areas, with a bewildering
web of interconnections. Taking all of these together, visual processing makes use of about half of the
brain. The other part of the brain that is greatly enlarged is the prefrontal cortex, whose functions are
difficult to summarize succinctly, but relate to planning, working memory, motivation, attention, and
executive control.
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from
the brain (the medulla specifically). The brain and spinal cord together make up the central nervous
system. The spinal cord extends down to the space between the first and second lumbar vertebrae; it
does not extend the entire length of the vertebral column. It is around 45 cm (18 in) in men and around
43 cm (17 in) long in women. The enclosing bony vertebral column protects the relatively shorter
spinal cord. The spinal cord functions primarily in the transmission of neural signals between the brain
and the rest of the body but also contains neural circuits that can independently control numerous
reflexes and central pattern generators. The spinal cord has three major functions: A. Serve as a
conduit for motor information, which travels down the spinal cord. B. Serve as a conduit for sensory
information, which travels up the spinal cord. C. Serve as a center for coordinating certain reflexes.
The spinal cord is the main pathway for information connecting the brain and peripheral nervous
system. The length of the spinal cord is much shorter than the length of the bony spinal column. The
human spinal cord extends from the medulla oblongata and continues through the conus medullaris
near the first or second lumbar vertebra, terminating in a fibrous extension known as the filum
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It is about 45 cm (18 in) long in men and around 43 cm (17 in) in women, ovoid-shaped, and is
enlarged in the cervical and lumbar regions. The cervical enlargement, located from C4 to T1, is
where sensory input comes from and motor output goes to the arms. The lumbar enlargement, located
between T9 and T12, handles sensory input and motor output coming from and going to the legs. You
should notice that the name is somewhat misleading. However, this region of the cord does indeed
have branches that extend to the lumbar region.
In cross-section, the peripheral region of the cord contains neuronal white matter tracts containing
sensory and motor neurons. Internal to this peripheral region is the gray, butterfly-shaped central
region made up of nerve cell bodies. This central region surrounds the central canal, which is an
anatomic extension of the spaces in the brain known as the ventricles and, like the ventricles, contains
cerebrospinal fluid.
The spinal cord has a shape that is compressed dorso-ventrally, giving it an elliptical shape. The cord
has grooves in the dorsal and ventral sides. The posterior median sulcus is the groove in the dorsal
side, and the anterior median fissure is the groove in the ventral side. Running down the center of the
spinal cord is a cavity, called the central canal.
The three meninges that cover the spinal cord—the outer dura mater, the arachnoid mater, and the
innermost pia mater—are continuous with that in the brainstem and cerebral hemispheres. Similarly,
cerebrospinal fluid is found in the subarachnoid space. The cord is stabilized within the dura mater by
the connecting denticulate ligaments, which extend from the enveloping pia mater laterally between
the dorsal and ventral roots. The dural sac ends at the vertebral level of the second sacral vertebra.
The spinal cord is protected by three layers of tissue, called spinal meninges, that surround the cord.
The dura mater is the outermost layer, and it forms a tough protective coating. Between the dura
mater and the surrounding bone of the vertebrae is a space, called the epidural space. The epidural
space is filled with adipose tissue, and it contains a network of blood vessels. The arachnoid is the
middle protective layer. Its name comes from the fact that the tissue has a spiderweb-like appearance.
The space between the arachnoid and the underlyng pia mater is called the subarachnoid space. The
subarachnoid space contains cerebrospinal fluid (CSF). The medical procedure known as a “spinal
tap” involves use of a needle to withdraw CSF from the subarachnoid space, usually from the lumbar
region of the spine. The pia mater is the innermost protective layer. It is very delicate and it is tightly
associated with the surface of the spinal cord.
Spinal cord segments
The human spinal cord is divided into 31 different segments. At every segment, right and left pairs of
spinal nerves (mixed; sensory and motor) form. Six to eight motor nerve rootlets branch out of right
and left ventro lateral sulci in a very orderly manner. Nerve rootlets combine to form nerve roots.
Likewise, sensory nerve rootlets form off right and left dorsal lateral sulci and form sensory nerve
roots. The ventral (motor) and dorsal (sensory) roots combine to form spinal nerves (mixed; motor and
sensory), one on each side of the spinal cord. Spinal nerves, with the exception of C1 and C2, form
inside intervertebral foramen (IVF). Note that at each spinal segment, the border between the central
and peripheral nervous system can be observed. Rootlets are a part of the peripheral nervous system.
In the upper part of the vertebral column, spinal nerves exit directly from the spinal cord, whereas in
the lower part of the vertebral column nerves pass further down the column before exiting. The
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terminal portion of the spinal cord is called the conus medullaris. The pia mater continues as an
extension called the filum terminale, which anchors the spinal cord to the coccyx. The cauda equina
(“horse’s tail”) is the name for the collection of nerves in the vertebral column that continue to travel
through the vertebral column below the conus medullaris. The cauda equina forms as a result of the
fact that the spinal cord stops growing in length at about age four, even though the vertebral column
continues to lengthen until adulthood. This results in the fact that sacral spinal nerves actually
originate in the upper lumbar region. The spinal cord can be anatomically divided into 31 spinal
segments based on the origins of the spinal nerves.
Each segment of the spinal cord is associated with a pair of ganglia, called dorsal root ganglia, which
are situated just outside of the spinal cord. These ganglia contain cell bodies of sensory neurons.
Axons of these sensory neurons travel into the spinal cord via the dorsal roots.
Ventral roots consist of axons from motor neurons, which bring information to the periphery from cell
bodies within the CNS. Dorsal roots and ventral roots come together and exit the intervertebral
foramina as they become spinal nerves.
The gray matter, in the center of the cord, is shaped like a butterfly and consists of cell bodies of
interneurons and motor neurons. It also consists of neuroglia cells and unmyelinated axons.
Projections of the gray matter (the “wings”) are called horns. Together, the gray horns and the gray
commissure form the “gray H.”
The white matter is located outside of the gray matter and consists almost totally of myelinated motor
and sensory axons. “Columns” of white matter carry information either up or down the spinal cord.
Within the CNS, nerve cell bodies are generally organized into functional clusters, called nuclei.
Axons within the CNS are grouped into tracts.
There are 33 (some EMS text say 25, counting the sacral as one solid piece) spinal cord nerve
segments in a human spinal cord:
8 cervical segments forming 8 pairs of cervical nerves (C1 spinal nerves exit spinal column
between occiput and C1 vertebra; C2 nerves exit between posterior arch of C1 vertebra and
lamina of C2 vertebra; C3-C8 spinal nerves through IVF above corresponding cervica vertebra,
with the exception of C8 pair which exit via IVF between C7 and T1 vertebra)
12 thoracic segments forming 12 pairs of thoracic nerves (exit spinal column through IVF
below corresponding vertebra T1-T12)
5 lumbar segments forming 5 pairs of lumbar nerves (exit spinal column through IVF, below
corresponding vertebra L1-L5)
5 (or 1) sacral segments forming 5 pairs of sacral nerves (exit spinal column through IVF,
below corresponding vertebra S1-S5)
3 coccygeal segments joined up becoming a single segment forming 1 pair of coccygeal nerves
(exit spinal column through the sacral hiatus).
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Because the vertebral column grows longer than the spinal cord, spinal cord segments do not
correspond to vertebral segments in adults, especially in the lower spinal cord. In the fetus, vertebral
segments do correspond with spinal cord segments. In the adult, however, the spinal cord ends around
the L1/L2 vertebral level, forming a structure known as the conus medullaris. For example, lumbar
and sacral spinal cord segments are found between vertebral levels T9 and L2.
Although the spinal cord cell bodies end around the L1/L2 vertebral level, the spinal nerves for each
segment exit at the level of the corresponding vertebra. For the nerves of the lower spinal cord, this
means that they exit the vertebral column much lower (more caudally) than their roots. As these
nerves travel from their respective roots to their point of exit from the vertebral column, the nerves of
the lower spinal segments form a bundle called the cauda equina.
There are two regions where the spinal cord enlarges:
Cervical enlargement - corresponds roughly to the brachial plexus nerves, which innervate the
upper limb. It includes spinal cord segments from about C4 to T1. The vertebral levels of the
enlargement are roughly the same (C4 to T1).
Lumbosacral enlargement - corresponds to the lumbosacral plexus nerves, which innervate the
lower limb. It comprises the spinal cord segments from L2 to S3 and is found about the
vertebral levels of T9 to T12.
The spinal cord is made from part of the neural tube during development. As the neural tube begins to
develop, the notochord begins to secrete a factor known as Sonic hedgehog or SHH. As a result, the
floor plate then also begins to secrete SHH, and this will induce the basal plate to develop motor
neurons. Meanwhile, the overlying ectoderm secretes bone morphogenetic protein (BMP). This
induces the roof plate to begin to secrete BMP, which will induce the alar plate to develop sensory
neurons. The alar plate and the basal plate are separated by the sulcus limitans.
Additionally, the floor plate also secretes netrins. The netrins act as chemoattractants to decussation of
pain and temperature sensory neurons in the alar plate across the anterior white commissure, where
they then ascend towards the thalamus.
Lastly, it is important to note that the past studies of Viktor Hamburger and Rita Levi-Montalcini in
the chick embryo have been further proven by more recent studies which demonstrated that the
elimination of neuronal cells by programmed cell death (PCD) is necessary for the correct assembly of
the nervous system.
Overall, spontaneous embryonic activity has been shown to play a role in neuron and muscle
development but is probably not involved in the initial formation of connections between spinal
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Somatosensory organization
Somatosensory organization is divided into the dorsal column-medial lemniscus tract (the
touch/proprioception/vibration sensory pathway) and the anterolateral system, or ALS (the
pain/temperature sensory pathway). Both sensory pathways use three different neurons to get
information from sensory receptors at the periphery to the cerebral cortex. These neurons are
designated primary, secondary and tertiary sensory neurons. In both pathways, primary sensory neuron
cell bodies are found in the dorsal root ganglia, and their central axons project into the spinal cord.
In the dorsal column-medial leminiscus tract, a primary neuron's axon enters the spinal cord and then
enters the dorsal column. If the primary axon enters below spinal level T6, the axon travels in the
fasciculus gracilis, the medial part of the column. If the axon enters above level T6, then it travels in
the fasciculus cuneatus, which is lateral to the fasiculus gracilis. Either way, the primary axon ascends
to the lower medulla, where it leaves its fasiculus and synapses with a secondary neuron in one of the
dorsal column nuclei: either the nucleus gracilis or the nucleus cuneatus, depending on the pathway it
took. At this point, the secondary axon leaves its nucleus and passes anteriorly and medially. The
collection of secondary axons that do this are known as internal arcuate fibers. The internal arcuate
fibers decussate and continue ascending as the contralateral medial lemniscus. Secondary axons from
the medial lemniscus finally terminate in the ventral posterolateral nucleus (VPL) of the thalamus,
where they synapse with tertiary neurons. From there, tertiary neurons ascend via the posterior limb of
the internal capsule and end in the primary sensory cortex.
The anterolateral system works somewhat differently. Its primary neurons enter the spinal cord and
then ascend one to two levels before synapsing in the substantia gelatinosa. The tract that ascends
before synapsing is known as Lissauer's tract. After synapsing, secondary axons decussate and ascend
in the anterior lateral portion of the spinal cord as the spinothalamic tract. This tract ascends all the
way to the VPL, where it synapses on tertiary neurons. Tertiary neuronal axons then travel to the
primary sensory cortex via the posterior limb of the internal capsule.
It should be noted that some of the "pain fibers" in the ALS deviate from their pathway towards the
VPL. In one such deviation, axons travel towards the reticular formation in the midbrain. The reticular
formation then projects to a number of places including the hippocampus (to create memories about
the pain), the centromedian nucleus (to cause diffuse, non-specific pain) and various parts of the
cortex. Additionally, some ALS axons project to the periaqueductal gray in the pons, and the axons
forming the periaqueductal gray then project to the nucleus raphe magnus, which projects back down
to where the pain signal is coming from and inhibits it. This helps control the sensation of pain to
some degree.
Motor organization
The corticospinal tract serves as the motor pathway for upper motor neuronal signals coming from the
cerebral cortex and from primitive brainstem motor nuclei.
Cortical upper motor neurons originate from Brodmann areas 1, 2, 3, 4, and 6 and then descend in the
posterior limb of the internal capsule, through the crus cerebri, down through the pons, and to the
medullary pyramids, where about 90% of the axons cross to the contralateral side at the decussation of
the pyramids. They then descend as the lateral corticospinal tract. These axons synapse with lower
motor neurons in the ventral horns of all levels of the spinal cord. The remaining 10% of axons
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descend on the ipsilateral side as the ventral corticospinal tract. These axons also synapse with lower
motor neurons in the ventral horns. Most of them will cross to the contralateral side of the cord (via
the anterior white commissure) right before synapsing.
The midbrain nuclei include four motor tracts that send upper motor neuronal axons down the spinal
cord to lower motor neurons. These are the rubrospinal tract, the vestibulospinal tract, the tectospinal
tract and the reticulospinal tract. The rubrospinal tract descends with the lateral corticospinal tract, and
the remaining three descend with the anterior corticospinal tract.
The function of lower motor neurons can be divided into two different groups: the lateral corticospinal
tract and the anterior cortical spinal tract. The lateral tract contains upper motor neuronal axons which
synapse on dorsal lateral (DL) lower motor neurons. The DL neurons are involved in distal limb
control. Therefore, these DL neurons are found specifically only in the cervical and lumbosaccral
enlargements within the spinal cord. There is no decussation in the lateral corticospinal tract after the
decussation at the medullary pyramids.
the proprioception of lower limb is something different from upper limb & upper trunk.there is a 4
neuron pathaway for lower limbs proprioception.this pathway initiaLLY follow with dorsal spino
cerebellar path way. the pathway proprioceptive receptors of lower limb -> peripheral process ->
dorsal root ganglion -> central process -> clarks column -> 2nd order neuron -> medulla oblogata
(neucleus z of broadal) -> 3rd order neuron -> VPL of thalamus -> 4th order neuron -> posterior limb
of internal capsule -> corona radiata -> sensory area of cerebrum.
The anterior corticospinal tract descends ipsilaterally in the anterior column, where the axons emerge
and either synapse on lower ventromedial (VM) motor neurons in the ventral horn ipsilaterally or
descussate at the anterior white commissure where they synapse on VM lower motor neurons
contralaterally . The tectospinal, vestibulospinal and reticulospinal descend ipsilaterally in the anterior
column but do not synapse across the anterior white commissure. Rather, they only synapse on VM
lower motor neurons ipsilaterally. The VM lower motor neurons control the large, postural muscles of
the axial skeleton. These lower motor neurons, unlike those of the DL, are located in the ventral horn
all the way throughout the spinal cord.
Spinocerebellar tracts
Proprioceptive information in the body travels up the spinal cord via three tracts. Below L2, the
proprioceptive information travels up the spinal cord in the ventral spinocerebellar tract. Also known
as the anterior spinocerebellar tract, sensory receptors take in the information and travel into the spinal
cord. The cell bodies of these primary neurons are located in the dorsal root ganglia. In the spinal cord,
the axons synapse and the secondary neuronal axons decussate and then travel up to the superior
cerebellar peduncle where they decussate again. From here, the information is brought to deep nuclei
of the cerebellum including the fastigial and interposed nuclei.
From the levels of L2 to T1, proprioceptive information enters the spinal cord and ascends
ipsilaterally, where it synapses in Clarke's nucleus. The secondary neuronal axons continue to ascend
ipsilaterally and then pass into the cerebellum via the inferior cerebellar peduncle. This tract is known
as the dorsal spinocerebellar tract.
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From above T1, proprioceptive primary axons enter the spinal cord and ascend ipsilaterally until
reaching the accessory cuneate nucleus, where they synapse. The secondary axons pass into the
cerebellum via the inferior cerebellar peduncle where again, these axons synapse on cerebellar deep
nuclei. This tract is known as the cuneocerebellar tract.
Motor information travels from the brain down the spinal cord via descending spinal cord tracts.
Descending tracts involve two neurons: the upper motor neuron (UMN) and lower motor neuron
(LMN). A nerve signal travels down the upper motor neuron until it synapses with the lower motor
neuron in the spinal cord. Then, the lower motor neuron conducts the nerve signal to the spinal root
where efferent nerve fibers carry the motor signal toward the target muscle. The descending tracts are
composed of white matter. There are several descending tracts serving different functions. The
corticospinal tracts (lateral and anterior) are responsible for coordinated limb movements.
Spinal cord injuries can be caused by trauma to the spinal column (stretching, bruising, applying
pressure, severing, laceration, etc.). The vertebral bones or intervertebral disks can shatter, causing the
spinal cord to be punctured by a sharp fragment of bone. Usually, victims of spinal cord injuries will
suffer loss of feeling in certain parts of their body. In milder cases, a victim might only suffer loss of
hand or foot function. More severe injuries may result in paraplegia, tetraplegia, or full body paralysis
(called Quadriplegia) below the site of injury to the spinal cord.
Damage to upper motor neuron axons in the spinal cord results in a characteristic pattern of ipsilateral
deficits. These include hyperreflexia, hypertonia and muscle weakness. Lower motor neuronal damage
results in its own characteristic pattern of deficits. Rather than an entire side of deficits, there is a
pattern relating to the myotome affected by the damage. Additionally, lower motor neurons are
characterized by muscle weakness, hypotonia, hyporeflexia and muscle atrophy.
Spinal shock and neurogenic shock can occur from a spinal injury. Spinal shock is usually temporary,
lasting only for 24–48 hours, and is a temporary absence of sensory and motor functions. Neurogenic
shock lasts for weeks and can lead to a loss of muscle tone due to disuse of the muscles below the
injured site.
The two areas of the spinal cord most commonly injured are the cervical spine (C1-C7) and the lumbar
spine (L1-L5). (The notation C1, C7, L1, L5 refer to the location of a specific vertebra in either the
cervical, thoracic, or lumbar region of the spine.)
A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in
response to a stimulus. In most contexts, in particular those involving humans, reflex actions are
mediated via the reflex arc; this is not always true in other animals, nor does it apply to casual uses of
the term 'reflex'.
A reflex is a rapid, involuntary response to a stimulus. A reflex arc is the pathway traveled by the
nerve impulses during a reflex. Most reflexes are spinal reflexes with pathways that traverse only the
spinal cord. During a spinal reflex, information may be transmitted to the brain, but it is the spinal
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cord, not the brain, that is responsible for the integration of sensory information and a response
transmitted to motor neurons. Some reflexes are cranial reflexes with pathways through cranial nerves
and the brainstem.
A reflex arc involves the following components, shown in Figure 1:
The receptor is the part of the neuron (usually a dendrite) that detects a stimulus.
The sensory neuron transmits the impulse to the spinal cord.
The integration center involves one synapse (monosynaptic reflex arc) or two or more synapses
(polysynaptic reflex arc) in the gray matter of the spinal cord. In polysynaptic reflex arcs, one
or more interneurons in the gray matter constitute the integration center.
A motor neuron transmits a nerve impulse from the spinal cord to a peripheral region.
An effector is a muscle or gland that receives the impulse from the motor neuron. In somatic
reflexes, the effector is skeletal muscle. In autonomic (visceral) reflexes, the effector is
smooth or cardiac muscle, or a gland.
Figure 1. A reflex arc.
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Some examples of reflexes follow:
A stretch reflex is a monosynaptic reflex that is a response to a muscle that has been stretched
(the knee-jerk reflex is an example). When receptors in muscles, called muscle spindles,
detect changes in muscle length, they stimulate, through a reflex arc, the contraction of a
muscle. Stretch reflexes help maintain posture by stimulating muscles to regain normal body
A flexor (withdrawal) reflex is a polysynaptic reflex that causes a limb to be withdrawn when
it encounters pain (refer to Figure 1).
A monosynaptic reflex is, typically, a reflex that does not involve the brain. See Figure 2.
There is no association neuron in the spinal cord; therefore, information does not go to the
brain. An example of a monosynaptic reflex is the patellar reflex, sometimes called the kneejerk reflex.
Figure 2. The patellar reflex.
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Reaction time
For a reflex, reaction time or latency is the time from the onset of a stimulus until the organism
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In animals, reaction time to visual stimuli is typically 150 to 300 milliseconds.
Human reflexes
Tendon reflexes
The deep tendon reflexes provide information on the integrity of the central and peripheral nervous
system. Generally, decreased reflexes indicate a peripheral problem, and lively or exaggerated reflexes
a central one.
Biceps reflex
Brachioradialis reflex
Extensor digitorum reflex
Triceps reflex
Patellar reflex or knee-jerk reflex
Ankle jerk reflex (Achilles reflex)
Plantar reflex or Babinski reflex
While the reflexes above are stimulated mechanically, the term H-reflex refers to the analogous reflex
stimulated electrically, and Tonic vibration reflex for those stimulated by vibration.
Reflexes usually only observed in human infants
Newborn babies have a number of other reflexes which are not seen in adults, referred to as primitive
reflexes. These include:
Asymmetrical tonic neck reflex (ATNR)
Grasp reflex
Hand-to-mouth reflex
Moro reflex, also known as the startle reflex
Rooting reflex
Symmetrical tonic neck reflex (STNR)
Tonic labyrinthine reflex (TLR)
Other reflexes
Other reflexes found in the central nervous system include:
Abdominal reflexes
Anocutaneous reflex
Cremasteric reflex
Mammalian diving reflex
Muscular defense
Scratch reflex
Startle reflex
Withdrawal reflex
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Many of these reflexes are quite complex requiring a number of synapses in a number of different
nuclei in the CNS (e.g., the escape reflex). Others of these involve just a couple of synapses to function
(eg., the withdrawal reflex). Processes such as breathing, digestion, and the maintenance of the
heartbeat can also be regarded as reflex actions, according to some definitions of the term.
The Psychology of Reflex Action
I. The Problem of Reflex Action. Extremely anomalous is the position of reflex action in the domain
of psychology. At least psychologists seem to be doubtful concerning the orientation of such behavior
in the general psychological field and hesitant as to the exact attitude they should take toward reflex
action. 'traditionally, psychologists have been able to grant no more than that reflexes are exclusively
physiological processes, because psychologists have always considered knowing as the essential
datum of their science. To accommodate the fact that reflex action actually constitutes a part of the
response equipment of organisms, as the biological influence upon psychology made apparent,
reflexes, though not considered to be psychological phenomena, were tolerated as the motor
accompaniments or the motor conditions of mental states.
In recent years a number of important conditions have conspired to bring about essential
modifications in our attitudes toward reflex action, and especially influential in this connection is the
fostering of the conviction that reflexes play a larger part in our total adjustments than was ever before
realized. Prominent also among these conditions were the discoveries concerning the conditioning of
reflexes. So recent have been these discoveries that at the present moment, lacking the necessary
perspective, we are unable to realize precisely how great the changes are which they have effected in
the domain of psychology, although it is apparent of course to everyone that important changes have
taken place.
Another equally significant and by no means unrelated condition is the fact that psychologists in
general are drifting away from the idea that a psychological datum is exclusively or even primarily a
knowing fact in the sense of some psychic stuff or mental function, toward a more organic position.
How great the change of front toward reflexes has been may be observed at a glance in the view now
current and gaining ground that all psychological facts are based upon and developed from reflex
action. Poignancy is added to this change of front when we reflect upon the great gulf which
psychologists once considered I o separate physiological behavior from elaborate 'knowing activities.
As important as reflex actions undoubtedly are it
(20) yet seems that we are going too fast and too far in our newer emphasis upon such behavior.
Because there exists apparently so much uncertainty in the attitudes toward reflex action, the
following study of reflexes is undertaken, with the aim of offering some suggestions toward the
redefinition of these interesting and important types of behavior.
II. Distinction Between the Psychological and Physiological Attitudes toward Reflexes. Although, as
we have endeavored to suggest in the preceding paragraph, psychologists have traditionally held
themselves aloof from reflexes, because the latter were presumed to be entirely physiological,
strangely enough it is owing in great part to the investigations of physiologists that the awed for a
closer study and understanding of reflexes by the psychologist has manifested itself.
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How strange in fact it is that the physiologist's study of the conditioning of reflexes should induce
the psychologist to recognize that a reflex action can and must be looked upon as a response to a
stimulus, that is to say, an adjustment act, can be readily appreciated when we observe how great the
differences are between the psychological and physiological attitudes toward reflex action. What is
then the difference in the two attitudes? Merely this: that while according to the psychologist a reflex
must be looked upon as a special adjustment of the organism as a whole, for the physiologist a reflex
action is the operation of an autonomous system of particular parts of an organism. Now if this
distinction is valid it is obvious that in order to reach an accurate description of reflex behavior this
differentiation must be kept in mind.
Because the physiologist, while studying reflexes, is primarily interested in the functioning of
neural structures, and secondarily in the activity of glands and muscles, he is disposed to look upon
such behavior, as well as other types of responses which he studies, as constant mechanisms entirely
independent of the surrounding stimuli. From this fact arises the distinction long current in
psychological literature between the so-called physiological and sensation reflexes, the former being
presumably completely autonomous and without the controlling influence of awareness. Accordingly
the psychologist assumed that typical reflexes are exemplified by the visceral activities. Since on the
whole, therefore, a reflex action for the physiologist consists of the innervating activity of a segmental
neural apparatus, a limited extension and flexion of muscles, and the localized action of glands and
nothing more, we must look upon the physiological description of a reflex action as an abstraction,
wholly unsuitable for use by psychologists.
From the psychological standpoint, as we have suggested, a reflex action is a definite adaptation act
and upon such a basis
( 21) is just as much a psychological datum as is thinking and knowing. Strictly speaking, the
psychological organism under ordinary circumstances cannot act otherwise than as a psychological
organism, and this refers to all activities, although for some purposes we might consider the individual
performing isolated reactions such as merely digestion, etc. But these situations are exactly analogous
to those accidental circumstances such as being struck by an automobile in which instance the
individual may function as a mere physical object. To be entirely precise at this point we mean to
point out that as a general principle, our exogenous reflex activities are stimulated to action by objects
and events about us and operate as adaptational mechanisms in exactly the same sense as any
psychological act.
In general, then, we may take as our standard for the differentiation between psychological and
physiological reactions, a criterion which we verily believe to be in the main reliable, the question
whether an act is or is not an organismic [1] response to stimulating circumstances. Now in order that
an act should be considered a genuine organismic response we must be able to trace its arousal to
some effect produced upon the person by some external object or some need for adaptation existing in
the organism itself. Accordingly, as we might expect, no sharp lines of division mark off the internal
from the external reflex stimuli. A food object operates precisely as does the hunger reflex (gastric
contractions) in the arousal of salivary reflexes. Similarly from the standpoint of effecting an action in
the person there is no functional difference between another person (opposite sex) and genital reflexes
in the acting person when each serves as a type of stimulus to elicit (other) sexual reflexes. Whether
the stimulus be endogenous or exogenous the reaction which it calls forth is an adaptation of the
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individual in just the same way that a habit or thought reaction is. On the whole, we find the reflex to
be (and this is why it is a psychological act) an interconnection between organisms and specific things;
or in better words, reflexes are the operation of reciprocally interacting stimuli and responses. Thus
when I see a dish of apples my salivary reflexes may begin to operate, while a dish of peaches may not
have the same effect at all. Now here the physiological attitude, according to which the problem of the
reflex begins and ends entirely within the organism, contrasts with the psychological study in that the
latter is concerned with (1)the means whereby the end-effect, which is the secretion or muscular
contraction, is initiated, and (2) how the act is dependent upon the reactional characteristics of the
specific individual performing the act. The psychologist cannot afford to overlook the fact
(22) that reflexes are very strictly conditioned and that upon the type of stimulus object which elicits
the reaction depend the intensity and range of the behavior.
Persuasive as an argument for the organsmic character of reflex action is the testimony of the
physiologist himself. Asserts Sherrington, than whom no investigator is more qualified to speak in this
matter, that reflexes are fractional pieces split off from an animal's total behavior which are artificially
though conveniently treated apart [2] , and that a simple reflex is probably a purely :abstract
conception, a convenient if not a probable fiction. [3] Furthermore, even the experimental physiologist
finds it necessary to declare "that the reflex reaction cannot be really intelligible to the physiologist
until he knows its aim."[4] And so the physiologist considers as an essential part of the investigation
of reflex phenomena the eliciting of their purposes. This does not mean at all the indulgence in any
factually baseless speculation, but merely involves looking upon a reflex action as a fact in its
adaptational perspective. When the operation of a reflex mechanism occurs, it is necessary in the
interests of a fair understanding of it, to include as many as possible of its essential features. Among
such essential features we may mention the influence upon the reflex action of the location of the
stimulus-the local sign of reflexes, as Sherrington calls it.
If the experimental physiologist acknowledges what we are pleased to call the definite
psychological character of reflex action, certainly the psychologist may well pause to reconsider his
habitual descriptions of such behavior. Let us hasten to add in unequivocal terms that to adopt the
psychological standpoint of studying reflex action means not at all that our study will lose one iota of
its objective character. On the contrary, such a method of study will add completeness as well as
definiteness to our descriptions. In plainer words, the psychological standpoint implies that we shall
look upon the reflex response as well as upon every other act that falls within our purview, as the
adjustment of a psychological machine, in the sense that we shall correlate the acts of the organism
with the coincidental surrounding conditions.
Is it necessary to add, in view of our discussion and our calling to witness the experimental
physiologist, that there is no actual conflict between the physiologist and the psychologist? No such
conflict exists in fact, since each worker is merely interested in a different phase of the same series of
events. While the psychologist is interested in the total action of the person to some definite stimulus,
the physiologist is interested
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( 23) in the workings of the reflex mechanism as they operate within the organism itself. What we are
desirous of showing is that when the psychologist is satisfied to duplicate the work of the physiologist
then he cannot hope to do justice to the psychological facts in the case.
If our distinction between psychological and physiological behavior is valid, we find in it a
compelling warning not to confuse reflex behavior with the truncated activities of injured and partially
destroyed organisms. All experimental animals such as the decerebrate pigeons of Flourens and
Schrader, Goltz's dogs and other laboratory animals exhibit atypical forms of behavior which cannot
be fairly taken as examples of reflex responses. How valid this point is will come out later in our
discussion of the differences between normal responses of animals and human beings.
III. The Nature of a Reflex Action. From the psychological standpoint, then, we must look upon a
reflex action as a specific sort of behavior segment and this means, as we have indicated, that we must
not only investigate the response mechanisms but the stimulating circumstances as well. Now the
special reflex characteristic of the reflex type of behavior segment is that there is only one reaction
system in it. To be more specific the reflex activity, although the adjustment of a complex animal or
person, is a simple and immediate final response to a directly presented stimulus. Obvious it is then
that there are no precurrent or anticipatory reactions in reflex segments of behavior such as we find in
our complex behavior segments, in which the final act is preceded not only by a definite attention set
but also by another reaction which we may call a free perceptual or ideational act, and still other sorts
of responses.[5] It is the absence of the anticipatory or precurrent responses which justifies the
statement that reflexes involve no foresight of the end or knowing by the organism with respect to
what is to take place before the response occurs or what is in fact transpiring at the moment of action.
But what of the complicated reflex behavior in which apparently several adaptations are taking place?
Upon investigation we find as a matter of fact that such behavior can be analyzed into a series of
behavior segments; that is to say we can analyze the behavior into a series of stimulus and response
coordinations. And here, as is not the case elsewhere, we have a chain effect. One final response
serves as the stimulus for the next reaction and so on throughout the series no matter what its length.
From the unitary character of the response in the reflex segment of behavior follows the fact that
reflex action is abso
( 24) -lutely unintelligent action. And this statement holds true no matter how complexly conditioned
is the response act. For the character of intelligence is an essential contribution of the precurrent
reaction systems. The latter are means of conditioning actions so that they can serve to adapt the
organism in a very precise way to a particular total stimulating circumstance. Needless it is to contrast
this prescient type of conditioning of one part of a response by another phase of the reaction, with the
simple conditioning of a total reaction by the stimulus, as is the case in reflex behavior. The most
complex conditioning of the latter sort, while demonstrating an awareness on the part of the reacting
organism, merits in no degree the ascription of intelligence.
As a consequence of the unitary character of the reflex response it appears to possess the following
specific characteristics, namely (1) relative automaticity, (2) constancy, (3) permanency, and (4)
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(1) Relative Automaticity. Since there is but one immediate movement or one essential secretory act,
then the reflex response must perforce appear as practically automatic. An evidence of the
automaticity of the reflex is the fact that it occurs and reoccurs in practically the same way no matter
what the person is doing at the time or where he is when the stimulus is presented. It is obvious, of
course, that when the reflex behavior segment is connected with other segments the total behavior
situation of the person may appear different, although the individual reflex act remains the same.
Generally speaking we might consider that the mere presence of an adequate stimulus, whether
primary or secondary, will throw the reaction system into automatic operation.
It is needless to add perhaps that the automaticity of reflex action is relative in the sense that no
psychological reaction can be wholly without spontaneity. What is meant of course is that relative to
other responses the reflexes are immediate and direct consequences of the stimulation. There is, in
fact, a very close relationship between the stimulus and the response, and no variation through
interpolated responses is possible between the appearance of the stimulus object or situation and the
final adjustment.
(2) Constancy. The constancy of reflex actions is a fact which follows from the function which they
perform in the various adaptation situations of the organism or person. Reflex behavior of the simpler
sort adapts the person to the simple maintenance situations in which he is found, such as shielding
ourselves from immediate noxious stimuli and nourishing ourselves in order to grow. Note that in the
trophic reflexes, for example, the mere presence of the food objects at certain strat-
( 25) -egic points of contact with the organism (at pillar of fautes, for deglutition) brings about the
action; also in the shelter reactions, the pin prick, the hot or cold object must be in immediate contact
with the organism. Now all of these food and shelter conditions are constant factors in the
surroundings of the individual and consequently the reflex adaptations remain constant in their
functional and morphological character, although as we have intimated, in the human being reflexes
may become organized with other behavior segments. It is possible also that the reaction system as a
whole in reflex behavior segments may become slightly modified because of changes in the size and
tonicity of the organic apparatus, although the general character of the reaction remains constant.
(3) Permanency. Since reflexes are elementary forms of responses adapting the organism to
permanent specific conditions they are permanent factors in the reactional equipment of the organism.
Moreover, reflex reaction systems do not become integrated and modified to become phases of larger
and more complex reaction systems. They remain simple reflexes. We have already indicated that
complex reflex adjustments consist of numerous repetitions of a particular reaction system of which
the preceding members of the series serve as stimuli to the following ones. In short, a serial reflex is
merely a series of behavior segments and not an integration of reflexes into more complex behavior.
(4) Localization. The comparative simplicity of reflex reaction systems and the definiteness of their
operation permit us to look upon them as partial acts. As a result, it appears as though the organism
operates in limited segments when functioning reflexly. Thus we speak of an eye or hand reflex. This
partial functioning is not an actual fact, however, for it is a biological and psychological impossibility
for the organism to act unless it acts as a whole. When we withdraw our hand from a hot object with
which it accidentally comes into contact we obviously react as a complete organism. Similarly every
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reflex no matter how great a change it produces in the person's relations to his surroundings may be
for practical purposes circumscribed and localized in a comparatively limited area.
IV. The Analysis of a Reflex Reaction System. Since from our standpoint the reflex reaction system
is a typical example of the ordinary unit of psychological activity it would be unnecessary to single it
out for analytic description were it not for the fact that reflexes are frequently and always fallaciously
presumed to be different in principle from other forms of behavior. To us it hardly seems possible that
such a difference should exist and as it is entirely unlikely that any asserted dif-
( 26) -ference will lie in the glandular, muscular er neural mechanisms involved in reflexes and other
action types, we may therefore confine our analysis to what would conventionally be called the mental
phases or accompaniments of reflexes.
Let us be understood then, as forthwith declaring that whatever factors are present in psychological
responses of whatever description are found also in reflex behavior. And if psychological phenomena
may properly be partitioned into cognitive, conative and affective factors, these factors are found in
reflexes no less than in any other behavior segments.
(1)And first let us consider the cognitive factor. Every reflex action involves a definite
discrimination of stimuli, although the discriminative factor is more pronounced in some reflexes than
in others, a condition, however, which reflexes share with all types of psychological behavior. If
evidence is needed to prove the presence of a cognitive element in reflexes we need only refer to the
fact that in common with all psychological responses, reflexes require their specific adequate stimuli
to put them into operation. A hot object will call out the reflex, while a warm object will not produce
such an effect. Again, the conditioning of reflex behavior constitutes excellent testimony to its
psychological character.[6]
What precisely is cognition then? It is necessary to specify that by cognition we refer to the fact that
different objects elicit differential responses from the reacting person or organism. Clearly, in the case
of such comparatively simple responses as reflexes the differential reactions will be aroused not so
much by complex objects as by simpler qualities of such objects, or in many cases the differential
response may be elicited by a condition rather than by any specific quality.
Obviously, we must all agree with those who assert that when we perform a reflex reaction we do
not know just what is taking place, for in such a reaction there is lacking the verbal response systems
which among other factors strikingly represent the knowing element. This absence of overt knowing,
however, in no sense militates against the fact that a reflex action is a differential reaction or a
cognitive process. As we are planning to indicate in a later section of this paper, the entire general
prejudice against regarding reflexes as psychological processes,
( 27) as well as the particular bias against looking upon reflexes as involving cognitive factors, have
their roots in the acceptance of an unsatisfactory conception of cognition. This conception implies that
knowing is something separated from the adjustmental act. We believe that all difficulties involved in
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the ascription of cognition to reflex action are dissipated when we recall that by such action we mean
in the final analysis an adaptation of the complete organism.
(2) In similar fashion we may find in reflex action an affective factor also. Otherwise stated, an
analysis of such behavior reveals a change produced in the organism which we may well call a feeling
condition. Glandular and visceral activities are aroused indicating that not only has the stimulus object
been acted upon by the organism, but also that the action has extended to itself. Here again it is
entirely superfluous to suppose that in any sense the person overtly reports to himself that he feels thus
and so; the feeling situation or affective response is merely a sort of internal response involving
relatively more the visceral organs than the external skeletal muscles. Naturally the diffusion and
intensity of the visceral disturbances will correlate with the violence of the response to the urgency
and pressure of the stimulus.
(3) In much the same manner can we analyze in reflexes the conative factor, by which is meant the
attention change from one stimulus to another. In no reflex action, of course, is there present the
deliberate precurrent change of position or attitude by which the individual prepares himself for
adaptation to a new stimulus. Hence, if the term be allowed, the attention factors in all the various
reflex action systems are involuntary, that is to say the person exhibits more or less violent jerky
movements in shifting his adjustments to new stimuli.
Once more we repeat that throughout this entire analysis of reflex reaction systems we refer to the
behavior of psychological organisms or persons. To those of our reactions which are merely biological
responses, namely tropisms, and we cannot well doubt that we occasionally perform such behavior,
these descriptions which we have offered do not at all apply. In the interests of accurate description we
cannot be too careful at this point, for since the psychological organism is obviously a biological
organism as well, it consequently is sometimes, albeit very seldom, thrown back upon what we must
call biological or tropismic modes of response.
V. Reflexes Are Not Neural Mechanisms. If our description and analysis of reflex behavior segments
are corresponsive with the facts in the case, then it is manifest that our interpretation of reflex action is
in conflict with and must replace the prac-
( 28) -tically universal belief that these forms of behavior are merely specific forms of neural arcs or
circuits. In clearer words, the essential thing about a reflex action is supposed to be a particular
concatenation of neurons, usually described as preformed patterns in the nervous system. Probably the
most fundamental error in the neural theory of reflexes is that the neural apparatus is in some sense
presumed to be the cause of the muscular movement and glandular action which constitute the
observable results of the reflex action. In the neuronic theory apparently the neural circuit replaces the
soul or consciousness as cause of a given adaptation. Credence is lent this view when we consider that
as a matter of historical fact the so-called spinal reflex was sometimes considered to be the exclusive
reflex type of action, while at other periods the spinal reflexes were presumed to be the typical if not
the exclusive reflex responses.
In addition to the general difficulty which is involved here, of neglecting most of our reflex
reactions, for we probably have as many cerebral as spinal reflexes, another question arises equally
fundamental for the whole of physiological psychology. Does the neural apparatus control the muscles
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any more than the muscles and glands control the neural apparatus? Is it not a fact that the specific
pathways involved in any reaction are involved because certain muscles or glands need to function?
The writer is firm in his disbelief in the functional priority of any system to any other. In fact, to our
way of thinking no such priority exists or can exist in the ordinary circumstances of behavior. [7]
Especially clear becomes the problem of the supremacy of the neural apparatus when we consider the
activities in which the muscle spindles are the primary receptors, for in such activity we have a
circular process from which it is extremely difficult to analyze out prior or posterior phases of the
reaction. This action in our opinion minimizes the general view of the primacy of the neural apparatus
in any type of reflex action.
Is it not closer to fact to affirm that the neural, muscular, glandular as well as all other action phases
of any behavior are simultaneous in their functioning and that no system is prior to or more important
than any other? What actually happens in every psychological behavior is that the organism performs
an act of which all the component systems are phases, in the sense that they constitute factors of a total
response. In their aggregate these phases constitute an adaptation to some object or situation. But
apparently we have dissipated the cause of the adjustment. What, it is asked, if not the neural
( 29) apparatus, conditions how the muscles, glands and other phases of the reflex should act?
To this we answer: what in fact could be the cause of which the total (neural, muscular, glandular)
adjustment is the effect but the stimulus object or situation, for in general what other observable
causes of our actions are evident? Again let us stress that a reflex action represents a differential mode
of behavior, neural, muscular, glandular, etc., which the organism has acquired in the course of its
development and which now operates when its adequate stimulus is presented. The failure of reflex
descriptions is largely owing to the fact that psychologists do not recognize the dependence of reflex
action upon actual stimulation, exactly as is the case in any other psychological action. When we do
appreciate the relationship between the stimulus and the total unitary response then we can cheerfully
dispense with the causative character of the neural apparatus.
Now here another objection may be anticipated. How can we argue that the neural factor is not
primary in importance nor prior in time when we know that as a matter of fact the essentially
conductive function of the neural apparatus requires some time to operate, no matter how brief? Such
an argument, we reply, rests upon a misconception which can be obviated by a closer observation of
facts. Of a surety when we experiment upon a neuro-muscular coordination we may with perfect
propriety disengage logically the conducting from the contracting mechanism, but when we do
perform such a logical analysis we must not forget that such an experiment implies the falsism that the
mechanisms are both inactive at the inception of the experiment, when in fact both nervous and
muscular mechanisms are functioning before the experiment is started, since the organism is never at
rest. Further, this objection implies that a single neural impulse can be in fact isolated and that it can
be I traced from a receptor to a muscle or gland. Now it is incontrovertible that a psychological
organism is constantly in action and therefore neural impulses are discharging uninterruptedly over all
the tracts in synchronous harmony with muscular, glandular and other types of processes. What in
actuality happens when we present the organism with a stimulus is a redistribution of action, an
emphasis of other features of the person than were prominent when the new stimulus appeared, in
short there is a refocussing of the individual upon a new stimulus.
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By way of emphasizing our hypothesis that if a reflex action in a psychological datum it is a
segment of behavior, i. e., a stimulus and a response, we mean to deny as stated before that and a
reflex is a partial reaction in any sense. In especial, we mean to controvert the three typical forms of
the decurtation theory
(30) concerning reflexes which are found in psychological and physiological literature. (1)Reflexes are
not primarily neural circuits in the sense of concatenated neurons or the operation of such circuits. (2)
Neither are they exclusively activities of the complete nervous system. (3) Nor are they merely
neuromuscular or neuroglandular responses. Again, we return to the charge: reflexes are complete or
organismic adaptations to specific or adequate stimulating objects and circumstances. Lethal is the
blow dealt to all neuronic abstractions by the fundamental facts of neural physiology. No question
exists at all but that the nervous system functions as a unified whole,[8] and while the neural
abstraction may be useful for experimental purposes, as our quotations from Sherrington indicate,
physiologists and neurologists are not insensitive to the factitious character of the neural circuits. To
the writer it appears most extraordinary that psychologists who are not benefited in the slightest by
neural abstractions but on the contrary are seriously hampered by them in their studies, still persist in
their employment, whereas even the physiologist uses them only as convenient fictions.[9]
From the experimental work on neural physiology we believe that we derive substantial support for
our organismic view of reflexes. What do we learn from spinal and decerebrate animals? Certainly not
that organisms can function in parts. On the contrary, what we learn from laboratory animals is that a
truncated organism can perform comparatively simple activities. This fact is amply demonstrated in
the classic descriptions of Flourens, Bouillaud, Schrader, and others [10] , when we forget their futile
arguments about "consciousness" and its seat. Especially well brought out is this fact of truncated
action in Munk's distinction between sensorial and psychic blindness.[11] It is because animals are
simpler in their organization that the vivisectional experiments can be performed upon them without
destroying entirely their capacity to act. By no means must we believe that a transsection or
extirpation indicates that the animal can function in parts because some of its behavior is mental and
some merely neural. No, the fact is that experiments can only be made up to the point of not disturbing
the intrinsic functional organization of the animal. For this reason, exper-
( 31) -iments on the higher apes or the human being produce either "shock" or death. But as long as
the functional organization of the animal remains undisturbed throughout all the mutilations it is as
much a psychological organism as it ever was. No other view would ever have been held but for the
assumption by most workers that consciousness was a force or power separate from, but paralleling
exclusively, the cerebral functions, or was coordinate with other neural functions, as Pfl?Goltz, and
Lewes believed.
We find in the reflex controversy,[12] as well as in the facts which the contending parties sought to
interpret, considerable evidence for our contention concerning the unitary character of psychological
behavior. Both the mechanical and spontaneity arguments are of course partial views, as the facts
employed in them amply testify, and are not nearly as much descriptions of those facts as they are
metaphysical interpretations. Both views are frankly based upon a psychoneural dualism, the existence
of which we unqualifiedly deny. That the organismic theory is sound may be further seen from the fact
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that in the human organism much reconstitution and substitution of behavior may occur during and
because of the degeneration of cortical tissue, [13] although the extreme intensity of the general
functional organization is such that few liberties can be taken with the individual because of
destroying the animal's organization.[14]
VI. The Origin of Reflex Behavior. From the standpoint of genesis, reflex responses are unique
among the permanent behavior equipment of human beings, in that they may be considered as the
earliest and most intrinsic of all the types of responses. The simplest of them are organized and operate
considerably before the completion of gestation. Exactly does this fact comport with the function and
general behavior conditions of these comparatively simple but utterly essential activities. Reflex
behavior is essentially life-maintaining activity and therefore is most intimately related to and
( 32) upon the biological structures of organisms. Hence, reflexes absolutely must begin to operate
from the very inception of the organism's life; in fact the reflex reaction systems may be said to be
inherited (if this can be said of any response system) along with the specific organs which have a part
in their operation. So elementary and primary are some of the simple reflex responses that as a general
rule the impression is prevalent that all reflex reactions are congenital and that none of them are
acquired in the life of the person. Such an assumption is not strictly correct. To account for the
essentially adaptive character of such basic and undeveloped reactions which are not inappropriately
named protective, defensive, avoiding, and seeking responses, we must fall back upon some sort of
natural selection hypothesis.
Need we add that neither the mode of origin of reflexes nor their essential features of permanency
and constancy deprive such responses of any specific psychological character? Let it be noted that the
psychological domain comprises behavior covering wide ranges of complexity and effectiveness, but
significant is the point that all these types of reactions are determinate responses to stimuli, whether
the adaptation involved be complex and imply much previous contact with its stimulus or whether the
reaction be fairly simple and occur while the organism is in primary contact with the stimulus calling
out the act in question.
It is possible that the basic character and primitive origin of the reflex responses contribute no small
share to the constancy and permanency of these reactions. That is to say, as long as the type of
organism remains unmodified and as long as the reciprocal stimulating circumstances remain the
same, then there is no need for the variation in the response system.
VII. Distinction Between Human and Animal Reflexes. Because of the comparative simplicity of
reflex action it is doubtless true that the slightest variation exists between human and infrahuman
behavior at this point. And yet if we were to overlook the enormous differences that after all exist
between human and animal reflexes we should do irreparable damage to our observations as well as
our interpretations. For there are great differences even between the various reflex actions of the
human individual, depending upon size, weight, health, and maturity, which cannot be neglected in
any analysis of behavior, especially if we are to attain exactitude in our descriptions. From the
existence of the different reflexes in the human species it follows that there must be extreme variations
in the behavior of the individuals of the human and infrahuman developments.
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In support of the proposition of the wide variation between human and infrahuman reflexes two
sorts of considerations
( 33) suggest themselves. First, not only the general biological differences between the two species of
animals but also the specific neural disparity argue conclusively for great diversities between the two
types of reflexes. The second and more striking consideration is the general fact of behavior
equipment. If our hypothesis is valid that a reflex act is a specific type of organismic adaptation, then
clearly the behavior must be colored by the total reactional condition of the individual. In other words,
the reflex action in the human being must be a function of the entire behavior equipment of the person
[15] , and the specific surrounding circumstances. Now obviously the behavior equipment of the
person is so different from that of any infrahuman animal that the reflex behavior as well as any other
class of action will be very different in the two cases.
Whatever argument is offered for the continuity of the two series of reflexes must perforce be based
upon continuity in biological development. Now such continuity, it must be observed, does not imply
any similarity in specific acts of psychological behavior; rather the argument for continuity overlooks
all actual facts of concrete behavior in favor of a general developmental or descent hypothesis. Such a
neglect of the specific adjustment inevitably results in error. To illustrate, it was only because of a lack
of interest in actual adjustments that the believers in continuity attempted to make of spinal reflexes
the typical reflex action to the exclusion of cerebral reflexes, and moreover, they believed this in
disregard of the fact that even when reflexes are considered as neural mechanisms they are cortically
controlled and modified.[16] In view of the cortical control of reflex action who can deny the
distinction between reflexes which we are attempting to make? In concluding this section of our paper
we might suggest that our distinction bet weep human and infrahuman reflexes in no wise interferes
with the biological continuity doctrine. For the logical implication of our hypothesis is that only a
degree of difference exists between any two levels of psychological action. In consequence, it is our
argument that animals are not different from human I beings in lacking memory, thought and
language, as the text books would have it, but only in the capacity to respond with simpler memory,
thought and language reactions. But note, that in all cases of behavior the needs of psychology dictate
a careful and accurate differentiation and description of responses.
( 34)
VIII. Types of Reflex Action. For practical purposes we might classify reflex reaction systems into at
least five types, partially upon the basis of their organization and especially upon the kind of contact
which they effect with their ordinary stimuli. In general, reflexes may be adjustments to conditions
(1)within the organism, or (2) to changes surrounding the individual or (3) to both of these at once.
The first type we may name the interoceptive reflexes and we may,mention as illustrations of such
responses the stomach and intestinal reflexes, etc., or expressed differently, responses in which these
phases of the organism play a prominent part.
On the other hand, reflexes which are primarily adjustors of the person to outside stimuli we may
call exteroceptive actions. Here we may analyze two types which we will name localized and general
exteroceptive reflexes respectively. In the former type the response appears to be localized in a
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definite way and involves primarily external skeletal mechanisms. As examples of these reflex actions
we may name the hand, foot or body withdrawal responses to heat or pain objects, the knee jerk,
turning the head toward a flash or sound, etc. The latter type, i. e. general reflexes, contrasts with the
local responses in that a larger phase of the organism is saliently involved and also in the fact that the
visceral and glandular factors may dominate the segment of behavior. As examples of the general
exteroceptive reflexes we may quote writhing, trembling, shivering, etc.
The third class of reflex which adapts the individual to both external and internal stimuli we will
call combination reflexes; these we may likewise analyze into two types, local and general. The
former type would comprise adjustments of a more or less restricted sort, although on the whole the
reactions would be more complex than those in our second class. Among the localized combination
reflexes we may enumerate the sexual and salivary responses. In this class both the local and general
responses may involve much glandular activity although the latter involves so much more of the
visceral and glandular factors that we may refer to some of them at least as feeling reflexes. Illustrative
of the general combination reflexes are the "startle" and "start" responses which are frequently
confused with feeling and emotion reactions, and which in some cases constitute the simplest form of
attention acts.
It is plain, of course, as we have indeed suggested, that the specific reaction systems in these
different types of reflexes will be integrated from specific factors. For example,. the principal
interoceptive reflexes as a rule will include mainly both muscle and glandular factors while the
exteroceptive reflexes involve primarily the skeletal muscles. Again, practically all
( 35) the interoceptive and some of the exteroceptive reactions will involve the sympathetic nervous
apparatus in a prominent way, while the exteroceptive reactions will involve mainly the central
nervous system. A prominent exception to this rule is the iris reflex in which the muscles involved are
innervated by the sympathetic system. In such comparatively simple reactions as reflexes the
discriminating factors would naturally be, as we have already seen, the simplest found in any reaction.
In all of these cases, to be sure, the discriminating factor strictly speaking is nothing more than the
occurrence of a simple differential response to its specific stimulus-object. As in all cases of
classifying reactions the divisions and subdivisions that we have made represent only attempts to order
behavior and not the separation of unequivocally different responses. For this reason no classification
can avoid many overlappings and the value of any classification may be judged most adequately by
the criterion of whether it suggests the likenesses and overlappings of behavior types or whether it
serves to obscure such similari l ies and transcursions of the classes.
IX. The Stimulation and Conditioning of Reflex Action. A radical change in the view concerning
what constitutes the stimulus for reflexes is implied in the acceptance of the organismic hypothesis.
For if we assent to the view that reflexes are adjustments of the individual some of which are very
complex, then we can no longer entertain the notion that they are aroused to action by merely simple
thermal, light or sound radiation. Aside from the general confusion which this notion implies between
the media of stimulations and stimuli objects or situations ,i1 such a view in the domain of reflexes
excludes all but the simplest situations as stimuli.
Let us notice then that reflex action is stimulated as are all other kinds of responses by objects of
various sorts, and by circumstances and situations. To be plainer, human reflex Factions are rapid and
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localized responses to things, persons, and conditions. Now this way of describing the reflex situation
allows for the fact that the whole person is acting and not a single part of him, which is of course an
impossibility. Moreover this mode of analysis forestalls the tendency to overlook any type of reflex
response, since we may be entirely certain that the class of reflex action is large enough to include
more than the very simplest avoidance responses. How complex and varied the stimuli for reflex
actions actually are appears clearly an in the consideration that objects in particular settings will elicit
( 36) the salivary reflexes while the same objects in different settings or different objects in the same
settings will not bring out the reflex adjustment. What more complex stimulus can there be than the
actions which operate while we are reflecting upon or reading of some person or event which excites
us to sex or hunger functioning? Extremely informing also are the observations concerning the reflex
changes in the person while under the subtle social and sex stimulation of other persons. Especially
important here are the complex social objects and situations which constitute the stimuli for intricate
human reflexes of all sorts. As among such social situations we may refer to games and gatherings of
persons of the same or opposite sex which arouse sex and hunger responses of various degrees of
diffusion. Again, we are familiar with the revulsion responses which dead or live animals produce in
us when touched or seen; these are all complex reflex responses representing functions of the total
reaction equipment of the person to customary stimuli which are therefore social in nature.
In the last mentioned reflex adjustment as well as in many others we meet with the very important
conditioning activities influencing the adaptation of the person to his surroundings. Thus, for example,
the nauseous visceral responses to dead animals may have become definitely attached to this new or
accessory stimulus at some specific time and under particular circumstances. The early stages in
training an infant to perform proper excretory behavior is in great part a process of attaching reflexes
already present and functioning to a new eliciting stimulus.[18] Especially subject to the conditioning
process are the combination reflexes, since the internally stimulated act can be variously transferred to
and from the coordinately stimulating external object. So involved are the conditioning processes that
in many cases it truly appears that the reflexes have become integrated into more complex forms of
behavior, although as a matter of fact this type of response remains practically in its original condition
throughout all of its complication by attachment to various new forms of stimuli.
X. Reflex Action as Stimuli and as Behavior Setting. So intimately related are the reflexes with the
total behavior of the organism that they constitute the stimuli to many of our reactions. Because of this
intimate relation, however, the reflexes are frequently overlooked and their importance unsuspected.
As a consequence psychologists are frequently guilty of the assumption that mysterious powers bring
about various reactions, whereas a careful study reveals that the reactions in
( 37) question have definite stimuli in the reflex responses. There can be little doubt that reflex
stimulation is responsible for much of our action which we call diffuse feeling-responses and moods
and that they compose elements in our complex social acts; acts of love, pity, revenge, etc. may be due
in large measure to reflex stimulation.
To state it otherwise, the reflex stimulations comprise some of the facts referred to as mixed
motives in complex responses. Can we deny that it is through the accessory stimulation of reflexes, in
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addition to thoughts and memories aroused by tales of cruelty and violence, that we are induced to add
our contribution to alleviate the suffering of which the stories inform us? In further illustration of
reflex stimulation we may quote the ways in which our reflex responses color our reactions to
aesthetic objects. Thus through the operation of these by-play reflex reactions we are stimulated to
read human and personal qualities into physical objects. stimulating us. In many instances the
additional stimulation by reflexes may also supply energy and alacrity te the behavior. This latter fact
has been celebrated in the statement that artistic production is the sublimation of sex impulses. While
the term sublimation hardly represents a psychological process, still it does serve to emphasize the
prevalence and power of reflex stimulation. In all these cases, reflexes serve as adjunct or additional
stimuli and as a result of their operation the individual may act in an entirely different way than if he
were stimulated only by the original object. Suggestive it is to note that the way we respond reflexly to
things and persons in turn stimulates our first impression reactions to those things and persons. In
other words, reflexes of a complex sort are called out in us by things and people and then when we
attempt to formulate a judgment of these things and people, or if we merely are prepared for some
future response, the delayed action is based and dependent upon reflex responses.
Furthermore, reflexes may function as combination stimuli-responses in delayed reactions, which
function in general is made possible by the fact that series of reflex responses intervene between the
presentation of the stimulus and the final adjustment, and thus function in many cases to keep alive the
effect of the stimulation until the final response occurs. The combination reflex act-stimulus is
undoubtedly a primary basis of the wants and desires as psychological facts. One is forcibly led to this
view by the consideration of the place of sexual and hunger-digestive reflexes in food and sex wants
and desires. So potent are the reflex actions as stimuli, that we can elicit from a careful study of the
differential frequency and intensity of such reactions much valuable information concerning the type
differences between individuals which are usually consid-
( 38) ered as temperament, disposition, etc.; such characteristics being marked by actions which in a
large measure are stimulated by reflex responses.
And finally, the visceral and glandular reflex behavior of persons may serve not as direct stimuli to
actions but rather as the setting of responses. That is to say, these reflex responses serve as influences
of behavior affecting the general condition of the person. As the setting of stimuli, visceral reflexes
determine whether or not certain stimuli shall be potent and call out a reaction. Thus for example,
during the operation of hunger reflexes it is more difficult than otherwise to attend to one's work; in
other words, the sensitivity to exacting stimuli is decreased; in other cases the functioning of reflexes
may serve to increase one's sensitivity to particular stimuli and in consequence the person will respond
more readily to surrounding objects.
XI. Reflex Action and Instinctive Behavior. Very prevalent is the view that reflex actions are closely
related to instinctive behavior and especially is this assumption made in order to provide a solid
foundation for instincts. In detail, instincts are presumed to be combinations or chains of reflexes, and
since the latter are supposed to be specific neural pathways the conventionally teleological instinct
achieves a factual support. From our standpoint, however, the relationship requires reformulation,
since we cannot assign to instincts any sort of teleological character nor can we consider reflexes to be
merely neural mechanisms.
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Instinctive actions we consider to be behavior segments of a different order and type than those of
reflexes and the main difference is that the instinctive behavior segments contain more than one
reaction system.[19] Common to both instincts and reflexes, however, is the fact that both acts
constitute definite final adjustments although an instinct segment of behavior contains a pattern of
several reaction systems. Because the instinct segment of behavior does contain more than a single
reaction system, it partakes of a series of definite characteristics not found in the reflex segments.
These characteristics we may enumerate as follows: (1) spontaneity and variability, (2) modifiability,
and (3) integration.
(1) By spontaneity and variability we mean to refer to the greater adaptability which instinctive
behavior exhibits than do reflexes. The latter may be conditioned in various ways; that is to say, the
simpler reaction system can be differently attached to a stimulus, but the response factor itself does not
vary. In
( 39) the case of the instinctive reaction on the contrary, the members of the pattern may rearrange
themselves in modification of the pattern.
This arrangement is made possible by the fact that while the total pattern may be stimulated by a
definite appropriate stimulus, say some interoceptive reflex, the individual reaction systems in the
pattern are aroused to action by other surrounding stimuli, namely, objects and conditions; so that each
has some autonomy and the whole pattern is spontaneous. Accordingly, t he instinctive behavior act is
the more adaptable response when the adjustment conditions are more variable. This type of
instinctive adjustment in which the member reaction systems are subject to rearrangement is the most
spontaneous that we can observe. The reactions which are less spontaneous are so because the member
reaction systems of the segment are only very little stimulated by the surroundings and more by the
preceding members of the pattern series. It was this kind of instinct, no doubt, which gave rise to the
notion that instincts are chains of reflexes. When there is only one, or at most only a few surrounding
stimuli conditioning the instinctive behavior, (hen the reaction as a whole will be more rigid and
conform to a type.
(2) Since the individual reaction systems in an instinct behavior segment are correlated with specific
external stimuli it is entirely probable that the auxiliary stimuli may become more and more effective
as factors in the total response, thus modifying in a specific way the total instinct act and making it
more serviceable to the organism in its particular surroundings. This modifiability contrasts with the
permanence of the reflex behavior segment which cannot of course be modified in any essenI i tial
(3) Another intrinsic characteristic of instinct acts is the fact that when conditions allow and make
necessary they become integrated into more complex reactions. It is this fact of integration above all
which marks off the human from the infrahuman instinctive behavior and also distinctly differentiates
instincts from reflexes. While the reflex activities remain practically as they originally appear, instinct
behavior becomes developed into more complex forms of responses. And so it h: a happens that in the
animal domain the instinct reactions become an integrated to only a slight degree because the
conditions are not conducive to any considerable development. In the human individual on the other
hand, the behavior conditions are so complex that there are very few instinct responses to begin with
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and I these few become integrated into larger reactions and disappear. By the same token the reflexes
which originally comprise
(40) elements in the behavior equipment of the individual remain with him in practically the original
number and condition. We might repeat here once more that reflexes become modified only by
conditioning, that is to say, the correlated stimuli may vary and consequently the whole behavior
segment becomes altered but not the response factor or reaction system itself. It is consequently clear
that in our comparison of . reflexes with instincts, animal instincts must be understood to be the facts
discussed, while in the case of reflexes both animal and human actions may be considered as the
subject matter in question.
Superfluous it would seem now to add that in our entire discussion we referred not in a single
sentence to instincts in the sense of a purpose or impulse in the individual to do various kinds of acts;
no such impulses, we firmly believe, exist in any sense .[20] Many times we have implied that we are
discussing only action mechanisms which are constituted exclusively of definite adjustment acts
conditioned by stimulating circumstances with which they are coordinated.
XII. Reflexes and Tropismic Action. As a final consideration of reflex action we may place it in
comparison with tropismic or purely biological action. Since there is no strict convention governing
the use of the term tropism, let us be understood to exclude the criterion, that such action does not
involve systematized nervous tissue. For our hypothesis concerning the adjustments of organisms does
not permit us to seek in the mechanisms of organisms for the exclusive conditions of behavior. Now
observe that very prominently is the comparatively simple mechanism of the tropismic action
correlated with the sensitivity of the organism to its surroundings. As a matter of fact it is possible to
differentiate between reflex actions as typical psychological responses, and tropisms, on the basis of
the relationship of the organisms to the surroundings when they are performing either one or the other
type of action.
Although it is entirely probable that the difference between tropisms and reflexes is merely a
variation in developmental complexity, still we can specify particular adjustmental differences. For
example, tropisms as responses, while entirely disproportional to the exciting condition in the
expenditure of energy, in form and type of movement are still constant. This constancy of movement
is a function of
( 41) definite organic structures operating as a whole in the manner which is referred to as irritability,
and correlates exactly with some stimulating condition in the surroundings. This external condition is
practically an undifferentiated condition and never an object with its specific qualities. Moreover, the
reactions are such as maintain the present status and condition of the individual's organization
exclusively by means of metabolic functioning. From this standpoint it is easy to see why we must
look upon tropismic action as relatively simple responses to surrounding conditions.
Reflexes as psychological activities, on the other hand, are specific responses to particular objects
and conditions. Such behavior, as we have seen, constitutes differential responses and exhibits a subtle
interdependence of stimuli and responses. By virtue of the person's possession of numerous
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characteristically different reflex action systems to which many different specific objects and
conditions are coordinate, the individual is selectively sensitive to many features of the milieu. That
this essential psychological character of reflex actions has been overlooked may be accounted for by
the fact that many of the specific differences between reflexes and tropisms seem to fall away when
we compare them both with the higher developed and more complex adaptations which we may call
volitional responses. Such an overlooking of the definite psychological character of reflex action is
exceedingly unfortunate, of course, since as a matter of fact reflex responses can be shown to partake
in some fashion of practically all the typical characteristics of the complex psychological reactions.
XIII. Conclusion. In conclusion, we might suggest that our study of reflex action finds its most
important feature in the general psychological problem which it raises. How shall we look upon
psychological phenomena? Shall we consider t them as definite autonomous facts in nature or shall we
look upon them as merely epiphenomenal attachments to such facts? Or, again, shall we try to make
psychological facts into physiological actions, because presumably psychological activities are not
concrete or simple enough to handle without changing them into neural terms? In our study the
conclusion we reach is that unless we consider reflexes as well as every other type of reaction as
definite psychological facts, and not physiological acts, we cannot hope to understand them. That
psychological acts are just as definite and just as real as any other kind of fact investigation has amply
revealed. A definite criterion for a psychological fact we have discovered in the intricate
interconnection between a stimulus and a total reaction of an organism.
( 42) Applying this criterion to reflexes we have found that such behavior must be considered as
definite psychological phenomena, and further, we find that to study reflex actions as definite
psychological facts not only enables us better to understand them but to appreciate the place they take
in the adaptations of the organism, both as responses to specific stimuli and their settings, and as
themselves stimuli and reactionial backgrounds for our more complex behavior.
The meninges is the system of membranes which envelops the central nervous system. The meninges
consist of three layers: the dura mater, the arachnoid mater, and the pia mater. The primary function of
the meninges and of the cerebrospinal fluid is to protect the central nervous system.
Dura mater
The dura mater (also rarely called meninx fibrosa, or pachymeninx) is a thick, durable membrane,
closest to the skull. It consists of two layers, the periosteal layer which lies closest to the calvaria, and
the inner meningeal layer which lies closer to the brain. It contains larger blood vessels which split
into the capillaries in the pia mater. It is composed of dense fibrous tissue, and its inner surface is
covered by flattened cells like those present on the surfaces of the pia mater and arachnoid. The dura
mater is a sac which envelops the arachnoid and has been modified to serve several functions. The
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dura mater surrounds and supports the large venous channels (dural sinuses) carrying blood from the
brain toward the heart.
Pia mater
The pia or pia mater is a very delicate membrane. It is the meningeal envelope which firmly adheres to
the surface of the brain and spinal cord. As such it follows all the minor contours of the brain (gyri and
sulci). It is a very thin membrane composed of fibrous tissue covered on its outer surface by a sheet of
flat cells thought to be impermeable to fluid. The pia mater is pierced by blood vessels which travel to
the brain and spinal cord, and its capillaries are responsible for nourishing the brain.
The subarachnoid space is the space which normally exists between the arachnoid and the pia mater,
which is filled with cerebrospinal fluid.
Normally, the dura mater is attached to the skull, or to the bones of the vertebral canal in the spinal
cord. The arachnoid is NOT attached to the dura mater, while the pia mater is attached to the central
nervous system tissue. When the dura mater and the arachnoid separate through injury or illness, the
space between them is the subdural space.
There are three types of hemorrhage involving the meninges
A subarachnoid hemorrhage is acute bleeding under the arachnoid; it may occur spontaneously or as
a result of trauma.
A subdural hematoma is a hematoma (collection of blood) located in a separation of the arachnoid
from the dura mater. The small veins which connect the dura mater and the arachnoid are torn, usually
during an accident, and blood can leak into this area.
An epidural hematoma similarly may arise after an accident or spontaneously.
Other medical conditions which affect the meninges include meningitis (usually from fungal, bacterial,
or viral infection) and meningiomas arising from the meninges or from tumors formed elsewhere in
the body which metastasize to the meninges.
Cerebrospinal fluid (CSF), Liquor cerebrospinalis, is a clear bodily fluid that occupies the
subarachnoid space and the ventricular system around and inside the brain and spinal cord. In essence,
the brain "floats" in it.
The CSF occupies the space between the arachnoid mater (the middle layer of the brain cover,
meninges), and the pia mater (the layer of the meninges closest to the brain). It constitutes the content
of all intra-cerebral (inside the brain, cerebrum) ventricles, cisterns, and sulci (singular sulcus), as well
as the central canal of the spinal cord.
It acts as a "cushion" or buffer for the cortex, providing a basic mechanical and immunological
protection to the brain inside the skull.It is produced in the choroid plexus
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CSF is produced in the brain by modified ependymal cells in the choroid plexus (approx. 50-70%),
and the remainder is formed around blood vessels and along ventricular walls. It circulates from the
lateral ventricles to the foramen of Monro, third ventricle, aqueduct of Sylvius, fourth ventricle,
foramina of Magendie and Luschka; subarachnoid space over brain and spinal cord; reabsorption into
venous sinus blood via arachnoid granulations.
It had been thought that CSF returns to the vascular system by entering the dural venous sinuses via
the arachnoid granulations or villi. However, some have suggested that CSF flow along the cranial
nerves and spinal nerve roots allow it into the lymphatic channels; this flow may play a substantial
role in CSF reabsorbtion, in particular in the neonate, in which arachnoid granulations are sparsely
distributed. The flow of CSF to the nasal submucosal lymphatic channels through the cribiform plate
seems to be specially important.
Amount and constitutionReference ranges in CSF
The CSF is produced at a rate of 500 ml/day. Since the brain can contain only 135 to 150 ml, large
amounts are drained primarily into the blood through arachnoid granulations in the superior sagittal
sinus. Thus the CSF turns over about 3.7 times a day. This continuous flow into the venous system
dilutes the concentration of larger, lipoinsoluble molecules penetrating the brain and CSF.
The CSF contains approximately 0.3% plasma proteins, or approximately 15 to 40 mg/dL, depending
on sampling site.[4] CSF pressure ranges from 80 to 100 mmH2O (780–980 Pa or 4.4–7.3 mmHg) in
newborns, and < 200 mmH20 (1.94 kPa) in normal children and adults, with most variations due to
coughing or internal compression of jugular veins in the neck.
There are quantitative differences in the distributions of a number of proteins in the CSF. In general,
globular proteins and albumin are in lower concentration in ventricular CSF compared to lumbar or
cisternal fluid.
CSF serves four primary purposes:
Buoyancy: The actual mass of the human brain is about 1400 grams; however the net weight of the
brain suspended in the CSF is equivalent to a mass of 25 grams. The brain therefore exists in neutral
buoyancy, which allows the brain to maintain its density without being impaired by its own weight,
which would cut off blood supply and kill neurons in the lower sections without CSF.
Protection: CSF protects the brain tissue from injury when jolted or hit. In certain situations such as
auto accidents or sports injuries, the CSF cannot protect the brain from forced contact with the skull
case, causing hemorrhaging, brain damage, and sometimes death.
Chemical stability: CSF flows throughout the inner ventricular system in the brain and is absorbed
back into the bloodstream, rinsing the metabolic waste from the central nervous system through the
blood-brain barrier. This allows for homeostatic regulation of the distribution of neuroendocrine
factors, to which slight changes can cause problems or damage to the nervous system. For example,
high glycine concentration disrupts temperature and blood pressure control, and high CSF pH causes
dizziness and syncope.
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Prevention of brain ischemia: The prevention of brain ischemia is made by decreasing the amount of
CSF in the limited space inside the skull. This decreases total intracranial pressure and facilitates
blood perfusion.
Pathology and laboratory diagnosis
When CSF pressure is elevated, cerebral blood flow may be constricted. When disorders of CSF flow
occur, they may therefore affect not only CSF movement but also craniospinal compliance and the
intracranial blood flow, with subsequent neuronal and glial vulnerabilities. The venous system is also
important in this equation. Infants and patients shunted as small children may have particularly
unexpected relationships between pressure and ventricular size, possibly due in part to venous pressure
dynamics. This may have significant treatment implications, but the underlying pathophysiology needs
to be further explored.
CSF connections with the lymphatic system have been demonstrated in several mammalian systems.
Preliminary data suggest that these CSF-lymph connections form around the time that the CSF
secretory capacity of the choroid plexus is developing (in utero). There may be some relationship
between CSF disorders, including hydrocephalus and impaired CSF lymphatic transport.
CSF can be tested for the diagnosis of a variety of neurological diseases. It is usually obtained by a
procedure called lumbar puncture. Removal of CSF during lumbar puncture can cause a severe
headache after the fluid is removed, because the brain hangs on the vessels and nerve roots, and
traction on them stimulates pain fibers. The pain can be relieved by intrathecal injection of sterile
isotonic saline. Lumbar puncture is performed in an attempt to count the cells in the fluid and to detect
the levels of protein and glucose. These parameters alone may be extremely beneficial in the diagnosis
of subarachnoid hemorrhage and central nervous system infections (such as meningitis). Moreover, a
CSF culture examination may yield the microorganism that has caused the infection. By using more
sophisticated methods, such as the detection of the oligoclonal bands, an ongoing inflammatory
condition (for example, multiple sclerosis) can be recognized. A beta-2 transferrin assay is highly
specific and sensitive for the detection for, e.g., CSF leakage.
Lumbar puncture
Lumbar puncture can also be performed to measure the intracranial pressure, which might be
increased in certain types of hydrocephalus. However a lumbar puncture should never be performed if
increased intracranial pressure is suspected because it could lead to brain herniation and ultimately
This fluid has an importance in anesthesiology. Baricity refers to the density of a substance compared
to the density of human cerebral spinal fluid. Baricity is used in anesthesia to determine the manner in
which a particular drug will spread in the intrathecal space.
Glial cells, commonly called neuroglia or simply glia (Greek for "glue"), are non-neuronal cells that
maintain homeostasis, form myelin, and provide support and protection for the brain's neurons. In the
human brain, there is roughly one glia for every neuron with a ratio of about two neurons for every
three glia in the cerebral gray matter.
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As the Greek name implies, glia are commonly known as the glue of the nervous system; however,
this is not fully accurate. The four main functions of glial cells are to surround neurons and hold them
in place, to supply nutrients and oxygen to neurons, to insulate one neuron from another, and to
destroy pathogens and remove dead neurons. They also modulate neurotransmission.
Some glial cells function primarily as the physical support for neurons. Others regulate the internal
environment of the brain, especially the fluid surrounding neurons and their synapses, and nutrify
neurons. During early embryogeny, glial cells direct the migration of neurons and produce molecules
that modify the growth of axons and dendrites. Recent research indicates that glial cells of the
hippocampus and cerebellum participate in synaptic transmission, regulate the clearance of
neurotransmitters from the synaptic cleft, release factors such as ATP, which modulate presynaptic
function, and even release neurotransmitters themselves. Unlike the neuron, which is, in general,
considered permanently post-mitotic glial cells are capable of mitosis.
In the past, glia had been considered to lack certain features of neurons. For example, glial cells were
not believed to have chemical synapses or to release neurotransmitters. They were considered to be the
passive bystanders of neural transmission. However, recent studies have shown this to be untrue[4].
For example, astrocytes are crucial in clearance of neurotransmitter from within the synaptic cleft,
which provides distinction between arrival of action potentials and prevents toxic build-up of certain
neurotransmitters such as glutamate (excitotoxicity). It is also thought that glia play a role in
Alzheimer's disease. Furthermore, at least in vitro, astrocytes can release neurotransmitter glutamate in
response to certain stimulation. Another unique type of glial cell, the oligodendrocyte precursor cells
or OPCs, have very well-defined and functional synapses from at least two major groups of neurons.
The only notable differences between neurons and glial cells are neurons' possession of axons and
dendrites, and capacity to generate action potentials.
Glia ought not to be regarded as 'glue' in the nervous system as the name implies; rather, they are more
of a partner to neurons. They are also crucial in the development of the nervous system and in
processes such as synaptic plasticity and synaptogenesis. Glia have a role in the regulation of repair of
neurons after injury. In the CNS, glia suppresses repair. Glial cells known as astrocytes enlarge and
proliferate to form a scar and produce inhibitory molecules that inhibit regrowth of a damaged or
severed axon. In the PNS, glial cells known as Schwann cells promote repair. After axonal injury,
Schwann cells regress to an earlier developmental state to encourage regrowth of the axon. This
difference between PNS and CNS raises hopes for the regeneration of nervous tissue in the CNS. For
example a spinal cord may be able to be repaired following injury or severance.
Microglia are like specialized macrophages capable of phagocytosis that protect neurons of the central
nervous system. They are derived from hematopoietic precursors rather than ectodermal tissue; they
are commonly categorized as such because of their supportive role to neurons.
These cells comprise approximately 15% of the total cells of the central nervous system. They are
found in all regions of the brain and spinal cord. Microglial cells are small relative to macroglial cells,
with changing shapes and oblong nuclei. They are mobile within the brain and multiply when the brain
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is damaged. In the healthy central nervous system, microglia processes constantly sample all aspects
of their environment (neurons, macroglia and blood vessels).
Capacity to divide
Glia retain the ability to undergo cell division in adulthood, whereas most neurons cannot. The view is
based on the general deficiency of the mature nervous system in replacing neurons after an injury,
such as a stroke or trauma, while very often there is a profound proliferation of glia, or gliosis near or
at the site of damage. However, detailed studies found no evidence that 'mature' glia, such as
astrocytes or oligodendrocytes, retain the ability of mitosis. Only the resident oligodendrocyte
precursor cells seem to keep this ability after the nervous system matures. On the other hand, there are
a few regions in the mature nervous system, such as the dentate gyrus of the hippocampus and the
subventricular zone, where generation of new neurons can be observed.
Embryonic development
Most glia are derived from ectodermal tissue of the developing embryo, in particular the neural tube
and crest. The exception is microglia, which are derived from hemopoietic stem cells. In the adult,
microglia are largely a self-renewing population and are distinct from macrophages and monocytes,
which infiltrate the injured and diseased CNS.
In the central nervous system, glia develop from the ventricular zone of the neural tube. These glia
include the oligodendrocytes, ependymal cells, and astrocytes. In the peripheral nervous system, glia
derive from the neural crest. These PNS glia include Schwann cells in nerves and satellite glial cells in
Glia were discovered in 1856 by the pathologist Rudolf Virchow in his search for a 'connective tissue'
in the brain.
The human brain contains roughly equal numbers of glial cells and neurons with 84.6 billion glia and
86.1 billion neurons. The ratio differs between its different parts. The glia/neuron ratio in the cerebral
cortex is 3.72(60.84 billion glia; 16.34 billion neurons) while that of the cerebellum is only 0.23
(16.04 billion glia; 69.03 billion neurons). The ratio in the cerebral cortex gray matter is 1.48 (the
white matter part has few neurons). The ratio of the basal ganglia, diencephalon and brainstem
combined is 11.35.
Most cerebral cortex glia are oligodendrocytes (75.6%) then astrocytes (17.3%) and least for microglia
The amount of brain tissue that is made up of glia cells increases with brain size: the nematode brain
contains only a few glia, a fruitfly's brain is 25% glia, that of a mouse, 65%, a human, 90%, and an
elephant, 97%.
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The human brain is the center of the human nervous system and is a highly complex organ. Enclosed
in the cranium, it has the same general structure as the brains of other mammals, but is over three
times as large as the brain of a typical mammal with an equivalent body size. Most of the expansion
comes from the cerebral cortex, a convoluted layer of neural tissue that covers the surface of the
forebrain. Especially expanded are the frontal lobes, which are associated with executive functions
such as self-control, planning, reasoning, and abstract thought. The portion of the brain devoted to
vision is also greatly enlarged in human beings.
Brain evolution, from the earliest shrewlike mammals through primates to hominids, is marked by a
steady increase in encephalization, or the ratio of brain to body size. The human brain has been
estimated to contain 50–100 billion (1011) neurons, of which about 10 billion (1010) are cortical
pyramidal cells. These cells pass signals to each other via as many as 1000 trillion (1015) synaptic
The brain monitors and regulates the body's actions and reactions. It continuously receives sensory
information, and rapidly analyzes this data and then responds, controlling bodily actions and functions.
The brainstem controls breathing, heart rate, and other autonomic processes. The neocortex is the
center of higher-order thinking, learning, and memory. The cerebellum is responsible for the body's
balance, posture, and the coordination of movement.
In spite of the fact that it is protected by the thick bones of the skull, suspended in cerebrospinal fluid,
and isolated from the bloodstream by the blood-brain barrier, the delicate nature of the human brain
makes it susceptible to many types of damage and disease. The most common forms of physical
damage are closed head injuries such as a blow to the head, a stroke, or poisoning by a wide variety of
chemicals that can act as neurotoxins. Infection of the brain is rare because of the barriers that protect
it, but is very serious when it occurs. The human brain is also susceptible to degenerative disorders,
such as Parkinson's disease, multiple sclerosis, and Alzheimer's disease. A number of psychiatric
conditions, such as schizophrenia and depression, are widely thought to be caused at least partially by
brain dysfunctions, although the nature of such brain anomalies is not well understood.
Bisection of the head of an adult man, showing the cerebral cortex and underlying white matter with a
size (volume) of around 1130 cubic centimetres (cm3) in women and 1260 cm3 in men, although there
is substantial individual variation. Men with the same body height and body surface area as women
have on average 100g heavier brains, although these differences do not correlate in any simple way
with gray matter neuron counts or with overall measures of cognitive performance. Neanderthals had
larger brains at adulthood than present-day humans. The brain is very soft, having a consistency
similar to soft gelatin or firm tofu. Despite being referred to as "grey matter", the live cortex is
pinkish-beige in color and slightly off-white in the interior. At the age of 20, a man has around
176,000 km and a woman, about 149,000 km of myelinated axons in their brains.
General features
The cerebral hemispheres form the largest part of the human brain and are situated above most other
brain structures. They are covered with a cortical layer with a convoluted topography. Underneath the
cerebrum lies the brainstem, resembling a stalk on which the cerebrum is attached. At the rear of the
brain, beneath the cerebrum and behind the brainstem, is the cerebellum, a structure with a
horizontally furrowed surface that makes it look different from any other brain area. The same
structures are present in other mammals, although the cerebellum is not so large relative to the rest of
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the brain. As a rule, the smaller the cerebrum, the less convoluted the cortex. The cortex of a rat or
mouse is almost completely smooth. The cortex of a dolphin or whale, on the other hand, is more
convoluted than the cortex of a human.
The dominant feature of the human brain is corticalization. The cerebral cortex in humans is so large
that it overshadows every other part of the brain. A few subcortical structures show alterations
reflecting this trend. The cerebellum, for example, has a medial zone connected mainly to subcortical
motor areas, and a lateral zone connected primarily to the cortex. In humans the lateral zone takes up a
much larger fraction of the cerebellum than in most other mammalian species. Corticalization is
reflected in function as well as structure. In a rat, surgical removal of the entire cerebral cortex leaves
an animal that is still capable of walking around and interacting with the environment. In a human,
comparable cerebral cortex damage produces a permanent state of coma. The amount of association
cortex, relative to the other two categories, increase dramatically as one goes from simpler mammals,
such as the rat and the cat, to more complex ones, such as the chimpanzee and the human.
The cerebral cortex is essentially a sheet of neural tissue, folded in a way that allows a large surface
area to fit within the confines of the skull. Each cerebral hemisphere, in fact, has a total surface area of
about 1.3 square feet. Anatomists call each cortical fold a sulcus, and the smooth area between folds a
gyrus. Most human brains show a similar pattern of folding, but there are enough variations in the
shape and placement of folds to make every brain unique. Nevertheless, the pattern is consistent
enough for each major fold to have a name, for example, the "superior frontal gyrus", "postcentral
sulcus", or "trans-occipital sulcus". Deep folding features in brain such as the inter-hemispheric and
lateral fissure, and the insular cortex are present in almost all normal subjects.
Cortical divisions
Four lobes
Outwardly, the cerebral cortex is nearly symmetrical, with left and right hemispheres. Anatomists
conventionally divide each hemisphere into four "lobes", the frontal lobe, parietal lobe, occipital lobe,
and temporal lobe. This categorization does not actually arise from the structure of the cortex itself:
the lobes are named after the bones of the skull that overlie them. There is one exception: the border
between the frontal and parietal lobes is shifted backward to the central sulcus, a deep fold that marks
the line where the primary somatosensory cortex and primary motor cortex come together.
Functional divisions
Researchers who study the functions of the cortex divide it into three functional categories of regions,
or areas. One consists of the primary sensory areas, which receive signals from the sensory nerves and
tracts by way of relay nuclei in the thalamus. Primary sensory areas include the visual area of the
occipital lobe, the auditory area in parts of the temporal lobe and insular cortex, and the somatosensory
area in the parietal lobe. A second category is the primary motor area, which sends axons down to
motor neurons in the brainstem and spinal cord. This area occupies the rear portion of the frontal lobe,
directly in front of the somatosensory area. The third category consists of the remaining parts of the
cortex, which are called the association areas. These areas receive input from the sensory areas and
lower parts of the brain and are involved in the complex process that we call perception, thought, and
decision making.
Different parts of the cerebral cortex are involved in different cognitive and behavioral functions. The
differences show up in a number of ways: the effects of localized brain damage, regional activity
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patterns exposed when the brain is examined using functional imaging techniques, connectivity with
subcortical areas, and regional differences in the cellular architecture of the cortex. Anatomists
describe most of the cortex—the part they call isocortex—as having six layers, but not all layers are
apparent in all areas, and even when a layer is present, its thickness and cellular organization may
vary. Several anatomists have constructed maps of cortical areas on the basis of variations in the
appearance of the layers as seen with a microscope. One of the most widely used schemes came from
Brodmann, who split the cortex into 51 different areas and assigned each a number (anatomists have
since subdivided many of the Brodmann areas). For example, Brodmann area 1 is the primary
somatosensory cortex, Brodmann area 17 is the primary visual cortex, and Brodmann area 25 is the
anterior cingulate cortex.
Many of the brain areas Brodmann defined have their own complex internal structures. In a number of
cases, brain areas are organized into "topographic maps", where adjoining bits of the cortex
correspond to adjoining parts of the body, or of some more abstract entity. A simple example of this
type of correspondence is the primary motor cortex, a strip of tissue running along the anterior edge of
the central sulcus, shown in the image to the right. Motor areas innervating each part of the body arise
from a distinct zone, with neighboring body parts represented by neighboring zones. Electrical
stimulation of the cortex at any point causes a muscle-contraction in the represented body part. This
"somatotopic" representation is not evenly distributed, however. The head, for example, is represented
by a region about three times as large as the zone for the entire back and trunk. The size of a zone
correlates to the precision of motor control and sensory discrimination possible. The areas for the lips,
fingers, and tongue are particularly large, considering the proportional size of their represented body
In visual areas, the maps are retinotopic—that is, they reflect the topography of the retina, the layer of
light-activated neurons lining the back of the eye. In this case too the representation is uneven: the
fovea—the area at the center of the visual field—is greatly overrepresented compared to the periphery.
The visual circuitry in the human cerebral cortex contains several dozen distinct retinotopic maps,
each devoted to analyzing the visual input stream in a particular way .The primary visual cortex
(Brodmann area 17), which is the main recipient of direct input from the visual part of the thalamus,
contains many neurons that are most easily activated by edges with a particular orientation moving
across a particular point in the visual field. Visual areas farther downstream extract features such as
color, motion, and shape.
In auditory areas, the primary map is tonotopic. Sounds are parsed according to frequency (i.e., high
pitch vs. low pitch) by subcortical auditory areas, and this parsing is reflected by the primary auditory
zone of the cortex. As with the visual system, there are a number of tonotopic cortical maps, each
devoted to analyzing sound in a particular way.
Within a topographic map there can sometimes be finer levels of spatial structure. In the primary
visual cortex, for example, where the main organization is retinotopic and the main responses are to
moving edges, cells that respond to different edge-orientations are spatially segregated from one
Each hemisphere of the brain interacts primarily with one half of the body, but for reasons that are
unclear, the connections are crossed: the left side of the brain interacts with the right side of the body,
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and vice versa.[citation needed] Motor connections from the brain to the spinal cord, and sensory
connections from the spinal cord to the brain, both cross the midline at brainstem levels. Visual input
follows a more complex rule: the optic nerves from the two eyes come together at a point called the
optic chiasm, and half of the fibers from each nerve split off to join the other. The result is that
connections from the left half of the retina, in both eyes, go to the left side of the brain, whereas
connections from the right half of the retina go to the right side of the brain. Because each half of the
retina receives light coming from the opposite half of the visual field, the functional consequence is
that visual input from the left side of the world goes to the right side of the brain, and vice versa. Thus,
the right side of the brain receives somatosensory input from the left side of the body, and visual input
from the left side of the visual field—an arrangement that presumably is helpful for visuomotor
The two cerebral hemispheres are connected by a very large nerve bundle called the corpus callosum,
which crosses the midline above the level of the thalamus. There are also two much smaller
connections, the anterior commisure and hippocampal commisure, as well as many subcortical
connections that cross the midline. The corpus callosum is the main avenue of communication
between the two hemispheres, though. It connects each point on the cortex to the mirror-image point in
the opposite hemisphere, and also connects to functionally related points in different cortical areas.
In most respects, the left and right sides of the brain are symmetrical in terms of function. For
example, the counterpart of the left-hemisphere motor area controlling the right hand is the righthemisphere area controlling the left hand. There are, however, several very important exceptions,
involving language and spatial cognition. In most people, the left hemisphere is "dominant" for
language: a stroke that damages a key language area in the left hemisphere can leave the victim unable
to speak or understand, whereas equivalent damage to the right hemisphere would cause only minor
impairment to language skills.
A substantial part of our current understanding of the interactions between the two hemispheres has
come from the study of "split-brain patients"—people who underwent surgical transection of the
corpus callosum in an attempt to reduce the severity of epileptic seizures. These patients do not show
unusual behavior that is immediately obvious, but in some cases can behave almost like two different
people in the same body, with the right hand taking an action and then the left hand undoing it. Most
such patients, when briefly shown a picture on the right side of the point of visual fixation, are able to
describe it verbally, but when the picture is shown on the left, are unable to describe it, but may be
able to give an indication with the left hand of the nature of the object shown.
It should be noted that the differences between left and right hemispheres are greatly overblown in
much of the popular literature on this topic. The existence of differences has been solidly established,
but many popular books go far beyond the evidence in attributing features of personality or
intelligence to the left or right hemisphere dominance.
During the first 3 weeks of gestation, the human embryo's ectoderm forms a thickened strip called the
neural plate. The neural plate then folds and closes to form the neural tube. This tube flexes as it
grows, forming the crescent-shaped cerebral hemispheres at the head, and the cerebellum and pons
towards the tail.
Neuroscientists, along with researchers from allied disciplines, study how the human brain works.
Such research has expanded considerably in recent decades. The "Decade of the Brain", an initiative of
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the United States Government in the 1990s, is considered to have marked much of this increase in
Information about the structure and function of the human brain comes from a variety of experimental
methods. Most information about the cellular components of the brain and how they work comes from
studies of animal subjects, using techniques described in the brain article. Some techniques, however,
are used mainly in humans, and therefore are described here.
By placing electrodes on the scalp it is possible to record the summed electrical activity of the cortex,
in a technique known as electroencephalography (EEG).[16] EEG measures mass changes in
population synaptic activity from the cerebral cortex, but can only detect changes over large areas of
the brain, with very little sensitivity for sub-cortical activity. EEG recordings can detect events lasting
only a few thousandths of a second. EEG recordings have good temporal resolution, but poor spatial
Apart from measuring the electric field around the skull it is possible to measure the magnetic field
directly in a technique known as magnetoencephalography (MEG). This technique has the same
temporal resolution as EEG but much better spatial resolution, although not as good as MRI. The
greatest disadvantage of MEG is that, because the magnetic fields generated by neural activity are very
weak, the method is only capable of picking up signals from near the surface of the cortex, and even
then, only neurons located in the depths of cortical folds (sulci) have dendrites oriented in a way that
gives rise to detectable magnetic fields outside the skull.
Structural and functional imaging
There are several methods for detecting brain activity changes by three-dimensional imaging of local
changes in blood flow. The older methods are SPECT and PET, which depend on injection of
radioactive tracers into the bloodstream. The newest method, functional magnetic resonance imaging
(fMRI), has considerably better spatial resolution and involves no radioactivity. Using the most
powerful magnets currently available, fMRI can localize brain activity changes to regions as small as
one cubic millimeter. The downside is that the temporal resolution is poor: when brain activity
increases, the blood flow response is delayed by 1–5 seconds and lasts for at least 10 seconds. Thus,
fMRI is a very useful tool for learning which brain regions are involved in a given behavior, but gives
little information about the temporal dynamics of their responses. A major advantage for fMRI is that,
because it is non-invasive, it can readily be used on human subjects.
Effects of brain damage
A key source of information about the function of brain regions is the effects of damage to them. In
humans, strokes have long provided a "natural laboratory" for studying the effects of brain damage.
Most strokes result from a blood clot lodging in the brain and blocking the local blood supply, causing
damage or destruction of nearby brain tissue: the range of possible blockages is very wide, leading to a
great diversity of stroke symptoms. Analysis of strokes is limited by the fact that damage often crosses
into multiple regions of the brain, not along clear-cut borders, making it difficult to draw firm
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In human beings, it is the left hemisphere that usually contains the specialized language areas. While
this holds true for 97% of right-handed people, about 19% of left-handed people have their language
areas in the right hemisphere and as many as 68% of them have some language abilities in both the left
and the right hemisphere. [citation needed] The two hemispheres are thought to contribute to the
processing and understanding of language: the left hemisphere processes the linguistic meaning of
prosody (or, the rhythm, stress, and intonation of connected speech), while the right hemisphere
processes the emotions conveyed by prosody. Studies of children have shown that if a child has
damage to the left hemisphere, the child may develop language in the right hemisphere instead. The
younger the child, the better the recovery. So, although the "natural" tendency is for language to
develop on the left, human brains are capable of adapting to difficult circumstances, if the damage
occurs early enough.
The first language area within the left hemisphere to be discovered is Broca's area, named after Paul
Broca, who discovered the area while studying patients with aphasia, a language disorder. Broca's area
doesn't just handle getting language out in a motor sense, though. It seems to be more generally
involved in the ability to process grammar itself, at least the more complex aspects of grammar. For
example, it handles distinguishing a sentence in passive form from a simpler subject-verb-object
sentence — the difference between "The boy was hit by the girl" and "The girl hit the boy."
The second language area to be discovered is called Wernicke's area, after Carl Wernicke, a German
neurologist who discovered the area while studying patients who had similar symptoms to Broca's area
patients but damage to a different part of their brain. Wernicke's aphasia is the term for the disorder
occurring upon damage to a patient's Wernicke's area.
Wernicke's aphasia does not only affect speech comprehension. People with Wernicke's aphasia also
have difficulty recalling the names of objects, often responding with words that sound similar, or the
names of related things, as if they are having a hard time recalling word associations.
Clinically, death is defined as an absence of brain activity as measured by EEG. Injuries to the brain
tend to affect large areas of the organ, sometimes causing major deficits in intelligence, memory,
personality, and movement. Head trauma caused, for example, by vehicle or industrial accidents, is a
leading cause of death in youth and middle age. In many cases, more damage is caused by resultant
edema than by the impact itself. Stroke, caused by the blockage or rupturing of blood vessels in the
brain, is another major cause of death from brain damage.
Other problems in the brain can be more accurately classified as diseases than as injuries.
Neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, motor neurone disease,
and Huntington's disease are caused by the gradual death of individual neurons, leading to diminution
in movement control, memory, and cognition.
Mental disorders, such as clinical depression, schizophrenia, bipolar disorder and post-traumatic stress
disorder may involve particular patterns of neuropsychological functioning related to various aspects
of mental and somatic function. These disorders may be treated by psychotherapy, psychiatric
medication or social intervention and personal recovery work; the underlying issues and associated
prognosis vary significantly between individuals.
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Some infectious diseases affecting the brain are caused by viruses and bacteria. Infection of the
meninges, the membrane that covers the brain, can lead to meningitis. Bovine spongiform
encephalopathy (also known as "mad cow disease") is deadly in cattle and humans and is linked to
prions. Kuru is a similar prion-borne degenerative brain disease affecting humans. Both are linked to
the ingestion of neural tissue, and may explain the tendency in human and some non-human species to
avoid cannibalism. Viral or bacterial causes have been reported in multiple sclerosis and Parkinson's
disease, and are established causes of encephalopathy, and encephalomyelitis.
Many brain disorders are congenital, occurring during development. Tay-Sachs disease, fragile X
syndrome, and Down syndrome are all linked to genetic and chromosomal errors. Many other
syndromes, such as the intrinsic circadian rhythm disorders, are suspected to be congenital as well.
Normal development of the brain can be altered by genetic factors, drug use, nutritional deficiencies,
and infectious diseases during pregnancy.
Certain brain disorders are treated by neurosurgeons, while others are treated by neurologists and
Visualization of a diffusion tensor imaging (DTI) measurement of a human brain. Depicted are
reconstructed axon tracts that run through the mid-sagittal plane. Especially prominent are the Ushaped fibers that connect the two hemispheres through the corpus callosum (the fibers come out of
the image plane and consequently bend towards the top) and the fiber tracts that descend toward the
spine (blue, within the image plane).
Brain metabolism normally is completely dependent upon blood glucose as an energy source, since
fatty acids do not cross the blood-brain barrier. During times of low glucose (such as fasting), the
brain will instead use ketone bodies for fuel. The brain does not store any glucose in the form of
glycogen, in contrast, for example, to skeletal muscle.
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian
brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and
consciousness. It is constituted of up to six horizontal layers, each of which has a different
composition in terms of neurons and connectivity. The human cerebral cortex is 2–4 mm (0.08–0.16
inches) thick.
In preserved brains, it has a gray color, hence the name "gray matter". In contrast to gray matter that is
formed from neurons and their unmyelinated fibers, the white matter below them is formed
predominantly by myelinated axons interconnecting neurons in different regions of the cerebral cortex
with each other and neurons in other parts of the central nervous system.
The surface of the cerebral cortex is folded in large mammals, such that more than two-thirds of it in
the human brains is buried in the grooves, called "sulci". The phylogenetically most recent part of the
cerebral cortex, the neocortex (also called isocortex), is differentiated into six horizontal layers; the
more ancient part of the cerebral cortex, the hippocampus (also called archicortex), has at most three
cellular layers, and is divided into subfields. Neurons in various layers connect vertically to form small
microcircuits, called columns. Different neocortical architectonic fields are distinguished upon
variations in the thickness of these layers, their predominant cell type and other factors such as
neurochemical markers.
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The cerebral cortex develops from the most anterior part of the neural plate, a specialized part of the
embryonic ectoderm. The neural plate folds and closes to form the neural tube. From the cavity inside
the neural tube develops the ventricular system, and, from the epithelial cells of its walls, the neurons
and glia of the nervous system. The most anterior (frontal) part of the neural tube, the telencephalon,
gives rise to the cerebral hemispheres and cortex.
Cortical neurons are generated within the ventricular zone, next to the ventricles. At first, this zone
contains "progenitor" cells, which divide to produce glial and neuronal cells . The glial fibers produced
in the first divisions of the progenitor cells are radially oriented, spanning the thickness of the cortex
from the ventricular zone to the outer, pial surface, and provide scaffolding for the migration of
neurons outwards from the ventricular zone. The first divisions of the progenitor cells are symmetric,
which duplicates the total number of progenitor cells at each mitotic cycle. Then, some progenitor
cells begin to divide asymmetrically, producing one postmitotic cell that migrates along the radial glial
fibers, leaving the ventricular zone, and one progenitor cell, which continues to divide until the end of
development, when it differentiates into a glial cell or an ependymal cell. The migrating daughter cells
become the pyramidal neurons of the cerebral cortex.
The layered structure of the mature cerebral cortex is formed during development. The first pyramidal
neurons generated migrate out of the ventricular zone and subventricular zone, together with CajalRetzius cells form the preplate. Next, a cohort of neurons migrating into the middle of the preplate
divides this transient layer into the superficial marginal zone, which will become layer one of the
mature neocortex, and the subplate, forming a middle layer called the cortical plate. These cells will
form the deep layers of the mature cortex, layers five and six. Later born neurons migrate radially into
the cortical plate past the deep layer neurons, and become the upper layers (two to four). Thus, the
layers of the cortex are created in an inside-out order. The only exception to this inside-out sequence
of neurogenesis occurs in the layer I of primates, in which, contrary to rodents, neurogenesis continues
throughout the entire period of corticogenesis.
Laminar pattern
The different cortical layers each contain a characteristic distribution of neuronal cell types and
connections with other cortical and subcortical regions. One of the clearest examples of cortical
layering is the Stria of Gennari in the primary visual cortex. This is a band of whiter tissue that can be
observed with the naked eye in the fundus of the calcarine sulcus of the occipital lobe. The Stria of
Gennari is composed of axons bringing visual information from the thalamus into layer four of visual
Staining cross-sections of the cortex to reveal the position of neuronal cell bodies and the intracortical
axon tracts allowed neuroanatomists in the early 20th century to produce a detailed description of the
laminar structure of the cortex in different species. After the work of Korbinian Brodmann (1909), the
neurons of the cerebral cortex are grouped into six main layers, from outside (pial surface) to inside
(white matter):
The molecular layer I, which contains few scattered neurons and consists mainly of extensions of
apical dendritic tufts of pyramidal neurons and horizontally-oriented axons, as well as glial cells.
Some Cajal-Retzius and spiny stellate neurons can be found here. Inputs to the apical tufts are thought
to be crucial for the ‘‘feedback’’ interactions in the cerebral cortex involved in associative learning
and attention. While it was once thought that the input to layer I came from the cortex itself, it is now
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realized that layer I across the cerebral cortex mantle receives substantial input from ‘‘matrix’’ or Mtype thalamus cells (in contrast to ‘‘core’’ or C-type that go to layer IV).
The external granular layer II, which contains small pyramidal neurons and numerous stellate neurons
The external pyramidal layer III, which contains predominantly small and medium-size pyramidal
neurons, as well as non-pyramidal neurons with vertically-oriented intracortical axons; layers I
through III are the main target of interhemispheric corticocortical afferents, and layer III is the
principal source of corticocortical efferents
The internal granular layer IV, which contains different types of stellate and pyramidal neurons, and is
the main target of thalamocortical afferents from thalamus type C neurons. as well as intrahemispheric corticocortical afferents
The internal pyramidal layer V, which contains large pyramidal neurons (such as the Betz cells in the
primary motor cortex); it is the principal source of subcortical efferents
The multiform layer VI, which contains few large pyramidal neurons and many small spindle-like
pyramidal and multiform neurons; layer VI sends efferent fibers to the thalamus, establishing a very
precise reciprocal interconnection between the cortex and the thalamus. These connections are both
excitatory and inhibitory. Neurons send excitatory fibers to neurons in the thalamus and also from
collateral to them ones via the thalamic reticular nucleus that inhibit these thalamus neurons or ones
adjacent to them. Since the inhibitory output is reduced by cholinergic input to the cerebral cortex, this
provides the brainstem with adjustable "gain control for the relay of lemnsical inputs".
t is important to note that the cortical layers are not simply stacked one over the other; there exist
characteristic connections between different layers and neuronal types, which span all the thickness of
the cortex. These cortical microcircuits are grouped into cortical columns and minicolumns, the latter
of which have been proposed to be the basic functional units of cortex. In 1957, Vernon Mountcastle
showed that the functional properties of the cortex change abruptly between laterally adjacent points;
however, they are continuous in the direction perpendicular to the surface. Later works have provided
evidence of the presence of functionally distinct cortical columns in the visual cortex (Hubel and
Wiesel, 1959), auditory cortex and associative cortex.
Cortical areas that lack a layer IV are called agranular. Cortical areas that have only a rudimentary
layer IV are called dysgranular. Information processing within each layer is determined by different
temporal dynamics with that in the layers II/III having a slow 2Hz oscillation while that in layer V
having a fast 10–15 Hz one.
Connections of the cerebral cortex
The cerebral cortex is connected to various subcortical structures such as the thalamus and the basal
ganglia, sending information to them along efferent connections and receiving information from them
via afferent connections. Most sensory information is routed to the cerebral cortex via the thalamus.
Olfactory information, however, passes through the olfactory bulb to the olfactory cortex (piriform
cortex). The vast majority of connections are from one area of the cortex to another rather than to
subcortical areas; Braitenberg and Schüz (1991) put the figure as high as 99%.
The cortex is commonly described as comprising three parts: sensory, motor, and association areas.
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Sensory areas
The sensory areas are the areas that receive and process information from the senses. Parts of the
cortex that receive sensory inputs from the thalamus are called primary sensory areas. The senses of
vision, audition, and touch are served by the primary visual cortex, primary auditory cortex and
primary somatosensory cortex. In general, the two hemispheres receive information from the opposite
(contralateral) side of the body. For example the right primary somatosensory cortex receives
information from the left limbs, and the right visual cortex receives information from the left visual
field. The organization of sensory maps in the cortex reflects that of the corresponding sensing organ,
in what is known as a topographic map. Neighboring points in the primary visual cortex, for example,
correspond to neighboring points in the retina. This topographic map is called a retinotopic map. In the
same way, there exists a tonotopic map in the primary auditory cortex and a somatotopic map in the
primary sensory cortex. This last topographic map of the body onto the posterior central gyrus has
been illustrated as a deformed human representation, the somatosensory homunculus, where the size
of different body parts reflects the relative density of their innervation. Areas with lots of sensory
innervation, such as the fingertips and the lips, require more cortical area to process finer sensation.
Motor areas
The motor areas are located in both hemispheres of the cortex. They are shaped like a pair of
headphones stretching from ear to ear. The motor areas are very closely related to the control of
voluntary movements, especially fine fragmented movements performed by the hand. The right half of
the motor area controls the left side of the body, and vice versa.
Two areas of the cortex are commonly referred to as motor:
Primary motor cortex, which executes voluntary movements
Supplementary motor areas and premotor cortex, which select voluntary movements.
In addition, motor functions have been described for:
Posterior parietal cortex, which guides voluntary movements in space
Dorsolateral prefrontal cortex, which decides which voluntary movements to make according to
higher-order instructions, rules, and self-generated thoughts.
Association areas
Association areas function to produce a meaningful perceptual experience of the world, enable us to
interact effectively, and support abstract thinking and language. The parietal, temporal, and occipital
lobes - all located in the posterior part of the cortex - organize sensory information into a coherent
perceptual model of our environment centered on our body image. The frontal lobe or prefrontal
association complex is involved in planning actions and movement, as well as abstract thought. In the
past it was theorized that language abilities are localized in the left hemisphere in areas 44/45, the
Broca's area, for language expression and area 22, the Wernicke's area, for language reception.
However, language is no longer limited to easily identifiable areas. More recent research suggests that
the processes of language expression and reception occur in areas other than just the perisylvian
structures, such as the prefrontal lobe, basal ganglia, cerebellum, pons, caudate nucleus, and others.
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Based on the differences in lamination the cerebral cortex can be classified into two major groups:
Isocortex (homotypical cortex), the part of the cortex with six layers.
Allocortex (heterotypical cortex) the part of the cortex with less than six layers (varying in number).
The allocortex includes the olfactory cortex and the hippocampus.
Auxiliary classes are:
Mesocortex, classification between isocortex and allocortex where layers 2, 3, and 4 are merged
Proisocortex, Brodmann areas 24, 25, 32
Periallocortex, comprising cortical areas adjacent to allocortex.
Based on supposed developmental differences the following classification also appears:
Neocortex or Neopallium, which corresponds to the isocortex.
Archicortex, which phylogenetically is the oldest cortex
In addition, cortex may be classified on the basis of gross topographical conventions into four lobes:
Temporal Cortex
Occipital Cortex
Parietal Cortex
Frontal Cortex
Cortical thickness
With magnetic resonance brain scanners, it is possible to get a measure for the thickness of the human
cerebral cortex and relate it to other measures. The thickness of different cortical areas varies. In
general, sensory cortex is thinner and motor cortex is thicker. One study has found some positive
association between the cortical thickness and intelligence. Another study has found that the
somatosensory cortex is thicker in migraine sufferers.
It is a pair of nuclear masses which form the basal ganglia, along with the subthalamic nucleus and the
substantia nigra. It may also refer to both the basal ganglia and internal capsule collectively.
According to the 1917 version of Gray's Anatomy, it is the combination of the lentiform nucleus (also
known as the lenticular nucleus) and the caudate nucleus
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From lateral to medial, the corpus striatum is composed of the external capsule (white matter), the
lentiform nucleus (gray matter), the internal capsule (white matter), and the caudate nucleus (gray
matter). The alternating white and gray matter give it a striated appearance.
The corpus striatum has received its name from the striped appearance which a section of its anterior
part presents, in consequence of diverging white fibers being mixed with the gray substance which
forms its chief mass.
A part of the corpus striatum is imbedded in the white substance of the hemisphere, and is therefore
external to the ventricle; it is termed the extraventricular portion, or the lenticular nucleus.
The remainder, however, projects into the ventricle, and is named the intraventricular portion, or the
caudate nucleus.
The thalamus is a midline paired symmetrical structure within the brains of vertebrates, including
humans. It is situated between the cerebral cortex and midbrain, both in terms of location and
neurological connections. Its function includes relaying sensation, special sense and motor signals to
the cerebral cortex, along with the regulation of consciousness, sleep and alertness. The thalamus
surrounds the third ventricle. It is the main product of the embryonic diencephalon.
Location and topography
The thalamus is the largest structure in the diencephalon, the part of the brain situated between the
midbrain (mesencephalon) and forebrain (telencephalon). Anatomically, the thalamus is perched on
top of the brainstem, near the center of the brain, in a position to send nerve fibers out to the cerebral
cortex in all directions. The diencephalon includes also the dorsally located epithalamus (essentially
the habenula and annexes) and the perithalamus (prethalamus formerly described as ventral thalamus)
containing the zona incerta and the "reticulate nucleus" (not the reticular, term of confusion). Due to
their different ontogenetic origins, the epithalamus and the perithalamus are formally distinguished
from the thalamus proper.
In humans, the two halves of the thalamus are prominent bulb-shaped masses, about 5.7 cm in length,
located obliquely (about 30°) and symmetrically on each side of the third ventricle.
The thalamus comprises a system of lamellae (made up of myelinated fibers) separating different
thalamic subparts. Other areas are defined by distinct clusters of neurons, such as the periventricular
gray, the intralaminar elements, the "nucleus limitans", and others. These latter structures, different in
structure from the major part of the thalamus, have been grouped together into the allothalamus as
opposed to the isothalamus. This distinction simplifies the global description of the thalamus.
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Arterial supply
The thalamus derives its blood supply from a number of arteries including polar and paramedian
arteries, inferolateral (thalamogeniculate) arteries, and posterior (medial and lateral) choroidal arteries.
These are all branches of the posterior cerebral artery.
The thalamus has multiple functions. It is generally believed to act as a relay between a variety of
subcortical areas and the cerebral cortex. In particular, every sensory system (with the exception of the
olfactory system) includes a thalamic nucleus that receives sensory signals and sends them to the
associated primary cortical area. For the visual system, for example, inputs from the retina are sent to
the lateral geniculate nucleus of the thalamus, which in turn projects to the primary visual cortex (area
V1) in the occipital lobe. The thalamus is believed to both process sensory information as well as
relaying it—each of the primary sensory relay areas receives strong "back projections" from the
cerebral cortex. Similarly the medial geniculate nucleus acts as a key auditory relay between the
inferior colliculus of the midbrain and the primary auditory cortex, and the ventral posterior nucleus is
a key somatosensory relay, which sends touch and proprioceptive information to the primary
somatosensory cortex.
The thalamus also plays an important role in regulating states of sleep and wakefulness. Thalamic
nuclei have strong reciprocal connections with the cerebral cortex, forming thalamo-cortico-thalamic
circuits that are believed to be involved with consciousness. The thalamus plays a major role in
regulating arousal, the level of awareness, and activity. Damage to the thalamus can lead to permanent
Many different functions are linked to various regions of the thalamus. This is the case for many of the
sensory systems (except for the olfactory system), such as the auditory, somatic, visceral, gustatory
and visual systems where localized lesions provoke specific sensory deficits. A major role of the
thalamus is devoted to "motor" systems. This has been and continues to be a subject of interest for
investigators. VIm, the relay of cerebellar afferences, is the target of stereotactians particularly for the
improvement of tremor. The role of the thalamus in the more anterior pallidal and nigral territories in
the basal ganglia system disturbances is recognized but still poorly understood. The contribution of the
thalamus to vestibular or to tectal functions is almost ignored. The thalamus has been thought of as a
"relay" that simply forwards signals to the cerebral cortex. Newer research suggests that thalamic
function is more selective.
Cerebrovascular accidents (strokes) can cause thalamic syndrome, which results in a contralateral
hemianaesthesia, burning or aching sensation on one half of a body (painful anaesthesia) often
accompanied by mood swings. Ischemia of the territory of the paramedian artery, if bilateral, causes
serious troubles including akinetic mutism accompanied or not by oculomotor troubles. It is also
related to Thalamocortical Dysrhythmia.
The thalamic complex is composed of the perithalamus (or prethalamus, previously also known as
ventral thalamus), the zona limitans intrathalamica (ZLI) and the thalamus (dorsal thalamus).
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The ZLI is a transverse boundary located between the perithalamus and the functional distinct
thalamus. Besides its morphological characteristics, it bears the hallmarks of a signalling centre. Fate
mapping experiments in chicks have shown that the ZLI is cell lineage restricted at its boundaries and
therefore can be termed a true developmental compartment in the forebrain.
Besides morphological characteristics, the ZLI is the only structure in the alar plate of the neural tube
that expresses signaling molecules.
In mice, the function of signaling at the ZLI has not been addressed directly due to a complete absence
of the diencephalon in Shh mutants.
Studies in chicks have shown that Shh is both necessary and sufficient for thalamic gene induction.
In zebrafish, it was shown that the expression of two Shh genes, shh-a and shh-b (formerly described
as twhh) mark the ZLI territory, and that Shh signaling is sufficient for the molecular differentiation of
both the prethalamus and the thalamus but is not required for their maintenance and Shh signaling
from the ZLI/alar plate is sufficient for the maturation of prethalamic and thalamic territory while
ventral Shh signals are dispensable.
In humans, a common genetic variation in the promotor region of the serotonin transporter (the SERTlong and -short allele: 5-HTTLPR) has been shown to affect the development of several regions of the
thalamus in adults. People who inherit two short alleles (SERT-ss) have more neurons and a larger
volume in the pulvinar and possibly the limbic regions of the thalamus. Enlargement of the thalamus
provides an anatomical basis for why people who inherit two SERT-ss alleles are more vulnerable to
major depression, posttraumatic stress disorder, and suicide.
The hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of
functions. One of the most important functions of the hypothalamus is to link the nervous system to
the endocrine system via the pituitary gland (hypophysis).
The hypothalamus is located below the thalamus, just above the brain stem. In the terminology of
neuroanatomy, it forms the ventral part of the diencephalon. All vertebrate brains contain a
hypothalamus. In humans, it is roughly the size of an almond.
The hypothalamus is responsible for certain metabolic processes and other activities of the Autonomic
Nervous System. It synthesizes and secretes neurohormones, often called hypothalamic-releasing
hormones, and these in turn stimulate or inhibit the secretion of pituitary hormones. The hypothalamus
controls body temperature, hunger, thirst, fatigue, and circadian cycles.
The hypothalamus is a complex region in the brain of humans, and even small nuclei within the
hypothalamus are involved in many different functions. The paraventricular nucleus for instance
contains oxytocin and vasopressin (also called antidiuretic hormone) neurons which project to the
posterior pituitary, but also contains neurons that regulate ACTH and TSH secretion from the anterior
pituitary, as well as gastric reflexes, maternal behavior, blood pressure, feeding, immune responses,
and temperature.
The hypothalamus co-ordinates many hormonal and behavioural circadian rhythms, complex patterns
of neuroendocrine outputs, complex homeostatic mechanisms, and many important behaviours.
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The hypothalamus must therefore respond to many different signals, some of which are generated
externally and some internally. It is thus richly connected with many parts of the central nervous
system, including the brainstem reticular formation and autonomic zones, the limbic forebrain
(particularly the amygdala, septum, diagonal band of Broca, and the olfactory bulbs, and the cerebral
The hypothalamus is responsive to:
Light: daylength and photoperiod for regulating circadian and seasonal rhythms
Olfactory stimuli, including pheromones
Steroids, including gonadal steroids and corticosteroids
Neurally transmitted information arising in particular from the heart, the stomach, and the
reproductive tract
Autonomic inputs
Blood-borne stimuli, including leptin, ghrelin, angiotensin, insulin, pituitary hormones, cytokines,
plasma concentrations of glucose and osmolarity etc.
Invading microorganisms by increasing body temperature, resetting the body's thermostat upward.
Olfactory stimuli
Olfactory stimuli are important for sex and neuroendocrine function in many species. For instance if a
pregnant mouse is exposed to the urine of a 'strange' male during a critical period after coitus then the
pregnancy fails (the Bruce effect). Thus during coitus, a female mouse forms a precise 'olfactory
memory' of her partner which persists for several days. Pheromonal cues aid synchronisation of
oestrus in many species; in women, synchronised menstruation may also arise from pheromonal cues,
although the role of pheromones in humans is doubted by many.
Blood-borne stimuli
Peptide hormones have important influences upon the hypothalamus, and to do so they must evade the
blood-brain barrier. The hypothalamus is bounded in part by specialized brain regions that lack an
effective blood-brain barrier; the capillary endothelium at these sites is fenestrated to allow free
passage of even large proteins and other molecules. Some of these sites are the sites of neurosecretion
- the neurohypophysis and the median eminence. However others are sites at which the brain samples
the composition of the blood. Two of these sites, the subfornical organ and the OVLT (organum
vasculosum of the lamina terminalis) are so-called circumventricular organs, where neurons are in
intimate contact with both blood and CSF. These structures are densely vascularized, and contain
osmoreceptive and sodium-receptive neurons which control drinking, vasopressin release, sodium
excretion, and sodium appetite. They also contain neurons with receptors for angiotensin, atrial
natriuretic factor, endothelin and relaxin, each of which is important in the regulation of fluid and
electrolyte balance. Neurons in the OVLT and SFO project to the supraoptic nucleus and
paraventricular nucleus, and also to preoptic hypothalamic areas. The circumventricular organs may
also be the site of action of interleukins to elicit both fever and ACTH secretion, via effects on
paraventricular neurons.
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It is not clear how all peptides that influence hypothalamic activity gain the necessary access. In the
case of prolactin and leptin, there is evidence of active uptake at the choroid plexus from blood into
CSF. Some pituitary hormones have a negative feedback influence upon hypothalamic secretion; for
example, growth hormone feeds back on the hypothalamus, but how it enters the brain is not clear.
There is also evidence for central actions of prolactin and TSH.
The hypothalamus functions as a type of thermostat for the body. It sets a desired body temperature,
and stimulates either heat production and retention to raise the blood temperature to a higher setting,
or sweating and vasodilation to cool the blood to a lower temperature. All fevers result from a raised
setting in the hypothalamus; elevated body temperatures due to any other cause are classified as
hyperthermia. Rarely, direct damage to the hypothalamus, such as from a stroke, will cause a fever;
this is sometimes called a hypothalamic fever. However, it is more common for such damage to cause
abnormally low body temperatures.
The hypothalamus contains neurons that react strongly to steroids and glucocorticoids – (the steroid
hormones of the adrenal gland, released in response to ACTH). It also contains specialised glucosesensitive neurons (in the arcuate nucleus and ventromedial hypothalamus), which are important for
appetite. The preoptic area contains thermosensitive neurons; these are important for TRH secretion.
Neural inputs
The hypothalamus receives many inputs from the brainstem; notably from the nucleus of the solitary
tract, the locus coeruleus, and the ventrolateral medulla. Oxytocin secretion in response to suckling or
vagino-cervical stimulation is mediated by some of these pathways; vasopressin secretion in response
to cardiovascular stimuli arising from chemoreceptors in the carotid body and aortic arch, and from
low-pressure atrial volume receptors, is mediated by others. In the rat, stimulation of the vagina also
causes prolactin secretion, and this results in pseudo-pregnancy following an infertile mating. In the
rabbit, coitus elicits reflex ovulation. In the sheep, cervical stimulation in the presence of high levels
of estrogen can induce maternal behavior in a virgin ewe. These effects are all mediated by the
hypothalamus, and the information is carried mainly by spinal pathways that relay in the brainstem.
Stimulation of the nipples stimulates release of oxytocin and prolactin and suppresses the release of
LH and FSH.
Cardiovascular stimuli are carried by the vagus nerve, but the vagus also conveys a variety of visceral
information, including for instance signals arising from gastric distension to suppress feeding. Again
this information reaches the hypothalamus via relays in the brainstem.
A cross section of the monkey hypothalamus displays 2 of the major hypothalamic nuclei on either
side of the fluid-filled 3rd ventricle
The outputs of the hypothalamus can be divided into two categories: neural projections, and endocrine
Neural projections
Most fiber systems of the hypothalamus run in two ways (bidirectional).
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Projections to areas caudal to the hypothalamus go through the medial forebrain bundle, the
mammillotegmental tract and the dorsal longitudinal fasciculus.
Projections to areas rostral to the hypothalamus are carried by the mammillothalamic tract, the fornix
and terminal stria.
Projections to areas of the sympathetic motor system (lateral horn spinal segments T1-L2/L3 of the)
are carried by the hypothalamospinal tract and they activate the sympathetic motor pathway
Endocrine hormones
The hypothalamus affects the endocrine system and governs emotional behavior, such as anger and
sexual activity. Most of the hypothalamic hormones generated are distributed to the pituitary via the
hypophyseal portal system.[10] The hypothalamus maintains homeostasis; this includes a regulation of
blood pressure, heart rate, and temperature.
Control of food intake
The extreme lateral part of the ventromedial nucleus of the hypothalamus is responsible for the control
of food intake. Stimulation of this area causes increased food intake. Bilateral lesion of this area
causes complete cessation of food intake. Medial parts of the nucleus have a controlling effect on the
lateral part. Bilateral lesion of the medial part of the ventromedial nucleus causes hyperphagia and
obesity of the animal. Further lesion of the lateral part of the ventromedial nucleus in the same animal
produces complete cessation of food intake.
There are different hypotheses related to this regulation:
Lipostatic hypothesis - this hypothesis holds that adipose tissue produces a humoral signal that is
proportionate to the amount of fat and acts on the hypothalamus to decrease food intake and increase
energy output. It has been evident that a hormone leptin acts on the hypothalamus to decrease food
intake and increase energy output.
Gutpeptide hypothesis - gastrointestinal hormones like Grp, glucagons, CCK and others claimed to
inhibit food intake. The food entering the gastrointestinal tract triggers the release of these hormones
which acts on the brain to produce satiety. The brain contains both CCK-A and CCK-B receptors.
Glucostatic hypothesis - the activity of the satiety center in the ventromedial nuclei is probably
governed by the glucose utilization in the neurons. It has been postulated that when their glucose
utilization is low and consequently when the arteriovenous blood glucose difference across them is
low, the activity across the neurons decrease. Under these conditions, the activity of the feeding center
is unchecked and the individual feels hungry. Food intake is rapidly increased by intraventricular
administration of 2-deoxyglucose therefore decreasing glucose utilization in cells.
Thermostatic hypothesis - according to this hypothesis, a decrease in body temperature below a given
set point stimulates appetite, while an increase above the set point inhibits appetite.
Sexual dimorphism
Several hypothalamic nuclei are sexually dimorphic, i.e. there are clear differences in both structure
and function between males and females.
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Some differences are apparent even in gross neuroanatomy: most notable is the sexually dimorphic
nucleus within the preoptic area, which is present only in males. However most of the differences are
subtle changes in the connectivity and chemical sensitivity of particular sets of neurons.
The importance of these changes can be recognised by functional differences between males and
females. For instance, males of most species prefer the odor and appearance of females over males,
which is instrumental in stimulating male sexual behavior. If the sexually dimorphic nucleus is
lesioned, this preference for females by males diminishes. Also, the pattern of secretion of growth
hormone is sexually dimorphic, and this is one reason why in many species, adult males are much
larger than females
Responses to ovarian steroids
Other striking functional dimorphisms are in the behavioral responses to ovarian steroids of the adult.
Males and females respond differently to ovarian steroids, partly because the expression of estrogensensitive neurons in the hypothalamus is sexually dimorphic, i.e. estrogen receptors are expressed in
different sets of neurons.
Estrogen and progesterone can influence gene expression in particular neurons or induce changes in
cell membrane potential and kinase activation, leading to diverse non-genomic cellular functions.
Estrogen and progesterone bind to their cognate nuclear hormone receptors, which translocate to the
cell nucleus and interact with regions of DNA known as hormone response elements (HREs) or get
tethered to another transcription factor's binding site. Estrogen receptor (ER) has been shown to
transactivate other transcription factors in this manner, despite the absence of an estrogen response
element (ERE) in the proximal promoter region of the gene. ERs and progesterone receptors (PRs) are
generally gene activators, with increased mRNA and subsequent protein synthesis following hormone
Male and female brains differ in the distribution of estrogen receptors, and this difference is an
irreversible consequence of neonatal steroid exposure. Estrogen receptors (and progesterone receptors)
are found mainly in neurons in the anterior and mediobasal hypothalamus, notably:
the preoptic area (where LHRH neurons are located)
the periventricular nucleus (where somatostatin neurons are located)
the ventromedial hypothalamus (which is important for sexual behavior).
Gonadal steroids in neonatal life of rats
In neonatal life, gonadal steroids influence the development of the neuroendocrine hypothalamus. For
instance, they determine the ability of females to exhibit a normal reproductive cycle, and of males
and females to display appropriate reproductive behaviors in adult life.
If a female rat is injected once with testosterone in the first few days of postnatal life (during the
"critical period" of sex-steroid influence), the hypothalamus is irreversibly masculinized; the adult rat
will be incapable of generating an LH surge in response to estrogen (a characteristic of females), but
will be capable of exhibiting male sexual behaviors (mounting a sexually receptive female).
By contrast, a male rat castrated just after birth will be feminized, and the adult will show female
sexual behavior in response to estrogen (sexual receptivity, lordosis behavior).
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Androgens in primates
In primates, the developmental influence of androgens is less clear, and the consequences are less
understood. Within the brain, testosterone is aromatized to (estradiol), which is the principal active
hormone for developmental influences. The human testis secretes high levels of testosterone from
about week 8 of fetal life until 5–6 months after birth (a similar perinatal surge in testosterone is
observed in many species), a process that appears to underlie the male phenotype. Estrogen from the
maternal circulation is relatively ineffective, partly because of the high circulating levels of steroidbinding proteins in pregnancy.
Human sexual orientation and the hypothalamus
According to D.F. Swaab, "Neurobiological research related to sexual orientation in humans is only
just gathering momentum, but the evidence already shows that humans have a vast array of brain
differences, not only in relation to gender, but also in relation to sexual orientation." Specifically, there
are similarities between the hypothalamuses in heterosexual men (HeM) and homosexual women
(HoW) and also between homosexual men (HoM) and heterosexual women (HeW).
Swaab first reported on the relationship between sexual orientation in males and the hypothalamus's
"clock", the suprachiasmatic nucleus (SCN). In 1990, Swaab and Hofman reported that the SCN of
HoM was significantly larger than HeM. Then in 1995, Swaab et al. linked brain development to
sexual orientation by treating male rats both pre- and postnatally with ATD, a testosterone blocker in
the brain. This produced an enlarged SCN and bisexual behavior in the adult male rats. In 1991,
LeVay showed that part of the sexually dimorphic nucleus (SDN), the interstitial nuclei of the anterior
hypothalamus (INAH) 3, is twice as large in HeM as HoM and HeW.
In 2004 and 2006, two studies by Berglund, Lindström, and Savic used Positron Emission
Tomography (PET) to observe how the hypothalamus responds to smelling common odors, the scent
of testosterone found in male sweat, and the scent of estrogen found in female urine. These studies
showed that the hypothalamus of HeM and HoW both respond to estrogen. Also, the hypothalamus of
HoM and HeW both respond to testosterone. The hypothalamus of all four groups did not respond to
the common odors, which produced a normal olfactory response in the brain.
Other influences upon hypothalamic development
Sex steroids are not the only important influences upon hypothalamic development; in particular, prepubertal stress in early life determines the capacity of the adult hypothalamus to respond to an acute
stressor. Unlike gonadal steroid receptors, glucocorticoid receptors are very widespread throughout the
brain; in the paraventricular nucleus, they mediate negative feedback control of CRF synthesis and
secretion, but elsewhere their role is not well understood.
In biological anatomy, the mesencephalon (or midbrain) comprises the tectum (or corpora
quadrigemini), tegmentum, the ventricular mesocoelia (or "iter"), and the cerebral peduncles, as well
as several nuclei and fasciculi. Caudally the mesencephalon adjoins the pons (metencephalon) and
rostrally it adjoins the diencephalon (Thalamus, hypothalamus, et al.).
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During development, the mesencephalon forms from the middle of three vesicles that arise from the
neural tube to generate the brain. In mature human brains, the mesencephalon becomes the least
differentiated, from both its developmental form and within its own structure, among the three
vesicles. The mesencephalon is considered part of the brain stem. Its substantia nigra is closely
associated with motor system pathways of the basal ganglia.
The human mesencephalon is archipallian in origin, meaning its general architecture is shared with the
most ancient of vertebrates. Dopamine produced in the substantia nigra plays a role in motivation and
habituation of species from humans to the most elementary animals such as insects.
Corpora quadrigemina
The corpora quadrigemina ("quadruplet bodies") are four solid optic lobes on the dorsal side of
cerebral aqueduct, where the superior posterior pair are called the superior colliculi and the inferior
posterior pair are called the inferior colliculi. The four solid optic lobes help to decussate several fibres
of the optic nerve. However some fibers also show ipsilateral arrangement (i.e. they run parallel on the
same side without decussating.) The superior colliculus is involved with saccadic eye movements;
while the inferior is a synapsing point for sound information. The trochlear nerve comes out of the
posterior surface of the midbrain, below the inferior colliculus.
Cerebral peduncle
The cerebral peduncles are paired structures, present on the ventral side of cerebral aqueduct, and they
further carry tegmentum on the dorsal side and cresta or pes on the ventral side, and both of them
accommodate the corticospinal tract fibres, from the internal capsule (i.e. ascending + descending
tracts = longitudinal tract.) the middle part of cerebral peduncles carry substantia nigra (also called
"Black Matter") which is a type of basal nucleus. It is the only part of the brain that carries melanin
Between the peduncles is the interpeduncular fossa, which is a cistern filled with cerebrospinal fluid.
The oculomotor nerve comes out between the peduncles, and the trochlear nerve is visible wrapping
around the outside of the peduncles.
Cross-section through the midbrain
The midbrain is usually sectioned at the level of the superior and inferior colliculi.
A cross-section at the level of the superior colliculus shows the red nucleus, the nuclei of the
oculomotor nerve (and associated Edinger-Westphal nucleus), as well as the substantia nigra.
The substantia nigra is still present at inferior colliculus level. Also apparent are the trochlear nerve
nucleus, and the decussation of the superior cerebellar peduncles.
The cerebral aqueduct runs through the midbrain, and is the communication between the third and
fourth ventricle.
As a mnemonic the mesencephalic cross-section resembles a bear (or teddy bear) upside down with
the two red nuclei as the eyes and the crus cerebri as the ears.
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inferior colliculi
superior colliculi
cerebral peduncle
midbrain tegmentum
crus cerebri
substantia nigra
The pons (Latin for "bridge"), sometimes pons Varolii (after Costanzo Varolio, a 16th-century Italian
anatomist and surgeon) is a structure located on the brain stem. It is superior to (up from) the medulla
oblongata, inferior to (down from) the midbrain, and ventral to (in front of) the cerebellum. In humans
and other bipeds this means it is above the medulla, below the midbrain, and anterior to the
cerebellum. Its white matter includes tracts that conduct signals from the cerebrum down to the
cerebellum and medulla, and tracts that carry the sensory signals up into the thalamus.
The pons measures about 2.5 cm in length. Most of it appears as a broad anterior bulge rostral to the
medulla. Posteriorly, it consists mainly of two pairs of thick stalks called cerebellar peduncles. They
connect the cerebellum to the pons and midbrain.
The pons contains nuclei that relay signals from the cerebrum to the cerebellum, along with nuclei that
deal primarily with sleep, respiration, swallowing, bladder control, hearing, equilibrium, taste, eye
movement, facial expressions, facial sensation, and posture.
Within the pons is the pneumotaxic center, a nucleus in the pons that regulates the change from
inspiration to expiration.
Embryonic development
During embryonic development the embryonic metencephalon develops into two structures: the pons
and the cerebellum.
Cranial nerve nuclei
A number of cranial nerve nuclei are present in the pons:
mid-pons: The chief or pontine nucleus of the trigeminal nerve sensory nucleus (V)
mid-pons: the motor nucleus for the trigeminal nerve (V)
lower down in the pons: abducens nucleus (VI)
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lower down in the pons: facial nerve nucleus (VII)
lower down in the pons: vestibulocochlear nuclei (vestibular nuclei and cochlear nuclei) (VIII)
The functions of these four nerves include sensory roles in hearing, equilibrium, and taste, and in
facial sensations such as touch and pain; as well as motor roles in eye movement, facial expressions,
chewing, swallowing, urination, and the secretion of saliva and tears.
Related diseases
Central pontine myelinosis, a demyelination disease that causes difficulty with sense of balance,
walking, sense of touch, swallowing and speaking. In a clinical setting it is often associated with
transplant. Undiagnosed it can lead to death or locked-in syndrome.
The cerebellum (Latin for little brain) is a region of the brain that plays an important role in motor
control. It is also involved in some cognitive functions such as attention and language, and probably in
some emotional functions such as regulating fear and pleasure responses, but it is its function in
movement that is most clearly understood. The cerebellum does not initiate movement, but it
contributes to coordination, precision, and accurate timing. It receives input from sensory systems and
from other parts of the brain and spinal cord, and integrates these inputs to fine tune motor activity.
Because of this fine-tuning function, damage to the cerebellum does not cause paralysis, but instead
produces disorders in fine movement, equilibrium, posture, and motor learning.
Anatomically, the cerebellum has the appearance of a separate structure attached to the bottom of the
brain, tucked underneath the cerebral hemispheres. The surface of the cerebellum is covered with
finely spaced parallel grooves, in striking contrast to the broad irregular convolutions of the cerebral
cortex. These parallel grooves conceal the fact that the cerebellum is actually a continuous thin layer
of neural tissue (the cerebellar cortex), tightly folded in the style of an accordion. Within this thin
layer are several types of neurons with a highly regular arrangement, the most important being
Purkinje cells and granule cells. This complex neural network gives rise to a massive signalprocessing capability, but almost the entirety of its output is directed to a set of small deep cerebellar
nuclei lying in the interior of the cerebellum.
In addition to its direct role in motor control, the cerebellum also is necessary for several types of
motor learning, most notably learning to adjust to changes in sensorimotor relationships. Several
theoretical models have been developed to explain sensorimotor calibration in terms of synaptic
plasticity within the cerebellum. Most of them derive from early models formulated by David Marr
and James Albus, which were motivated by the observation that each cerebellar Purkinje cell receives
two dramatically different types of input: on one hand, thousands of inputs from parallel fibers, each
individually very weak; on the other hand, input from one single climbing fiber, which is however so
strong that a single climbing fiber action potential will reliably cause a target Purkinje cell to fire a
burst of action potentials. The basic concept of the Marr-Albus theory is that the climbing fiber serves
as a "teaching signal", which induces a long-lasting change in the strength of synchronously activated
parallel fiber inputs. Observations of long-term depression in parallel fiber inputs have provided
support for theories of this type, but their validity remains controversial.
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At the level of large-scale anatomy, the cerebellum consists of a tightly folded and crumpled layer of
cortex, with white matter underneath, several deep nuclei embedded in the white matter, and a fluidfilled ventricle at the base. At the microscopic level, each part of the cerebellar cortex consists of the
same small set of neuronal elements, laid out with a highly stereotyped geometry. At an intermediate
level, the cerebellum and its auxiliary structures can be decomposed into several hundred or thousand
independently functioning modules called "microzones" or "microcompartments".
The cerebellum is located at the bottom of the brain, with the large mass of the cerebral cortex above it
and the portion of the brainstem called the pons in front of it. It is separated from the overlying
cerebrum by a layer of leathery dura mater; all of its connections with other parts of the brain travel
through the pons. Anatomists classify the cerebellum as part of the metencephalon, which also
includes the pons; the metencephalon in turn is the upper part of the rhombencephalon or "hindbrain".
Like the cerebral cortex, the cerebellum is divided into two hemispheres; it also contains a narrow
midline zone called the vermis. A set of large folds are conventionally used to divide the overall
structure into 10 smaller "lobules". Because of its large number of tiny granule cells, the cerebellum
contains more neurons than the rest of the brain put together, but it only takes up 10% of total brain
The unusual surface appearance of the cerebellum conceals the fact that most of its volume is made
up of a very tightly folded layer of gray matter, the cerebellar cortex. It has been estimated that if the
human cerebellar cortex were completely unfolded, it would give rise to a layer of neural tissue about
1 meter long and averaging 5 centimeters wide—a total surface area of about 500 square cm, packed
within a volume of dimensions 6 cm × 5 cm × 10 cm. Underneath the gray matter of the cortex lies
white matter, made up largely of myelinated nerve fibers running to and from the cortex. Embedded
within the white matter—which is sometimes called the arbor vitae (Tree of Life) because of its
branched, tree-like appearance in cross-section—are four deep cerebellar nuclei, composed of gray
Based on surface appearance, three lobes can be distinguished in the cerebellum, called the
flocculonodular lobe, anterior lobe (above the primary fissure), and posterior lobe (below the primary
fissure). These lobes divide the cerebellum from rostral to caudal (in humans, top to bottom).
Functionally, however, there is a more important distinction along the medial-to-lateral dimension.
Leaving out the flocculonodular part, which has distinct connections and functions, the cerebellum can
be parsed functionally into a medial sector called the spinocerebellum and a larger lateral sector called
the cerebrocerebellum. A narrow strip of protruding tissue along the midline is called the vermis
(Latin for "worm").
The smallest region, the flocculonodular lobe, is often called the vestibulocerebellum. It is the oldest
part in evolutionary terms (archicerebellum) and participates mainly in balance and spatial orientation;
its primary connections are with the vestibular nuclei, although it also receives visual and other
sensory input. Damage to it causes disturbances of balance and gait.
The medial zone of the anterior and posterior lobes constitutes the spinocerebellum, also known as
paleocerebellum. This sector of the cerebellum functions mainly to fine-tune body and limb
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movements. It receives proprioception input from the dorsal columns of the spinal cord (including the
spinocerebellar tract) as well as from the trigeminal nerve, as well as from visual and auditory
systems. It sends fibres to deep cerebellar nuclei which in turn project to both the cerebral cortex and
the brain stem, thus providing modulation of descending motor systems.
The lateral zone, which in humans is by far the largest part, constitutes the cerebrocerebellum, also
known as neocerebellum. It receives input exclusively from the cerebral cortex (especially the parietal
lobe) via the pontine nuclei (forming cortico-ponto-cerebellar pathways), and sends output mainly to
the ventrolateral thalamus (in turn connected to motor areas of the premotor cortex and primary motor
area of the cerebral cortex) and to the red nucleus
There is disagreement about the best way to describe the functions of the lateral cerebellum: it is
thought to be involved in planning movement that is about to occur, in evaluating sensory information
for action, and probably in a number of purely cognitive functions as well.
Cellular components
Two types of neuron play dominant roles in the cerebellar circuit: Purkinje cells and granule cells.
Three types of axons also play dominant roles: mossy fibers and climbing fibers (which enter the
cerebellum from outside), and parallel fibers (which are the axons of granule cells). Functionally, there
are two main pathways through the cerebellar circuit, originating from mossy fibers and climbing
fibers, both terminating in the deep cerebellar nuclei.
Mossy fibers project directly to the deep nuclei, but also give rise to the pathway: mossy fiber →
granule cells → parallel fibers → Purkinje cells → deep nuclei. Climbing fibers project to Purkinje
cells and also send collaterals directly to the deep nuclei. The mossy fiber and climbing fiber inputs
each carry fiber-specific information; the cerebellum also receives dopaminergic, serotonergic,
noradrenergic, and cholinergic inputs that presumably perform global modulation.
The cerebellar cortex is divided into three layers. At the bottom lies the thick granular layer, densely
packed with granule cells, along with much smaller numbers of interneurons, mainly Golgi cells. In
the middle lies the Purkinje layer, a narrow zone that contains only the cell bodies of Purkinje cells. At
the top lies the molecular layer, which contains the flattened dendritic trees of Purkinje cells, along
with the huge array of parallel fibers penetrating the Purkinje cell dendritic trees at right angles. This
outermost layer of the cerebellar cortex also contains two types of inhibitory interneurons, stellate
cells and basket cells. Both stellate and basket cells form GABAergic synapses onto Purkinje cell
Purkinje cells
Purkinje cells are among the most distinctive neurons in the brain, and also among the earliest types to
be recognized—they were first described by the Czech anatomist Jan Evangelista Purkyně in 1837.
They are distinguished by the shape of the dendritic tree: the dendrites branch very profusely, but are
severely flattened in a plane perpendicular to the cerebellar folds. Thus, the dendrites of a Purkinje cell
form a dense planar net, through which parallel fibers pass at right angles. The dendrites are covered
with dendritic spines, each of which receives synaptic input from a parallel fiber. Purkinje cells
receive more synaptic inputs than any other type of cell in the brain—estimates of the number of
spines on a single human Purkinje cell run as high as 200,000. The large, spherical cell bodies of
Purkinje cells are packed into a narrow layer (one cell thick) of the cerebellar cortex, called the
Purkinje layer. After emitting collaterals that innervate nearby parts of the cortex, their axons travel
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into the deep cerebellar nuclei, where they make on the order of 1,000 contacts each with several types
of nuclear cells, all within a small domain. Purkinje cells use GABA as their neurotransmitter, and
therefore exert inhibitory effects on their targets.
Purkinje cells form the heart of the cerebellar circuit, and their large size and distinctive activity
patterns have made it relatively easy to study their response patterns in behaving animals using
extracellular recording techniques. Purkinje cells normally emit action potentials at a high rate even in
the absence of synaptic input. In awake, behaving animals, mean rates averaging around 40 Hz are
typical. The spike trains show a mixture of what are called simple and complex spikes. A simple spike
is a single action potential followed by a refractory period of about 10 msec; a complex spike is a
stereotyped sequence of action potentials with very short inter-spike intervals and declining
amplitudes.[citation needed] Physiological studies have shown that complex spikes (which occur at
baseline rates around 1 Hz and never at rates much higher than 10 Hz) are reliably associated with
climbing fiber activation, while simple spikes are produced by a combination of baseline activity and
parallel fiber input. Complex spikes are often followed by a pause of several hundred msec during
which simple spike activity is suppressed.
Granule cells
Cerebellar granule cells, in contrast to Purkinje cells, are among the smallest neurons in the brain.
They are also easily the most numerous neurons in the brain: in humans, estimates of their total
number average around 50 billion, which means that about 3/4 of the brain's neurons are cerebellar
granule cells. Their cell bodies are packed into a thick layer at the bottom of the cerebellar cortex. A
granule cell emits only four to five dendrites, each of which ends in an enlargement called a dendritic
claw. These enlargements are sites of excitatory input from mossy fibers and inhibitory input from
Golgi cells.
The thin, unmyelinated axons of granule cells rise vertically to the upper (molecular) layer of the
cortex, where they split in two, with each branch traveling horizontally to form a parallel fiber; the
splitting of the vertical branch into two horizontal branches gives rise to a distinctive "T" shape. A
parallel fiber runs for an average of 3 mm in each direction from the split, for a total length of about 6
mm (about 1/10 of the total width of the cortical layer). As they run along, the parallel fibers pass
through the dendritic trees of Purkinje cells, contacting one of every 3–5 that they pass, making a total
of 80–100 synaptic connections with Purkinje cell dendritic spines.[4] Granule cells use glutamate as
their neurotransmitter, and therefore exert excitatory effects on their targets.
Granule cells receive all of their input from mossy fibers, but outnumber them 200 to 1 (in humans).
Thus, the information in the granule cell population activity state is the same as the information in the
mossy fibers, but recoded in a much more expansive way. Because granule cells are so small and so
densely packed, it has been very difficult to record their spike activity in behaving animals, so there is
little data to use as a basis of theorizing. The most popular concept of their function was proposed by
David Marr, who suggested that they could encode combinations of mossy fiber inputs. The idea is
that with each granule cell receiving input from only 4–5 mossy fibers, a granule cell would not
respond if only a single one of its inputs was active, but would respond if more than one were active.
This combinatorial coding scheme would potentially allow the cerebellum to make much finer
distinctions between input patterns than the mossy fibers alone would permit.
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Mossy fibers
Mossy fibers enter the granular layer from their points of origin, many arising from the pontine nuclei,
others from the spinal cord, vestibular nuclei, etc. In the human cerebellum, the total number of mossy
fibers has been estimated at about 200 million. These fibers form excitatory synapses with the granule
cells and the cells of the deep cerebellar nuclei. Within the granular layer, a mossy fiber generates a
series of enlargements called rosettes. The contacts between mossy fibers and granule cell dendrites
take place within structures called glomeruli. Each glomerulus has a mossy fiber rosette at its center,
and up to 20 granule cell dendritic claws contacting it. Terminals from Golgi cells infiltrate the
structure and make inhibitory synapses onto the granule cell dendrites. The entire assemblage is
surrounded by a sheath of glial cells. Each mossy fiber sends collateral branches to several cerebellar
folia, generating a total of 20–30 rosettes; thus a single mossy fiber makes contact with an estimated
400–600 granule cells.
Climbing fibers
Purkinje cells also receive input from the inferior olivary nucleus (IO) on the contralateral side of the
brainstem, via climbing fibers. Although the IO lies in the medulla oblongata, and receives input from
the spinal cord, brainstem, and cerebral cortex, its output goes entirely to the cerebellum. A climbing
fiber gives off collaterals to the deep cerebellar nuclei before entering the cerebellar cortex, where it
splits into about 10 terminal branches, each of which innervates a single Purkinje cell. In striking
contrast to the 100,000-plus inputs from parallel fibers, each Purkinje cell receives input from exactly
one climbing fiber; but this single fiber "climbs" the dendrites of the Purkinje cell, winding around
them and making a total of up to 300 synapses as it goes. The net input is so strong that a single action
potential from a climbing fiber is capable of producing an extended complex spike in the Purkinje cell:
a burst of several spikes in a row, with diminishing amplitude, followed by a pause during which
activity is suppressed. The climbing fiber synapses cover the cell body and proximal dendrites; this
zone is devoid of parallel fiber inputs.
Climbing fibers fire at low rates, but a single climbing fiber action potential induces a burst of several
action potentials in a target Purkinje cell (a complex spike). The contrast between parallel fiber and
climbing fiber inputs to Purkinje cells (over 100,000 of one type versus exactly one of the other type)
is perhaps the most provocative feature of cerebellar anatomy, and has motivated much of the
theorizing. In fact, the function of climbing fibers is the most controversial topic concerning the
cerebellum. There are two schools of thought, one following Marr and Albus in holding that climbing
fiber input serves primarily as a teaching signal, the other holding that its function is to shape
cerebellar output directly. Both views have been defended in great length in numerous publications. In
the words of one review, "In trying to synthesize the various hypotheses on the function of the
climbing fibers, one has the sense of looking at a drawing by Escher. Each point of view seems to
account for a certain collection of findings, but when one attempts to put the different views together,
a coherent picture of what the climbing fibers are doing does not appear. For the majority of
researchers, the climbing fibers signal errors in motor performance, either in the usual manner of
discharge frequency modulation or as a single announcement of an 'unexpected event'. For other
investigators, the message lies in the degree of ensemble synchrony and rhythmicity among a
population of climbing fibers."
Deep nuclei
The deep nuclei of the cerebellum are clusters of gray matter lying within the white matter at the core
of the cerebellum. They are, with the minor exception of the nearby vestibular nuclei, the sole sources
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of output from the cerebellum. These nuclei receive collateral projections from mossy fibers and
climbing fibers, as well as inhibitory input from the Purkinje cells of the cerebellar cortex. The three
nuclei (dentate, interpositus, and fastigial) each communicate with different parts of the brain and
cerebellar cortex. The fastigial and interpositus nuclei belong to the spinocerebellum. The dentate
nucleus, which in mammals is much larger than the others, is formed as a thin, convoluted layer of
gray matter, and communicates exclusively with the lateral parts of the cerebellar cortex. The
flocculonodular lobe is the only part of the cerebellar cortex that does not project to the deep nuclei—
its output goes to the vestibular nuclei instead.
The majority of neurons in the deep nuclei have large cell bodies and spherical dendritic trees with a
radius of about 400 μm, and use glutamate as their neurotransmitter. These cells project to a variety of
targets outside the cerebellum. Intermixed with them are a lesser number of small cells, which use
GABA as neurotransmitter and project exclusively to the inferior olivary nucleus, the source of
climbing fibers. Thus, the nucleo-olivary projection provides an inhibitory feedback to match the
excitatory projection of climbing fibers to the nuclei. There is evidence that each small cluster of
nuclear cells projects to the same cluster of olivary cells that send climbing fibers to it; there is strong
and matching topography in both directions.
When a Purkinje cell axon enters one of the deep nuclei, it branches to make contact with both large
and small nuclear cells, but the total number of cells contacted is only about 35 (in cats)—conversely,
a single deep nuclear cell receives input from approximately 860 Purkinje cells (again in cats).
From the viewpoint of gross anatomy, the cerebellar cortex appears to be a homogeneous sheet of
tissue, and from the viewpoint of microanatomy, all parts of this sheet appear to have the same internal
structure. There are, however, a number of respects in which the structure of the cerebellum is
compartmentalized. There are large compartments that are generally known as zones; these can be
decomposed into smaller compartments known as microzones.
The strongest clues to the function of the cerebellum have come from examining the consequences of
damage to it. Animals and humans with cerebellar dysfunction show, above all, problems with motor
control. They continue to be able to generate motor activity, but it loses precision, producing erratic,
uncoordinated, or incorrectly timed movements. A standard test of cerebellar function is to reach with
the tip of the finger for a target at arm's length: a healthy person will move the fingertip in a rapid
straight trajectory, while a person with cerebellar damage will reach slowly and erratically, with many
mid-course corrections. Deficits in non-motor functions are more difficult to detect. Thus, the general
conclusion reached decades ago is that the basic function of the cerebellum is not to initiate
movements, or to decide which movements to execute, but rather to calibrate the detailed form of a
Prior to the 1990s it was almost universally believed that the function of the cerebellum was purely
motor-related, but newer findings have brought that view strongly into question. Functional imaging
studies have shown cerebellar activation in relation to language, attention, and mental imagery;
correlation studies have shown interactions between the cerebellum and non-motoric areas of the
cerebral cortex; and a variety of non-motor symptoms have been recognized in people with damage
that appears to be confined to the cerebellum.
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The most salient symptoms of cerebellar dysfunction are motor-related—the specific symptoms
depend on which part of the cerebellum is involved and how it is disrupted. Damage to the
flocculonodular lobe (the vestibular part) may show up as a loss of equilibrium and in particular an
altered walking gait, with a wide stance that indicates difficulty in balancing. Damage to the midline
portion may disrupt whole-body movements, while damage localized more laterally is more likely to
disrupt fine movements of the hands or limbs. Damage to the upper part of the cerebellum tends to
cause gait impairments and other problems with leg coordination; damage to the lower part is more
likely to cause uncoordinated or poorly aimed movements of the arms and hands, as well as
difficulties in speed. This complex of motor symptoms is called "ataxia". To identify cerebellar
problems, the neurological examination includes assessment of gait (a broad-based gait being
indicative of ataxia), finger-pointing tests and assessment of posture. If cerebellar dysfunction is
indicated, a magnetic resonance imaging scan can be used to obtain a detailed picture of any structural
alterations that may exist.
The list of medical problems that can produce cerebellar damage is long: it includes stroke;
hemorrhage; tumors; alcoholism; physical trauma such as gunshot wounds; and chronic degenerative
conditions such as olivopontocerebellar atrophy. Some forms of migraine headache may also produce
temporary dysfunction of the cerebellum, of variable severity.
The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar
contexts where no ambiguity will result, it is often referred to as simply the medulla. The medulla
contains the cardiac, respiratory, vomiting and vasomotor centers and deals with autonomic functions,
such as breathing, heart rate and blood pressure.
The medulla is often thought of as being in two parts:
an open part or superior part where the dorsal surface of the medulla is formed by the fourth ventricle.
a closed part or inferior part where the metacoel lies within the medulla.
Between the anterior median sulcus and the anterolateral sulcus
The region between the anterior median sulcus and the anterolateral sulcus is occupied by an elevation
on either side known as the pyramid of medulla oblongata. This elevation is caused by the
corticospinal tract.
In the lower part of the medulla some of these fibers cross each other thus obliterating the anterior
median fissure. This is known as the decussation of the pyramids.
Some other fibers that originate from the anterior median fissure above the decussation of the
pyramids and run laterally across the surface of the pons are known as the external arcuate fibers.
Between the anterolateral and posterolateral sulci
The region between the anterolateral and posterolateral sulci in the upper part of the medulla is marked
by a swelling known as the Olivary body.
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It is caused by a large mass of gray matter known as the inferior olivary nucleus.
Between the posterior median sulcus and the posterolateral sulcus
The posterior part of the medulla between the posterior median sulcus and the posterolateral sulcus
contains tracts that enter it from the posterior funiculus of the spinal cord. These are the fasciculus
gracilis, lying medially next to the midline, and the fasciculus cuneatus, lying laterally.
These fasciculi end in rounded elevations known as the gracile and the cuneate tubercles. They are
caused by masses of gray matter known as the nucleus gracilis and the nucleus cuneatus.
Just above the tubercles, the posterior aspect of the medulla is occupied by a triangular fossa, which
forms the lower part of the floor of the fourth ventricle. The fossa is bounded on either side by the
inferior cerebellar peduncle, which connects the medulla to the cerebellum.
Lower part
The lower part of the medulla, immediately lateral to the fasciculus cuneatus, is marked by another
longitudinal elevation known as the tuberculum cinereum.
It is caused by an underlying collection of gray matter known as the spinal nucleus of the trigeminal
The gray matter of this nucleus is covered by a layer of nerve fibers that form the spinal tract of the
trigeminal nerve.
The base of the medulla is defined by the commissural fibers, crossing over from the ipsilateral side in
the spinal cord to the contralateral side in the brain stem; below this is the spinal.
The medulla oblongata controls autonomic functions, and relays nerve signals between the brain and
spinal cord. It is also responsible for controlling several major points and autonomic functions of the
respiration ----- chemoreceptors
cardiac center ----- sympathetic, parasympathetic system
vasomotor center---- baroreceptors
reflex centers of vomiting, coughing, sneezing, and swallowing
Blood supply
Blood to the medulla is supplied by a number of arteries.
Anterior spinal artery: The anterior spinal artery supplies the whole medial part of the medulla
oblongata. A blockage (such as in a stroke) will injure the pyramidal tract, medial lemniscus, and the
hypoglossal nucleus. This causes a syndrome called medial medullary syndrome.
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Posterior inferior cerebellar artery (PICA): The posterior inferior cerebellar artery, a major branch of
the vertebral artery, supplies the posterolateral part of the medulla, where the main sensory tracts run
and synapse. (As the name implies, it also supplies some of the cerebellum.)
Direct branches of the vertebral artery: The vertebral artery supplies an area between the other two
main arteries, including the nucleus solitarius and other sensory nuclei and fibers. Lateral medullary
syndrome can be caused by occlusion of either the PICA or the vertebral arteries.
Cranial Nerves Functions
There are twelve pairs of cranial nerves, all but two of which originate in the brain stem. These nerves
facilitate operation of the senses and some muscles' of sense organs.
Cranial Nerves I and II
Cranial nerves I and II are the olfactory and optic nerves, respectively. These are the only two cranial
nerves that do not originate in the brain stem. The olfactory nerve originates in the part of the brain
called the olfactory bulb. It functions in controlling the sense of smell (olfaction). The optic nerve
originates in the eye and carries information to the brain for vision. These nerves can be tested very
simply. In the case of cranial nerve I, a simple test to see if the subject can identify certain distinct
smells like coffee, garlic or lemon. Cranial nerve II can be tested using the standard eye chart.
Cranial Nerves III, IV and VI
These are the nerves that control movement. Cranial nerve III, the oculomotor nerve, is the nerve that
controls eye movement and pupil constriction. Cranial nerves IV and VI also control eye movement.
They are called, in order, the Trochlear nerve and the Abducens nerve. All three may be tested by
holding one finger up and asking the subject to keep his head still and track the movement of your
finger from side to side and up and down. To check pupil constriction, look at the pupils in bright and
then in dim light. The pupils should be smaller in bright light and larger in dim.
Cranial Nerve V
Cranial nerve V is the trigeminal nerve and controls the touch and pain sensations from the face and
head. The trigeminal nerve also controls the muscles used for chewing. To test this nerve, ask the
subject to close his jaws as if biting down on gum. To test facial sensation, touch different parts of her
face with something soft or blunt and ask her to name the places you are touching. Do not place
anything in the mouth.
Cranial Nerves VII and VIII
These cranial nerves are the facial nerve (VII) and vestibulocochlear nerve (VIII). The facial nerve
controls taste on the front two-thirds of the tongue. It also receives information from the ear and
controls the muscles of facial expression. This can be tested by asking the subject to smile or frown.
Taste can be tested with something sweet or salty placed on the front part of the tongue. The
vestibulocochlear nerve controls hearing and balance. To test, determine how far away the subject can
hear a particular sound. It is not safe to test balance except in an actual clinical test.
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Cranial Nerves IX and X
The glossopharyngeal nerve (IX) controls some functions of swallowing as well as taste on the back
third of the tongue. It conducts information from the tongue, tonsils and pharynx (the cavity that
connects the mouth and nasal passages with the esophagus). Observe the subject as he drinks and
swallows water. Taste may be tested in the previously described fashion except the salty or sweet
substance should be placed on the back of the tongue. The vagus nerve (X) controls pain/touch
sensations of digestion, heart rate and some glands. Do not test the vagus nerve.
Cranial Nerve XI
The spinal accessory nerve controls the muscles of head movement. In order to test cranial nerve XI,
place your hands lightly on the sides of the subjects head and ask her to move it in all directions.
Cranial Nerve XII
The hypoglossal nerve serves to control the muscles of the tongue. To test, ask the subject to stick out
his tongue and wiggle it in all directions.
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian
brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and
consciousness. It is constituted of up to six horizontal layers, each of which has a different
composition in terms of neurons and connectivity. The human cerebral cortex is 2–4 mm (0.08–0.16
inches) thick.
In preserved brains, it has a gray color, hence the name "gray matter". In contrast to gray matter that is
formed from neurons and their unmyelinated fibers, the white matter below them is formed
predominantly by myelinated axons interconnecting neurons in different regions of the cerebral cortex
with each other and neurons in other parts of the central nervous system.
The surface of the cerebral cortex is folded in large mammals, such that more than two-thirds of it in
the human brains is buried in the grooves, called "sulci". The phylogenetically most recent part of the
cerebral cortex, the neocortex (also called isocortex), is differentiated into six horizontal layers; the
more ancient part of the cerebral cortex, the hippocampus (also called archicortex), has at most three
cellular layers, and is divided into subfields. Neurons in various layers connect vertically to form small
microcircuits, called columns. Different neocortical architectonic fields are distinguished upon
variations in the thickness of these layers, their predominant cell type and other factors such as
neurochemical markers.Contents
The cerebral cortex develops from the most anterior part of the neural plate, a specialized part of the
embryonic ectoderm. The neural plate folds and closes to form the neural tube. From the cavity inside
the neural tube develops the ventricular system, and, from the epithelial cells of its walls, the neurons
and glia of the nervous system. The most anterior (frontal) part of the neural tube, the telencephalon,
gives rise to the cerebral hemispheres and cortex.
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Cortical neurons are generated within the ventricular zone, next to the ventricles. At first, this zone
contains "progenitor" cells, which divide to produce glial and neuronal cells . The glial fibers produced
in the first divisions of the progenitor cells are radially oriented, spanning the thickness of the cortex
from the ventricular zone to the outer, pial surface, and provide scaffolding for the migration of
neurons outwards from the ventricular zone. The first divisions of the progenitor cells are symmetric,
which duplicates the total number of progenitor cells at each mitotic cycle. Then, some progenitor
cells begin to divide asymmetrically, producing one postmitotic cell that migrates along the radial glial
fibers, leaving the ventricular zone, and one progenitor cell, which continues to divide until the end of
development, when it differentiates into a glial cell or an ependymal cell. The migrating daughter cells
become the pyramidal neurons of the cerebral cortex.
The layered structure of the mature cerebral cortex is formed during development. The first pyramidal
neurons generated migrate out of the ventricular zone and subventricular zone, together with CajalRetzius cells form the preplate. Next, a cohort of neurons migrating into the middle of the preplate
divides this transient layer into the superficial marginal zone, which will become layer one of the
mature neocortex, and the subplate, forming a middle layer called the cortical plate. These cells will
form the deep layers of the mature cortex, layers five and six. Later born neurons migrate radially into
the cortical plate past the deep layer neurons, and become the upper layers (two to four). Thus, the
layers of the cortex are created in an inside-out order. The only exception to this inside-out sequence
of neurogenesis occurs in the layer I of primates, in which, contrary to rodents, neurogenesis continues
throughout the entire period of corticogenesis.
Laminar pattern
The different cortical layers each contain a characteristic distribution of neuronal cell types and
connections with other cortical and subcortical regions. One of the clearest examples of cortical
layering is the Stria of Gennari in the primary visual cortex. This is a band of whiter tissue that can be
observed with the naked eye in the fundus of the calcarine sulcus of the occipital lobe. The Stria of
Gennari is composed of axons bringing visual information from the thalamus into layer four of visual
Staining cross-sections of the cortex to reveal the position of neuronal cell bodies and the intracortical
axon tracts allowed neuroanatomists in the early 20th century to produce a detailed description of the
laminar structure of the cortex in different species. After the work of Korbinian Brodmann (1909), the
neurons of the cerebral cortex are grouped into six main layers, from outside (pial surface) to inside
(white matter):
The molecular layer I, which contains few scattered neurons and consists mainly of extensions of
apical dendritic tufts of pyramidal neurons and horizontally-oriented axons, as well as glial cells[4].
Some Cajal-Retzius and spiny stellate neurons can be found here. Inputs to the apical tufts are thought
to be crucial for the ‘‘feedback’’ interactions in the cerebral cortex involved in associative learning
and attention. While it was once thought that the input to layer I came from the cortex itself, it is now
realized that layer I across the cerebral cortex mantle receives substantial input from ‘‘matrix’’ or Mtype thalamus cells (in contrast to ‘‘core’’ or C-type that go to layer IV).
The external granular layer II, which contains small pyramidal neurons and numerous stellate neurons
The external pyramidal layer III, which contains predominantly small and medium-size pyramidal
neurons, as well as non-pyramidal neurons with vertically-oriented intracortical axons; layers I
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through III are the main target of interhemispheric corticocortical afferents, and layer III is the
principal source of corticocortical efferents
The internal granular layer IV, which contains different types of stellate and pyramidal neurons, and is
the main target of thalamocortical afferents from thalamus type C neurons. as well as intrahemispheric corticocortical afferents
The internal pyramidal layer V, which contains large pyramidal neurons (such as the Betz cells in the
primary motor cortex); it is the principal source of subcortical efferents
The multiform layer VI, which contains few large pyramidal neurons and many small spindle-like
pyramidal and multiform neurons; layer VI sends efferent fibers to the thalamus, establishing a very
precise reciprocal interconnection between the cortex and the thalamus. These connections are both
excitatory and inhibitory. Neurons send excitatory fibers to neurons in the thalamus and also from
collateral to them ones via the thalamic reticular nucleus that inhibit these thalamus neurons or ones
adjacent to them. Since the inhibitory output is reduced by cholinergic input to the cerebral cortex, this
provides the brainstem with adjustable "gain control for the relay of lemnsical inputs".
It is important to note that the cortical layers are not simply stacked one over the other; there exist
characteristic connections between different layers and neuronal types, which span all the thickness of
the cortex. These cortical microcircuits are grouped into cortical columns and minicolumns, the latter
of which have been proposed to be the basic functional units of cortex. In 1957, Vernon Mountcastle
showed that the functional properties of the cortex change abruptly between laterally adjacent points;
however, they are continuous in the direction perpendicular to the surface. Later works have provided
evidence of the presence of functionally distinct cortical columns in the visual cortex (Hubel and
Wiesel, 1959), auditory cortex and associative cortex.
Cortical areas that lack a layer IV are called agranular. Cortical areas that have only a rudimentary
layer IV are called dysgranular. Information processing within each layer is determined by different
temporal dynamics with that in the layers II/III having a slow 2Hz oscillation while that in layer V
having a fast 10–15 Hz one.
Connections of the cerebral cortex
The cerebral cortex is connected to various subcortical structures such as the thalamus and the basal
ganglia, sending information to them along efferent connections and receiving information from them
via afferent connections. Most sensory information is routed to the cerebral cortex via the thalamus.
Olfactory information, however, passes through the olfactory bulb to the olfactory cortex (piriform
cortex). The vast majority of connections are from one area of the cortex to another rather than to
subcortical areas; Braitenberg and Schüz (1991) put the figure as high as 99%.
The cortex is commonly described as comprising three parts: sensory, motor, and association areas.
Sensory areas
The sensory areas are the areas that receive and process information from the senses. Parts of the
cortex that receive sensory inputs from the thalamus are called primary sensory areas. The senses of
vision, audition, and touch are served by the primary visual cortex, primary auditory cortex and
primary somatosensory cortex. In general, the two hemispheres receive information from the opposite
(contralateral) side of the body. For example the right primary somatosensory cortex receives
information from the left limbs, and the right visual cortex receives information from the left visual
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field. The organization of sensory maps in the cortex reflects that of the corresponding sensing organ,
in what is known as a topographic map. Neighboring points in the primary visual cortex, for example,
correspond to neighboring points in the retina. This topographic map is called a retinotopic map. In the
same way, there exists a tonotopic map in the primary auditory cortex and a somatotopic map in the
primary sensory cortex. This last topographic map of the body onto the posterior central gyrus has
been illustrated as a deformed human representation, the somatosensory homunculus, where the size
of different body parts reflects the relative density of their innervation. Areas with lots of sensory
innervation, such as the fingertips and the lips, require more cortical area to process finer sensation.
Motor areas
The motor areas are located in both hemispheres of the cortex. They are shaped like a pair of
headphones stretching from ear to ear. The motor areas are very closely related to the control of
voluntary movements, especially fine fragmented movements performed by the hand. The right half of
the motor area controls the left side of the body, and vice versa.
Two areas of the cortex are commonly referred to as motor:
Primary motor cortex, which executes voluntary movements
Supplementary motor areas and premotor cortex, which select voluntary movements.
In addition, motor functions have been described for:
Posterior parietal cortex, which guides voluntary movements in space
Dorsolateral prefrontal cortex, which decides which voluntary movements to make according to
higher-order instructions, rules, and self-generated thoughts.
Association areas
Association areas function to produce a meaningful perceptual experience of the world, enable us to
interact effectively, and support abstract thinking and language. The parietal, temporal, and occipital
lobes - all located in the posterior part of the cortex - organize sensory information into a coherent
perceptual model of our environment centered on our body image. The frontal lobe or prefrontal
association complex is involved in planning actions and movement, as well as abstract thought. In the
past it was theorized that language abilities are localized in the left hemisphere in areas 44/45, the
Broca's area, for language expression and area 22, the Wernicke's area, for language reception.
However, language is no longer limited to easily identifiable areas. More recent research suggests that
the processes of language expression and reception occur in areas other than just the perisylvian
structures, such as the prefrontal lobe, basal ganglia, cerebellum, pons, caudate nucleus, and others.
Based on the differences in lamination the cerebral cortex can be classified into two major
-Isocortex (homotypical cortex), the part of the cortex with six layers.
-Allocortex (heterotypical cortex) the part of the cortex with less than six layers (varying in number).
The allocortex includes the olfactory cortex and the hippocampus.
Auxiliary classes are:
-Mesocortex, classification between isocortex and allocortex where layers 2, 3, and 4 are merged
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-Proisocortex, Brodmann areas 24, 25, 32
-Periallocortex, comprising cortical areas adjacent to allocortex.
Based on supposed developmental differences the following classification also appears:
-Neocortex or Neopallium, which corresponds to the isocortex.
-Archicortex, which phylogenetically is the oldest cortex
In addition, cortex may be classified on the basis of gross topographical conventions into four
Temporal Cortex
Occipital Cortex
Parietal Cortex
Frontal Cortex
Cortical thickness
With magnetic resonance brain scanners, it is possible to get a measure for the thickness of the human
cerebral cortex and relate it to other measures. The thickness of different cortical areas varies. In
general, sensory cortex is thinner and motor cortex is thicker. One study has found some positive
association between the cortical thickness and intelligence. Another study has found that the
somatosensory cortex is thicker in migraine sufferers.
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Module 2
In sensory coding, sensory events are converted from one form of energy (light, heat, etc.) into a
nerve (electro-chemical) impulse. the conversion is known as transduction.
light -> retina -> optic nerve
sound -> eardrum -> middle ear bones -> cochlea -> auditory nerve
pressure/heat -> receptors (pacinian, ruffini, meissner's corpuscles, merkel receptors, free nerve
endings) -> dorsal root ganglion cells -> spinal cord
chemical -> taste buds -> peripheral nerves
chemical -> olfactory nerve endings
Human sensory system
The Human sensory system consists of the following sub-systems:
-Visual system consists of the photoreceptor cells, optic nerve, and V1.
-Auditory system
-Somatosensory system consists of the receptors, transmitters (pathways) leading to S1, and S1 that
experiences the sensations labelled as touch or pressure, temperature (warm or cold), pain (including
itch and tickle), and the sensations of muscle movement and joint position including posture,
movement, and facial expression (collectively also called proprioception).
-Gustatory system
-Olfactory system
Human sensory receptors are:
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A chemosensor, also known as chemoreceptor, is a sensory receptor that transduces a chemical signal
into an action potential. Or, more generally, a chemosensor detects certain chemical stimuli in the
There are two main classes of the chemosensor: direct and distance.
Examples of distance chemoreceptors are:
olfactory receptor neurons in the olfactory system
neurons in the vomeronasal organ that detect pheromones
Examples of direct chemoreceptors include
Taste buds in the gustatory system
Carotid bodies and aortic bodies detect changes primarily in oxygen. They also sense increases in CO2
partial pressure and decreases in arterial pH, but to a lesser degree than for O2.
The chemoreceptor trigger zone is an area of the medulla in the brain that receives inputs from bloodborne drugs or hormones, and communicates with the vomiting center.
Cellular antennae
Within the biological and medical disciplines, recent discoveries have noted that primary cilia in many
types of cells within eukaryotes serve as cellular antennae. These cilia play important roles in
chemosensation. The current scientific understanding of primary cilia organelles views them as
"sensory cellular antennae that coordinate a large number of cellular signaling pathways, sometimes
coupling the signaling to ciliary motility or alternatively to cell division and differentiation."
Systems affected
Breathing rate
Chemoreceptors detect the levels of carbon dioxide in the blood. To do this, they monitor the
concentration of hydrogen ions in the blood, which decrease the pH of the blood. This is a direct
consequence of an increase in carbon dioxide concentration, because carbon dioxide becomes carbonic
acid in an aqueous environment.
The response is that the respiratory centre (in the medulla), sends nervous impulses to the external
intercostal muscles and the diaphragm, via the intercostal nerve and the phrenic nerve, respectively, to
increase breathing rate and the volume of the lungs during inhalation.
Chemoreceptors which affect breathing rate are broken down into two categories.
central chemoreceptors are located on the ventrolateral surface of medulla oblongata and detect
changes in pH of cerebrospinal fluid. They do not respond to a drop in oxygen, and eventually
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peripheral chemoreceptors: Aortic body detects changes in blood oxygen and carbon dioxide, but not
pH, while carotid body detects all three. They do not desensitize. Their effect on breathing rate is less
than that of the central chemoreceptors.
Heart rate
Chemoreceptors in the medulla oblongata, carotid arteries and aortic arch, detect the levels of carbon
dioxide in the blood, in the same way as applicable in the Breathing Rate section.
In response to this high concentration, a nervous impulse is sent to the cardiovascular centre in the
medulla, which will then feedback to the sympathetic ganglia, increasing nervous impulses here, and
prompting the sinoatrial node to stimulate more contractions of the myogenic cardiac muscle,
increasing heart rate by causing the secretion of nor-adrenaline directly on to the sinoatrial node.
Sense organs
In taste sensation, the tongue is composed of 5 different taste buds: salty, sour, sweet, bitter, and
savory. The salty and sour tastes work directly through the ion channels, the sweet and bitter taste
work through G protein-coupled receptors, and the savoury sensation is activated by glutamate.
Noses in vertebrates and antennae in many invertebrates act as distance chemoreceptors. Molecules
are diffused through the air and bind to specific receptors on olfactory sensory neurons, activating an
opening ion channel via G-proteins.
When inputs from the environment are significant to the survival of the organism the input must be
detected. As all life processes are ultimately based on chemistry it is natural that detection and passing
on of the external input will involve chemical events. The chemistry of the environment is, of course,
relevant to survival, and detection of chemical input from the outside may well articulate directly with
cell chemicals.
For example: The emissions of a predator's food source, such as odors or pheromones, may be in the
air or on a surface where the food source has been. Cells in the head, usually the air passages or
mouth, have chemical receptors on their surface that change when in contact with the emissions. The
change does not stop there. It passes in either chemical or electrochemical form to the central
processor, the brain or spinal cord. The resulting output from the CNS (central nervous system) makes
body actions that will engage the food and enhance survival.
A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion. There are
four main types in the glabrous skin of humans: Pacinian corpuscles, Meissner's corpuscles, Merkel's
discs, and Ruffini corpuscles. There are also mechanoreceptors in the hairy skin, and the hair cells in
the cochlea are the most sensitive mechanoreceptors, transducing air pressure waves into sound.
Mechanism of sensation
Mechanoreceptors are primary neurons that respond to mechanical stimuli by firing action potentials.
Peripheral transduction is believed to occur in the end-organs.
In somatosensory transduction, the afferent neurons transmit the message through synapses in the
dorsal column nuclei, where the second order neurons send the signal to the thalamus and synapse
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with the third order neurons in the ventrobasal complex. The third order neurons then send the signal
to the somatosensory cortex.
More recent work has expanded the role of the cutaneous mechanoreceptors for feedback in fine motor
control . Single action potentials from RAI and PC afferents are directly linked to activation of related
hand muscles, whereas SAI activation does not trigger muscle activity.
The human work stemmed from Vallbo and Johansson's percutaneous recordings from human
volunteers in the late 1970s, Work in rhesus monkeys has found virtually identical mechanoreceptors
with the exception of Ruffini corpuscles which are not found in the monkey.
Cutaneous mechanoreceptors are located in the skin, like other cutaneous receptors. They are all
innervated by Aβ fibers, except the mechanorecepting free nerve endings, which are innervated by Aδ
fibers. They can be categorized both by morphology, by what kind of sensation they perceive and by
the rate of adaptation. Furthermore, they have different receptive field.
By morphology
Ruffini's end organ detects tension deep in the skin.
Meissner's corpuscle detects changes in texture (vibrations around 50 Hz); adapts rapidly.
Pacinian corpuscle detects rapid vibrations (about 200-300 Hz).
Merkel's disc detects sustained touch and pressure.
Mechanorecepting Free nerve endings (touch, pressure, stretch)
Hair follicle receptors are located in hair follicles and sense position changes of hairs.
By sensation
Cutaneous mechanoreceptors provide the senses of touch, pressure, vibration, proprioception and
The Slowly Adapting type 1 (SA1) mechanoreceptor, with the Merkel cell end-organ, underlies the
perception of form and roughness on the skin. They have small receptive fields and produce sustained
responses to static stimulation.
The Slowly Adapting type 2 (SA2) mechanoreceptors respond to skin stretch, but have not been
closely linked to either proprioceptive or mechanoreceptive roles in perception. They also produce
sustained responses to static stimulation, but have large receptive fields.
The Rapidly Adapting (RA) mechanoreceptor underlies the perception of flutter and slip on the skin.
They have small receptive fields and produce transient responses to the onset and offset of stimulation.
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Pacinian receptors underlie the perception of high frequency vibration. They also produce transient
responses, but have large receptive fields.
By rate of adaptation
Cutaneous mechanoreceptors can also be separated into categories based on their rates of adaptation.
When a mechanoreceptor receives a stimulus it begins to fire impulses or action potentials at an
elevated frequency (the stronger the stimulus the higher the frequency). The cell, however, will soon
“adapt” to a constant or static stimulus and the pulses will subside to a normal rate. Receptors that
adapt quickly (i.e. quickly return to a normal pulse rate) are referred to as ‘’phasic’’. Those receptors
that are slow to return to their normal firing rate are called ‘’tonic’’. Phasic mechanoreceptors are
useful in sensing such things as texture, vibrations, etc; whereas tonic receptors are useful for
temperature and proprioception among others.
Slowly adapting
Slowly adapting mechanoreceptors include Merkel and Ruffini corpuscle end-organs, some free nerve
Slowly adapting type I mechanoreceptors have multiple Merkel corpuscle end-organs.
Slowly adapting type II mechanoreceptors have single Ruffini corpuscle end-organs.
Intermediate adapting
Some free nerve endings are intermediate adapting.
Rapidly adapting
Rapidly adapting mechanoreceptors include Meissner corpuscle end-organs, Pacinian corpuscle endorgans, hair follicle receptors and some free nerve endings.
Rapidly adapting type I mechanoreceptors have multiple Meissner corpuscle end-organs.
Rapidly adapting type II mechanoreceptors (usually called Pacinian) have single Pacinian corpuscle
Receptive field
Cutaneous mechanoreceptors with small, accurate receptive fields are found in areas needing accurate
taction (e.g. the fingertips). In the fingertips and lips, innervation density of slowly adapting type I and
rapidly adapting type I mechanoreceptors are greatly increased. These two types of mechanoreceptors
have small discrete receptive fields and are thought to underlie most low threshold use of the fingers in
assessing texture, surface slip, and flutter. Mechanoreceptors found in areas of the body with less
tactile acuity tend to have larger receptive fields.
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Other mechanoreceptors than cutaneous ones include the hair cells, which are sensory receptors in the
vestibular system in the inner ear, where they contribute to the auditory system and equilibrioception.
There are also Juxtacapillary (J) receptors, which respond to events such as pulmonary edema,
pulmonary emboli, pneumonia, and barotrauma.
The Pacinian Corpuscle
Pacinian corpuscles are pressure receptors. They are located in the skin and also in various internal
organs. Each is connected to a sensory neuron. Because of its relatively large size, a single Pacinian
corpuscle can be isolated and its properties studied. Mechanical pressure of varying strength and
frequency is applied to the corpuscle by the stylus. The electrical activity is detected by electrodes
attached to the preparation.
Deforming the corpuscle creates a generator potential in the sensory neuron arising within it. This is a
graded response: the greater the deformation, the greater the generator potential. If the generator
potential reaches threshold, a volley of action potentials (also called nerve impulses) are triggered at
the first node of Ranvier of the sensory neuron.
Once threshold is reached, the magnitude of the stimulus is encoded in the frequency of impulses
generated in the neuron. So the more massive or rapid the deformation of a single corpuscle, the
higher the frequency of nerve impulses generated in its neuron.
The optimal sensitivity of Pacinian Corpuscle is 250 Hz and this is the frequency range generated
upon finger tips by textures made of features smaller than 200 µms.
Muscle Spindles and the Stretch Reflex
The knee jerk is a stretch reflex. Your physician taps you just below the knee with a rubber-headed
hammer. You respond with an involuntary kick of the lower leg. The hammer strikes a tendon that
inserts an extensor muscle in the front of the thigh into the lower leg. Tapping the tendon stretches the
thigh muscle. This activates stretch receptors within the muscle called muscle spindles. Each muscle
spindle consists of sensory nerve endings wrapped around special muscle fibers called spindle fibers
(also called intrafusal fibers) Stretching a spindle fiber initiates a volley of impulses in the sensory
neuron (a I-a neuron) attached to it. The impulses travel along the sensory axon to the spinal cord
where they form several kinds of synapses:
Some of the branches of the I-a axons synapse directly with alpha motor neurons
(1). These carry impulses back to the same muscle causing it to contract. The leg straightens. Some of
the branches of the I-a axons synapse with inhibitory interneurons in the spinal cord
(2). These, in turn, synapse with motor neurons leading back to the antagonistic muscle, a flexor in the
back of the thigh. By inhibiting the flexor, these interneurons aid contraction of the extensor.
(3). Still other branches of the I-a axons synapse with interneurons leading to brain centers, e.g., the
cerebellum, that coordinate body movements.
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A nociceptor is a sensory receptor that responds to potentially damaging stimuli by sending nerve
signals to the spinal cord and brain. This process, called nociception, usually causes the perception of
In mammals, nociceptors are sensory neurons that are found in any area of the body that can sense
pain either externally or internally. External examples are in tissues such as skin (cutaneous
nociceptors), cornea and mucosa. Internal nociceptors are in a variety of organs, such as the muscle,
joint, bladder, gut and continuing along the digestive tract. The cell bodies of these neurons are located
in either the dorsal root ganglia or the trigeminal ganglia. The trigeminal ganglia are specialized
nerves for the face, whereas the dorsal root ganglia associate with the rest of the body. The axons
extend into the peripheral nervous system and terminate in branches to form receptive fields.
Nociceptors develop from neural crest stem cells. The neural crest is responsible for a large part of
early development in vertebrates. More specifically it is responsible for development of the peripheral
nervous system. The neural crest stem cells split off from the neural tube as it closes, and nociceptors
grow from the dorsal part of this neural crest tissue. They form late during neurogenesis. Earlier
forming cells from this region can become non-pain sensing receptors; either proprioceptors or lowthreshold mechanoreceptors. All neurons derived from neural crest, including embryonic nociceptors,
express the TrkA nerve growth factor (NGF). However, transcription factors that determine the type of
nociceptor remain unclear.
Types and functions
The peripheral terminal of the mature nociceptor is where the noxious stimuli are detected and
transduced into electrical energy. When the electrical energy reaches a threshold value, an action
potential is induced and driven towards the central nervous system (CNS). This leads to the train of
events that allows for the conscious awareness of pain. The sensory specificity of nociceptors is
established by the high threshold only to particular features of stimuli. Only when the high threshold
has been reached by either chemical, thermal, or mechanical environments are the nociceptors
triggered. The majority of nociceptors are classified by which of the environmental modalities they
respond to. Some nociceptors respond to more than one of these modalities and are consequently
designated polymodal. Other nociceptors respond to none of these modalities (although they may
respond to stimulation under conditions of inflammation) and are referred to as sleeping or silent
Nociceptors have two different types of axons. The first are the Aδ fiber axons. They are myelinated
and can allow an action potential to travel at a rate of about 20 meters/second towards the CNS. The
other type is the more slowly conducting C fiber axons. These only conduct at speeds of around 2
meters/second. This is due to the light or non-myelination of the axon. As a result, pain comes in two
phases. The first phase is mediated by the fast-conducting Aδ fibers and the second part due to
(Polymodal) C fibers. The pain associated with the Aδ fibers can be associated to an initial extremely
sharp pain. The second phase is a more prolonged and slightly less intense feeling of pain as a result
from the damage. If there is massive or prolonged input to a C fiber there is progressive build up in the
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spinal cord dorsal horn. This phenomenon is similar to tetanus in muscles but is called wind-up. If
wind-up occurs there is a probability of increased sensitivity to pain.
Thermal nociceptors are activated by noxious heat or cold at various temperatures. There are specific
nociceptor transducers that are responsible for how and if the specific nerve ending responds to the
thermal stimulus. The first to be discovered was TRPV1, and it has a threshold that coincides with the
heat pain temperature of 42°C. Other temperature in the warm-hot range is mediated by more than one
TRP channel. Each of these channels express a particular C-terminal domain that corresponds to the
warm-hot sensitivity. The interactions between all these channels and how the temperature level is
determined to be above the pain threshold are unknown at this time. The cool stimuli are sensed by
TRPM8 channels. Its C-terminal domain differs from the heat sensitive TRPs. Although this channel
corresponds to cool stimuli, it is still unknown whether it also contributes in the detection of intense
cold. An interesting finding related to cold stimuli is that tactile sensibility and motor function
deteriorate while pain perception persists.
Mechanical nociceptors respond to excess pressure or mechanical deformation. They also respond to
incisions that break the skin surface. The reaction to the stimulus is processed as pain by the cortex,
just like chemical and thermal responses. Many times these mechanical nociceptors have polymodal
characteristics. So it is possible that some of the transducers for thermal stimuli are the same for
mechanical stimuli. The same is true for chemical stimuli, since TRPA1 appears to detect both
mechanical and chemical changes.
Chemical nociceptors have TRP channels that respond to a wide variety of spices commonly used in
cooking. The one that sees the most response and is very widely tested is Capsaicin. Other chemical
stimulants are environmental irritants like acrolein, a World War I chemical weapon and a component
of cigarette smoke. Besides from these external stimulants, chemical nociceptors have the capacity to
detect endogenous ligands, and certain fatty acid amines that arise from changes in internal tissues.
Like in thermal nociceptors, TRPV1 can detect chemicals like capsaicin and spider toxins.
Although each nociceptor can have a variety of possible threshold levels, some do not respond at all to
chemical, thermal or mechanical stimuli unless injury actually has occurred. These are typically
referred to as silent or sleeping nociceptors since their response comes only on the onset of
inflammation to the surrounding tissue.
Afferent nociceptive fibers (those that send information to, rather than from the brain) travel back to
the spinal cord where they form synapses in its dorsal horn. This nociceptive fiber (located in the
periphery) is a first order neuron. The cells in the dorsal horn are divided into physiologically distinct
layers called laminae. Different fiber types form synapses in different layers, and use either glutamate
or substance P as the neurotransmitter. Aδ fibers form synapses in laminae I and V, C fibers connect
with neurons in lamina II, Aβ fibers connect with lamina I, III, & V. After reaching the specific lamina
within the spinal cord, the first order nociceptive project to second order neurons and cross the
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midline. The second order neurons then send their information via two pathways to the thalamus: the
dorsal column medial-lemniscal system and the anterolateral system. The first is reserved more for
regular non-painful sensation, while the lateral is reserved for pain sensation. Upon reaching the
thalamus, the information is processed in the ventral posterior nucleus and sent to the cerebral cortex
in the brain. As there is an ascending pathway to the brain that initiates the conscious realization of
pain, there also is a descending pathway which modulates pain sensory. The brain can request the
release of specific hormones or chemicals that can have analgesic effects which can reduce or inhibit
pain sensation. The area of the brain that stimulates the release of these hormones is the hypothalamus.
This effect of descending inhibition can be shown by electrically stimulating the periaqueductal grey
area of the midbrain. The periaqueductal grey in turn projects to other areas involved in pain
regulation, such as the nucleus raphe magnus (which also receives similar afferents from the nucleus
reticularis paragigantocellularis (NPG). In turn the nucleus raphe magnus projects to the substantia
gelatinosa region of the dorsal horn and mediates the sensation of spinothalamic inputs. The
periaqueductal grey also contains opioid receptors which explains one of the mechanisms by which
opioids such as morphine and diacetylmorphine exhibit an analgesic effect.
Nociceptor neuron sensitivity is modulated by a large variety of mediators in the extracellular space.
Peripheral sensitization represents a form of functional plasticity of the nociceptor. The nociceptor can
change from being simply a noxious stimulus detector to a detector of non-noxious stimuli. The result
is that low intensity stimuli from regular activity, initiates a painful sensation. This is commonly
known as hyperalgesia. Inflammation is one common cause that results in the sensitization of
nociceptors. Normally hyperalgesia ceases when inflammation goes down, however, sometimes
genetic defects and/or repeated injury can result in allodynia: a completely non-noxious stimulus like
light touch causes extreme pain. Allodynia can also be caused when a nociceptor is damaged in the
peripheral nerves. This can result in deafferentation, which means the development of different central
processes from the surviving afferent nerve. With this situation, surviving dorsal root axons of the
nociceptors can make contact with the spinal cord, thus changing the normal input.
Due to historical understandings of pain, nociceptors are also called pain receptors. This usage is not
consistent with the modern definition of pain as a subjective experience.
A photoreceptor, or photoreceptor cell, is a specialized type of neuron (nerve cell) found in the eye's
retina that is capable of phototransduction. The great biological importance of photoreceptors is that as
cells they convert light (electromagnetic radiation) into the beginning of a chain of biological
processes. More specifically, the photoreceptor absorbs photons from the field of view, and through a
specific and complex biochemical pathway, signals this information through a change in its membrane
For hundreds of years, photoreceptors in vertebrates were thought to be of only two main classes. The
two classic photoreceptors are rods and cones, each contributing information used by the visual system
to form a representation of the visual world, sight.
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A third class of photoreceptors was discovered during the 1990s: the photosensitive ganglion cells.
These cells, found in the inner retina, have dendrites and long axons projecting to the protectum
(midbrain), the suprachiasmatic nucleus in the hypothalamus, and the lateral geniculate (thalamus).
There are major functional differences between the rods and cones. Cones are adapted to detect colors,
and function well in bright light; rods are more sensitive, but do not detect color well, being adapted
for low light. In humans there are three different types of cone - responding respectively to short
(blue), medium (green) and long (yellow-red) light. The human retina contains about 120 million rod
cells and 6 million cone cells. The number and ratio of rods to cones varies among species, dependent
on whether an animal is primarily diurnal or nocturnal. Certain owls have a tremendous number of
rods in their retinas — the eyes of the tawny owl are approximately 100 times more sensitive at night
than those of humans. There are about 1.3 million ganglion cells in the human visual system; 1 to 2%
of them are photosensitive.
A thermoreceptor is a sensory receptor, or more accurately the receptive portion of a sensory neuron,
that codes absolute and relative changes in temperature, primarily within the innocuous range. In the
mammalian peripheral nervous system warmth receptors are thought to be unmyelinated C-fibres (low
conduction velocity; reaches brain within a few seconds ), while those responding to cold have both Cfibers and thinly myelinated A delta fibers (faster conduction velocity; reaches brain within one
second). The adequate stimulus for a warm receptor is warming, which results in an increase in their
action potential discharge rate. Cooling results in a decrease in warm receptor discharge rate. For cold
receptors their firing rate increases during cooling and decreases during warming. Some cold receptors
also respond with a brief action potential discharge to high temperatures, i.e. typically above 45°C,
and this is known as a paradoxical response to heat. The mechanism responsible for this behavior has
not been determined. A special form of thermoreceptor is found in some snakes, the viper pit organ
and this specialized structure is sensitive to energy in the infrared part of the spectrum.
In mammals, temperature receptors innervate various tissues including the skin (as cutaneous
receptors), cornea and urinary bladder. Neurons from the pre-optic and hypothalamic regions of the
brain that respond to small changes in temperature have also been described, providing information on
core temperature. The hypothalamus is involved in thermoregulation, the thermoreceptors allowing
feed-forward responses to a predicted change in core body temperature in response to changing
environmental conditions.
Thermoreceptors have been classically described as having 'free' non-specialised endings; the
mechanism of activation in response to temperature changes is not completely understood.
Cold-sensitive thermoreceptors give rise to the sensations of cooling, cold and freshness. In the cornea
cold receptors are thought to respond with an increase in firing rate to cooling produced by
evaporation of lacrimal fluid 'tears' and thereby to elicit a reflex blink.
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Warm and cold receptors play a part in sensing innocuous environmental temperature. Temperatures
likely to damage an organism are sensed by sub-categories of nociceptors that may respond to noxious
cold, noxious heat or more than one noxious stimulus modality (i.e., they are polymodal). The nerve
endings of sensory neurons that respond preferentially to cooling are found in moderate density in the
skin but also occur in relatively high spatial density in the cornea, tongue, bladder, and facial skin. The
speculation is that lingual cold receptors deliver information that modulates the sense of taste; i.e.
some foods taste good when cold, while others do not.
Mechanism of transduction
This area of research has recently received considerable attention with the identification and cloning of
the Transient Receptor Potential (TRP) family of proteins. The transduction of temperature in cold
receptors is mediated in part by the TRPM8 channel. This channel passes a mixed inward cationic
(predominantly carried by Ca2+ ions) current of a magnitude that is inversely proportional to
temperature. The channel is sensitive over a temperature range spanning about 10-35°C. TRPM8 can
also be activated by the binding of an extracellular ligand. Menthol can activate the TRPM8 channel in
this way. Since the TRPM8 is expressed in neurones whose physiological role is to signal cooling,
menthol applied to various bodily surfaces evokes a sensation of cooling. The feeling of freshness
assocaiated with the activation of cold receptors by menthol, particularly those in facial areas with
axons in the trigeminal (V) nerve, accounts for its use in numerous toiletries including toothpaste,
shaving lotions, facial creams and the like. Another molecular component of cold transduction is the
temperature dependence of so-called leak channels which pass an outward current carried by
potassium ions. Some leak channels derive from the family of two-pore (2P) domain potassium
channels. Amongst the various members of the 2P-domain channels, some close quite promptly at
temperatures less than about 28°C (eg. TRAAK, TREK). Temperature also modulates the activity of
the Na+/K+-ATPase. The Na+/K+-ATPase is a P-type pump that extrudes 3Na+ ions in exchange for
2K+ ions for each hydrolytic cleavage of ATP. This results in a nett movement of positive charge out
of the cell, i.e. a hyperpolarizing current. The magnitude of this current is proportional to the rate of
pump activity. It has been suggested that it is the constellation of various thermally sensitive proteins
together in a neuron that gives rise to a cold receptor. This emergent property of the neuron is thought
to comprise, the expression of the aforementioned proteins as well as various voltage-sensitive
channels including the hyperpolarization-activated, cyclic nucleotide-gated (HCN) channel and the
rapidly activating and inactivating transient potassium channel (IKA).
Arousal is a physiological and psychological state of being awake or reactive to stimuli. It involves the
activation of the reticular activating system in the brain stem, the autonomic nervous system and the
endocrine system, leading to increased heart rate and blood pressure and a condition of sensory
alertness, mobility and readiness to respond.
There are many different neural systems involved in what is collectively known as the arousal system.
Four major systems originating in the brainstem, with connections extending throughout the cortex,
are based on the brain's neurotransmitters, acetylcholine, norepinephrine, dopamine, and serotonin.
When these systems are in action, the receiving neural areas become sensitive and responsive to
incoming signals.
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Arousal is important in regulating consciousness, attention, and information processing. It is crucial
for motivating certain behaviours, such as mobility, the pursuit of nutrition, the fight-or-flight response
and sexual activity (see Masters and Johnson's human sexual response cycle, where it is known as the
arousal phase). It is also very important in emotion, and has been included as a part of many influential
theories such as the James-Lange theory of emotion. According to Hans Eysenck, differences in
baseline arousal level lead people to be either extraverts or introverts. Later research suggest it is most
likely that extroverts and introverts have different arousability. Their baseline arousal level is the
same, but the response to stimulation is different.
The Yerkes-Dodson Law states that there is a relationship between arousal and task performance,
essentially arguing that there is an optimal level of arousal for performance, and too little or too much
arousal can adversely affect task performance. One interpretation of the Yerkes-Dodson Law is the
Easterbrook Cue-Utilisation hypothesis. Easterbrook states that an increase of arousal leads to a
decrease in number of cues that can be utilised.
In positive psychology, arousal is described as a response to a difficult challenge for which the subject
has moderate skills
Abnormally increased behavioral arousal
This is a state caused by withdrawal from alcohol or barbiturates, acute encephalitis, head trauma
resulting in coma, partial seizures in epilepsy, metabolic disorders of electrolyte imbalance, Intracranial space- occupying lesions, Alzheimer's disease, rabies, hemispheric lesions in stroke and
multiple sclerosis.
Anatomically this is a disorder of the limbic system, hypothalamus, temporal lobes, amygdala and
frontal lobes. It is not to be confused with mania.
Attention is the cognitive process of selectively concentrating on one aspect of the environment while
ignoring other things. Attention has also been referred to as the allocation of processing resources.
Selective visual attention
In cognitive psychology there are at least two models which describe how visual attention operates.
These models may be considered loosely as metaphors which are used to describe internal processes
and to generate hypotheses that are falsifiable. Generally speaking, visual attention is thought to
operate as a two-stage process. In the first stage, attention is distributed uniformly over the external
visual scene and processing of information is performed in parallel. In the second stage, attention is
concentrated to a specific area of the visual scene (i.e. it is focused), and processing is performed in a
serial fashion.
The first of these models to appear in the literature is the spotlight model. The term "spotlight" was
first used by David LaBerge, and was inspired by the work of William James who described attention
as having a focus, a margin, and a fringe. The focus is an area that extracts information from the visual
scene with a high-resolution, the geometric center of which being where visual attention is directed.
Surrounding the focus is the fringe of attention which extracts information in a much more crude
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fashion (i.e. low-resolution). This fringe extends out to a specified area and this cut-off is called the
The second model is called the zoom-lens model, and was first introduced in 1983. This model inherits
all properties of the spotlight model (i.e. the focus, the fringe, and the margin) but has the added
property of changing in size. This size-change mechanism was inspired by the zoom lens you might
find on a camera, and any change in size can be described by a trade-off in the efficiency of
processing. The zoom-lens of attention can be described in terms of an inverse trade-off between the
size of focus and the efficiency of processing: because attentional resources are assumed to be fixed,
then it follows that the larger the focus is, the slower processing will be of that region of the visual
scene since this fixed resource will be distributed over a larger area. It is thought that the focus of
attention can subtend a minimum of 1° of visual angle, however the maximum size has not yet been
Overt and covert attention
Attention may be differentiated according to its status as "overt" versus "covert." Overt attention is the
act of directing sense organs towards a stimulus source. Covert attention is the act of mentally
focusing on one of several possible sensory stimuli. Covert attention is thought to be a neural process
that enhances the signal from a particular part of the sensory panorama.
There are studies that suggest the mechanisms of overt and covert attention may not be as separate as
previously believed. Though humans and primates can look in one direction but attend in another,
there may be an underlying neural circuitry that links shifts in covert attention to plans to shift gaze.
For example, if individuals attend to the right hand corner field of view, movement of the eyes in that
direction may have to be actively suppressed.
The current view is that visual covert attention is a mechanism for quickly scanning the field of view
for interesting locations. This shift in covert attention is linked to eye movement circuitry that sets up a
slower saccade to that location.
Executive attention
Inevitably situations arise where it is advantageous to have cognition independent of incoming sensory
data or motor responses. There is a general consensus in psychology that there is an executive system
based in the frontal cortex that controls our thoughts and actions to produce coherent behavior. This
function is often referred to as executive function, executive attention, or cognitive control.
No exact definition has been agreed upon. However, typical descriptions involve maintaining
behavioral goals, and using these goals as a basis for choosing what aspects of the environment to
attend to and which action to select.
Neural correlates of attention
Most experiments show that one neural correlate of attention is enhanced firing. If a neuron has a
certain response to a stimulus when the animal is not attending to the stimulus, then when the animal
does attend to the stimulus, the neuron's response will be enhanced even if the physical characteristics
of the stimulus remain the same.
In a recent review, Knudsen, describes a more general model which identifies four core processes of
attention, with working memory at the center:
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Working memory temporarily stores information for detailed analysis.
Competitive selection is the process that determines which information gains access to working
Through top-down sensitivity control, higher cognitive processes can regulate signal intensity in
information channels that compete for access to working memory, and thus give them an advantage in
the process of competitive selection. Through top-down sensitivity control, the momentary content of
working memory can influence the selection of new information, and thus mediate voluntary control
of attention in a recurrent loop (endogenous attention).
At the neural network level, it is thought that processes like lateral inhibition mediate the process of
competitive selection.
The visual system is the part of the central nervous system which enables organisms to see, as well as
enabling several non-image forming photoresponse functions. It interprets information from visible
light to build a representation of the surrounding world. The visual system accomplishes a number of
complex tasks, including the reception of light and the formation of monocular representations; the
construction of a binocular perception from a pair of two dimensional projections; the identification
and categorization of visual objects; assessing distances to and between objects; and guiding body
movements in relation to visual objects. The psychological manifestation of visual information is
known as visual perception, a lack of which is called blindness. Non-image forming visual functions,
independent of visual perception, include the pupillary light reflex (PLR) and circadian
The visual system includes the eyes, the connecting pathways through to the visual cortex and other
parts of the brain. The illustration shows the mammalian system.
The eye is a complex biological device. The functioning of a camera is often compared with the
workings of the eye, mostly since both focus light from external objects in the field of view onto a
light-sensitive medium. In the case of the camera, this medium is film or an electronic sensor; in the
case of the eye, it is an array of visual receptors. With this simple geometrical similarity, based on the
laws of optics, the eye functions as a transducer, as does a CCD camera.
Light entering the eye is refracted as it passes through the cornea. It then passes through the pupil
(controlled by the iris) and is further refracted by the lens. The cornea and lens act together as a
compound lens to project an inverted image onto the retina.
The retina consists of a large number of photoreceptor cells which contain particular protein molecules
called opsins. In humans, two types of opsins are involved in conscious vision: rod opsins and cone
opsins. (A third type, melanopsin in some of the retinal ganglion cells (RGC), part of the body clock
mechanism, is probably not involved in conscious vision, as these RGC do not project to the lateral
geniculate nucleus (LGN) but to the pretectal olivary nucleus (PON).) An opsin absorbs a photon (a
particle of light) and transmits a signal to the cell through a signal transduction pathway, resulting in
hyperpolarization of the photoreceptor.
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In the visual system, retinal, technically called retinene1 or "retinaldehyde", is a light-sensitive
retinene molecule found in the rods and cones of the retina. Retinal is the fundamental structure
involved in the transduction of light into visual signals, i.e. nerve impulses in the ocular system of the
central nervous system. In the presence of light, the retinal molecule changes configuration and as a
result a nerve impulse is generated.
Optic nerve
Information flow from the eyes (top), crossing at the optic chiasma, joining left and right eye
information in the optic tract, and layering left and right visual stimuli in the lateral geniculate nucleus.
V1 in red at bottom of image. (1543 image from Andreas Vesalius' Fabrica)
The information about the image via the eye is transmitted to the brain along the optic nerve. Different
populations of ganglion cells in the retina send information to the brain through the optic nerve. About
90% of the axons in the optic nerve go to the lateral geniculate nucleus in the thalamus. These axons
originate from the M, P, and K ganglion cells in the retina, see above. This parallel processing is
important for reconstructing the visual world; each type of information will go through a different
route to perception. Another population sends information to the superior colliculus in the midbrain,
which assists in controlling eye movements (saccades) as well as other motor responses.
A final population of photosensitive ganglion cells, containing melanopsin, sends information via the
retinohypothalamic tract (RHT) to the pretectum (pupillary reflex), to several structures involved in
the control of circadian rhythms and sleep such as the suprachiasmatic nucleus (SCN, the biological
clock), and to the ventrolateral preoptic nucleus (VLPO, a region involved in sleep regulation).[11] A
recently discovered role for photoreceptive ganglion cells is that they mediate conscious and
unconscious vision – acting as rudimentary visual brightness detectors as shown in rodless coneless
Optic chiasm
The optic nerves from both eyes meet and cross at the optic chiasm, at the base of the hypothalamus of
the brain. At this point the information coming from both eyes is combined and then splits according
to the visual field. The corresponding halves of the field of view (right and left) are sent to the left and
right halves of the brain, respectively, to be processed. That is, the right side of primary visual cortex
deals with the left half of the field of view from both eyes, and similarly for the left brain. A small
region in the center of the field of view is processed redundantly by both halves of the brain.
Optic tract
Information from the right visual field (now on the left side of the brain) travels in the left optic tract.
Information from the left visual field travels in the right optic tract. Each optic tract terminates in the
lateral geniculate nucleus (LGN) in the thalamus.
Lateral geniculate nucleus
The lateral geniculate nucleus (LGN) is a sensory relay nucleus in the thalamus of the brain. The LGN
consists of six layers in humans and other primates starting from catarhinians, including
cercopithecidae and apes. Layers 1, 4, and 6 correspond to information from the contralateral (crossed)
fibers of the nasal visual field; layers 2, 3, and 5 correspond to information from the ipsilateral
(uncrossed) fibers of the temporal visual field. Layer one (1) contains M cells which correspond to the
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M (magnocellular) cells of the optic nerve of the opposite eye and are concerned with depth or motion.
Layers four and six (4 & 6) of the LGN also connect to the opposite eye, but to the P cells (color and
edges) of the optic nerve. By contrast, layers two, three and five (2, 3, & 5) of the LGN connect to the
M cells and P (parvocellular) cells of the optic nerve for the same side of the brain as its respective
LGN. Spread out, the six layers of the LGN are the area of a credit card and about three times its
thickness. The LGN is rolled up into two ellipsoids about the size and shape of two small birds' eggs.
In between the six layers are smaller cells that receive information from the K cells (color) in the
retina. The neurons of the LGN then relay the visual image to the primary visual cortex (V1) which is
located at the back of the brain (caudal end) in the occipital lobe in and close to the calcarine sulcus.
Optic radiation
The optic radiations, one on each side of the brain, carry information from the thalamic lateral
geniculate nucleus to layer 4 of the visual cortex. The P layer neurons of the LGN relay to V1 layer 4C
β. The M layer neurons relay to V1 layer 4C α. The K layer neurons in the LGN relay to large neurons
called blobs in layers 2 and 3 of V1.
There is a direct correspondence from an angular position in the field of view of the eye, all the way
through the optic tract to a nerve position in V1. At this juncture in V1, the image path ceases to be
straightforward; there is more cross-connection within the visual cortex.
Visual cortex
The visual cortex is the most massive system in the human brain and is responsible for processing the
visual image. It lies at the rear of the brain (highlighted in the image), above the cerebellum. The
region that receives information directly from the LGN is called the primary visual cortex, (also called
V1 and striate cortex). Visual information then flows through a cortical hierarchy. These areas include
V2, V3, V4 and area V5/MT (the exact connectivity depends on the species of the animal). These
secondary visual areas (collectively termed the extrastriate visual cortex) process a wide variety of
visual primitives. Neurons in V1 and V2 respond selectively to bars of specific orientations, or
combinations of bars. These are believed to support edge and corner detection. Similarly, basic
information about color and motion is processed here.
Visual association cortex
As visual information passes forward through the visual hierarchy, the complexity of the neural
representations increase. Whereas a V1 neuron may respond selectively to a line segment of a
particular orientation in a particular retinotopic location, neurons in the lateral occipital complex
respond selectively to complete object (e.g., a figure drawing), and neurons in visual association
cortex may respond selectively to human faces, or to a particular object.
Along with this increasing complexity of neural representation may come a level of specialization of
processing into two distinct pathways: the dorsal stream and the ventral stream (the Two Streams
hypothesis, first proposed by Ungerleider and Mishkin in 1982). The dorsal stream, commonly
referred to as the "where" stream, is involved in spatial attention (covert and overt), and communicates
with regions that control eye movements and hand movements. More recently, this area has been
called the "how" stream to emphasize its role in guiding behaviors to spatial locations. The ventral
stream, commonly referred as the "what" stream, is involved in the recognition, identification and
categorization of visual stimuli.
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However, there is still much debate about the degree of specialization within these two pathways,
since they are in fact heavily interconnected.
The vertebrate retina is a light sensitive tissue lining the inner surface of the eye. The optics of the eye
create an image of the visual world on the retina, which serves much the same function as the film in a
camera. Light striking the retina initiates a cascade of chemical and electrical events that ultimately
trigger nerve impulses. These are sent to various visual centers of the brain through the fibers of the
optic nerve.
In vertebrate embryonic development, the retina and the optic nerve originate as outgrowths of the
developing brain, so the retina is considered part of the central nervous system (CNS). It is the only
part of the CNS that can be visualized non-invasively.
The retina is a complex, layered structure with several layers of neurons interconnected by synapses.
The only neurons that are directly sensitive to light are the photoreceptor cells. These are mainly of
two types: the rods and cones. Rods function mainly in dim light and provide black-and-white vision,
while cones support daytime vision and the perception of colour. A third, much rarer type of
photoreceptor, the photosensitive ganglion cell, is important for reflexive responses to bright daylight.
Neural signals from the rods and cones undergo complex processing by other neurons of the retina.
The output takes the form of action potentials in retinal ganglion cells whose axons form the optic
nerve. Several important features of visual perception can be traced to the retinal encoding and
processing of light.
Anatomy of vertebrate retina
Section of retina.
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The vertebrate retina has ten distinct layers. From innermost to outermost, they include:
Inner limiting membrane - Müller cell footplates
Nerve fiber layer - Essentially the axons of the ganglion cell nuclei.
Ganglion cell layer - Layer that contains nuclei of ganglion cells and gives rise to optic nerve
Inner plexiform layer
Inner nuclear layer contains bipolar cells, which correspond to heat and touch sensory skin
receptors transmitting signals to the spinal cord or its continuation, the medulla.[1]
Outer plexiform layer - In the macular region, this is known as the Fiber layer of Henle.
Outer nuclear layer
External limiting membrane - Layer that separates the inner segment portions of the
photoreceptors from their cell nucleaus.
Photoreceptor layer - Rods / Cones
Retinal pigment epithelium
Of these the four main layers of the ten, from outside in: pigment epithelium, the photoreceptor layer,
bipolar cells, and finally, the ganglion cell layer.
Therefore, the optic nerve is less a nerve than a central tract, connecting the bipolars to the lateral
geniculate body, a visual relay station in the diencephalon (the rear of the forebrain). Additional
structures, not directly associated with vision, are found as outgrowths of the retina in some vertebrate
groups. In birds, the pecten is a vascular structure of complex shape that projects from the retina into
the vitreous humour; it supplies oxygen and nutrients to the eye, and may also aid in vision. Reptiles
have a similar, but much simpler, structure, referred to as the papillary cone.
Physical structure of human retina
In adult humans the entire retina is approximately 72% of a sphere about 22 mm in diameter. The
entire retina contains about 7 million cones and 75 to 150 million rods. An area of the retina is the
optic disc, sometimes known as "the blind spot" because it lacks photoreceptors. It appears as an oval
white area of 3 mm². Temporal (in the direction of the temples) to this disc is the macula. At its center
is the fovea, a pit that is most sensitive to light and is responsible for our sharp central vision. Human
and non-human primates possess one fovea as opposed to certain bird species such as hawks who
actually are bifoviate and dogs and cats who possess no fovea but a central band known as the visual
streak. Around the fovea extends the central retina for about 6 mm and then the peripheral retina. The
edge of the retina is defined by the ora serrata. The length from one ora to the other (or macula), the
most sensitive area along the horizontal meridian is about 3.2 mm.
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Retina's simplified axial organization. The retina is a stack of several neuronal layers. Light is
concentrated from the eye and passes across these layers (from left to right) to hit the photoreceptors
(right layer). This elicits chemical transformation mediating a propagation of signal to the bipolar and
horizontal cells (middle yellow layer). The signal is then propagated to the amacrine and ganglion
cells. These neurons ultimately may produce action potentials on their axons. This spatiotemporal
pattern of spikes determines the raw input from the eyes to the brain. (Modified from a drawing by
Ramón y Cajal.)
In section the retina is no more than 0.5 mm thick. It has three layers of nerve cells and two of
synapses, including the unique ribbon synapses. The optic nerve carries the ganglion cell axons to the
brain and the blood vessels that open into the retina. The ganglion cells lie innermost in the retina
while the photoreceptive cells lie outermost. Because of this counter-intuitive arrangement, light must
first pass through and around the ganglion cells and through the thickness of the retina, (including its
capillary vessels,not shown) before reaching the rods and cones. However it does not pass through the
epithelium or the choroid (both of which are opaque).
The white blood cells in the capillaries in front of the photoreceptors can be perceived as tiny bright
moving dots when looking into blue light. This is known as the blue field entoptic phenomenon (or
Scheerer's phenomenon).
Between the ganglion cell layer and the rods and cones there are two layers of neuropils where
synaptic contacts are made. The neuropil layers are the outer plexiform layer and the inner plexiform
layer. In the outer the rods and cones connect to the vertically running bipolar cells, and the
horizontally oriented horizontal cells connect to ganglion cells.
The central retina is cone-dominated and the peripheral retina is rod-dominated. In total there are
about seven million cones and a hundred million rods. At the centre of the macula is the foveal pit
where the cones are smallest and in a hexagonal mosaic, the most efficient and highest density. Below
the pit the other retina layers are displaced, before building up along the foveal slope until the rim of
the fovea or parafovea which is the thickest portion of the retina. The macula has a yellow
pigmentation from screening pigments and is known as the macula lutea. The area directly
surrounding the fovea has the highest density of rods converging on single bipolars. Since the cones
have a much lesser power of merging signals, the fovea allows for the sharpest vision the eye can
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Since there are about 150 million receptors and only 1 million optic nerve fibers, and the horizontal
action of the horizontal and amacrine cells can allow one area of the retina to control another (e.g., one
stimulus inhibiting another), so the messages are merged and mixed. In some lower vertebrates, (e.g.,
the pigeon) there is a "centrifugal" control of messages, that is, one layer can control another, or higher
regions of the brain can drive the retinal nerve cells, but in primates this does not occur.
An image is produced by the "patterned excitation" of the cones and rods in the retina. The excitation
is processed by the neuronal system and various parts of the brain working in parallel to form a
representation of the external environment in the brain.
The cones respond to bright light and mediate high-resolution colour vision during daylight
illumination (also called "photopic" vision). The rods are saturated at daylight levels and don't
contribute to pattern vision. However, rods do respond to dim light and mediate lower-resolution,
monochromatic vision under very low levels of illumination (called "scotopic" vision). The
illumination in most office settings falls between these two levels and is called "mesopic" vision. At
these light levels, both the rods and cones are actively contributing pattern information to that exiting
the eye. What contribution the rod information makes to pattern vision under these circumstances is
The response of cones to various wavelengths of light is called their "spectral sensitivity". In normal
human vision, the spectral sensitivity of a cone falls into one of three subgroups. These are often
called "red, green, and blue" cones but more accurately are short, medium, and long wavelength
sensitive cone subgroups. It is a lack of one or more of the cone subtypes that causes individuals to
have deficiencies in colour vision or various kinds of colour blindness. These individuals are not
"blind" to objects of a particular colour but experience the inability to distinguish between two groups
of colours that can be distinguished by people with normal vision. Humans have three different types
of cones (trichromatic vision) while most other mammals lack cones with red sensitive pigment and
therefore have poorer (dichromatic) colour vision. However, some animals have four spectral
subgroups, e.g., the trout adds an ultraviolet subgroup to short, medium and long subgroups that are
similar to humans. Some fish are sensitive to the polarization of light as well.
When light falls on a receptor it sends a proportional response synaptically to bipolar cells which in
turn signal the retinal ganglion cells. The receptors are also 'cross-linked' by horizontal cells and
amacrine cells, which modify the synaptic signal before the ganglion cells. Rod and cone signals are
intermixed and combine, although rods are mostly active in very poorly lit conditions and saturate in
broad daylight, while cones function in brighter lighting because they are not sensitive enough to work
at very low light levels.
Despite the fact that all are nerve cells, only the retinal ganglion cells and few amacrine cells create
action potentials. In the photoreceptors, exposure to light hyperpolarizes the membrane in a series of
graded shifts. The outer cell segment contains a photopigment. Inside the cell the normal levels of
cyclic guanosine monophosphate (cGMP) keep the Na+ channel open and thus in the resting state the
cell is depolarised. The photon causes the retinal bound to the receptor protein to isomerise to transretinal. This causes receptor to activate multiple G-proteins. This in turn causes the Ga-subunit of the
protein to bind and degrade cGMP inside the cell which then cannot bind to the Na+ cyclic nucleotidegated ion channels (CNGs). Thus the cell is hyperpolarised. The amount of neurotransmitter released
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is reduced in bright light and increases as light levels fall. The actual photopigment is bleached away
in bright light and only replaced as a chemical process, so in a transition from bright light to darkness
the eye can take up to thirty minutes to reach full sensitivity (see Adaptation (eye)).
In the retinal ganglion cells there are two types of response, depending on the receptive field of the
cell. The receptive fields of retinal ganglion cells comprise a central approximately circular area,
where light has one effect on the firing of the cell, and an annular surround, where light has the
opposite effect on the firing of the cell. In ON cells, an increment in light intensity in the centre of the
receptive field causes the firing rate to increase. In OFF cells, it makes it decrease. In a linear model,
this response profile is well described by a Difference of Gaussians and is the basis for edge detection
algorithms. Beyond this simple difference ganglion cells are also differentiated by chromatic
sensitivity and the type of spatial summation. Cells showing linear spatial summation are termed X
cells (also called "parvocellular", "P", or "midget" ganglion cells), and those showing non-linear
summation are Y cells (also called "magnocellular, "M", or "parasol" retinal ganglion cells), although
the correspondence between X and Y cells (in the cat retina) and P and M cells (in the primate retina)
is not as simple as it once seemed.
In the transfer of visual signals to the brain, the visual pathway, the retina is vertically divided in two,
a temporal (nearer to the temple) half and a nasal (nearer to the nose) half. The axons from the nasal
half cross the brain at the optic chiasma to join with axons from the temporal half of the other eye
before passing into the lateral geniculate body.
Although there are more than 130 million retinal receptors, there are only approximately 1.2 million
fibres (axons) in the optic nerve; a large amount of pre-processing is performed within the retina. The
fovea produces the most accurate information. Despite occupying about 0.01% of the visual field (less
than 2° of visual angle), about 10% of axons in the optic nerve are devoted to the fovea. The resolution
limit of the fovea has been determined at around 10,000 points. The information capacity is estimated
at 500,000 bits per second (for more information on bits, see information theory) without colour or
around 600,000 bits per second including colour.
Diseases and disorders
There are many inherited and acquired diseases or disorders that may affect the retina. Some of them
Retinitis pigmentosa is a group of genetic diseases that affect the retina and causes the loss of
night vision and peripheral vision.
Macular degeneration describes a group of diseases characterized by loss of central vision
because of death or impairment of the cells in the macula.
Cone-rod dystrophy (CORD) describes a number of diseases where vision loss is caused by
deterioration of the cones and/or rods in the retina.
In retinal separation, the retina detaches from the back of the eyeball. Ignipuncture is an
outdated treatment method. The term retinal detachment is used to describe a separation of the
neurosensory retina from the retinal pigment epithelium.[6] There are several modern treatment
methods for fixing a retinal detachment: pneumatic retinopexy, scleral buckle, cryotherapy,
laser photocoagulation and pars plana vitrectomy.
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Both hypertension and diabetes mellitus can cause damage to the tiny blood vessels that
supply the retina, leading to hypertensive retinopathy and diabetic retinopathy.
Retinoblastoma is a cancer of the retina.
Retinal diseases in dogs include retinal dysplasia, progressive retinal atrophy, and sudden
acquired retinal degeneration.
Diagnosis and treatment
A number of different instruments are available for the diagnosis of diseases and disorders affecting
the retina. An ophthalmoscope is used to examine the retina. Recently, adaptive optics has been used
to image individual rods and cones in the living human retina and a company based in Scotland have
engineered technology that allows physicians to observe the complete retina without any discomfort to
The electroretinogram is used to measure non-invasively the retina's electrical activity, which is
affected by certain diseases. A relatively new technology, now becoming widely available, is optical
coherence tomography (OCT). This non-invasive technique allows one to obtain a 3D volumetric or
high resolution cross-sectional tomogram of the retinal fine structure with histologic-quality.
Treatment depends upon the nature of the disease or disorder. Transplantation of retinas has been
attempted, but without much success. At MIT, The University of Southern California, and the
University of New South Wales, an "artificial retina" is under development: an implant which will
bypass the photoreceptors of the retina and stimulate the attached nerve cells directly, with signals
from a digital camera.
A photoreceptor, or photoreceptor cell, is a specialized type of neuron (nerve cell) found in the eye's
retina that is capable of phototransduction. The great biological importance of photoreceptors is that as
cells they convert light (electromagnetic radiation) into the beginning of a chain of biological
processes. More specifically, the photoreceptor absorbs photons from the field of view, and through a
specific and complex biochemical pathway, signals this information through a change in its membrane
For hundreds of years, photoreceptors in vertebrates were thought to be of only two main classes. The
two classic photoreceptors are rods and cones, each contributing information used by the visual system
to form a representation of the visual world, sight.
A third class of photoreceptors was discovered during the 1990s , the photosensitive ganglion cells.
These cells, found in the inner retina, have dendrites and long axons projecting to the protectum
(midbrain), the suprachiasmatic nucleus in the hypothalamus, and the lateral geniculate (thalamus).
There are major functional differences between the rods and cones. Cones are adapted to detect colors,
and function well in bright light; rods are more sensitive, but do not detect color well, being adapted
for low light. In humans there are three different types of cone - responding respectively to short
(blue), medium (green) and long (yellow-red) light. The human retina contains about 120 million rod
cells and 6 million cone cells. The number and ratio of rods to cones varies among species, dependent
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on whether an animal is primarily diurnal or nocturnal. Certain owls have a tremendous number of
rods in their retinas — the eyes of the tawny owl are approximately 100 times more sensitive at night
than those of humans. There are about 1.3 million ganglion cells in the human visual system; 1 to 2%
of them are photosensitive.
Described here are vertebrate photoreceptors. Invertebrate photoreceptors in organisms such as insects
and molluscs are different in both their morphological organization and their underlying biochemical
Rod and cone photoreceptors have the same complex structural formation. Closest to the visual field
(and farthest from the brain) is the axon terminal, which releases a neurotransmitter called glutamate
to bipolar cells. Farther back is the cell body, which contains the cell's organelles. Farther back still is
the inner segment, a specialized part of the cell full of mitochondria. The chief function of the inner
segment is to provide ATP (energy) for the sodium-potassium pump. Finally, closest to the brain (and
farthest from the field of view) is the outer segment, the part of the photoreceptor that absorbs light.
Outer segments are actually modified cilia that contain disks filled with opsin, the molecule that
absorbs photons, as well as voltage-gated sodium channels.
Anatomy of a Rod Cell
The membranous photoreceptor protein opsin contains a pigment molecule called retinal. In rod cells,
these together are called rhodopsin. In cone cells there are different types of opsins that combine with
retinal to form pigments called photopsins. Three different classes of photopsins in the cones react to
different ranges of light frequency, a differentation which eventually allows the visual system to
distinguish color. The function of the photoreceptor cell is to convert the light energy of the photon
into a form of energy communicable to the nervous system and readily usable to the organism: this
conversion is called signal transduction.
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The opsin found in the photosensitive ganglion cells of the retina that are involved in various reflexive
responses of the brain and body to the presence of (day)light, such as the regulation of circadian
rhythms, pupillary reflex and other non-visual responses to light, is called melanopsin. Atypical in
vertebrates, melanopsin functionally resembles invertebrate opsins. In structure, it is an opsin, a
retinylidene protein variety of G-protein-coupled receptor.
When light activates the melanopsin signaling system, the melanopsin-containing ganglion cells
discharge nerve impulses which are conducted through their axons to specific brain targets. These
targets include the olivary pretectal nucleus (a center responsible for controlling the pupil of the eye)
and, through the retinohypothalamic tract (RHT), the suprachiasmatic nucleus of the hypothalamus
(the master pacemaker of circadian rhythms). Melanopsin ganglion cells are thought to influence these
targets by releasing from their axon terminals the neurotransmitters glutamate and pituitary adenylate
cyclase activating polypeptide (PACAP).
The human visual system uses millions of photoreceptors. With the exception of melanopsincontaining photosensitive ganglion cells, ocular photoreceptors are the only neurons in humans
capable of phototransduction. All photoreceptors in humans are found either in the outer nuclear layer
in the retina at the back of each eye, while the bipolar and ganglion cells that transmit information
from photoreceptors to the brain are in front of them. This inverted arrangement significantly reduces
acuity, as light must travel through the axons and cell bodies of other neurons before reaching the
photoreceptors. The retina contains two specializations to deal with this issue. First, a region at the
center of the retina, called the fovea, containing only photoreceptors, is used for high visual acuity.
Second, each retina contains a blind spot, an area where axons from the ganglion cells can go back
through the retina to the brain.
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Normalized typical human cone (and rod) absorbances (not responses) to different wavelengths of
For hundreds of years humans were thought to have only two types of photoreceptors in the retina:
rods and cones. Both transduce light into a change in membrane potential through the same signal
transduction pathway (see below). However, they differ in the nature of the opsin they contain, and
their functions. Rods are used primarily to see at low levels of light, while cones are used to determine
color, depth, and intensity. Furthermore, there are three types of cones, which differ in the spectrum of
wavelengths of photons over which they absorb (see graph). Because cones respond to both the
wavelength and intensity of light, a single cone cannot tell color; instead, color vision requires
interactions of more than one type of cone (see below), primarily by comparing responses across
different cone types.
Phototransduction is the complex process whereby the energy of a photon is used to change the
inherent membrane potential of the photoreceptor. This change thereby signals to the nervous system
that light is in the visual field.
Activation of a photoreceptor cell is actually a hyperpolarization; when they are not being stimulated,
rods and cones depolarize and release glutamate continuously. In the dark, cells have a relatively high
concentration of cyclic guanosine 3'-5' monophosphate (cGMP), which opens ion channels (largely
sodium channels, though calcium can enter through these channels as well). The positive charges of
the ions that enter the cell down its electrochemical gradient change the cell's membrane potential,
cause depolarization, and lead to the release of the neurotransmitter glutamate. Glutamate can
depolarize some neurons and hyperpolarize others.
When light hits a photoreceptive pigment within the photoreceptor cell, the pigment changes shape.
The pigment, called iodopsin or rhodopsin, consists of large proteins called opsin (situated in the
plasma membrane), attached to a covalently-bound prosthetic group: an organic molecule called
retinal (a derivative of vitamin A). The retinal exists in the 11-cis-retinal form when in the dark, and
stimulation by light causes its structure to change to all-trans-retinal. This structural change causes it
to activate a regulatory protein called transducin, which leads to the activation of cGMP
phosphodiesterase, which breaks cGMP down into 5'-GMP. Reduction in cGMP allows the ion
channels to close, preventing the influx of positive ions, hyperpolarizing the cell, and stopping the
release of neurotransmitters.. The entire process by which light initiates a sensory response is called
visual phototransduction.
Dark current
Unstimulated (in the dark), cyclic-nucleotide gated channels in the outer segment are open because
cyclic GMP (cGMP) is bound to them. Hence, positively charged ions (namely sodium ions) enter the
photoreceptor, depolarizing it to about -40 mV (resting potential in other nerve cells is usually -65
mV). This depolarizing current is often known as dark current.
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Signal transduction pathway
The signal transduction pathway is the mechanism by which the energy of a photon signals a
mechanism in the cell that leads to its electrical polarization. This polarization ultimately leads to
either the transmittance or inhibition of a neural signal that will be fed to the brain via the optic nerve.
The steps, or signal transduction pathway, in the vertebrate eye's rod and cone photoreceptors are then:
The rhodopsin or iodopsin in the outer segment absorbs a photon, changing the configuration
of a retinal Schiff base cofactor inside the protein from the cis-form to the trans-form, causing
the retinal to change shape.
This results in a series of unstable intermediates, the last of which binds stronger to the G
protein in the membrane and activates transducin, a protein inside the cell. This is the first
amplification step - each photoactivated rhodopsin triggers activation of about 100 transducins.
(The shape change in the opsin activates a G protein called transducin.)
Each transducin then activates the enzyme cGMP-specific phosphodiesterase (PDE).
PDE then catalyzes the hydrolysis of cGMP. This is the second amplification step, where a
single PDE hydrolyses about 1000 cGMP molecules. (The enzyme hydrolyzes the second
messenger cGMP to GMP)
With the intracellular concentration of cGMP reduced, the net result is closing of cyclic
nucleotide-gated ion channels in the photoreceptor membrane because cGMP was keeping the
channels open. (Because cGMP acts to keep Na+ ion channels open, the conversion of cGMP
to GMP closes the channels.)
As a result, sodium ions can no longer enter the cell, and the photoreceptor hyperpolarizes (its
charge inside the membrane becomes more negative). (The closing of Na + channels
hyperpolarizes the cell.)
This change in the cell's membrane potential causes voltage-gated calcium channels to close.
This leads to a decrease in the influx of calcium ions into the cell and thus the intracellular
calcium ion concentration falls.
The lack of calcium means that less glutamate is released to the bipolar cell than before (see
below). (The decreased calcium level slows the release of the neurotransmitter glutamate,
which can either excite or inhibit the postsynaptic bipolar cells.)
Reduction in the release of glutamate means one population of bipolar cells will be depolarized
and a separate population of bipolar cells will be hyperpolarized, depending on the nature of
receptors (ionotropic or metabotropic) in the postsynaptic terminal (see receptive field).
Thus, a rod or cone photoreceptor actually releases less neurotransmitter when stimulated by light.
ATP provided by the inner segment powers the sodium-potassium pump. This pump is necessary to
reset the initial state of the outer segment by taking the sodium ions that are entering the cell and
pumping them back out.
Although photoreceptors are neurons, they do not conduct action potentials with the exception of the
ganglion cell photoreceptor.
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Phototransduction in rods and cones is unique in that the stimulus (in this case, light) actually reduces
the cell's response or firing rate, which is unusual for a sensory system where the stimulus usually
increases the cell's response or firing rate. However, this system offers several key advantages.
First, the classic (rod or cone) photoreceptor is depolarized in the dark, which means many sodium
ions are flowing into the cell. Thus, the random opening or closing of sodium channels will not affect
the membrane potential of the cell; only the closing of a large number of channels, through absorption
of a photon, will affect it and signal that light is in the visual field. Hence, the system is noiseless.
Second, there is a lot of amplification in two stages of classic phototransduction: one pigment will
activate many molecules of transducin, and one PDE will cleave many cGMPs. This amplification
means that even the absorption of one photon will affect membrane potential and signal to the brain
that light is in the visual field. This is the main feature which differentiates rod photoreceptors from
cone photoreceptors. Rods are extremely sensitive and have the capacity of registering a single photon
of light unlike cones. On the other hand, cones are known to have very fast kinetics in terms of rate of
amplification of phototransduction unlike rods.
Difference between rods and cones
Used for scotopic vision
Used for photopic vision
Very light sensitive; sensitive to scattered light
Not very light sensitive; sensitive to only direct
Loss causes night blindness
Loss causes legal blindness
Low visual acuity
High visual acuity; better spatial resolution
Not present in fovea
Concentrated in fovea
Slow response to light, stimuli added over time
Fast response to light, can perceive more rapid
changes in stimuli
Have more pigment than cones, so can detect lower Have less pigment than rods, require more
light levels
light to detect images
Stacks of membrane-enclosed disks are unattached to
Disks are attached to outer membrane
cell membrane directly
20 times more rods than cones in the retina
One type of photosensitive pigment
Three types of photosensitive pigment in
Confer achromatic vision
Confer color vision
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Photoreceptors do not signal color; they only signal the presence of light in the visual field.
A given photoreceptor responds to both the wavelength and intensity of a light source. For example,
red light at a certain intensity can produce the same exact response in a photoreceptor as green light of
a different intensity. Therefore, the response of a single photoreceptor is ambiguous when it comes to
To determine color, the visual system compares responses across a population of photoreceptors
(specifically, the three different cones with differing absorption spectra). To determine intensity, the
visual system computes how many photoreceptors are responding. This is the mechanism that allows
trichromatic color vision in humans and some other animals.
The rod and cone photoreceptors signal their absorption of photons through a release of the
neurotransmitter glutamate to bipolar cells at its axon terminal. Since the photoreceptor is depolarized
in the dark, a high amount of glutamate is being released to bipolar cells in the dark. Absorption of a
photon will hyperpolarize the photoreceptor and therefore result in the release of less glutamate at the
presynaptic terminal to the bipolar cell.
Every rod or cone photoreceptor releases the same neurotransmitter, glutamate. However, the effect of
glutamate differs in the bipolar cells, depending upon the type of receptor imbedded in that cell's
membrane. When glutamate binds to an ionotropic receptor, the bipolar cell will depolarize (and
therefore will hyperpolarize with light as less glutamate is released). On the other hand, binding of
glutamate to a metabotropic receptor results in a hyperpolarization, so this bipolar cell will depolarize
to light as less glutamate is released.
In essence, this property allows for one population of bipolar cells that gets excited by light and
another population that gets inhibited by it, even though all photoreceptors show the same response to
light. This complexity becomes both important and necessary for detecting color, contrast, edges, etc.
Further complexity arises from the various interconnections among bipolar cells, horizontal cells, and
amacrine cells in the retina. The final result is differing populations of ganglion cells in the retina, a
sub-population of which is also intrinsically photosensitive, using the photopigment melanopsin.
Ganglion cell (non-rod non-cone) photoreceptors
In 1991, Foster et al. discovered a non-rod non-cone photoreceptor in the eyes of mice, which was
shown to mediate circadian rhythms. These neuronal cells, called intrinsically photosensitive retinal
ganglion cells (ipRGC), are a small subset (~1-3%) of the Retinal Ganglion Cells located in the inner
retina, that is, in front of the rods and cones located in the outer retina. ipRGCs contain a
photopigment, melanopsin, which has an absorption peak of the light at a different wavelength
(~480 nm) than rods and cones. Beside circadian / behavioral functions, ipRGCs have a role in
initiating the pupil light reflex.
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Dennis Dacey with colleagues showed in a species of Old World monkey that giant ganglion cells
expressing melanopsin projected to the lateral geniculate nucleas. Previously only projections to the
midbrain (pre-tectal nucleas) and hypothalamus (suprachiasmatic nucleus) had been shown. However
a visual role for the receptor was still unsuspected and unproven.
In 2007 Farhan H. Zaidi and colleagues published their pioneering work using rodless coneless
humans. Current Biology subsequently announced in their 2008 editorial, commentary and despatches
to scientists and ophthalmologists, that the non-rod non-cone photoreceptor had been conclusively
discovered in humans using landmark experiments on rodless coneless humans by Zaidi and
colleagues . The workers found the identity of the non-rod non-cone photoreceptor in humans to be a
ganglion cell in the inner retina as had been shown previously in rodless coneless models in some
other mammals. The workers had tracked down patients with rare diseases wiping out classic rod and
cone photoreceptor function but preserving ganglion cell function. Despite having no rods or cones
the patients continued to exhibit circadian photoentrainment, circadian behavioural patterns,
melanopsin suppression, and pupil reactions, with peak spectral sensitivities to environmental and
experimental light matching that for the melanopsin photopigment. Their brains could also associate
vision with light of this frequency.
In humans the retinal ganglion cell photoreceptor contributes to conscious sight as well as to nonimage-forming functions like circadian rhythms, behaviour and pupil reactions. Since these cells
respond mostly to blue light, it has been suggested that they have a role in mesopic vision. Zaidi and
colleagues' work with rodless coneless human subjects hence also opened the door into image-forming
(visual) roles for the ganglion cell photoreceptor. It was discovered that there are parallel pathways for
vision - one classic rod and cone-based arising from the outer retina, the other a rudimentary visual
brightness detector arising from the inner retina and which seems to be activated by light before the
other. Classic photoreceptors also feed into the novel photoreceptor system, and colour constancy may
be an important role as suggested by Foster. The receptor could be instrumental in understanding
many diseases including major causes of blindness worldwide like glaucoma, a disease which affects
ganglion cells, and the study of the receptor offered potential as a new avenue to explore in trying to
find treatments for blindness. It is in these discoveries of the novel photoreceptor in humans and in the
receptors role in vision, rather than its non-image-forming functions, where the receptor may have the
greatest impact on society as a whole, though the impact of disturbed circadian rhythms is another area
of relevance to clinical medicine.
Most work suggests that the peak spectral sensitivity of the receptor is between 460 and 481 nm,
though a minority of groups reported it being lower as far as 420 nm. Steven Lockley et al. in 2003
showed that 460 nm wavelengths of light suppress melatonin twice as much as longer 555 nm light.
However, in more recent work by Farhan Zaidi et al., using rodless coneless human, it was found that
what consciously led to light perception was a very intense 481 nm stimuli - this means that the
receptor in visual terms enables some rudimentary vision maximally for blue light.
Photoreceptor Signals
When a photoreceptor "captures" a photon, the protein part of the photopigment ("opsin") can
change shape ("become activated", "be isomerized"). This is often notated as R -> R*.
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This change in the shape of the photopigment triggers a biochemical cascade (we'll skip the
details). This results in the hyperpolarization of the receptor (i.e., its charge decreases relative
to the outside).
In the dark (at rest), the photoreceptors are quite active, constantly releasing neurotransmitter.
After absorption of a photon, the resulting hyperpolarization decreases the amount of
neurotransmitter released. This means that light actually turns receptors off.
These hyperpolarizations are graded responses. Gradual increases in light intensity have
gradual effects on neurotransmitter release.
The rods and cones are connected to horizontal and bipolar cells. These cells then connect to
the retinal ganglion cells. Refer to detailed and more schematized figures.
Retinal Ganglion Cells
The retinal ganglion cells represent the output of the retina. They exhibit several important properties
that are characteristic of many visual neurons. Begin by reviewing the basic anatomy of a neuron: cell
body ("soma"), dendrites (which receive inputs from other cells), and the axon (which sends outputs to
other cells).
Action Potentials. As opposed to graded responses, ganglion cells fire action potentials. These
electrical "spikes" are the basic way that information is transmitted in the brain, and so we'll
spend some time understanding how they work.
Retinal ganglion cells have a negative resting potential (~ -70 mV). At this resting
potential, there is a tension between the concentrations and charges of sodium and
potassium ions inside out outside the neuron. Sodium ions would like to enter the
neuron, while potassium ions would like to leave it.
When the ganglion cell receives a supra-threshold level of input from the bipolar cells,
voltage-gated sodium channels suddenly open, and Na+ ions rush in.
This causes a sudden reversal of the charge (from negative to positive), and this
propogates down the body (axon) of the neuron. [diagram of an action potential].
Then, the sodium channels close, and voltage-gated potassium channels open, which
restores the cell to its (negative) resting potential.
Key points about action potentials:
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Spikes are all-or-none, discrete, stereotyped events.
A certain threshold-level input must be achieved in order to produce a spike.
Below that, no spikes.
Refractory period: after a neuron has fired an action potential, it cannot fire
another until some time has passed.
The firing rate of the neuron (e.g., spikes per second) is what represents
information about stimulus intensity within each neuron.
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Receptive Fields. Retinal ganglion cells fire action potentials in response to certain types of
retinal stimulation. The part of the retina that needs to be stimulated in order to elicit a spike is
the retinal ganglion cell's receptive field.
The part of the visual world that the neuron is responsive to (that it "sees").
What the visual stimulus needs to be in order to elicit spikes.
Most retinal ganglion cells have concentric (or center-surround) receptive fields.
[Retinal ganglion cell receptive field].
These receptive fields are divided into 2 parts (center/surround), one of which is
excitatory ("ON"), the other inhibitory ("OFF"). For an ON/OFF center/surround cell, a
spot of light shown on the inside (center) of the receptive field will elicit spikes, while
light falling on the outside ring (surround) will suppress firing below the baseline rate.
Opposite results for an OFF/ON cell.
In class, we will watch a film of the original researchers who first characterized
receptive fields (Hubel and Weisel) defining the receptive field of retinal ganglion
The receptive field of a neuron can be defined, more generally, as:
A perceptual consequence of these receptive field shapes can be seen in a Hermann
grid. When you stare at a set of dark squares separated by a grid of white lines, you will
see darkness at the intersections of the white lines.
Referring to the 2 following figures, note that at the intersections, there is
relatively more light (white) falling on the inhibitory surround of ON/OFF
receptive fields (compare to the amount of white falling on the inhibitory
surround of receptive fields that are not centered over intersections. This
relatively-larger amount of inhibition suppresses the firing of cells with
receptive fields that overlie the intersections, decreasing their firing rates, and
yielding the perception of darkness. [response at an intersection] [response off
an intersection]
However, note that the darkness disappears at an intersection that you stare at
Lateral Inhibition. Visual neurons do not simply "pipe" the output of the retina through the
visual pathways. Instad, the activity of a given neuron is affected by the activity of nearby
neurons. Lateral inhibition in the retinal ganglion cells is a prime example.
When a retinal ganglion cell fires action potentials, it also inhibits the firing of nearby
(lateral) ganglion cells.
Lateral inhibition performs edge (contrast) enhancement.
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A perceptual example of this edge enhancement: Mach bands. The borders between
light and dark parts of the image are exaggerated and appear as extra-light and extradark bars. Note that your perceptual experience can be predicted by the responses of the
retinal ganglion cells.
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The Representation of the Visual Field in the Brain.
As the pattern of light reflected off objects in the world enters the eye, it is flipped upsidedown(recall from last lecture).
This upside-down pattern of light is the retinal image.
The spatial structure of the retinal image is preserved as neurons from the retina connect to the
LGN, and is still preserved further along in the cortex.
Although you have two eyes, and slightly different images in each eye, the brain does not keep
the information from the two retinas separate. Instead, it splits the world into a left and right
visual field. Check out the figure to learn what the horizontal and vertical meridia are.
After leaving the retina, the outputs of each eye are split. The nasal (toward the nose) half of
each eye's visual field crosses from one side to the other at the optic chiasm. The temporal half
(towards the temple) remains on the same side as its eye-of-origin. This splitting and crossing
re-organizes the retinal outputs so that the left hemisphere processes information from the
right visual field, and the right hemisphere processes information from the left visual field.
Parallel Pathways and the LGN
Retinal ganglion cells actually come in 2 sorts: M (magnocellular, or parasol) and P
(parvocellular, or midget).
P cells also exhibit color-opponent responses: their firing is also dependent on the
wavelength of light in their receptive field. M cells do not exhibit color-opponency.
M cells make transient responses: they fire action potentials when a stimulus is
introduced, but quickly fade if the stimulus does not change. P cells, meanwhile, give
sustained responses to stimuli in their receptive field.
The division of M and P pathways becomes anatomically evident in the lateral geniculate
nucleus (LGN), a part of the thalamus that acts as a relay station between the retina and the
While the receptive fields of LGN neurons look a lot like retinal ganglion cells, the
LGN does re-organize these circuits into distinct layers.
There are 6 layers in the LGN. Bottom 2 layers are M cells, upper 4 are P cells. Layers
alternate eye of origin. [LGN anatomy]
Parallel pathways generally exhibit these 4 main characteristics:
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Physiologically/functionally distinct. For example, the M cells conduct neural signals
faster, while P cells represent more constant stimulus presence. A simple hypothesis is
that M cells contribute to fast/transient processing (visual motion perception, eye
movements) while P cells contribute more to recognition (object reccognition, face
recognition, etc).
Anatomically distinct. The dendritic trees of P cells are always smaller than the M
cells (remember that they're also called "midget" cells). Note that dendritic trees of both
types of cell get larger as you move from fovea to periphery.
Complete coverage (or nearly complete). Both M and P cells cover the entire retina.
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Recombine. The M and P cells are separated in the LGN (different layers) but
recombine in visual cortex (although some separation still exists).
Gross Anatomy of the Cortex
Before we discuss visual cortex in detail, let's stop and get oriented in the brain as a whole.
As you know, there are left and right hemispheres of the brain. They are connected by a tract of
fibers known as the corpus callosum.
In each hemisphere, there are 4 "lobes": frontal, temporal, parietal, occipital.
Visual information is processed primarily in the occipital lobes, but parallel pathways extend
into the temporal and parietal lobes as information-processing becomes increasingly
Spatial Organization in the Brain
The spatial organization of the brain often provides hints about what the brain does to transform
sensory input to useful information for the guidance of action and thought. Spatial organization can be
seen at many different levels:
1. Functional specialization: different types of information are processed in different parts of the
brain (with varying degrees of separation).
2. Columnar architecture: within a brain area, neurons with similar (or complimentary)
sensitivities lie close together, often in "columns" or "pinwheels".
3. Topography/Retinotopy: a "map" of the visual world (or, a map of the retina) is preserved in
many visual brain areas. E.g., adjacent points of the visual world/retinal image are mapped
onto (or processed by) adjacent neurons. Just as there is a "retinal image", there is a "neural
image" in each visual area. People who study the visual system often use the existence of
multiple retinotopic maps to localize different brain areas. A technique for localizing visual
areas in humans using retinotopy measurements was developed here at Stanford.
Primary Visual Cortex: V1
V1 has a topographic/retinotopic map of the visual world (see above). This means that there is
a "neural image" that retains the spatial layout of the pattern of light that falls on the retina.
This map has several interesting characteristics:
Remember that there are 2 V1s in each person (left and right hemispheres). Each V1
has a representation of the opposite half of the visual field (e.g., left V1 has a map of
the right visual field, and vice versa). Note that each V1 does not simply receive input
from the opposite eye. The outputs of each retina are split (left half/right half) and then
run through the LGN to the appropriate V1.This diagram of the visual fields is helpful.
Just as othe image of the world is inverted when projected onto the retina, the
retinotopic V1 map is upside down. As discussed earlier, the right hemisphere's V1 has
a topographic map of the left visual field, and vice versa.
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Cortical magnification: more cortical space is dedicated to the fovea than the periphery
(remember the higher density of photoreceptors in the fovea, hence clearer vision).
There are 3 main types of cells in primary visual cortex.
Simple: receptive fields often have a long, narrow bar of light (ON) and flanking (OFF)
parts. Other types are the opposite (responding to dark bars) or simply respond to a
light/dark edge. [simple cell receptive field types]
Complex: bars of light must be oriented correctly, but can appear anywhere in the
receptive field. Moving the bar through the field produces a sustained response.
Complex cells often show direction-selectivity: they fire more when the bar moves in
one direction, and are suppressed by motion in the opposite direction. [complex cell
End-stopped (formerly Hypercomplex): Many simple and complex cells exhibit length
summation: if an appropriate bar is placed in the visual field, they fire action potentials;
if the bar is made longer, they fire more, up to the extent of the full receptive field.
However, end-stopped cells increase their responses with increases in bar length up to a
limit that is smaller than the receptive field.
In class, we'll see more of Hubel and Weisel, this time defining the receptive field of
various V1 cells.
Architecture of V1
Ocular dominance columns: as you move parallel to the surface of V1, there are
alternating columns of cells that are driven predominantly by inputs to a single eye.
These alternations between left and right eye are the ocular dominance columns.[ocular
dominance columns]
Orientation columns: as you move perpendicular to the surface, the preferred
orientation of the cells changes gradually from horizontal to vertical and back again.
View a schematic of the ocular dominance and orientation columns together.
Defining and Separating Different Brain Areas
Brain areas can be differentiated according to 4 main criteria:
Function: physiology. Neurons in different parts of the brain are responsive to different aspects
of the stimulus (= do different things).
Architecture: microanatomy can differ widely across brain areas. For example, V1 is also
referred to as "striate cortex" because it has a series of stripes that run parallel to the surface;
these stripes end abruptly at the end of V1.
Connections: different areas feed forward and also receive backward-reaching connections
from distinct areas.
Topography: e.g., retinotopy. Each distinct visual area has its own retinotopic map.
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Secondary Visual Areas
There are approximately 30 visual areas after V1. The functional specialization hypothesis drives
much of the research about these areas. Some areas seem specialized for processing a certain aspect of
visual information. E.g., MT - motion, V4 - color (?).
Cortical areas dedicated to vision are densely interconnected, and can seem quite confusing at
first glance.
However, a more general organization is evident in a pair of parallel pathways.
What pathway. Temporal lobe; recognition of objects.
Where pathway. Parietal lobe; motion, spatial orientation, localization.
Visual acuity (VA) is acuteness or clearness of vision, especially form vision, which is dependent on
the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the
Visual acuity is a measure of the spatial resolution of the visual processing system and is usually
tested in a manner to optimise and standarise the conditions. To this end black symbols on a white
background are used (for maximum contrast) and a sufficient distance allowed to approximate infinity
in the way the lens attempts to focus. Twenty feet is essentially infinity from an optical perspective
(the difference in optical power required to focus at 20 feet versus infinity is only 0.164 diopters).
Whilst in an eye exam lenses of varying powers are used to precisely correct for refractive errors,
using a pinhole will largely correct for refractive errors and allow VA to be tested in other
circumstances. Letters are normally used (as in the classic Snellen chart) as most people will recognise
them but other symbols (such as a letter E facing in different directions) can be used instead.
It is the most common clinical measurement of visual function. In the term "20/20 vision" the
numerator refers to the distance in feet between the subject and the chart. The denominator is the
distance at which the lines that make up those letters would be separated by a visual angle of 1 arc
minute, which for the lowest line that is read by an eye with no refractive error (or the errors
corrected) is usually 20 feet. The metric equivalent is 6/6 vision where the distance is 6 meters. This
means that at 20 feet or 6 meters, a typical human eye, able to separate 1 arc minute, can resolve lines
with a spacing of about 1.75mm. 20/20 vision can be considered nominal performance for human
distance vision; 20/40 vision can be considered half that acuity for distance vision and 20/10 vision
would be twice normal acuity. The 20/x number does not directly relate to the eyeglass prescription
required to correct vision, because it does not specify the nature of the problem with the lens, only the
resulting performance. Instead an eye exam seeks to find the prescription that will provide at least
20/20 vision.
Physiology of visual acuity
In low light vision, there is low resolution despite the high sensitivity thereof. This is due to spatial
summation of rods, so 100 rods could merge into many bipolars, in turn converging on ganglion cells,
and the unit for resolution is very large, thus acuity being small. The farther a pattern of white and
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black lines is presented to a person, the less he can distinguish the lines, culminating to a distance
when the pattern is seen as a uniform gray. The angle subtended by the detail at minimum acuity is the
resolving power, and its reciprocal is the visual acuity. For example, a visual acuity of 1 subtends 1
minute on the retina, that of 2 is 1/2 minutes (30 seconds) of arc. Visual acuity is much better in bright
light than dim light, the former reaching 2 with a bright center and surrounding, the latter perhaps
having visual acuity of 0.04 (25 minutes on eye). In this case, the stimulus is 1.7 inches (4.4 cm) and a
distance of 20 ft (6 m).
Thus, visual acuity, or resolving power, is the property of cones. To resolve detail, the eye's optical
system has to project a focused image on the fovea, a region inside the macula having the highest
density of cone photoreceptors (the only kind of photoreceptors existing on the fovea), thus having the
highest resolution and best color vision. Acuity and color vision, despite being mediated by the same
cells, are different physiologic functions that do not interrelate except by position. Acuity and color
vision can be affected independently.
The grain of a photographic mosaic has just as limited resolving power as the "grain" of the retinal
mosaic. In order to see detail, two sets of receptors must be intervened by a middle set. The maximum
resolution is that 30 seconds of arc, corresponding to the foveal cone diameter or the angle subtended
at the nodal point of the eye. In order to get reception from each cone, as it would be if vision was on a
mosaic basis, the "local sign" must be obtained from a single cone via a chain of one bipolar, ganglion,
and lateral geniculate cell each. A key factor of obtaining detailed vision, however, is inhibition. This
is mediated by neurons such as the amacrine and horizontal cells, which functionally render the spread
or convergence of signals inactive. This tendency to one-to-one shuttle of signals is powered by
brightening of the center and its surroundings, which triggers the inhibition leading to a one-to-one
wiring. This scenario, however, is rare, as cones may connect to both midget and flat (diffuse)
bipolars, and amacrine and horizontal cells can merge messages just as easily as inhibit them.
Light travels from the fixation object to the fovea through an imaginary path called the visual axis.
The eye's tissues and structures that are in the visual axis (and also the tissues adjacent to it) affect the
quality of the image. These structures are: tear film, cornea, anterior chamber, pupil, lens, vitreous,
and finally the retina. The posterior part of the retina, called the retinal pigment epithelium (RPE) is
responsible for, among many other things, absorbing light that crosses the retina so it cannot bounce to
other parts of the retina. Interestingly, in many vertebrates, such as cats, where high visual acuity is
not a priority, there is a reflecting tapetum layer that gives the photoreceptors a "second chance" to
absorb the light, thus improving the ability to see in the dark. This is what causes an animal's eyes to
seemingly glow in the dark when a light is shone on them. The RPE also has a vital function of
recycling the chemicals used by the rods and cones in photon detection. If the RPE is damaged and
does not clean up this "shed" blindness can result.
As in a photographic lens, visual acuity is affected by the size of the pupil. Optical aberrations of the
eye that decrease visual acuity are at a maximum when the pupil is largest (about 8 mm), which occurs
in low-light conditions. When the pupil is small (1–2 mm), image sharpness may be limited by
diffraction of light by the pupil (see diffraction limit). Between these extremes is the pupil diameter
that is generally best for visual acuity in normal, healthy eyes; this tends to be around 3 or 4 mm.
If the optics of the eye were otherwise perfect, theoretically acuity would be limited by pupil
diffraction which would be a diffraction-limited acuity of 0.4 minutes of arc (minarc) or 20/8 acuity.
The smallest cone cells in the fovea have sizes corresponding to 0.4 minarc of the visual field, which
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also places a lower limit on acuity. The optimal acuity of 0.4 minarc or 20/8 can be demonstrated
using a laser interferometer that bypasses any defects in the eye's optics and projects a pattern of dark
and light bands directly on the retina. Laser interferometers are now used routinely in patients with
optical problems, such as cataracts, to assess the health of the retina before subjecting them to surgery.
The visual cortex is the part of the cerebral cortex in the posterior part of the brain responsible for
processing visual stimuli, called the occipital lobe. The central 10° of field (approximately the
extension of the macula) is represented by at least 60% of the visual cortex. Many of these neurons are
believed to be involved directly in visual acuity processing.
Proper development of normal visual acuity depends on an animal having normal visual input when it
is very young. Any visual deprivation, that is, anything interfering with such input over a prolonged
period, such as a cataract, severe eye turn or strabismus, or covering or patching the eye during
medical treatment, will usually result in a severe and permanent decrease in visual acuity in the
affected eye if not treated early in life. The decreased acuity is reflected in various abnormalities in
cell properties in the visual cortex. These changes include a marked decrease in the number of cells
connected to the affected eye as well as few cells connected to both eyes, resulting in a loss of
binocular vision and depth perception, or stereopsis. The period of time over which an animal is highly
sensitive to such visual deprivation is referred to as the critical period.
The eye is connected to the visual cortex by the optic nerve coming out of the back of the eye. The two
optic nerves come together behind the eyes at the optic chiasm, where about half of the fibers from
each eye cross over to the opposite side and join fibers from the other eye representing the
corresponding visual field, the combined nerve fibers from both eyes forming the optic tract. This
ultimately forms the physiological basis of binocular vision. The tracts project to a relay station in the
midbrain called the lateral geniculate nucleus, which is part of the thalamus, and then to the visual
cortex along a collection of nerve fibers called the optic radiations.
Any pathological process in the visual system, even in older humans beyond the critical period, will
often cause decreases in visual acuity. Thus measuring visual acuity is a simple test in accessing the
health of the eyes, the visual brain, or pathway to the brain. Any relatively sudden decrease in visual
acuity is always a cause for concern. Common causes of decreases in visual acuity are cataracts and
scarred corneas, which affect the optical path, diseases that affect the retina, such as macular
degeneration and diabetes, diseases affecting the optic pathway to the brain such as tumors and
multiple sclerosis, and diseases affecting the visual cortex such as tumors and strokes.
Though the resolving power depends on size and packing density of the photoreceptors, the neural
system of receptors must interpret this resolving power. As determined from various experiments on
the cat, different ganglion cells are tuned to different frequencies of detail, as from a grating, so some
ganglion cells have better acuity than others. In humans the results are the same, this time utilizing the
same method as well as a device to read electrical changes in the scalp.
Optical aspects
Besides the neural connections of the receptors, the optical system is an equally key player in retinal
resolution. In the ideal eye, the image of a diffraction grating, can subtend 0.5 micrometre on the
retina. This is certainly not the case, however, and furthermore the pupil can cause diffraction of the
light. Thus, black lines on a grating will be mixed with the intervening white lines to make a gray
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appearance. Defective optical issues (such as myopia) can render it worse, but suitable lenses can help.
Images (such as gratings) can be sharpened by lateral inhibition, i.e., more highly excited cells
inhibiting the less excited cells. A similar reaction is in the case of chromatic aberrations, in which the
color fringes around black-and-white objects are inhibited similarly.
Visual acuity expression
Visual acuity is often measured according to the size of letters viewed on a Snellen chart or the size of
other symbols, such as Landolt Cs or Tumbling E.
In some countries, acuity is expressed as a vulgar fraction, and in some as a decimal number.
Using the foot as a unit of measurement, (fractional) visual acuity is expressed relative to 20/20.
Otherwise, using the metre, visual acuity is expressed relative to 6/6. For all intents and purposes, 6/6
vision is equivalent to 20/20. In the decimal system, the acuity is defined as the reciprocal value of the
size of the gap (measured in arc minutes) of the smallest Landolt C that can be reliably identified. A
value of 1.0 is equal to 20/20.
LogMAR is another commonly used scale which is expressed as the logarithm of the minimum angle
of resolution. LogMAR scale converts the geometric sequence of a traditional chart to a linear scale. It
measures visual acuity loss; positive values indicate vision loss, while negative values denote normal
or better visual acuity. This scale is
rarely used clinically; it is more frequently used in statistical calculations because it provides a more
scientific equivalent for the traditional clinical statement of “lines lost” or “lines gained”, which is
valid only when all steps between lines are equal, which is not usually the case.
A visual acuity of 20/20 is frequently described as meaning that a person can see detail from 20 feet
away the same as a person with normal eyesight would see from 20 feet. If a person has a visual acuity
of 20/40, he is said to see detail from 20 feet away the same as a person with normal eyesight would
see it from 40 feet away. It is possible to have vision superior to 20/20: the maximum acuity of the
human eye without visual aids (such as binoculars) is generally thought to be around 20/10 (6/3)
however, recent test subjects have exceeded 20/8 vision. Some birds, such as hawks, are believed to
have an acuity of around 20/2; in this respect, their vision is much better than human eyesight.
When visual acuity is below the largest optotype on the chart, the reading distance is reduced until the
patient can read it. Once the patient is able to read the chart, the letter size and test distance are noted.
If the patient is unable to read the chart at any distance, he or she is tested as follows:
Abbreviation Definition
Counting Fingers CF
Hand Motion
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Ability to count fingers at a given distance.
Ability to distinguish a hand if it is moving or not in front of the
patient's face.
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Light Perception
Ability to perceive any light.
Inability to see any light. Total blindness.
Many humans have one eye that has superior visual acuity over the other.
The federal statute defines blindness as follows:
The term "blindness" means central visual acuity of 20/200 or less in the better eye with the use of a
correcting lens. An eye which is accompanied by a limitation in the fields of vision such that the
widest diameter of the visual field subtends an angle no greater than 20 degrees shall be considered for
purposes in this paragraph as having a central visual acuity of 20/200 or less.
Visual acuity is typically measured monocularly rather than binocularly with the aid of an optotype
chart for distant vision, an optotype chart for near vision, and an occluder to cover the eye not being
tested. The examiner may also occlude an eye by sliding a tissue behind the patient's eyeglasses, or
instructing the patient to use his or her hand. This latter method is typically avoided in professional
settings as it may inadvertently allow the patient to peek through his or her fingers, or press the eye
and alter the measurement when that eye is evaluated.
Place the chart at 20 feet (or 6 meters) and illuminate to 480 lux at that distance.
If the patient uses glasses, then the test is performed using them.
Place the occluder in front of the eye that is not being evaluated. The first evaluated eye is the
one that is believed to see less or the one the patient says that is seeing less.
Start first with the big optotypes and proceed to the smaller ones. The patient has to identify
every one on the line being presented and communicate it to the physician.
If the measurement is reduced (below 20/20) then the test using a pinhole should be done and
register the visual acuity using the pinhole. Both measures should be registered, with and
without using pinhole.
Change the occluder to the other eye and proceed again from the 4th step.
After both eyes have been evaluated in distant visual acuity, proceed to evaluate near visual
acuity placing a modified snellen chart for near vision (such as the Rosembaum chart) at 15.7
inches (or 40 centimeters). Then repeat the test from the 2nd step.
In some cases, binocular visual acuity will be measured, because usually binocular visual acuity is
slightly better than monocular visual acuity.
Measurement considerations
Visual acuity measurement involves more than being able to see the optotypes. The patient should be
cooperative, understand the optotypes, be able to communicate with the physician, and many more
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factors. If any of these factors is missing, then the measurement will not represent the patient's real
visual acuity.
Visual acuity is a subjective test meaning that if the patient is unwilling or unable to cooperate, the test
cannot be done. A patient being sleepy, intoxicated, or having any disease that can alter the patient's
consciousness or his mental status can make the measured visual acuity worse than it actually is.
Illiterate patients who cannot read letters and/or numbers will be registered as having very low visual
acuity if this is not known. Some of the patients will not tell the physician that they don't know the
optotypes unless asked directly about it. Brain damage can result in a patient not being able to
recognize printed letters, or being unable to spell them.
A motor inability can make a person respond incorrectly to the optotype shown and negatively affect
the visual acuity measurement.
Variables such as pupil size, background adaptation luminance, duration of presentation, type of
optotype used, interaction effects from adjacent visual contours (or “crowding") can all affect visual
acuity measurement.
Normal vision
Visual acuity depends upon how accurately light is focused on the retina (mostly the macular region),
the integrity of the eye's neural elements, and the interpretative faculty of the brain. Normal visual
acuity is frequently considered to be what was defined by Snellen as the ability to recognize an
optotype when it subtended 5 minutes of arc, that is Snellen's chart 20/20 feet, 6/6 meter, 1.00 decimal
or 0.0 logMAR. In humans, the maximum acuity of a healthy, emmetropic eye (and even ametropic
eyes with correctors) is approximately 20/16 to 20/12, so it is inaccurate to refer to 20/20 visual acuity
as "perfect" vision. 20/20 is the visual acuity needed to discriminate two points separated by 1 arc
minute—about 1/16 of an inch at 20 feet. This is because a 20/20 letter, E for example, has three limbs
and two spaces in between them, giving 5 different detailed areas. The ability to resolve this therefore
requires 1/5 of the letter's total arc, which in this case would be 1 minute. The significance of the
20/20 standard can best be thought of as the lower limit of normal or as a screening cutoff. When used
as a screening test subjects that reach this level need no further investigation, even though the average
visual acuity of healthy eyes is 20/16 to 20/12.
Some people may suffer from other visual problems, such as color blindness, reduced contrast, or
inability to track fast-moving objects and still have normal visual acuity. Thus, normal visual acuity
does not mean normal vision. The reason visual acuity is very widely used is that it is a test that
corresponds very well with the normal daily activities a person can handle, and evaluate their
impairment to do them.
Other measures of visual acuity
Normally visual acuity refers to the ability to resolve two separated points or lines, but there are other
measures of the ability of the visual system to discern spatial differences.
Vernier acuity measures the ability to align two line segments. Humans can do this with remarkable
accuracy. Under optimal conditions of good illumination, high contrast, and long line segments, the
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limit to vernier acuity is about 8 arc seconds or 0.13 arc minutes, compared to about 0.6 arc minutes
(20/12) for normal visual acuity or the 0.4 arc minute diameter of a foveal cone. Because the limit of
vernier acuity is well below that imposed on regular visual acuity by the "retinal grain" or size of the
foveal cones, it is thought to be a process of the visual cortex rather than the retina. Supporting this
idea, vernier acuity seems to correspond very closely (and may have the same underlying mechanism)
enabling one to discern very slight differences in the orientations of two lines, where orientation is
known to be processed in the visual cortex.
The smallest detectable visual angle produced by a single fine dark line against a uniformally
illuminated background is also much less than foveal cone size or regular visual acuity. In this case,
under optimal conditions, the limit is about 0.5 arc seconds, or only about 2% of the diameter of a
foveal cone. This produces a contrast of about 1% with the illumination of surrounding cones. The
mechanism of detection is the ability to detect such small differences in contrast or illumination, and
does not depend on the angular width of the bar, which cannot be discerned. Thus as the line gets
finer, it appears to get fainter but not thinner.
Stereoscopic acuity is the ability to detect tiny differences in depth with the two eyes. For more
complex targets, stereoacuity is similar to normal monocular visual acuity, or around 0.6-1.0 arc
minutes, but for much simpler targets, such as vertical rods, may be as low as only 2 arc seconds.
Although stereoacuity normally corresponds very well with monocular acuity, it may be very poor or
even absent even with normal monocular acuities. Such individuals typically have abnormal visual
development when they are very young, such as an alternating strabismus or eye turn, where both eyes
rarely or never point in the same direction and therefore do not function together.
A blind spot, also known as a scotoma, is an obscuration of the visual field. A particular blind spot
known as the blindspot, or physiological blind spot, or punctum caecum in medical literature is the
place in the visual field that corresponds to the lack of light-detecting photoreceptor cells on the optic
disc of the retina where the optic nerve passes through it. Since there are no cells to detect light on the
optic disc, a part of the field of vision is not perceived. The brain fills in with surrounding detail and
with information from the other eye, so the blind spot is not normally perceived.
Although all vertebrates have this blind spot, cephalopod eyes, which are only superficially similar, do
not. In them, the optic nerve approaches the receptors from behind, so it does not create a break in the
The first documented observation of the phenomenon was in the 1660s by Edme Mariotte in France.
At the time when it was generally thought that the point at which the optic nerve entered the eye
should actually be the most sensitive portion of the retina; however, Mariotte's discovery disproved
this theory.
Color blindness or color vision deficiency is the inability to perceive differences between some of
the colors that others can distinguish. It is most often of genetic nature, but may also occur because of
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eye, nerve, or brain damage, or exposure to certain chemicals. The English chemist John Dalton
published the first scientific paper on the subject in 1798, "Extraordinary facts relating to the vision of
colours",after the realization of his own color blindness. Because of Dalton's work, the condition was
often called daltonism, although this term is now used for a type of color blindness called
Color blindness is usually classed as a mild disability, but in certain situations, color blind individuals
have an advantage over those with normal color vision. There are some studies which conclude that
color blind individuals are better at penetrating certain color camouflages and it has been suggested
that this may be the evolutionary explanation for the surprisingly high frequency of congenital redgreen color blindness.
By cause
Color vision deficiencies can be classified as acquired or inherited.
Inherited: There are three types of inherited or congenital color vision deficiencies:
monochromacy, dichromacy, and anomalous trichromacy.
Monochromacy, also known as "total color blindness," is the lack of ability to
distinguish colors; caused by cone defect or absence. Monochromacy occurs when two
or all three of the cone pigments are missing and color and lightness vision is reduced
to one dimension.
Rod monochromacy (achromatopsia) is an exceedingly rare, nonprogressive inability to
distinguish any colors as a result of absent or nonfunctioning retinal cones. It is
associated with light sensitivity (photophobia), involuntary eye oscillations
(nystagmus), and poor vision.
Cone monochromacy is a rare total color blindness that is accompanied by relatively
normal vision, electoretinogram, and electrooculogram.
Dichromacy is a moderately severe color vision defect in which one of the three basic
color mechanisms is absent or not functioning. It is hereditary and, in the case of
Protanopia or Deuteranopia, sex-linked, affecting predominantly males. Dichromacy
occurs when one of the cone pigments is missing and color is reduced to two
Protanopia is a severe type of color vision deficiency caused by the complete absence
of red retinal photoreceptors. It is a form of dichromatism in which red appears dark. It
is hereditary, sex-linked, and present in 1% of males.
Deuteranopia is a color vision deficiency in which the green retinal photoreceptors are
absent, moderately affecting red-green hue discrimination. It is a form of dichromatism
in which there are only two cone pigments present. It is likewise hereditary and sexlinked.
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Tritanopia is a very rare color vision disturbance in which there are only two cone
pigments present and a total absence of blue retinal receptors.
1. Anomalous trichromacy is a common type of inherited color vision deficiency,
occurring when one of the three cone pigments is altered in its spectral sensitivity. This
results in an impairment, rather than loss, of trichromacy (normal three-dimensional
color vision).
Protanomaly is a mild color vision defect in which an altered spectral sensitivity of red
retinal receptors (closer to green receptor response) results in poor red-green hue
discrimination. It is hereditary, sex-linked, and present in 1% of males.
Deuteranomaly, caused by a similar shift in the green retinal receptors, is by far the
most common type of color vision deficiency, mildly affecting red-green hue
discrimination in 5% of males. It is hereditary and sex-linked.
Tritanomaly is a rare, hereditary color vision deficiency affecting blue-yellow hue
discrimination. Unlike most other forms, it is not sex-linked.
By clinical appearance
Based on clinical appearance, color blindness may be described as total or partial. Total color
blindness is much less common than partial color blindness. There are two major types of color
blindness: those who have difficulty distinguishing between red and green, and those who have
difficulty distinguishing between blue and yellow.
1. Total color blindness
2. Partial color blindness
Dichromacy (protanopia and deuteranopia)
Anomalous trichromacy (protanomaly and deuteranomaly)
Dichromacy (tritanopia)
Anomalous trichromacy (tritanomaly)
At one time the U.S. Army found that color blind people could spot "camouflage" colors that fooled
those with normal color vision. Humans are the only trichromatic primates with such a high
percentage of color blindness.
Another possible advantage might result from the presence of a tetrachromic female. Owing to Xchromosome inactivation, females who are heterozygous for anomalous trichromacy ought to have at
least four types of cone in their retinae. It is possible that this affords them an extra dimension of color
vision, by analogy to New World monkeys where heterozygous females gain trichromacy in a
basically dichromatic species.
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Color blindness can be inherited genetically. It is most commonly inherited from mutations on the X
chromosome but the mapping of the human genome has shown there are many causative mutations –
mutations capable of causing color blindness originate from at least 19 different chromosomes and 56
different genes (as shown online at the Online Mendelian Inheritance in Man (OMIM) database at
Johns Hopkins University).
Some of the inherited diseases known to cause color blindness are:
cone dystrophy
cone-rod dystrophy
achromatopsia (aka rod monochromatism, aka stationary cone dystrophy, aka cone
dysfunction syndrome)
blue cone monochromatism,
Leber's congenital amaurosis.
retinitis pigmentosa (initially affects rods but can later progress to cones and therefore color
Inherited color blindness can be congenital (from birth), or it can commence in childhood or
adulthood. Depending on the mutation, it can be stationary, that is, remain the same throughout a
person's lifetime, or progressive. As progressive phenotypes involve deterioration of the retina and
other parts of the eye, certain forms of color blindness can progress to legal blindness, i.e., an acuity of
6/60 or worse, and often leave a person with complete blindness.
Color blindness always pertains to the cone photoreceptors in retinas, as the cones are capable of
detecting the color frequencies of light.
About 8 percent of males, but only 0.5 percent of females, are color blind in some way or another,
whether it is one color, a color combination, or another mutation. The reason males are at a greater
risk of inheriting an X linked mutation is because males only have one X chromosome (XY, with the
Y chromosome being significantly shorter than the X chromosome), and females have two (XX); if a
woman inherits a normal X chromosome in addition to the one which carries the mutation, she will not
display the mutation. Men do not have a second X chromosome to override the chromosome which
carries the mutation. If 5% of variants of a given gene are defective, the probability of a single copy
being defective is 5%, but the probability that two copies are both defective is 0.05 × 0.05 = 0.0025, or
just 0.25%.
Other causes
Other causes of color blindness include brain or retinal damage caused by shaken baby syndrome,
accidents and other trauma which produce swelling of the brain in the occipital lobe, and damage to
the retina caused by exposure to ultraviolet light. Most ultraviolet light damage is caused during
childhood and this form of retinal degeneration is the leading cause of blindness in the world. Damage
often presents itself later on in life.
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Color blindness may also present itself in the spectrum of degenerative diseases of the eye, such as
age-related macular degeneration, and as part of the retinal damage caused by diabetes.
There are many types of color blindness. The most common are red-green hereditary photoreceptor
disorders, but it is also possible to acquire color blindness through damage to the retina, optic nerve, or
higher brain areas. Higher brain areas implicated in color processing include the parvocellular pathway
of the lateral geniculate nucleus of the thalamus, and visual area V4 of the visual cortex. Acquired
color blindness is generally unlike the more typical genetic disorders. For example, it is possible to
acquire color blindness only in a portion of the visual field but maintain normal color vision
elsewhere. Some forms of acquired color blindness are reversible. Transient color blindness also
occurs (very rarely) in the aura of some migraine sufferers.
The different kinds of inherited color blindness result from partial or complete loss of function of one
or more of the different cone systems. When one cone system is compromised, dichromacy results.
The most frequent forms of human color blindness result from problems with either the middle or long
wavelength sensitive cone systems, and involve difficulties in discriminating reds, yellows, and greens
from one another. They are collectively referred to as "red-green color blindness", though the term is
an over-simplification and is somewhat misleading. Other forms of color blindness are much more
rare. They include problems in discriminating blues from yellows, and the rarest forms of all, complete
color blindness or monochromacy, where one cannot distinguish any color from grey, as in a blackand-white movie or photograph.
Congenital color vision deficiencies are subdivided based on the number of primary hues needed to
match a given sample in the visible spectrum.
Monochromacy is the condition of possessing only a single channel for conveying information about
color. Monochromats possess a complete inability to distinguish any colors and perceive only
variations in brightness. It occurs in two primary forms:
Rod monochromacy, frequently called achromatopsia, where the retina contains no cone cells,
so that in addition to the absence of color discrimination, vision in lights of normal intensity is
difficult. While normally rare, achromatopsia is very common on the island of Pingelap, a part
of the Pohnpei state, Federated States of Micronesia, where it is called maskun: about 10% of
the population there has it, and 30% are unaffected carriers. The island was devastated by a
storm in the 18th century, and one of the few male survivors carried a gene for achromatopsia;
the population is now several thousand.
Cone monochromacy is the condition of having both rods and cones, but only a single kind of
cone. A cone monochromat can have good pattern vision at normal daylight levels, but will not
be able to distinguish hues. Blue cone monochromacy (X chromosome) is caused by a
complete absence of L- and M-cones (red and green). It is encoded at the same place as redgreen color blindness on the X chromosome. Peak spectral sensitivities are in the blue region
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of the visible spectrum (near 440 nm). They generally show nystagmus ("jiggling eyes"),
photophobia (light sensitivity), reduced visual acuity, and myopia (nearsightedness). Visual
acuity usually falls to the 20/50 to 20/400 range.
Protanopes, deuteranopes, and tritanopes are dichromats; that is, they can match any color they see
with some mixture of just two spectral lights (whereas normally humans are trichromats and require
three lights). These individuals normally know they have a color vision problem and it can affect their
lives on a daily basis. Protanopes and deuteranopes see no perceptible difference between red, orange,
yellow, and green. All these colors, that seem so different to the normal viewer, appear to be the same
color for this two percent of the population.
Anomalous trichromacy
Those with protanomaly, deuteranomaly, or tritanomaly are trichromats, but the color matches they
make differ from the normal. They are called anomalous trichromats. In order to match a given
spectral yellow light, protanomalous observers need more red light in a red/green mixture than a
normal observer, and deuteranomalous observers need more green. From a practical standpoint
though, many protanomalous and deuteranomalous people breeze through life with very little
difficulty doing tasks that require normal color vision. Some may not even be aware that their color
perception is in any way different from normal. The only problem they have is passing a color vision
Protanomaly and deuteranomaly can be readily observed using an instrument called an anomaloscope,
which mixes spectral red and green lights in variable proportions, for comparison with a fixed spectral
yellow. If this is done in front of a large audience of men, as the proportion of red is increased from a
low value, first a small proportion of people will declare a match, while most of the audience sees the
mixed light as greenish. These are the deuteranomalous observers. Next, as more red is added the
majority will say that a match has been achieved. Finally, as yet more red is added, the remaining,
protanomalous, observers will declare a match at a point where everyone else is seeing the mixed light
as definitely reddish.
Protanomaly (1% of males, 0.01% of females): Having a mutated form of the longwavelength (red) pigment, whose peak sensitivity is at a shorter wavelength than in the normal
retina, protanomalous individuals are less sensitive to red light than normal. This means that
they are less able to discriminate colors, and they do not see mixed lights as having the same
colors as normal observers. They also suffer from a darkening of the red end of the spectrum.
This causes reds to reduce in intensity to the point where they can be mistaken for black.
Protanomaly is a fairly rare form of color blindness, making up about 1% of the male
population. Both protanomaly and deuteranomaly are carried on the X chromosome.
Deuteranomaly (most common - 6% of males, 0.4% of females): Having a mutated form of
the medium-wavelength (green) pigment. The medium-wavelength pigment is shifted towards
the red end of the spectrum resulting in a reduction in sensitivity to the green area of the
spectrum. Unlike protanomaly the intensity of colors is unchanged. This is the most common
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form of color blindness, making up about 6% of the male population. The deuteranomalous
person is considered "green weak". For example, in the evening, dark green cars appear to be
black to Deuteranomalous people. Similar to the protanomates, deuteranomates are poor at
discriminating small differences in hues in the red, orange, yellow, green region of the
spectrum. They make errors in the naming of hues in this region because the hues appear
somewhat shifted towards red. One very important difference between deuteranomalous
individuals and protanomalous individuals is deuteranomalous individuals do not have the loss
of "brightness" problem.
Tritanomaly (equally rare for males and females [0.01% for both]): Having a mutated form of
the short-wavelength (blue) pigment. The short-wavelength pigment is shifted towards the
green area of the spectrum. This is the rarest form of anomalous trichromacy color blindness.
Unlike the other anomalous trichromacy color deficiencies, the mutation for this color
blindness is carried on chromosome 7. Therefore it is equally prevalent in both male & female
populations. The OMIM gene code for this mutation is 304000 “Colorblindness, Partial
Total color blindness
Achromatopsia is strictly defined as the inability to see color. Although the term may refer to acquired
disorders such as color agnosia and cerebral achromatopsia, it typically refers to congenital color
vision disorders (i.e. more frequently rod monochromacy and less frequently cone monochromacy).
In color agnosia and cerebral achromatopsia, a person cannot perceive colors even though the eyes are
capable of distinguishing them. Some sources do not consider these to be true color blindness, because
the failure is of perception, not of vision. They are forms of visual agnosia.
Red-green color blindness
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X-linked recessive inheritance
Those with protanopia, deuteranopia, protanomaly, and deuteranomaly have difficulty with
discriminating red and green hues. It is sex-linked: genetic red-green color blindness affects males
much more often than females, because the genes for the red and green color receptors are located on
the X chromosome, of which males have only one and females have two. Females (46, XX) are redgreen color blind only if both their X chromosomes are defective with a similar deficiency, whereas
males (46, XY) are color blind if their single X chromosome is defective.
The gene for red-green color blindness is transmitted from a color blind male to all his daughters who
are heterozygote carriers and are usually unaffected. In turn, a carrier woman has a fifty percent
chance of passing on a mutated X chromosome region to each of her male offspring. The sons of an
affected male will not inherit the trait from him, since they receive his Y chromosome and not his
(defective) X chromosome. Should an affected male have children with a carrier or colorblind woman,
their daughters may be colorblind by inheriting an affected X chromosome from each parent.
Because one X chromosome is inactivated at random in each cell during a woman's development, it is
possible for her to have four different cone types, as when a carrier of protanomaly has a child with a
deuteranomalic man. Denoting the normal vision alleles by P and D and the anomalous by p and d, the
carrier is PD pD and the man is Pd. The daughter is either PD Pd or pD Pd. Suppose she is pD Pd.
Each cell in her body expresses either her mother's chromosome pD or her father's Pd. Thus her redgreen sensing will involve both the normal and the anomalous pigments for both colors. Such females
are tetrachromats, since they require a mixture of four spectral lights to match an arbitrary light.
Blue-yellow color blindness
Those with tritanopia and tritanomaly have difficulty with discriminating blue and yellow hues.
Color blindness involving the inactivation of the short-wavelength sensitive cone system (whose
absorption spectrum peaks in the bluish-violet) is called tritanopia or, loosely, blue-yellow color
blindness. The tritanopes neutral point occurs near a yellowish 570 nm; green is perceived at shorter
wavelengths and red at longer wavelengths. Mutation of the short-wavelength sensitive cones is called
tritanomaly. Tritanopia is equally distributed among males and females. Jeremy H. Nathans (with the
Howard Hughes Medical Institute) proved that the gene coding for the blue receptor lies on
chromosome 7, which is shared equally by males and females. Therefore it is not sex-linked. This
gene does not have any neighbor whose DNA sequence is similar. Blue color blindness is caused by a
simple mutation in this gene.
There is generally no treatment to cure color deficiencies. However, certain types of tinted filters and
contact lenses may help an individual to better distinguish different colors. Optometrists can supply a
singular red-tint contact lens to wear on the non-dominant eye. This may enable the wearer to pass
some color blindness tests, but they have little practical use. The effect of wearing such a device is
akin to wearing red/blue 3D glasses and can take some time getting used to as certain wavelengths can
"jump" out and be overly represented. Additionally, computer software and cybernetic devices have
been developed to assist those with visual color difficulties such as an eyeborg, a "cybernetic eye" that
allows individuals with color blindness to hear sounds representing colors.
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The GNOME desktop environment provides colorblind accessibility using the gnome-mag and the
libcolorblind software. Using a gnome applet, the user may switch a color filter on and off choosing
from a set of possible color transformations that will displace the colors in order to disambiguate them.
The software enables, for instance, a color blind person to see the numbers in the ishihara test.
In September 2009, the journal Nature reported that researchers at the University of Washington and
University of Florida were able to give trichromatic vision to squirrel monkeys, which normally have
only dichromatic vision, using gene therapy.
Color blindness affects a significant number of people, although exact proportions vary among groups.
In Australia, for example, it occurs in about 8 percent of males and only about 0.4 percent of females.
Isolated communities with a restricted gene pool sometimes produce high proportions of color
blindness, including the less usual types. Examples include rural Finland, Hungary, and some of the
Scottish islands. In the United States, about 7 percent of the male population – or about 10.5 million
men – and 0.4 percent of the female population either cannot distinguish red from green, or see red
and green differently (Howard Hughes Medical Institute, 2006) . It has been found that more than 95
percent of all variations in human color vision involve the red and green receptors in male eyes. It is
very rare for males or females to be "blind" to the blue end of the spectrum.
Color vision is the capacity of an organism or machine to distinguish objects based on the
wavelengths (or frequencies) of the light they reflect, emit, or transmit. The nervous system derives
color by comparing the responses to light from the several types of cone photoreceptors in the eye.
These cone photoreceptors are sensitive to different portions of the visible spectrum. For humans, the
visible spectrum ranges approximately from 380 to 740 nm, and there are normally three types of
cones. The visible range and number of cone types differ between species.
A 'red' apple does not emit red light. Rather, it simply absorbs all the frequencies of visible light
shining on it except for a group of frequencies that is perceived as red, which are reflected. An apple is
perceived to be red only because the human eye can distinguish between different wavelengths. The
advantage of color, which is a quality constructed by the visual brain and not a property of objects as
such, is the better discrimination of surfaces allowed by this aspect of visual processing. In some
dichromatic substances (e.g. pumpkin seed oil) the color hue depends not only on the spectral
properties of the substance, but also on its concentration and the depth or thickness.
Wavelength and hue detection
Isaac Newton discovered that white light splits into its component colors when passed through a
prism, but that if those bands of colored light pass through another and rejoin, they make a white
beam. The characteristic colors are, from low to high frequency: red, orange, yellow, green, cyan,
blue, violet. Sufficient differences in frequency give rise to a difference in perceived hue; the just
noticeable difference in wavelength varies from about 1 nm in the blue-green and yellow wavelengths,
to 10 nm and more in the red and blue. Though the eye can distinguish up to a few hundred hues,
when those pure spectral colors are mixed together or diluted with white light, the number of
distinguishable chromaticities can be quite high.
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In very low light levels, vision is scotopic, meaning mediated by rod cells, and not detecting color
differences; the rods are maximally sensitive to wavelengths near 500 nm. In brighter light, such as
daylight, vision is photopic, in which case the cone cells of the retina mediate color perception, and the
rods are essentially saturated; in this region, the eye is most sensitive to wavelengths near 555 nm.
Between these regions is known as mesopic vision, in which case both rods and cones are providing
meaningful signal to the retinal ganglion cells. The shift in color perception across these light levels
gives rise to differences known as the Purkinje effect.
The perception of "white" is formed by the entire spectrum of visible light, or by mixing colors of just
a few wavelengths, such as red, green, and blue, or even by mixing just a pair of complementary
colors such as blue and yellow.
Physiology of color perception
Normalized response spectra of human cones, S, M, and L types, to monochromatic spectral stimuli,
with wavelength given in nanometers.
Perception of color is achieved in mammals through color receptors containing pigments with
different spectral sensitivities. In most primates closely related to humans there are three types of color
receptors (known as cone cells). This confers trichromatic color vision, so these primates, like humans,
are known as trichromats. Many other primates and other mammals are dichromats, and many
mammals have little or no color vision. Indeed, "mammals with color vision are rare," with most
mammals having rod-dominated retinas, and some having pure-rod ones.
The cones are conventionally labeled according to the ordering of the wavelengths of the peaks of
their spectral sensitivities: short (S), medium (M), and long (L) cone types, also sometimes referred to
as blue, green, and red cones. While the L cones are often referred to as the red receptors,
microspectrophotometry has shown that their peak sensitivity is in the greenish-yellow region of the
spectrum. Similarly, the S- and M-cones do not directly correspond to blue and green, although they
are often depicted as such (such as in the graph to the right). It is important to note that the RGB color
model is merely a convenient means for representing color, and is not directly based on the types of
cones in the human eye.
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The peak response of human color receptors varies, even amongst individuals with 'normal' color
vision; in non-human species this polymorphic variation is even greater, and it may well be adaptive.
Theories of color vision
Two complementary theories of color vision are the trichromatic theory and the opponent process
theory. The trichromatic theory, or Young–Helmholtz theory, proposed in the 19th century by Thomas
Young and Hermann von Helmholtz, as mentioned above, states that the retina's three types of cones
are preferentially sensitive to blue, green, and red. Ewald Hering proposed the opponent process
theory in 1872. It states that the visual system interprets color in an antagonistic way: red vs. green,
blue vs. yellow, black vs. white. We now know both theories to be correct, describing different stages
in visual physiology.
Cone cells in the human eye
Cone type Name Range
Peak wavelength
400–500 nm 420–440 nm
450–630 nm 534–545 nm
500–700 nm 564–580 nm
A range of wavelengths of light stimulates each of these receptor types to varying degrees. Yellowishgreen light, for example, stimulates both L and M cones equally strongly, but only stimulates S-cones
weakly. Red light, on the other hand, stimulates L cones much more than M cones, and S cones hardly
at all; blue-green light stimulates M cones more than L cones, and S cones a bit more strongly, and is
also the peak stimulant for rod cells; and blue light stimulates almost exclusively S-cones. Violet light
appears to stimulate both L and S cones to some extent, but M cones very little, producing a sensation
that is somewhat similar to magenta. The brain combines the information from each type of receptor to
give rise to different perceptions of different wavelengths of light.
The pigments present in the L and M cones are encoded on the X chromosome; defective encoding of
these leads to the two most common forms of color blindness. The OPN1LW gene, which codes for
the pigment that responds to yellowish light, is highly polymorphic (a recent study by Verrelli and
Tishkoff found 85 variants in a sample of 236 men [11]), so up to twenty percent of women have an
extra type of color receptor, and thus a degree of tetrachromatic color vision. Variations in OPN1MW,
which codes for the bluish-green pigment, appear to be rare, and the observed variants have no effect
on spectral sensitivity.
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Color in the human brain
Visual pathways in the human brain. The ventral stream (purple) is important in color recognition.
The dorsal stream (green) is also shown. They originate from a common source in the visual cortex.
Color processing begins at a very early level in the visual system (even within the retina) through
initial color opponent mechanisms. Opponent mechanisms refer to the opposing color effect of redgreen, blue-yellow, and light-dark. Visual information is then sent back via the optic nerve to the optic
chiasma: a point where the two optic nerves meet and information from the temporal (contralateral)
visual field crosses to the other side of the brain. After the optic chiasma the visual fiber tracts are
referred to as the optic tracts, which enter the thalamus to synapse at the lateral geniculate nucleus
(LGN). The LGN is segregated into six layers: two magnocellular (large cell) achromatic layers (M
cells) and four parvocellular (small cell) chromatic layers (P cells). Within the LGN P-cell layers there
are two chromatic opponent types: red vs. green and blue vs. green/red.
After synapsing at the LGN, the visual tract continues on back toward the primary visual cortex (V1)
located at the back of the brain within the occipital lobe. Within V1 there is a distinct band (striation).
This is also referred to as "striate cortex", with other cortical visual regions referred to collectively as
"extrastriate cortex". It is at this stage that color processing becomes much more complicated.
In V1 the simple three-color segregation begins to break down. Many cells in V1 respond to some
parts of the spectrum better than others, but this "color tuning" is often different depending on the
adaptation state of the visual system. A given cell that might respond best to long wavelength light if
the light is relatively bright might then become responsive to all wavelengths if the stimulus is
relatively dim. Because the color tuning of these cells is not stable, some believe that a different,
relatively small, population of neurons in V1 is responsible for color vision. These specialized "color
cells" often have receptive fields that can compute local cone ratios. Such "double-opponent" cells
were initially described in the goldfish retina by Nigel Daw; their existence in primates was suggested
by David H. Hubel and Torsten Wiesel and subsequently proven by Bevil Conway. As Margaret
Livingstone and David Hubel showed, double opponent cells are clustered within localized regions of
V1 called blobs, and are thought to come in two flavors, red-green and blue-yellow. Red-green cells
compare the relative amounts of red-green in one part of a scene with the amount of red-green in an
adjacent part of the scene, responding best to local color contrast (red next to green). Modeling studies
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have shown that double-opponent cells are ideal candidates for the neural machinery of color
constancy explained by Edwin H. Land in his retinex theory.
This image (when viewed in full size, 1000 pixels wide) contains 1 milion pixels, each of a different
color. The human eye can distinguish about 10 million different colors.
From the V1 blobs, color information is sent to cells in the second visual area, V2. The cells in V2 that
are most strongly color tuned are clustered in the "thin stripes" that, like the blobs in V1, stain for the
enzyme cytochrome oxidase (separating the thin stripes are interstripes and thick stripes, which seem
to be concerned with other visual information like motion and high-resolution form). Neurons in V2
then synapse onto cells in the extended V4. This area includes not only V4, but two other areas in the
posterior inferior temporal cortex, anterior to area V3, the dorsal posterior inferior temporal cortex,
and posterior TEO. (Area V4 was identified by Semir Zeki to be exclusively dedicated to color, but
this has since been shown not to be the case. Color processing in the extended V4 occurs in
millimeter-sized color modules called globs. This is the first part of the brain in which color is
processed in terms of the full range of hues found in color space.
Anatomical studies have shown that neurons in extended V4 provide input to the inferior temporal
lobe . "IT" cortex is thought to integrate color information with shape and form, although it has been
difficult to define the appropriate criteria for this claim. Despite this murkiness, it has been useful to
characterize this pathway (V1 > V2 > V4 > IT) as the ventral stream or the "what pathway",
distinguished from the dorsal stream ("where pathway") that is thought to analyze motion, among
many other features.
Chromatic adaptation
An object may be viewed under various conditions. For example, it may be illuminated by sunlight,
the light of a fire, or a harsh electric light. In all of these situations, human vision perceives that the
object has the same color: an apple always appears red, whether viewed at night or during the day. On
the other hand, a camera with no adjustment for light may register the apple as having varying color.
This feature of the visual system is called chromatic adaptation, or color constancy; when the
correction occurs in a camera it is referred to as white balance.
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Chromatic adaptation is one aspect of vision that may fool someone into observing a color-based
optical illusion, such as the same color illusion.
Though the human visual system generally does maintain constant perceived color under different
lighting, there are situations where the relative brightness of two different stimuli will appear reversed
at different illuminance levels. For example, the bright yellow petals of flowers will appear dark
compared to the green leaves in dim light while the opposite is true during the day. This is known as
the Purkinje effect, and arises because the peak sensitivity of the human eye shifts toward the blue end
of the spectrum at lower light levels.
Organisms with trichromaticism are called trichromats. Their retina contains three types of color
receptors (called cone cells in vertebrates) with different absorption spectra. In practice the number of
such receptor types may be greater than three, since different types may be active at different light
intensities. In vertebrates with three types of cone cells, at low light intensities the rod cells may
contribute to color vision, giving a small region of tetrachromacy in the color space.
Besides the cones, which detect light entering the eye, the biological basis of the opponent theory
involves two other types of cells: bipolar cells, and ganglion cells. Information from the cones is
passed to the bipolar cells in the retina, which may be the cells in the opponent process that transform
the information from cones. The information is then passed to ganglion cells, of which there are two
major classes: magnocellular, or large-cell layers, and parvocellular, or small-cell layers. Parvocellular
cells, or P cells, handle the majority of information about color, and fall into two groups: one that
processes information about differences between firing of L and M cones, and one that processes
differences between S cones and a combined signal from both L and M cones. The first subtype of
cells are responsible for processing red-green differences,and the second process blue-yellow
differences. P cells also transmit information about intensity of light (how much of it there is) due to
their receptive fields.
Vision is the result of both unconscious processes and explicit (controlled), conscious processes.
The Young–Helmholtz theory (proposed in the 19th century by Thomas Young and Hermann von
Helmholtz) is a theory of trichromatic color vision – the manner in which the photoreceptors in the
eyes of humans and other primates work to enable color vision. In 1802, Young postulated the
existence of three types of photoreceptors (now known as cone cells) in the eye, each of which was
sensitive to a particular range of visible light.
Hermann von Helmholtz developed the theory further in 1850: that the three types of cone
photoreceptors could be classified as short-preferring (blue), middle-preferring (green), and longpreferring (red), according to their response to the wavelengths of light striking the retina. The relative
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strengths of the signals detected by the three types of cones are interpreted by the brain as a visible
For instance, yellow light uses different proportions of red and green, but little blue, so any hue
depends on a mix of all three cones, for example, a strong blue, medium green, and low red.
Moreover, the intensity of colors can be changed without changing their hues, since intensity depends
on the frequency of discharge to the brain, as a blue-green can be brightened but retain the same hue.
The system is not perfect, as it does not distinguish yellow from a red-green mixture, but can
powerfully detect subtle environmental changes.
The existence of cells sensitive to three different wavelength ranges was first shown in 1956 by
Gunnar Svaetichin. In 1983 it was validated in human retinas in an experiment by Dartnall,
Bowmaker, and Mollon, who obtained microspectrophotopic readings of single eye cone cells. Earlier
evidence for the theory had been obtained by looking at light reflected from the retinas of living
humans, and absorption of light by retinal cells removed from corpses.
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1. Scheider, A.M. & Tatshis, B.(1998), Physiological Psychology(3rd ed), Random House,
2. Leukal ,F.(2000), Introduction of Physiological Psychology(3 rd ed), CBS Publishers,
New Delhi.
3. http://www.brocku.ca/MeadProject/Kantor/Kantor_1922_d.html
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