School of Distance Education C01 Physiological...School of Distance Education PHYSIOLOGICAL PSYCHOLOGY 7 The Cranial Nerves The spinal cord mediates information transmission between

PHYSIOLOGICAL PSYCHOLOGY

Complementary Course of B.Sc. Counselling Psychology

II Semester

(CUCBCSS - 2014 Admission onwards)

UNIVERSITY OF CALICUT

SCHOOL OF DISTANCE EDUCATION

Calicut University P.O. Malappuram, Kerala, India 673 635

School of Distance Education

PHYSIOLOGICAL PSYCHOLOGY 2

UNIVERSITY OF CALICUT

SCHOOL OF DISTANCE EDUCATION

Complementary Course of B.Sc. Counselling Psychology

PHYSIOLOGICAL PSYCHOLOGY

Semester II

Prepared by: Smt. Abidha Kurukkan

M.Sc (Psychology), M.Ed

Research Scholar

University of Calicut

Layout: Computer Section, SDE

©

Reserved



CONTENTS PAGES

MODULE – I 5

MODULE – II 17

MODULE – III 36





Module 1

PERIPHERAL NERVOUS SYSTEM

The Nervous System

The nervous system can be simply described as collection of neurons which are arranged to

work in a coordinated function. One of the most important functions of the nervous system is

to process incoming information in such a way that appropriate mental and motor responses

will occur. The nervous system is composed of three major parts: the sensory input portion,

the central nervous system (or integrative portion), and the motor output portion. Sensory

receptors detect the state of the body or the state of the surroundings. For example, the eyes

are sensory organs that give one a visual image of the surrounding area. The ears also are

sensory organs. The central nervous system is composed of the brain and spinal cord. The

brain can store information, generate thoughts, create ambition, and determine reactions that

the body performs in response to the sensations. Appropriate signals are then transmitted

through the motor output portion of the nervous system to carry out one’s desires. More than

99 per cent of all sensory information is discarded by the brain as irrelevant and unimportant.

But, when important sensory information excites the mind, it is immediately channeled into

proper integrative and motor regions of the brain to cause desired responses. This channeling

and processing of information is called the integrative function of the nervous system. Thus,

if a person places a hand on a hot stove, the desired instantaneous response is to lift the hand.

And other associated responses follow, such as moving the entire body away from the stove,

and perhaps even shouting with pain.

Classification of nervous system

The human nervous system can be divided into two main parts: the central nervous system

(CNS) and the peripheral nervous system (PNS). The former consists of the brain and spinal

cord, while the latter is the rest of the nervous system (all the neural (nerve) tracts that lie

outside these central tissues and connect to the rest of the body). The brain and spinal cord

carry out the bulk of the complex processing, while the peripheral acts as a sort of buffer

between the central nervous system and the outside world. The PNS is connected to the CNS

and most of these connections are made via the spinal cord. The PNS is further subdivided

into two parts, the somatic nervous system and the autonomic nervous system (ANS), the

former responsible for somato sensation and conscious/purposeful action, while the latter is

responsible for "vegetative" processes. The somatic nervous system consists of nerves that

serve the muscles and sensory receptors, and the ANS is made up of nerves that serve the

smooth muscles of the internal organs. The autonomic division can also be divided into two

systems, the sympathetic and parasympathetic, which carry out the opposing processes of

arousal and relaxation.



PERIPHERAL NERVOUS SYSTEM

For all its power, the brain still depends on the peripheral nervous system to enable it to

perceive the outside world and to tell the body to carry out its commands. The role of the

peripheral nervous system is to carry sensory information from the body to the spinal cord

and brain and bring back to the body commands for appropriate responses; that are relay

between the central nervous system on one hand and the body surface, skeletal muscles, and

internal organs on the other hand.

The peripheral nervous system contains three structural divisions: the cranial nerves, the

spinal nerves, and the autonomic nervous system. Together, the cranial nerves and spinal

nerves comprise the somatic nervous system. The somatic nervous system includes the

motor neurons that operate the skeletal muscles and the sensory neurons that bring

information into CNS from the body and the outside world and returns commands to the

muscles. The autonomic nervous system controls the actions of many glands and organs and

that controls and regulates the internal organs without any conscious recognition or effort.

The autonomic nervous system controls smooth muscle (stomach, blood vessels, etc.), the

glands and the heart and other organs. It is made up of two antagonistic (opposing) sets of

neuronal tracts, known as the sympathetic and parasympathetic nervous systems, as well as

a third neuronal network, known as the enteric nervous system, which involves the digestive

organs. From the functional perspective, the PNS can be divided into the somatic nervous

system and the autonomic nervous system.

Nerve pair Functions

I Olfactory nerve

Carrying information about smell to the brain

II Optic nerve

Carrying information from the eyes to the brain

III Oculomotor nerve

Controls muscles of the eye

IV Trochlear nerve Controls the muscles of the eye

V Trigeminal nerve

Controls chewing movements and provides feedback

regarding facial expression

VI Abducens nerve Controls the muscles of the eye

VII Facial nerve

Produces muscle movement in facial expressions and

that carries taste information back to the brain

VIII Auditory nerve

Carries information from the inner ear to the brain

IX Glossopharyngeal

nerve

Manages both sensory and motor functions in the

throat

X Vagus nerve

Serves the heart, liver, and digestive tract



The Cranial Nerves

The spinal cord mediates information transmission between the brain and regions of body

below the neck. However, communication between the brain and other regions of the head is

not via this route. Rather, information travels via a series of special nerves, termed cranial

nerves. Cranial nerves are twelve pairs of nerves that exit the brain as part of the peripheral

nervous system. They serve the region of the head and neck. Three of the cranial nerves carry

only sensory information. These are the olfactory nerve (I), the optic nerve (II), and the

auditory nerve (VIII). Five of the nerves carry only motor information. The muscles of the

eyes are controlled by the oculomotor nerve (III), the trochlear nerve (IV), and the abducens

nerve (VI). The spinal accessory nerve (XI) controls the muscles remaining nerves have

mixed sensory and motor functions. The trigeminal nerve (V) controls chewing movements

but also provides some feedback regarding facial expression. The facial nerve (VII) produces

facial expressions and carries the sensation of taste. The glossopharyngeal nerve (IX)

performs both sensory and motor functions for the throat. Finally, the long-distance fibers of

the vagus nerve (X) provide input and receive sensation from the heart, liver, and digestive

tract.

The Spinal Nerves

31 pairs of spinal nerves exit the spinal cord to provide sensory and motor pathways to the

torso, arms, and legs. Each spinal nerve is also known as a mixed nerve, because it contains

a sensory, or afferent, nerve (a means toward the CNS in this case, as in access) and a motor,

or efferent, nerve (e means away from the CNS, as in exit). The mixed nerves travel together

to the part of the body they serve. This makes a great deal of practical sense. The nerves that

are bringing you sensory information from your hand are adjacent to the nerves that tell your

hand to move. Damage to a mixed nerve is likely to reduce both sensation and motor control

for a particular part of the body.

Upon leaving the spinal cord itself, the spinal nerves enjoy the protection of only two layers

of meninges: the dura mater and pia mater. CSF does not surround the spinal nerves. Afferent

roots arise from the dorsal part of the spinal cord, whereas efferent roots arise from the

ventral part. Once outside the cord, the dorsal afferent root swells into the dorsal spinal

ganglion (A collection of cell bodies of afferent nerves located just outside the spinal cord),

which contains the cell bodies of the afferent nerves that process information about touch,

temperature, and other body senses from the periphery. Beyond the dorsal spinal ganglion,

the dorsal and ventral roots join to form a mixed nerve. Afferent (sensory) nerves contain

both myelinated and unmyelinated fibers, whereas efferent (motor) nerves are all myelinated

in the adult. Myelin is a substance that insulates nerve fibers and increases the speed with

which they can transmit messages. Myelinated fibers in both systems tend to be very large

and very fast. Among the sensations carried by myelinated afferent fibers is the first, sharp

experience of pain. Small unmyelinated afferent fibers are responsible for that dull, achy

feeling that follows injury.

XI Spinal accessory

nerve

Controls

The muscles of the neck

XII Hypoglossal nerve

Responsible for movement of the tongue



The Autonomic Nervous System

The autonomic nervous system (ANS) is a branch of the nervous system which is concerned

with regulating the internal state of the organism. The main reason for considering it within a

psychology text is because it plays an important role in the control of emotional behaviours.

For example, it has a major role in the ‘fight or flight’ response that occurs when we are

faced with a dangerous situation.

ANS regulates the basic visceral processes needed for the maintenance of normal bodily

functions. It operates independently of voluntary control, although certain events, such as

stress, fear, sexual excitement, and alterations in the sleep-wake cycle, change the level of

autonomic activity. The autonomic system usually is defined as a motor system that

innervates three major types of tissue: cardiac muscle, smooth muscle, and glands. However,

it also relays visceral sensory information to the central nervous system and processes it so

that alterations can be made in the activity of specific autonomic motor outflows, such as

those that control the heart, blood vessels, and other visceral organs. It also stimulates the

release of certain hormones involved in energy metabolism (e.g., insulin, glucagon, and

epinephrine [also called adrenaline]) or cardiovascular functions (e.g., renin and vasopressin).

These integrated responses maintain the normal internal environment of the body in an

equilibrium state called homeostasis. The autonomic system consists of two major divisions:

the sympathetic nervous system and the parasympathetic nervous system. These often

function in antagonistic ways.

Structure of the ANS

Sympathetic and parasympathetic branches

The ANS serves those functions that are not under voluntary control. The ANS serves many

of the internal organs of our body. It is composed of two subsystems, the sympathetic

nervous system and the parasympathetic nervous system. These two branches work in a

complementary way to regulate the balance of the internal environment. For example, activity

in the sympathetic nervous system serves to increase the heart rate, whereas activity in the

parasympathetic nervous system serves to reduce the heart rate. The sympathetic nervous

system mainly activates systems. The parasympathetic nervous system mainly calms things

down. At all preganglionic synapses of both systems the neurotransmitter is acetylcholine. At

the target organ synapses the neurotransmitter substances in the sympathetic system is

predominantly norepinephrine. In the parasympathetic system it is predominantly

acetylcholine.

Sympathetic nervous system

It is the division of the autonomic nervous system that coordinates arousal. Sympathetic

(thoracolumbar) division activates the body under conditions of stress and emergency and is

called the fight-or-flight system. Sympathetic responses include dilated pupils, increased

heart and respiratory rates, increased blood pressure, dilation of the bronchioles of the lungs,

increased blood glucose levels, and sweating. During exercise or flight, sympathetic

vasoconstriction shunts blood from the skin and digestive viscera to the heart, brain, and

skeletal muscles. The sympathetic nervous system has been elegantly designed to cope with

emergencies. It prepares the body for action. Human beings have two basic ways of dealing

with an emergency. We can run, or we can fight. As a result, the sympathetic nervous system

is known as our fight-or-flight system. You probably know all too well what this feels like

because you probably have had a close call or two while driving your car. In this type of

emergency, our hearts race, our breathing is rapid, the palms of our hands get sweaty, our

faces are pale, and we are mentally alert and focused. All of these behaviors have been



refined through millions of years of evolution to keep you alive when faced with an

emergency. The sympathetic nervous system prepares the body for fighting or fleeing by

shutting down low-priority systems and putting blood and oxygen into the most necessary

parts of the body. Salivation and digestion are put on standby. If you’re facing a hungry lion

on the Serengeti Plain, you don’t need to worry about digesting your lunch unless you survive

the encounter. Your heart and lungs operate to provide extra oxygen, which is fed to the

large-muscle groups. Blood vessels near the skin’s surface are constricted to channel blood to

the large-muscle groups. Aside from giving you that pale look, you enjoy the added benefit of

not bleeding very badly should you be cut. With the increased blood flow to the brain, mental

alertness is at a peak.

The sympathetic nervous system is configured for a simultaneous, coordinated response to

emergencies. Axons from neurons in the thoracic and lumbar segments of the spinal cord

communicate with a series of ganglia just outside the cord known as the sympathetic chain.

Fibers from cells in the sympathetic chain then communicate with target organs. Because the

messages from the spinal neurons reach the sympathetic chain through fibers of equal length,

they arrive at about the same time. Consequently, input from the sympathetic chain arrives at

all of the target organs simultaneously. This coordinated response is essential for survival. It

wouldn’t be efficient for the heart to get a delayed message in the case of an emergency.

Because the same organs receive input from both the sympathetic and parasympathetic

systems, it is important for the organs to have a way to identify the source of the input.

This is accomplished through the types of chemical messengers used by the two systems.

Both the sympathetic and parasympathetic systems communicate with cells in ganglia outside

the spinal cord, which then form a second connection with a target organ. Both systems use

the chemical messenger acetylcholine (ACh) to communicate with their ganglia. At the target

organ, the parasympathetic nervous system continues to use acetylcholine. The sympathetic

nervous system, however, switches to another chemical messenger, norepinephrine, to

communicate with target organs. The only exception is the connection between the

sympathetic nerves and the sweat glands, where acetylcholine is still used. This system of

two chemical messengers provides a clear method of action at the target organ. If the heart,

for instance, is stimulated by acetylcholine, it will react by slowing. If it receives stimulation

from norepinephrine, it will speed up. Survival depends on not having any ambiguities,

mixed messages, or possibility of error.

The Parasympathetic Nervous System

It is the division of the autonomic nervous system responsible for rest and energy storage.

Parasympathetic (cranio sacral) division (resting and digesting system) is active under

normal, ordinary (nonstressful) conditions. It conserves body energy and maintains body

activities at basal levels. The effects include pupillary constriction, glandular secretion,

increased digestive tract mobility, and muscle actions leading to elimination of feces and

urine. During times of sympathetic nervous system activity, the body is expending rather than

storing energy. Obviously, the sympathetic nervous system can’t run continuously, or the

body would run out of resources. The job of the parasympathetic nervous system is to provide

rest, repair, and energy storage.

Whereas the neurons for the sympathetic nervous system are found in the thoracic and lumbar

regions of the spinal cord, the neurons for the parasympathetic nervous system are found

above and below these regions, in the brain and sacral divisions of the spinal cord,

specifically. This is the origin of the name parasympathetic. Para means around, and the

neurons of the parasympathetic nervous system are around those of the sympathetic nervous

system, like brackets or parentheses. After exiting the brain and sacral spinal cord,



parasympathetic axons do not synapse with a chain, as was the case with the sympathetic

axons. Instead, they travel some distance to locations near their target organs, where the

parasympathetic ganglia are located. Because timing is not as important to parasympathetic

activity as it is to sympathetic activity, the coordination provided by a chain is not necessary.

The table shows the differences in actions of sympathetic and parasympathetic divisions of

autonomic nervous system.

Actions of the ANS

ORGAN SYMPATHETIC ACTION PARASYMPATHETIC

ACTION

Eye

Mouth

Lungs

Heart

Sweat glands

Intestines

Stomach

Liver

Adrenal glands

Skin

Bladder

Penis

Dilates pupils

Inhibits salivation

Relaxes airways

Increases heart rate

Increases sweating

NO ACTION

Inhibits digestion

Stimulates glucose release

Stimulates adrenaline release

Constricts blood vessels

Relaxes bladder

Stimulates ejaculation

Constricts pupils

Stimulates salivation

Constricts airways

Decreases heart rate

NO ACTION

Dilates blood vessels

Stimulates digestion

NO ACTION

NO ACTION

Dilates blood vessels

Contracts bladder

Stimulates erection

Central control of the ANS

The brain structure that plays the greatest role in managing the autonomic nervous system is

the hypothalamus. The pathways to and from the hypothalamus are exceedingly complex.

Many structures involved with emotion have the potential to affect the hypothalamus and,

indirectly then, the autonomic nervous system. As a result, the responses of our internal

organs are tightly connected with our emotional behaviors, leading to the many common

physical symptoms we experience as a result of our emotions.

The hypothalamus, in turn, connects with the midbrain tegmentum and to the reticular

formation in particular. Damage to the midbrain in the vicinity of the red nucleus produces a

wide variety of autonomic disturbances, probably due to interruptions to large fiber pathways

that descend from these areas to the autonomic neurons of the lower brainstem and spinal

cord.

Functions and psychology of ANS

The contraction (or relaxation) of muscles and the secretion of glands is an overt expression

of the functional activity of the nervous system. These actions are mediated through the

somatic motor system and the autonomic (visceral) nervous system. The somatic motor

system innervates the voluntary (skeletal, striated) muscles, whereas the autonomic nervous

system influences the activities of involuntary (smooth) muscles, cardiac (heart) muscle, and

glands. The autonomic nervous system is often called the general visceral efferent system or



vegetative motor system because the effectors are associated with the visceral systems over

which only minimal, if any, direct conscious control can be exerted. The general role of the

autonomic nervous system (ANS) is to influence those visceral activities that are directed

toward maintaining a relatively stable internal environment. For example, functional

expressions of the activity of the ANS are the maintenance of (1) blood pressure

commensurate with the demands of the organism and (2) a constant body temperature. These

two systems are not independent; they interact. With a drop in body temperature, the somatic

motor system responds by generating heat through contraction of voluntary muscles, and the

ANS simultaneously stimulates the constriction of cutaneous blood vessels to reduce

radiational heat loss.

In this section we will examine the role of the ANS in two differing scenarios. The first ishow

the ANS contributes to the formation of ulcers. The second is the role of the ANS in the

‘fight or flight’ situation.

‘Executive stress’ and the formation of ulcers

Most people are familiar with the view that high-powered executives are more likely to

develop stomach ulcers due to the stresses of their job. A study that tried to address this was

conducted by Brady et al. (1958). In this experiment two monkeys were put in chairs. Both

monkeys had one foot attached to an electrode that could deliver an electric shock. One of the

monkeys (the passive monkey) had no way of avoiding the shock but the other monkey (the

executive monkey) could press a lever to prevent a shock being delivered. If the executive

monkey pressed the lever then both monkeys were spared a shock on that trial. If the monkey

failed to press the lever in time then both monkeys received a shock. In this way the

executive monkey was in total control of shock avoidance.

Brady et al. found that the executive monkey developed ulcers whereas the passive monkey

did not. They had predicted this because they claimed that the executive monkey was under

greater stress due to having the responsibility not just for its own shock but also for the other

monkey’s shock. However, this conclusion may be an oversimplification, and a number of

criticisms of the experiment suggest why. In the study the monkeys used as executives were

all proven good learners and so they mastered the association between pressing the lever and

preventing the shock in just a few hours or less. Once learned neither monkey then ever

received a shock. One might argue, therefore, that the only difference between the monkeys

was the high level of physical exertion endured by the executive monkey. Perhaps this caused

the ulcers.

In another experiment by Foltz and Millett (1964) the executive monkey experiment was

repeated but this time a new, naive executive was introduced every few weeks. It was noticed

that when a new executive was brought in it took a few hours for it to learn the task and

during this time the passive monkey would become highly agitated. In this scenario it was the

passive monkey that developed stomach ulcers. Foltz and Millett suggested that the passive

monkeys developed ulcers because of the high arousal levels associated with the unavoidable

shock.

So how might we explain the association between stress and the development of stomach

ulcers? Recent evidence suggests that it is not what happens during the stressful period that

causes ulcer formation but what happens during the period immediately afterwards. During

the stressful period levels of sympathetic nervous system activity are high. After the stress is

over, the parasympathetic nervous system rebounds with a high level of activity. One of the

functions of the parasympathetic nervous system is to cause a release of digestive juices. If

there is nothing in the stomach to be digested then the digestive juices will damage the walls



of the stomach and intestines and cause ulcers. It would seem, therefore, that one good thing

to do after a stressful situation has passed is to eat so that the digestive juices will be used up.

Fight or flight

A theory advanced by Walter B. Cannon, according to which animal and human organisms in

situations requiring that they either fight or flee are provided with a check and drive

mechanism that put them in readiness to respond with undivided energy output; the

mechanism is characterized by increased sympathetic nervous system activity,

including increased catecholamine production withassociated increases in blood pressure, hea

rt and respiratory rates, and skeletal muscle blood flow. Thus an internal reaction makes

possible external behavior in response to danger. For example, imagine that you are a little

mouse and you spot a cat in the near distance. Imagine also that it has spotted you and starts

to give chase. You have two choices. You can either stand, and try to defend yourself or you

can run like crazy. If you are a sensible mouse you will choose the latter but either way your

sympathetic nervous system will be called into action. Recollecting the outcomes of

sympathetic activity described in the table, we can take each one in turn and analyze whether

or not it is a useful function to aid running away.

• The pupils dilate. This allows plenty of light to enter the eye so that the animal can see

where it is going.

• The mouth dries out. The last thing you need to worry about is anything to do with digesting

your food. If you don’t survive it will hardly matter. Hence you must make all sources of

energy available for fleeing.

• The airways are relaxed. This helps you to breath more easily so that you can get more

oxygen into the blood. When you are doing a strenuous activity (like running) your muscles

will use up oxygen more quickly.

• Heart rate increases. Again, this enables blood to be pumped around the body more quickly

to speed up the supply of oxygen to where it is needed.

• Increased sweating. The increase in activity will increase the heat that the body is

generating. To counter this sweating deposits fluid onto the skin’s surface where it will

evaporate and cool the body down.

• Digestion by the stomach ceases. As with salivating, the last place you need to expend

energy at a time like this is in digesting food.

• Glucose release from the liver is stimulated. Once again this is about energy. As glucose is

the major source of energy for cells there needs to be a plentiful supply in the blood. That

which has previously been stored in the liver is now released.

• Adrenaline is released from the adrenal medulla. This is a hormone that is released into

the blood where it has a general mobilising effect.

• Blood vessels to the skin constrict. If any injury is sustained whilst fleeing the lowered

blood supply to the skin will prevent excessive bleeding.

• The bladder relaxes. The bladder is a muscle that is usually contracted until you wish to

urinate. The energy for this is more useful elsewhere and so the bladder relaxes and urination

takes place.

• Ejaculation. Obviously, the sympathetic nervous system’s control over ejaculation is not of

functional value at this time.

We can see that all except one of the actions of the sympathetic nervous system help the

animal to spring into action. If the scenario called for standing and fighting (e.g. during a

territorial battle) then these same considerations would hold true. For this reason, whether



fighting or fleeing, the body shuts down unwanted processes and mobilises oxygen and

energy to where it might be needed.

The polygraph

The polygraph was invented in 1921 by John Augustus Larson, a medical student at the

University of California at Berkeley and a police officer of the Berkeley Police Department in

Berkeley, California. Many members of the scientific community consider polygraphy to be

pseudoscience.

Polygraphy (popularly referred to as a lie detector) is the process which is used in medical

practice for comprehensive study of functioning of different body systems with particular

reference to circulation, respiration and peripheral nervous response. A polygraph measures

and records several physiological indices such as blood pressure, pulse, respiration and skin

conductivity while the subject is asked and answers a series of questions. This technology has

been attempted in forensic investigation process. The basis of its application is the fact that

mental excitation or stimulation there is alteration of these body functions due to autonomic,

particularly sympathetic excitation; that is the deceptive answers will produce physiological

responses that can be differentiated from those associated with non-deceptive answers.

Basing on this principle, polygraph, which indicates the functioning levels of the above noted

systems, has been used to know whether a suspect or an accused of a case is deceptive while

facing interrogations during the investigation, so that subsequent investigation process can be

channeled through right way. For this purpose, the persons to be so examined with the help of

a polygraph should be so done in his complete physical and mental relaxation stage, without

any factor acting on him to influence the responses, except which should naturally occur

while giving a deceiving or false reply.

Procedure of interrogation and questioning to the subject:

The subject to be examined is to be prepared without any premedication. The preparation is

more a mental preparation than otherwise. Certain subjects are naturally unsuitable for this

test, for instance, subjects with psychotic personality, over reactive personality, drug addicts;

persons suffering from gross abnormality of any of these three conditions and persons who

are by nature deceptive, restless and non co-operative. These subjects require special

preparation and need time to be fit for the test. They are not suitable for ready examination.

Preparation of the subject (who is suitable for ready examination): the person is subjected to

pre-examination interview during which its purpose, aim, the process of polygraph

examination to be followed, should be explained to him to his optimum understanding. For

satisfactory result of the test, the tester should have the knowledge of the incident. The

subject should be informed that, he would be asked certain questions, and he is to answer the

questions as „yes‟ or „no‟. For this questions will be of suggestive in nature. The subject has

nothing to be apprehensive about any wrong study and interpretation of the polygraphic test.

But if he deceives then, that will be reflected in the test. In the second stage he should be

made acquainted with the questions and he has to understand the questions well so as to give

„yes‟ or „no‟ answers. Ideally, not more than 10 questions should be asked to him in the

same sitting.

Materials & Methods

The person is made to sit on a chair and the accessories of the instrument are properly

attached on different parts of the body. An arm cuff is placed around the arm for recording

blood pressure and pulse rate and pulse features. An elastic belt is placed around the chest to

measure the rate and amplitude of respiration with deviations and an electrode connection is

placed, one on the tip of one side index finger for recording galvanic skin reaction (Galvanic



current is used for the purpose). The response is recorded graphically on a single paper from

where different adverse responses, the intensity of responses, and the time and extent of

exciting reaction, can be studied. All these measurements are recorded simultaneously in the

form of traces on a graph paper individually. These recordings on a graph paper, collectively,

are known as PolyGram. It is evaluated to find out whether during the lie detection test the

subject experienced emotional stress from any of the questions asked, or showed no reaction.

Application and utility:

Since the development of polygraph, it has been widely applied in criminal investigation by

the police. However, of late, the polygraph has also been used elsewhere and for other

purposes: Recruitment of police and other personal. Apart from the police department the

federal bureau of investigation and the department of defence, banks and other organizations

are also utilizing the lie detector as an aid for investigation undertaken by them.

The big business and industrial concerns use the lie detector for checking the honesty of their

employees. Specific quality of polygraph and allied deception tests can briefly be

summarized as follows: 1. It can detect deception.

2. The guilty can be induced to confess to his crime.

3. It can discriminate between the innocent and the guilty.

4. It can replace the third degree methods used in interrogations.

5. It can narrow down the field of inquiry for the police.

6. It can check the veracity of the statement of a witness.

7. It is an effective tool to ascertain and check the honesty of candidates or employees.

Autonomic balance/ homeostasis

The complementary and reciprocal interactions of the sympathetic and parasympathetic

branches of the autonomic nervous system, which together, create a state of homeostasis or

balance. The human organism consists of trillions of cells all working together for the

maintenance of the entire organism. While cells may perform very different functions, all the

cells are quite similar in their metabolic requirements. Maintaining a constant internal

environment with all that the cells need to survive (oxygen, glucose, mineral ions, waste

removal, and so forth) is necessary for the well-being of individual cells and the well-being of

the entire body. The varied processes by which the body regulates its internal environment

are collectively referred to as homeostasis.

Homeostasis in a general sense refers to stability, balance or equilibrium. It is the body's

attempt to maintain a constant internal environment. Maintaining a stable internal

environment requires constant monitoring and adjustments as conditions change. This

adjusting of physiological systems within the body is called homeostatic regulation.

Homeostatic regulation involves three parts or mechanisms: 1) the receptor, 2) the control

center and 3) the effector. The receptor receives information that something in the

environment is changing. The control center or integration center receives and processes

information from thereceptor. And lastly, the effector responds to the commands of

the control center by either opposing or enhancing the stimulus. This is an ongoing process

that continually works to restore and maintain homeostasis. For example, in regulating body

temperature there are temperature receptors in the skin, which communicate information to

the brain, which is the control center, and the effector is our blood vessels and sweat glands

in our skin.



Because the internal and external environment of the body is constantly changing and

adjustments must be made continuously to stay at or near the set point, homeostasis can be

thought of as a synthetic equilibrium.

Since homeostasis is an attempt to maintain the internal conditions of an environment by

limiting fluctuations, it must involve a series of negative feedback loops.

Positive and Negative Feedback:

When a change of variable occurs, there are two main types of feedback to which the system

reacts:

Negative feedback: a reaction in which the system responds in such a way as to

reverse the direction of change. Since this tends to keep things constant, it allows the

maintenance of homeostasis. For instance, when the concentration of carbon dioxide

in the human body increases, the lungs are signaled to increase their activity and expel

more carbon dioxide. Thermoregulation is another example of negative feedback.

When body temperature rises, receptors in the skin and the hypothalamus sense a

change, triggering a command from the brain. This command, in turn, effects the

correct response, in this case a decrease in body temperature.

1. Positive feedback: a response is to amplify the change in the variable. This has a

destabilizing effect, so does not result in homeostasis. Positive feedback is less

common in naturally occurring systems than negative feedback, but it has its

applications. For example, in nerves, a threshold electric potential triggers the

generation of a much larger action potential. Blood clotting in which the platelets

process mechanisms to transform blood liquid to solidify is an example of positive

feedback loop. Another example is the secretion of oxytocin which provides a

pathway for the uterus to contract, leading to child birth.

Review Questions

1. The PNS is connected to CNS via ------

a) Limbic system

b) ANS

c) Spinal cord

d) Medulla

2. One division of ANS is

a) Somatic nervous system

b) Sympathetic nervous system

c) Cranial nerves

d) Spinal nerves

3. Function of Trochlear nerve is

a) Carrying information about smell to the brain

b) Carries information from the inner ear to the brain

c) Controls the muscles of the eye

d) Manages both sensory and motor functions in the throat

4. -------nerve serves the heart, liver and digestive tract.

a) Glossopharyngeal nerve

b) Vagus nerve

c) Spinal accessory nerve

d) Hypoglossal nerve



5. --------- has the major role in fight or flight response

a) Sympathetic nervous system

b) Parasympathetic nervous system

c) Somatic nervous system

d) Spinal cord

6. Which functions are controlled by the sympathetic nervous system?

7. Explain the role of ANS in regulating the balance of the internal environment.

8. Briefly explain the functions of twelve pairs of cranial nerves.

9. What is the principle behind polygraph test? How it works? 10. What are the physiological reactions found in an individual during an emergency

situation?

----------------------------



Module II

The Central Nervous System The central nervous system includes the brain and spinal cord. The peripheral nervous system

contains all the nerves that exit the brain and spinal cord, carrying sensory and motor

messages to and from the other parts of the body. The tissue of the CNS is encased in bone,

but the tissue of the PNS is not.

Although the neurons in both the CNS and PNS are essentially similar, there are some

differences between the two systems. As we saw previously, the CNS is covered by three

layers of membranes, whereas the PNS is covered by only two. Cerebrospinal fluid circulates

within the layers covering the CNS but not within the PNS. In addition, damage to the CNS is

considered permanent, whereas recovery can occur in the PNS.

The reason that psychologists need to know about the workings of the central nervous system

is that most of our psychological behaviour is underpinned by our physiological makeup

(most notably the workings of our brains). The central nervous system is the most important

part of our nervous system and is involved in all psychological activity.

The Spinal Cord The spinal cord is a long cylinder of nerve tissue that extends from the medulla, the most

caudal structure of the brain, down to the first lumbar vertebra (vertebral column, the bones

of the spinal column that protect and enclose the spinal cord). The neurons making up the

spinal cord are found in the upper two thirds of the vertebral column. The spinal cord is

shorter than the vertebral column because the cord itself stops growing before the bones in

the vertebral column do. Running down the center of the spinal cord is the central canal.

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 nerves exit between the bones of the vertebral column. The bones are cushioned

from one another with disks. If any of these disks degenerate, pressure is exerted on the

adjacent spinal nerves, producing a painful pinched nerve. Based on the points of exit, we

divide the spinal cord into 31 segments, it controls head, neck, and arms. We refer to the neck

brace used after a whiplash injury as a cervical collar. Below the eight cervical nerves are

the 12 thoracic nerves, which serve most of the torso. Five lumbar nerves come next,

serving the lower back and legs. The five sacral nerves serve the backs of the legs and the

genitals. Finally, we have the single coccygeal nerve. Although the spinal cord weighs only 2

percent as much as the brain, it is responsible for several essential functions. The spinal cord

is the original information superhighway.

When viewed in a horizontal section much of the cord appears white. White matter

(An area of neural tissue primarily made up of myelinated axons) is made up of nerve fibers

known as axons, the parts of neurons that carry signals to other neurons. The tissue looks

white due to a fatty material known as myelin, which covers most human axons. When the

tissue is preserved for study, the myelin repels staining and remains white, looking much like

the fat on a steak. These large bundles or tracts of axons are responsible for carrying

information to and from the brain. Axons from sensory neurons that carry information about

touch, position, pain, and temperature travel up the dorsal parts of the spinal cord. Axons

from motor neurons, responsible for movement, travel in the ventral parts of the cord.



There appears to be a gray butterfly or letter H shape in the center of the cord. Gray

matter consists of areas primarily made up of cell bodies. The tissue appears gray because

the cell bodies absorb some of the chemicals used to preserve the tissue, which stains them

gray. The neurons found in the dorsal horns of the H receive sensory input, whereas neurons

in the ventral horns of the H pass motor information on to the muscles. These ventral horn

cells participate in either voluntary movement or spinal reflexes.

Without any input from the brain, the spinal cord neurons are capable of some

important reflexes. The knee jerk, or patellar reflex, that your doctor checks by tapping your

knee, is an example of one type of spinal reflex. This reflex is managed by only two neurons.

One neuron processes sensory information coming to the cord from muscle stretch receptors.

This neuron communicates with a spinal motor neuron that responds to input by contracting a

muscle, causingyour foot to kick. Spinal reflexes also protect us from injury. If you touch

something hot or step on something sharp, your spinal cord produces a withdrawal reflex.

You immediately pull your body away from the source of the pain. This time, three neurons

are involved: a sensory neuron, a motor neuron, and an interneuron between them. Because

so few neurons are involved, the withdrawal reflex produces very rapid movement. The

spinal cord also manages a number of more complex postural reflexes that help us stand and

walk. These reflexes allow us to shift our weight automatically from one leg to the other.

Damage to the spinal cord results in loss of sensation (of both the skin and internal organs)

and loss of voluntary movement in parts of the body served by nerves located below the

damaged area. Some spinal reflexes are usually retained. Muscles can be stimulated, but they

are not under voluntary control. A person with cervical damage is a quadriplegic (quad

meaning “four,” indicating loss of control over all four limbs). All sensation and ability to

move the arms, legs, and torso are lost. A person with lumbar-level damage is a paraplegic.

Use of the arms and torso is maintained, but sensation and movement in the lower torso and

legs are lost. In all cases of spinal injury, bladder and bowel functions are no longer under

voluntary control, as input from the brain to the sphincter muscles does not occur. Currently,

spinal damage is considered permanent, but significant progress is being made in repairing

the spinal cord. The main functions of spinal cord are conveying sensory information to

brain, conveying motor information to PNS and reflexively integrates sensory and motor

information (i.e. decides what to do without asking the brain for help).

Reflex behavior A reflex forms the basis of a relatively straightforward and automatic reaction (or ‘response’)

that is triggered by a stimulus. Each reflex is found in all members of the species, unless there

is malfunction, e.g. every dog salivates to meat in its mouth. We automatically move a limb

away from a damagingly hot object. We close our eyes when an object comes rapidly towards

us. As a result of how the nervous system is constructed, reflexes ‘just happen’ when an

appropriate stimulus is presented. We do not need to think about producing them. The genes

of an animal help to determine a nervous system that is equipped with a number of fitness-

enhancing reflexes.

From functional and evolutionary perspectives, reflexes provide ready-made ‘built-in’

answers to common problems that have presented themselves throughout the evolution of the

species. They are an economical means of operating. For example, all animals have a reflex

that reacts rapidly to damaging stimuli, such as sharp objects touching the skin. Humans

cannot afford to engage sophisticated but very slow conscious processing with finding

creative and original solutions to such a problem. This would not be cost-effective. By



contrast, other problems cannot be solved on the basis of ‘ready-made’ solutions and they

need to engage our conscious processing.

Without any input from the brain, the spinal cord neurons are capable of some important

reflexes. The knee jerk, or patellar reflex is an example of one type of spinal reflex. This

reflex is managed by only two neurons. One neuron processes sensory information coming to

the cord from muscle stretch receptors. This neuron communicates with a spinal motor

neuron that responds to input by contracting a muscle, causing your foot to kick. Spinal

reflexes also protect us from injury. If you touch something hot or step on something sharp,

your spinal cord produces a withdrawal reflex. You immediately pull your body away from

the source of the pain. This time, three neurons are involved: a sensory neuron, a motor

neuron, and an interneuron between them. Because so few neurons are involved, the

withdrawal reflex produces very rapid movement. The spinal cord also manages a number of

more complex postural reflexes that help us stand and walk. These reflexes allow us to shift

our weight automatically from one leg to the other.

Damage to the spinal cord results in loss of sensation (of both the skin and internal organs)

and loss of voluntary movement in parts of the body served by nerves located below the

damaged area. Some spinal reflexes are usually retained. Muscles can be stimulated, but they

are not under voluntary control.

SPINAL REFLEX ARCS

Reflex responses are mediated by neuronal linkages called reflex arcs or loops. The structure

of a spinal somatic reflex arc can be summarized in the following manner. (1) A sensory

receptor responds to an environmental stimulus. (2) An afferent fiber conveys signals through

the peripheral nerves to the gray matter of the spinal cord. (3a) In the simplest reflex arc, the



afferent root enters the spinal cord and synapses directly with lower motoneurons

(monosynaptic reflex). (3b) In more complex, and more common, reflex arcs, the afferent

root synapses with interneurons, which, in turn, synapse with lower motoneurons

(polysynaptic reflex). (4) A lower motoneuron transmits impulses to effectors striated

voluntary (skeletal) muscles.

Spinal reflexes are also classified as (1) segmental, (2) intersegmental, or (3) suprasegmental

reflexes. A segmental reflex comprises neurons associated with one or even a few spinal

segments. An intersegmental reflex consists of neurons associated with several to many

spinal segments. A suprasegmental reflex involves neurons in the brain that influence the

reflex activity in the spinal cord.

Reflexes in which the sensory receptor is in the muscle spindle of any muscle group are

known as myotatic, stretch, or deep tendon reflexes (DTR). These are intrasegmental reflexes.

Examples are (1) the biceps reflex tapping the biceps brachii tendon results in flexion of the

forearm at the elbow, (2) the triceps reflex—tapping the triceps tendon results in extension of

the forearm at the elbow, (3) the quadriceps reflex (knee jerk—tapping of the quadriceps

tendon results in extension of the leg at the knee, and (4) the triceps sural reflex (ankle

jerk)—tapping of the Achilles tendon results in plantar flexion of the foot.

Reflexes in which the sensory receptor is the Golgi tendon organ (GTO), located in a tendon

at its junction with a muscle, are known as Golgi tendon reflexes. A third kind of reflex, with

sensory receptors variously located, is a flexor reflex. In this reflex, for example, the upper

extremity withdraws from a noxious stimulus such as a hot stove. The reflex comprises (1)

sensory receptors, (2) afferent neurons, (3) spinal interneurons, (4) alpha motoneurons, and

(5) voluntary muscles. The flexor reflex is a protective reflex initiated by a diverse group of

receptors in the skin, muscles, joints, and viscera and conveyed by A-delta and C pain fibers,

as well as group III and IV fibers (called flexor reflex afferents [FRA]s). Intense stimulation

can elevate the level of excitability within the spinal cord to a point at which a crossed reflex

is evoked, with such responses as leaning or jumping away from a stimulus.

Brain

The brain essentially serves as the body’s information processing centre. It receives signals

from sensory neurons (nerve cell bodies and their axons and dendrites) in the central and

peripheral nervous systems, and in response it generates and sends new signals that instruct

the corresponding parts of the body to move or react in some way. It also integrates signals

received from the body with signals from adjacent areas of the brain, giving rise to perception

and consciousness.

The brain weighs about 1,500 grams (3 pounds) and constitutes about 2 percent of total body

weight. It consists of three major divisions: (1) the massive paired hemispheres of the

cerebrum, (2) the brainstem, consisting of the thalamus, hypothalamus, epithalamus,

subthalamus, midbrain, pons, and medulla oblongata, and (3) the cerebellum.



Structural outline of brain:

Early in embryological development, the brain divides into three parts: the hindbrain,

midbrain (or mesencephalon), and forebrain. Together, the hindbrain and midbrain make

up the brainstem. Later in embryological development, the midbrain makes no further

divisions, but the hindbrain divides into the myelencephalon, or medulla (The most caudal

part of the hindbrain), and the metencephalon. Cephalon refers to the head. We will begin

our study of the brain with the hindbrain, which is located just above the spinal cord.

The Hindbrain: It is the most caudal division of the brain, including the medulla, pons, and cerebellum.

The Myelencephalon (Medulla)

The gradual swelling of tissue above the cervical spinal cord marks the most caudal portion

of the brain, the myelencephalon, or medulla. The medulla, like the spinal cord, contains

large quantities of white matter. The vast majority of all information passing to and from

higher structures of the brain must still pass through the medulla. Instead of the butterfly

appearance of the gray matter in the spinal cord, the medulla contains a number of nuclei, or

collections of cell bodies with a shared function. These nuclei are suspended within the white

matter of the medulla. Some of these nuclei contain cell bodies whose axons make up several

of the cranial nerves serving the head and neck area. Other nuclei manage essential functions

such as breathing, heart rate, and blood pressure. Damage to the medulla is typically fatal due

to its control over these vital functions. Along the midline of the upper medulla, we see the

caudal portion of a structure known as the reticular formation. The reticular formation is a

complex collection of nuclei that runs along the midline of the brainstem from the medulla up

into the midbrain. The structure gets its name from the Latin reticulum, or network. The

reticular formation plays an important role in the regulation of sleep and arousal.

The Metencephalon

Pons and Cerebellum

The metencephalon contains two major structures, the pons and the cerebellum. The ponslies

immediately rostral to the medulla. Pons means “bridge” in Latin, and one of the roles of the



pons is to form connections between the medulla and higher brain centers as well as with the

cerebellum.

As in the medulla, large fiber pathways with embedded nuclei are found in the pons. Among

the important nuclei found at this level of the brainstem are the cochlear nucleus and the

vestibular nucleus. The fibers communicating with these nuclei arise in the inner ear. The

cochlear nucleus receives information about sound, and the vestibular nucleus receives

information about the position and movement of the head. This vestibular input helps us keep

our balance (or makes us feel motion sickness on occasion).

The reticular formation, which begins in the medulla, extends through the pons and on into

the midbrain. The pons contains a number of other important nuclei that have wide-ranging

effects on the activity of the rest of the brain. Nuclei located within the pons are necessary for

the production of rapid-eye-movement (REM) sleep. The raphe nuclei and the locus

coeruleus project widely to the rest of the brain and influence mood, states of arousal, and

sleep. The functions of pons 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, and the secretion of saliva and tears.

The second major part of the metencephalon is the cerebellum. The cerebellum looks almost

like a second little brain attached to the dorsal surface of the brainstem. Its name, cerebellum,

actually means little brain” in Latin. The use of “little” is misleading because the cerebellum

actually contains more nerve cells (neurons) than the rest of the brain combined. When

viewed with a sagittal section, the internal structure of the cerebellum resembles a tree. White

matter, or axons, forms the trunk and branches, while gray matter, or cell bodies, forms the

leaves. The traditional view of the cerebellum emphasizes its role in coordinating voluntary

movements, maintaining muscle tone, and regulating balance. Input from the spinal cord tells

the cerebellum about the current location of the body in three- dimensional space. Input from

the cerebral cortex, by way of the pons, tells the cerebellum about the movements you intend

to make. The cerebellum then processes the sequences and timing of muscle movements

required to carry out the plan.

Considerable data support this role for the cerebellum in movement. Damage to the

cerebellum affects skilled movements, including speech production. Because the cerebellum

is one of the first structures affected by the consumption of alcohol, most sobriety tests, such

as walking a straight line or pointing in a particular direction, are actually tests of cerebellar

function. Along with the previously mentioned vestibular system, the cerebellum contributes

to the experience of motion sickness.

More contemporary views see the cerebellum as responsible for much more than balance and

motor coordination. In spite of its lowly position in the hindbrain, the cerebellum is involved

in some of our more sophisticated processing of information. In the course of evolution, the

size of the cerebellum has kept pace with increases in the size of the cerebral cortex. One of

the embedded nuclei of the cerebellum, the dentate nucleus, has become particularly large in

monkeys and humans. A part of the dentate nucleus, known as the neodentate, is found only

in humans. In addition to language difficulties, patients with cerebellar damage also

experience subtle deficits in cognition and perception. The cerebellum also participates in

learning. Also 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-motor 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. In cases of autism, a disorder in which language, cognition, and social awareness

are severely afflicted, the most reliable anatomical marker is an abnormal cerebellum.



Although neuroscientists do not agree on its exact function, most theories propose a

cerebellum that can use past experience to make corrections and automate behaviors, whether

they involve motor systems or not.

The Midbrain It is the division of the brain lying between the hindbrain and forebrain. The midbrain, or

mesencephalon has a dorsal or top half known as the tectum, or “roof,” and a ventral, or

bottom half, known as the tegmentum, or “covering.” In the midbrain, cerebrospinal fluid is

contained in a small channel at the midline known as the cerebral aqueduct. The cerebral

aqueduct separates the tectum from the tegmentum and links the third and fourth ventricles.

Although the midbrain is relatively small compared with the other portions of the brainstem,

it still contains a complex array of nuclei. Surrounding the cerebral aqueduct are cell bodies

known as periaqueductal gray (peri means around). Periaqueductal gray appears to play an

important role in our perception of pain. There are large numbers of receptors in the

periaqueductal gray that respond to opiates such as morphine and heroin. Electrical

stimulation of this area provides considerable relief from pain.

The midbrain also contains the most rostral portion of the reticular formation and a number of

nuclei associated with cranial nerves. Several important motor nuclei are also found at this

level of the brainstem, including the red nucleus and the substantia nigra. The red nucleus,

which is located within the reticular formation, communicates motor information between the

spinal cord and the cerebellum. The substantia nigra, whose name literally means “black

stuff” due to the pigmentation of the structure, is closely connected with the basal ganglia of

the forebrain. Degeneration of the substantia nigra occurs in Parkinson’s disease, which is

characterized by difficulty moving.

On the dorsal surface of the midbrain are four prominent bumps. The upper pair is known as

the superior colliculi. The superior colliculi receive input from the optic nerves leaving the

eye. Although the colliculi are part of the visual system, they are unable to tell you what

you’re seeing. Instead, these structures allow us to make visually guided movements, such as

pointing in the direction of a visual stimulus. They also participate in a variety of visual

reflexes, including changing the size of the pupils of the eye in response to light conditions.

The other pair of bumps is known as the inferior colliculi. These structures are involved with

hearing, or audition. The inferior colliculi are one stop along the pathway from the ear to the

auditory cortex. These structures are involved with auditory reflexes such as turning the head

in the direction of a loud noise. The inferior colliculi also appear to participate in the

localization of sounds in the environment by comparing the timing of the arrival of sounds at

the two ears.

Some Important Structures in the Brainstem

Brainstem

Location

Important Structures Functions

Medulla

Reticular formation Arousal

Cranial nerve nuclei Various

Pons

Reticular formation (continuing) Arousal


Cochlear nucleus Audition



Vestibular nucleus

Balance,

position

Raphe nucleus Sleep and arousal

Locus coeruleus

Sleep and arousal

Cerebellum

Balance, motor

coordination,

cognition

Midbrain

Reticular formation (continuing) Arousal


Periaqueductal gray Pain

Red nucleus Motor

Substantia nigra Motor

Superior colliculi Vision

Inferior colliculi Audition

The Forebrain It is the division of the brain containing the diencephalon and the telencephalon.

The forebrain contains the most advanced and most recently evolved structures of the brain.

Like the hindbrain, the forebrain divides again later in embryological development. The two

resulting divisions are the diencephalon and the telencephalon. The diencephalon contains the

thalamus and hypothalamus, which are located at the midline just above the mesencephalon

or midbrain. The telencephalon contains the bulk of the symmetrical left and right cerebral

hemispheres.

The Thalamus and Hypothalamus The diencephalon is located at the rostral end of the brainstem. The upper portion of the

diencephalon consists of the thalamus. We actually have two thalamic nuclei, one on either

side of the midline. These structures appear to be just about in the middle of the brain, as

viewed in a midsagittal section. Inputs from most of our sensory systems converge on the

thalamus, which then forwards the information on to the cerebral cortex for further

processing. It appears that the thalamus does not change the nature of the sensory

information; so much as it filters the information passed along to the cortex, depending on the

organism’s state of arousal. The cerebral cortex, in turn, forms large numbers of connections

with the thalamus.

The exact purpose of this cortical input to the thalamus remains a mystery. In addition to its

role in sensation, the thalamus is also involved with states of arousal and consciousness.

Damage to the thalamus typically results in coma, and disturbances in circuits linking the

thalamus and cerebral cortex are involved in some seizures. The thalamus has also been

implicated in learning and memory.

Functions of thalamus:



The thalamus has multiple functions. It may be thought of as a kind of hub of information. It

is generally believed to act as a relay between different 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 visual cortex in

the occipital lobe. The thalamus is believed to both process sensory information as well as

relay it—each of the primary sensory relay areas receives strong feedback connections from

the cerebral cortex. Similarly the medial geniculate nucleus acts as a keyauditory 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,

formingthalamo-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 coma.

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. 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. The thalamus is functionally connected to the hippocampus as part of the

extended hippocampal system at the thalamic anterior nuclei with respect to spatial memory

and spatial sensory datum they are crucial for human episodic memory and rodent event

memory. There is support for the hypothesis that thalamic regions connection to particular

parts of the mesio-temporal lobe provide differentiation of the functioning of recollective and

familiarity memory.

Just below the thalamus is the hypothalamus. The name hypothalamus literally means

“below the thalamus.” The hypothalamus is a major regulatory center for such behaviors as

eating, drinking, sex, biorhythms, and temperature control. Rather than being a single,

homogeneous structure, the hypothalamus is a collection of nuclei. For example, the

ventromedial nucleus of the hypothalamus (VMH) participates in the regulation of feeding

behavior. The suprachiasmatic nucleus receives input from the optic nerve and helps set daily

rhythms according to the rising of the sun. The hypothalamus is directly connected to the

pituitary gland, from which many important hormones are released. Finally, the

hypothalamus directs the autonomic nervous system, the portion of the peripheral nervous

system that controls our glands and organs.

The hypothalamus is the structure where most of our homeostatic control takes place. For

example, nuclei within the hypothalamus are critically involved in eating and satiety,

drinking, and temperature regulation. In addition to its role in homeostasis, the hypothalamus

has a major role in the control of biological rhythms. We have an internal body clock that

operates on a circadian rhythm of approximately 24 hours. This biological clock synchronizes

our sleep–waking cycle and also the release of a wide range of hormones. The clock is

located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Since destruction of the



SCN completely destroys the circadian rhythm, it is safe to say that this is a clearly localized

function. Sexual functioning comprises a large range of separate, but coordinated, elements.

These include hormonal control, olfaction, and other sensory systems, as well as a number of

higher functions.

Nevertheless, we can still consider localization of function here as each of these elements is

relatively localized to a small region of the brain. For example, whilst hormonal control is

localized in the hypothalamus and the pituitary gland, male sexual behavior is controlled by

the medial preoptic area (just rostral to the hypothalamus) and female sexual behavior is

controlled by the ventromedial nucleus of the hypothalamus.

The hypothalamus plays an important role in controlling the release of hormones. It has direct

control over the release of many pituitary hormones and also contains a number of detectors

for circulating hormones. The hypothalamus sends neuronal projections to the posterior

pituitary gland. These come from two hypothalamic nuclei called the supraoptic nucleus and

the paraventricular nucleus. The hormones released by the posterior pituitary are actually

synthesized in the hypothalamic neurons and are transported down the axon to the posterior

pituitary. Once there, the release of these hormones is controlled by neuronal activity in these

hypothalamic nuclei.

The connection between the hypothalamus and the anterior pituitary is by way of a rich

vascular system called the hypothalamic–pituitary portal system. Neuronal cells in the

hypothalamus are called neurosecretory cells because they produce and release hormones that

are secreted directly into the blood from the ends of their axons. Many of the hormones

released by the hypothalamus are called releasing hormones as they, in turn, cause the release

of hormones from the anterior pituitary. Examples of these are corticotropinreleasing

hormone (CRH) and growth-hormone-releasing hormone (GRH).

The Basal Ganglia

Several nuclei make up the basal ganglia, which participate in motor control. A ganglion

(ganglia is plural) is a general term for a collection of cell bodies. These nuclei include the

caudate nucleus, the putamen, the globus pallidus, and the subthalamic nucleus (which gets

its name from its location “sub,” or below, the thalamus). Because these structures are so

closely connected with the substantia nigra of the midbrain, some anatomists include the

substantia nigra as part of the basal ganglia. Also associated with the basal ganglia is the

nucleus accumbens, which plays an important role in the experience of reward.

The basal ganglia are an important part of our motor system. Degeneration of the basal

ganglia, which occurs in Parkinson’s disease and in Huntington’s disease, produces

characteristic disorders of movement. The basal ganglia have also been implicated in a

number of psychological disorders, including attention deficit/ hyperactivity disorder

(ADHD) and obsessive-compulsive disorder (OCD).

Although located in the diencephalon, the hypothalamus is often included in the limbic

system. We are obviously emotional when it comes to eating, drinking, and sex. The

hypothalamus also produces our so-called fightor- flight response to emergencies. Electrical

stimulation to parts of the hypothalamus can produce pleasure, rage, and fear as well as

predatory behavior.

Corpus striatum

The striatum, also known as the neostriatum or striate nucleus, is a subcortical part of

the forebrain. It receives input from the cerebral cortex and is the primary input to the basal

ganglia system. In all primates, the striatum is divided by a white matter tract called

the internal capsule into two sectors called the caudate nucleus and the putamen.



The lenticular nucleus refers to the putamen together with the globus pallidus. Functionally,

the striatum helps coordinate motivation with body movement. Body movements can be as

simple as controlling fine-motor functions or as complex as inhibiting one's behavior

depending upon social interactions.

Functions:

The striatum is best known for its role in the planning and modulation of movement

pathways, but is also potentially involved in a variety of other cognitive processes

involving executive function, such as working memory. Metabotropic dopamine receptors are

present both on spiny neurons and on cortical axon terminals. Second messenger cascades

triggered by activation of these dopamine receptors can modulate pre- and postsynaptic

function, both in the short term and in the long term. In humans, the striatum is activated by

stimuli associated with reward, but also by aversive, novel, unexpected, or intense stimuli,

and cues associated with such events. fMRI evidence suggests that the common property

linking these stimuli, to which the striatum is reacting, is salienceunder the conditions of

presentation. A number of other brain areas and circuits are also related to reward, such as

frontal areas. The striatum is also associated with novelty-related decision-making behaviors.

Functional maps of the striatum reveal interactions with widely distributed regions of the

cerebral cortex important to a diverse range of functions.

The ventral tegmental dopaminergic neurons that innervate portions of the striatum are the

primary site of rewarding feeling. Intracranial stimulation studies first done by James

Olds and collaborators in the 1950s showed that implants in this brain area will elicit bar

pressing from rats for many hours at a time. Interference with dopamine neurotransmission

impairs behavioral reward processes and their underlying neuronal mechanisms.

The Limbic System

Different anatomists propose different sets of forebrain structures for inclusion in the limbic

system. Limbic means border and describes the location of these structures on the margins of

the cerebral cortex. The limbic system (or paleomammalian brain) is a complex set of brain

structures located on both sides of the thalamus, right under the cerebrum. It is not a separate

system but a collection of structures from the telencephalon, diencephalon,

and mesencephalon. It includes the olfactory bulbs, hippocampus, amygdala, anterior

thalamic nuclei, fornix, columns of fornix, mammillary body, septum pellucidum, habenular

commissure, cingulate gyrus, parahippocampal gyrus, limbic cortex, and

limbic midbrain areas.

The limbic system supports a variety of functions

including adrenaline flow, emotion, behavior, motivation, long-term memory, and olfaction.

Emotional life is largely housed in the limbic system, and it has a great deal to do with the

formation of memories.

The hippocampus, named after the Greek word for “seahorse,” curves around within the

cerebral hemispheres from close to the midline out to the tip of the temporal lobe. The

hippocampus participates in learning and memory. Damage to the hippocampus in both

hemispheres produces a syndrome known as anterograde amnesia. People with this type of

memory loss have difficulty forming new long-term declarative memories, which are

memories for facts, language, and personal experience. In studies of patients with

hippocampal damage, it was found that memories formed prior to the damage remained

relatively intact; however, the patients were able to learn and remember procedures for

solving a puzzle requiring multiple steps.



The amygdala plays important roles in fear, rage, and aggression. In addition, the amygdala

interacts with the hippocampus during the encoding and storage of emotional memories.

Damage to the amygdala specifically interferes with an organism’s ability to respond

appropriately to dangerous situations.

In laboratory studies, rats with damaged amygdalas were unable to learn to fear tones that

reliably predicted electric shock. Rhesus monkeys with damaged amygdalas were overly

friendly with unfamiliar monkeys, a potentially dangerous way to behave in a species that

enforces strict social hierarchies. Stimuli that normally elicit fear in monkeys, such as rubber

snakes or unfamiliar humans, failed to do so in monkeys with lesions in their amygdalas. In

humans, autism, which produces either extreme and inappropriate fear and anxiety or a

complete lack of fear, might involve abnormalities of the amygdale.

The cingulate cortex is a fold of cortical tissue on the inner surface of the cerebral

hemispheres. “Cingulum” means “belt” in Latin. The cingulate cortex contains an unusual

and possibly recently evolved class of nerve cells known as Von Economo neurons. Von

Economo neurons are found only in the great apes and humans and might, therefore, have

considerable significance for the recent evolution of intelligent behavior. The cingulate cortex

is further divided into anterior and posterior sections. The anterior cingulate cortex (ACC)

exerts some influence over autonomic functions but has received the greatest attention from

neuroscientists for its apparent roles in decision-making, error detection, emotion,

anticipation of reward, and empathy.

The septal area is located anterior to the thalamus and hypothalamus. Electrical stimulation

of this area is usually experienced as pleasurable, whereas lesions in this area produce

uncontrollable rage and attack behaviors. On one unforgettable occasion, a rat with a septal

lesion jumped at my face when I leaned over to pick it up (my apologies to those of you who

are phobic about rodents).

Other structures often included in the limbic system are the olfactory bulbs, which are

located at the base of the forebrain. These structures receive and process information about

smell. If our sense of smell were not at all emotional, the perfume industry would probably

go out of business.

The Cortex

The outer covering of the cerebral hemispheres is known as the cortex, from the Latin word

for “bark.” Like the bark of a tree, the cerebral cortex is a thin layer of gray matter that varies

from 1.5 mm to 4 mm in thickness in different parts of the brain. Unlike the spinal cord, the

cerebral hemispheres are organized with gray matter on the outside and white matter on the

inside. Below the thin layers of cortical cell bodies are vast fiber pathways that connect the

cortex with the rest of the nervous system.

The cerebral cortex has a wrinkled appearance somewhat like the outside of a walnut. The

hills of the cortex are referred to as gyri (plural of gyrus), and the valleys are known as sulci

(plural of sulcus). A particularly large sulcus is usually called a fissure. Why is the cerebral

cortex so wrinkled? This feature of the cortex provides more surface area for cortical cells.

We have limited space within the skull for brain tissue, and the wrinkled surface of the cortex

allows us to pack in more neurons than we could otherwise. If stretched out flat, the human

cortex would cover an area of about 2½ square feet. Just as we ball up a piece of paper to

save space in our wastebasket, the sulci and gyri of the brain allow us to fit more tissue into

our heads. The degree of wrinkling, or convolution, is related to how advanced a species is.

Our brains are much more convoluted than a sheep’s brain, for instance, and the sheep’s brain

is more convoluted than a rat’s brain.



The cells of the cerebral cortex are organized in layers. The number, organization, and size of

the layers vary somewhat throughout the cortex. In most parts of the cortex, there are six

distinct layers, which are numbered from the outermost layer toward the center of the brain.

Layer I has no cell bodies at all. Instead, it is made up of the nerve fibers of cells forming

connections with other layers. Layers II and IV contain large numbers of small cells known

as granule cells. Layers III and V are characterized by large numbers of the triangular-

shaped pyramidal cells. These layers usually provide most of the output from an area of

cortex to other parts of the nervous system.Layer VI has many types of neurons, which merge

into the white matter that lies below the cortical layers.

There are a number of systems for dividing the cerebral cortex. A simpler approach divides

the cortex into four sections known as lobes. The lobes are actually named after the skull

bones that lie above them. The most rostral of the lobes is the frontal lobe. The caudal

boundary of the frontal lobe is marked by the central sulcus. On the other side of the central

sulcus, we find the parietal lobe. In the ventral direction, the frontal lobe is separated from

the temporal lobe by the lateral sulcus. At the very back of the cortex is the occipital lobe.

Separating the two cerebral hemispheres along the dorsal midline is the longitudinal fissure.

These areas of the cortex are so large that many different functions are located in each lobe.

Broca’s area, located in the left frontal lobe, is concerned with speech production, whereas

Wernicke’s area, located in the left temporal lobe, is concerned with speech comprehension.

The frontal lobes, especially the orbitofrontal cortex, receive inputs from various brain

locations involved in emotion. It sends fibers to the hippocampus, the amygdala, and the

lateral hypothalamus, amongst other structures. Destruction of the frontal lobes has a calming

effect. This led to the development of lobotomy, cutting the fiber tracts connecting the frontal

lobes with other parts of the brain, as a treatment for anxiety, depression, and obsessive-

compulsive behavior. The orbitofrontal cortex plays a role in reward mechanisms, and seems

to mediate the of use emotion to direct judgments. The parietal lobe contains much of what is

referred to as the association cortex. It contains many of the regions of the brain where

information is integrated after it has been processed for its initial perceptual qualities. The

temporal lobes are implicated in hearing and memory functions. Indeed, damage to the

temporal lobes is associated with a form of amnesia.

In general, we can divide the functional areas of the cortex into three categories:

sensory cortex, motor cortex, and association cortex. The sensory cortex processes incoming

information from the sensory systems. Different areas of the sensory cortex are found in the

occipital, temporal, and parietal lobes. The occipital lobe contains the primary visual cortex.

The primary auditory cortex is located in the temporal lobe. The postcentral gyrus of the

parietal lobe contains the primary somatosensory cortex, which is the highest level of

processing for information about touch, pain, position, and temperature. The postcentral

gyrus gets its name from its location directly caudal (“post” means after) to the central sulcus,

which divides the frontal and parietal lobes. The motor areas of the cortex provide the highest

level of command for voluntary movements. The primary motor cortex is located in the

precentral gyrus of the frontal lobe.

Some areas of the cortex have neither specific motor nor specific sensory functions. These

areas are known as association cortex. Association means connection. In other words, these

are the areas we have available for connecting and integrating sensory and motor functions.

The right and left cerebral hemispheres are linked by a special branch of white matter known

as the corpus callosum and by the much smaller anterior commissure.

Localization of Function in the Cortex



In addition to the sensory and motor functions identified earlier, we can localize a number of

specific functions in areas of the cerebral cortex. In many cases, these functions appear to be

managed by cortex on either the left or right hemisphere. In addition to being the location of

the primary motor cortex, the frontal lobe participates in a number of higher-level cognitive

processes such as the planning of behavior, attention, and judgment. Two important

structures within the frontal lobes are the dorsolateral prefrontal cortex, located to the top and

side of the frontal lobes, and the orbitofrontal cortex, located above and behind the eyes.

These areas of the frontal lobes maintain extensive, reciprocal connections with the limbic

system, the basal ganglia, and other parts of the cortex. The dorsolateral prefrontal cortex is

involved in executive functions such as attention and working memory and the planning of

behavior, whereas the orbitofrontal cortex is involved in impulse control.

One of the classical methods for identifying brain functions is to consider cases in which the

area of interest has been damaged. Possibly the most dramatic case of frontal lobe damage is

that of the unfortunate Phineas Gage, a railroad worker in the middle 1800s. While Gage was

preparing to blow up some rock, a spark set off his gunpowder and blew an iron tamping rod

through his head, entering below his left eye and exiting through the top of his skull. A

reconstruction of the rod’s pathway through Gage’s skull. Miraculously, Gage survived the

accident. He was not the same man, however, according to his friends. Prior to his accident,

Gage appears to have been responsible, friendly, and polite. After his accident, Gage had

difficulty holding a job and was profane and irritable. His memory and reason were intact,

but his personality was greatly changed for the worse.

Gage’s results are consistent with modern findings of frontal lobe damage. People with

damage to the dorsolateral prefrontal cortex experience apathy, personality change, and the

lack of ability to plan. People with damage to the orbitofrontal cortex experience emotional

disturbances and impulsivity. As we will see in our discussion of mental disorders, several

types of psychopathology involve the frontal lobes. Some people with schizophrenia show

lower than normal activity in the frontal lobe. Because children with attention

deficit/hyperactivity disorder are usually very impulsive and have short attention spans, it has

been suggested that they, too, suffer from underactivity of the frontal lobes. Finally, people

who show extreme antisocial behavior, including serial murderers, frequently show damage

to the orbitofrontal cortex.

In 1935, Yale researchers Carlyle Jacobsen and John Fulton reported evidence indicating that

chimpanzees with frontal lobe damage experienced a reduction in negative emotions. After

listening to a presentation by Fulton, Portuguese neurologist Egaz Moniz advocated the use

of frontal lobotomies (A surgical procedure in which a large portion of the frontal lobe is

separated from the rest of the brain) with human patients. During the 1940s and 1950s, more

than 10,000 frontal lobotomies were performed to reduce fear and anxiety in mental patients

and in some people without major disorders. The physicians stopped doing lobotomies (very

few are done today) because they recognized the tremendous negative side effects of the

procedure, but that would not be entirely accurate. Lobotomies were largely discontinued

when major antipsychotic medications were discovered. With the new drugs, the lobotomies

were no longer considered necessary.

Supporting Cells The neuroglia (also known as glia or glial cells) protect, surround and nourish the neurons.

There are several types of glia, each with specialized functions. Unlike most neurons, which

lose their ability to divide after maturity, glia can reproduce freely and therefore be replaced

regularly.



Neuroglia (glia) Neuroglia (glia) are the supportive cells that surround the cell bodies, dendrites, and axons of

neurons in both the CNS and PNS. In mammals, glia outnumber neurons by 10–50 times

depending on the region of the CNS. They constitute about one-half of the total volume of the

human brain. Neuroglia (nerve glue) were originally considered to have relatively passive

and limited roles within the CNS, but now are known to function at high rates of metabolic

activity. During development, neuroglia guide migrating neuronal precursors from the

neuroepithelium to their destinations, where patterns of neuronal circuitry are formed

Throughout life, they are important in the maintenance and sustenance of neurons and their

circuits and the release of trophic factors.

Classification of Glial Cells

Like neurons, glial cells resist a rigid classification. Those of the CNS include astrocytes

(astroglia), oligodendrocytes (oligodendroglia), microglia, and ependymal cells. The astroglia

and ligodendroglia are called macroglia. The“glial” cells of the PNS consist of Schwann cells

that surround nerve fibers and perineuronal satellite cells surrounding the cell body. These

cell types, now considered to be functionally indistinguishable, are collectively called

neurolemma cells. All of these cells except microglia are derived from ectoderm. The

Schwann cells and satellite cells originate from the neural crests derived from ectoderm.

Microglia are mainly of mesodermal origin. Glial cells can divide mitotically throughout life

in contrast to neurons, which do not. In the CNS, the neurons and glial cells are separated

from each other by an extracellular fluid in the 10- to 20-nm-wide intercellular

space comprising 15–20% of the brain’s volume. The glial cells have no synapses, do not

generate action potentials, and presumably are not directly involved in information

processing.

Multiple Roles of Glia

The following are some of the essential roles in which glial cells are actively involved: (1)

Glia provide the organized scaffolds that give the CNS structural support for neurons and

their circuitry. (2) Certain glial cells (radial glia) are critical in guiding the developing

neurons during migration from their sites of origin to their correct destination and in directing

the paths for the outgrowth of their axons. (3) Glial cells produce growth and trophic factors

that are key elements in CNS regeneration and plasticity. (4) Oligodendrocytes and Schwann

cells produce myelin sheaths. (5) Microglia function (a) as scavengers removing debris

produced following injury or neuronal death and (b) in the immune surveillance of the CNS.

(6) Astrocytes are important for maintaining homeostasis of the microenvironment of the

extracellular fluid for neuronal function by buffering the pH and regulating the potassium ion

concentrations. They act as bridges that shuttle nutrients from the capillaries to the neurons.

(7) Glial cells are involved in the production of cerebrospinal fluid (CSF) and of extracellular

fluids that coat, support and protect neurons. (8) Glia proliferate to form astrocytic scars to

repair nervous tissue following injury (reactive gliosis).

Astrocytes

Astroglia constitute a heterogeneous morphologic and functional population occupying the

spaces surrounding each CNS neuron. There are protoplasmic astrocytes in the gray matter

and fibrous astrocytes in white matter. Others include Bergmann cells in the cerebellum,

Muller’s cells in the retina, pinealocytes in the pineal gland, and pituicytes located in the

posterior lobe of the pituitary gland. These cells contain 8- to 10-nmwide microfilaments

composed of polymerized strands of glia fibrillary acetic protein (GFAP), a specific



biochemical marker for astrocytes that can be revealed through immunohistochemistry. Thus,

astroglia can be distinguished from neurons for diagnostic purposes.

The cell bodies and processes of astrocytes are interconnected by gap junctions to form a

matrix in which the neurons are embedded and separated from each other. In addition,

astrocytes can act synergistically as a functional syncytium, allowing for the interchange of

ions and molecules between the astrocytes and the extracellular fluids.

Both glial cells and neurons have negative membrane potentials, indicating that their cell

membranes are permeable to potassium ions. Because their cell membranes have only a few

potassium channels, astrocytes do not generate action potentials. Each astrocyte could have

several processes that extend among the neurons before terminating in different places as

expansions called end-feet that (1) form a jacket in contact with the basement membrane of a

capillary, (2) are juxta posed with the free surfaces of the cell bodies and dendrites and

envelop the synapses, thereby insulating synapses from each other, (3) come into contact with

the pia mater of the pial-glial limiting membrane adjacent to the subarachnoid space, and (4)

make contact with ependymal cells of the ventricular system. Astrocytes store and transfer

metabolites such as glucose from the capillaries to the neurons, and they take up excess

potassium from the extracellular potassium sinks via potassium channels. Additionally,

following intense neuronal activity, they take up glutamate and neurotoxins that accumulate

in the extracellular spaces and synaptic clefts. Following the uptake of excess potassium ions

from focal high-concentration sinks, the astrocytes can then transfer the excess ions via their

gap junctions to regions within the astrocytic syncytium, where the potassium ion

concentration is lower (known as spatial buffering). This prevents the spreading depression

that results from the presence of high extracellular concentrations of potassium ions that can

trigger excessive neuronal depolarization. In essence, astrocytes have roles in regulating and

maintaining the homeostatic composition of the extracellular fluid (ionic microenvironment

and pH) essential to the normal functioning of the neurons of the CNS. The ability of these

cells to divide throughout life could explain, in part, why tumors of astrocytic origin are the

most common CNS tumors. Astrocytes might secrete such neurotrophins as NGF and BDNF,

which are important in promoting the survival of some neurons.

Oligodendrocytes

These CNS cells are the equivalents of Schwann cells of the PNS. They are the cells that

make and maintain CNS myelin. There are two types of oligodendroglia: perineuronal

satellite cells, which are closely associated with cell bodies and dendrites in the gray matter,

and (2) interfascicular cells, which are involved in myelination of axons in white matter. The

numerous processes of Individual oligodendrocytes form the myelinated internodes for as

many as 70 axons. Asnoted earlier for peripheral nerve axons, the myelin sheath is a

continuous layering of spiral lamellae of the oligodendroglial plasma membrane. Myelination

of many axons commences prenatally. Most pathways in the human brain are not fully

myelinated until 2 years after birth. Oligodendrocytes can participate in the remyelination

that can occur following acute or chronic demyelination. This so-called spontaneous

remyelination takes place in such diseases as multiple sclerosis and could explain the clinical

improvement observed in different demyelinating diseases.

Microglia

Microglia exist as (1) resting microglial cells in normal CNS (called resident brain

macrophages), which can become converted into (2) activated or reactive nonphagocytic

microglia capable of producing cytokines that become (3) phagocytic microglia

(macrophages). Other sources of macrophages are monocytes (its precursor cell in the blood)

and meningial and perivascular cells of CNS blood vessels. They become scavengers after



being activated by foreign bodies, brain injury, degradation products, or inflammation and

thus function as phagocytes to remove debris from the CNS. The resident microglia are small

cells that comprise from 5% to 10 % of all glial cells. They contain lysosomes and vesicles

characteristic of macrophages, only sparse ER, and a few cytoskeletal fibers. They are found

in the CNS as parenchymal microglia, in the choroid plexus, and in the circumventricular

organs. Microglia could participate in shaping of neuronal circuits by actively eliminating

“extra” axon collateral branches without affecting the viability of the neuron itself.

Microglia are the representatives of the immune system, with activated microglia having a

key role in immune processing in the CNS. They are the cornerstones for the interaction

between the domains of the CNS (neurological) and the peripheral immune system (non-

neurological). Microglia join astrocytes in responding to immune factors and are associated

with the synthesis of growth factors and adhesion molecules. They can produce and secrete

cytokines—the soluble proteins associated with the magnitude of the inflammatory and

immune response. Glia can participate in autoimmune disease process by being able to act as

antigen-presenting cells and, thus, they serve in the immune surveillance of the CNS.

In brief, microglia are dynamic immunocompetent cells that function as vigilant ever-present

guardians protecting the vital and vulnerable brain and spinal cord. Microglia can be imaged

in vivo by positron-emitting tomography (PET) as a means of evaluating microglial

activation following a stroke in humans. In this procedure, PET scanning images

benzodiazepine receptors of the activated microglia.

Ependyma

The ependymal cells are simple cuboidal glial cells that line the central canal of the spinal

cord and the ventricles of the brain, including a layer of the tela choroidea. They are involved

in the production of cerebrospinal fluid (CSF). These cells are ciliated in the embryonic

stages of humans. The ependymal cells and the adjacent astrocyte end-feet comprise a brain–

CFS interface The cellular elements of this interface along with that of the pial–glial

membrane on the surface of the brain and spinal cord permit (are not barriers) the exchange

of substances between the CSF and the CNS. In the floor of the third ventricle are patches of

specialized ependymal cells called tanycytes (elongated cell) with basal processes that extend

through the neuropil to terminate with end-feet on blood vessels and neurons. They have a

role in transporting substances between the ventricles and blood. One suggestion is that they

transport molecules from the CSF to hypothalamic neurons involved with the regulation of

gonadotropic hormone release from the pituitary gland.

Schwann Cells and Satellite Cells

Schwann cells of peripheral nerves and perineuronal satellite cells of sensory and autonomic

ganglia in the PNS are the equivalent of the three types of CNS supportive cell (astroglia,

oligodendroglia, and microglia). Except for differences in location, Schwann cells and

satellite cells are indistinguishable from each other and, hence, are collectively called

neurolemma cells. Like astroglia, neurolemma cells both (1) enclose and separate

unmyelinated nerve fibers from each other and (2) are located in the interneuronal space

between neurons. Like oligodendroglia, they produce myelin sheaths around axons and a few

cell bodies of ganglia. Like microglia, Schwann cells can become phagocytes in response to

nerve injury and inflammation. Unlike glial cells, the Schwann cells secrete collagen,

laminin, and fibronectin (extracellular adhesive proteins). These proteins are the main

constituents of the basal lamina and extraneuronal matrix and also of the basement lamina

that surrounds the cell membrane of axons. The Schwann cells invest the peripheral neurons

and, thus, are effective in isolating their immediate environment from the extraneuronal space

of about 20 mm intercalated between a Schwann cell and a neuron.



FLUID ENVIRONMENT OF THE BRAIN

The brain and spinal cord can only function in a chemically stable homeostatic fluid

environment. This comprises (1) the interstitial fluid bathing the neurons, glia, and blood

vessels within the central nervous system and (2) the CSF. These two fluids are essentially

similar in composition.

Cerebrospinal fluid (CSF) The meninges are three layers of connective tissue that surround and protect the soft brain

and spinal cord. Cerebrospinal fluid (CSF) passes between two of the layers of the meninges

and, thus, slowly circulates over the entire perimeter of the central nervous system (CNS).

CSF also flows through the ventricles (One of four hollow spaces within the brain that

contain cerebrospinal fluid). Within the lining of the ventricles, the choroid plexus (The

lining of the ventricles, which secretes the cerebrospinal fluid ) converts material from the

nearby blood supply into cerebrospinal fluid. CSF is very similar in composition to the clear

plasma of the blood. It is a crystal clear, colorless solution that looks like water and is found

in the ventricular system and the subarachnoid space. It consists of water, small amounts of

protein, gases in solution (oxygen and carbon dioxide), sodium, potassium, magnesium, and

chloride ions, glucose, and a few white cells (mostly lymphocytes). Because of its weight and

composition,

CSF essentially floats the brain within the skull. This has several advantages. If you bump

your head, the fluid acts like a cushion to soften the blow to your brain. In addition, neurons

respond to appropriate input, not to pressure on the brain. Pressure can often cause neurons to

fire in maladaptive ways, such as when a tumor causes seizures by pressing down on a part of

the brain. By floating the brain, the cerebrospinal fluid prevents neurons from responding to

pressure and providing false information.

The CSF, formed primarily by a combination of capillary filtration and active epithelial

secretion, serves two major functional roles:

1. Physical Support. By acting as a “water jacket” surrounding the brain and by providing

buoyancy for it, the CSF protects, supports, and keeps the brain afloat in a sea of fluid.

2. Homeostasis. The CSF of the ventricles and the subarachnoid space comprises a pool to

which some of the endogenous water-soluble products, including unwanted substances, drain

by diffusion from extracellular fluids of the brain to the ventricles and subarachnoid space.

CSF circulates through the central canal (The small midline channel in the spinal cord that

contains cerebrospinal fluid) of the spinal cord and four ventricles in the brain: the two lateral

ventricles, one in each hemisphere, and the third and fourth ventricles in the brainstem. The

fourth ventricle is continuous with the central canal of the spinal cord, which runs the length

of the cord at its midline.

Below the fourth ventricle, there is a small opening that allows the CSF to flow into the

ubarachnoid space that surrounds both the brain and spinal cord. New CSF is made

constantly, with the entire supply being turned over about three times per day. The old CSF is

reabsorbed into the blood supply at the top of the head. Because there are several narrow

sections in this circulation system, blockages sometimes occur. This condition, known as

hydrocephalus, is apparent at birth in affected infants. Hydrocephalus literally means water

on the brain. Left untreated, hydrocephalus can cause mental retardation, as the large quantity

of CSF prevents the normal growth of the brain. Currently, however, hydrocephalus can be

treated by the installation of a shunt to drain off excess fluid. When the baby is old enough,

surgery can be used to repair the obstruction. Some adults also experience blockages of the

CSF circulatory system. They, too, must be treated with shunts and/or surgery.



CSF moves through a completely self-contained and separate circulation system that never

has direct contact with the blood supply. Because the composition of the cerebrospinal fluid

is often important in diagnosing diseases, a spinal tap is a common, though extremely

unpleasant, procedure. In a spinal tap, the physician withdraws some fluid from the

subarachnoid space through a needle.

Review questions

1. Cerebellum is a part of

a) Midbrain

b) Hindbrain

c) Parietal lobe

d) Temporal lob

2. ------------means little brain

a) Medulla

b) Hypothalamus

c) Cerebellum

d) Thalamus

3. Circadian rhythm is controlled by

a) Thalamus

b) Neuroglia

c) Hypothalamus

d) amygdala

4. Explain the spinal reflex arc.

5. What are the two structures contained in metencephalon?, describe their function.

6. Give a short note on neuroglia.

7. What are the functional roles of CSF.

8. Explain the differing roles of four lobes of cortex in human functioning.



Module III

SENSORY PROCESSES

Primates, including human beings, experience a dramatically colorful world. In contrast, the

visual world of dogs features only the blues, yellows, and grays. Some nocturnal mammal

species probably do not see color at all. Our ability to see many colors helps us to find better

food such as younger or tastier leaves of fruits.

Visual perception involves active processes, which depend upon bottom-up factors (signals

arising from light falling on the eye) and top-down factors (e.g. memories and expectations).

A set of cells at the back of the eye converts light energy into electrical signals. This provides

the bottom-up factor common to all visual perception and action. Cells in the eye also do

some processing of information as well as transmitting it towards the brain.

The figure exemplifies some processing that the CNS does on raw sensory input. It

distinguishes between what is detected by the early stages of the visual system and perception

of the world. In the case of the Kanizsa triangle people perceive a white triangle but it is

illusory. If you examine the physical stimulus, you will see that there are no full sides to the

triangle. Rather, any sides ‘seen’ are extrapolations by the brain. This illustrates that

perception is much more than seeing exactly what stimulates the eye. It is also dependent

upon context and involves extrapolation beyond the physical image.

Light is the stimulus in case of vision. Light and sound are characterized by

wavelength and frequency. (However, whereas sound needs a medium through which to pass,

e.g. air, light can pass through a vacuum.) Corresponding to variations in wavelength of light

(physical stimulus) is the spectrum of colours that we perceive (the psychological

dimension). For example, we usually describe light having a wavelength of 690 nanometres

(nm) as red. Strictly speaking, red is a psychological quality, albeit one usually associated

with a particular physical stimulus. The light emitted by, or reflected from, an object in part

determines perception. By exploiting this input and stored information, the visual system

extracts what is invariant about the world.

Characteristics of Light

Visible light, or the energy we can see, is one form of electromagnetic radiation produced

by the sun. Electromagnetic radiation can be described as moving waves of energy.

Wavelength, or the distance between successive peaks of waves, is decoded by the visual

system either as color or as shades of gray. The amplitude of light waves refers to the height

of each wave, which is translated by the visual system as brightness. Large-amplitude waves

are perceived as bright, and low amplitude waves are perceived as dim. Electromagnetic

radiation can also be described as the movement of tiny, indivisible particles known as

photons. Photons always travel at the same speed (the so-called speed of light), but they can

vary in the amount of energy they possess. It is this variation in energy levels among photons

that gives us waves with different wavelengths and amplitudes.

The Advantages of Light as a Stimulus



Electromagnetic energy and visible light in particular, has features that make it a valuable

source of information. First, electromagnetic energy is abundant in our universe. Second,

because electromagnetic energy travels very quickly, there is no substantial delay between an

event and an organism’s ability to see the event. Finally, electromagnetic energy travels in

fairly straight lines, minimizing the distortion of objects. What we see is what we get,

literally.

The Electromagnetic Spectrum

The light from the sun contains a mixture of wavelengths and appears white to the human

eye. Placing a prism in sunlight will separate the individual wavelengths, which we see as

individual colors. Light shining through water droplets is affected the same way, producing

the rainbows we enjoy seeing after a rainstorm. Light that is visible to humans occupies a

very small part of the electromagnetic spectrum. The range of electromagnetic energy visible

to humans falls between 400 and 700 nanometers (nm). Shorter wavelengths, approaching

400 nm, are perceived by humans as violet and blue, whereas longer wavelengths,

approaching 700 nm, are perceived as red.

Absorption, Reflection, and Refraction

Objects can absorb, reflect, or refract electromagnetic radiation. In some cases, an object’s

physical characteristics will absorb or retain certain wavelengths. In other cases, light is

reflected from the surface of objects, or bent back toward the source. Most of the light energy

entering the eye has been reflected from objects in the environment. Absorption and

reflection determine the colors we see. The color of an object is not some intrinsic

characteristic of the object but, rather, the result of the wavelengths of light that are

selectively absorbed and reflected by the object. Instead of saying that my sweater is red, it is

more accurate to say that my sweater has physical characteristics that reflect long

wavelengths of visible light (perceived as red) and absorb shorter wavelengths. “Light-

colored” clothing keeps us cooler because materials perceived as white or light-colored

reflect more electromagnetic energy. “Dark” clothing keeps us warmer because these

materials absorb more electromagnetic energy. You can easily demonstrate this concept by

timing the melting of ice cubes in sunlight when one ice cube is covered by a white piece of

cloth and the other by a black piece of cloth.

Air and water refract, or change the direction of, traveling waves of light in different ways.

Because human eyes developed for use in air, they don’t work as well underwater. To see

clearly underwater, we need goggles or a face mask to maintain a bubble of air next to the

eye. Consequently, even though our bodies are underwater, our eyes remain exposed to light

that has been refracted by air, and they function normally.

Features of Light as a Stimulus:

Feature Significance

Wavelength Distance between peaks of the waves; determines the perceived color of objects.

Amplitude Height of the wave; determines brightness.

Absorption Objects that absorb more visible light energy appear dark-colored.

Reflection

Objects that reflect more visible light energy appear light-colored. We perceive the

reflected wavelengths as the color of an object.

Refraction Refraction, as by air and water molecules, changes the direction of light.



General Principles of Perception

Each receptor is specialized to absorb one kind of energy and transduce (convert) it into an

electrochemical pattern in the brain. For example, visual receptors can absorb and sometimes

respond to as little as one photon of light and transduce it into a receptor potential, a local

depolarization or hyperpolarization of a receptor membrane. The strength of the receptor

potential determines the amount of excitation or inhibition the receptor delivers to the next

neuron on the way to the brain. After all the information from millions of receptors reaches

the brain, the brain make sense to it.

From Neuronal Activity to Perception

The main point is that our brain’s activity does not duplicate the objects that we see. For

example, when we see a table, the representation of the top of the table does not have to be on

the top of our retina or on the top of our head. Consider an analogy to computers: When a

computer stores a photograph, the top of the photograph does not have to be toward the top of

the computer’s memory bank.

Visual Attention

Of all the stimuli striking your retina at any moment, you attend to only a few. A stimulus can

grab our attention by its size, brightness, or movement, but we can also voluntarily direct our

attention to one stimulus or another in what is called a “top-down” process that is, one

governed by other cortical areas, principally the frontal and parietal cortex. The difference

between attended and unattended stimuli pertains to the amount and duration of activity in a

cortical area. While we are increasing our brain’s response to the attended stimulus, the

responses to other stimuli decrease. Also, if we are told to pay attention to color or motion,

activity increases in the areas of our visual cortex responsible for color or motion perception.

In fact, activity increases in those areas even before the stimulus. Somehow the instructions

prime those areas, so that they can magnify their responses to any appropriate stimulus.

Law of Specific Nerve Energies

An important aspect of all sensory coding is which neurons are active. Impulses in one

neuron indicate light, whereas impulses in another neuron indicate sound. In 1838, Johannes

Müller described this insight as the law of specific nerve energies. Müller held that whatever

excites a particular nerve establishes a special kind of energy unique to that nerve. In modern

terms, any activity by a particular nerve always conveys the same kind of information to the

brain. We can state the law of specific nerve energies another way: No nerve has the option

of sending the message “high C note” at one time, “bright yellow” at another time, and

“lemony scent” at yet another. It sends only one kind of message—action potentials. The

brain somehow interprets the action potentials from the auditory nerve as sounds, those from

the olfactory nerve as odors, and those from the optic nerve as light. Admittedly, the word

“somehow” glosses over a deep mystery, but the idea is that some experiences are given. You

don’t have to learn how to perceive green; a certain pattern of activity in particular produces

that experience automatically.

Here is a demonstration: If you rub your eyes, you may see spots or flashes of light even in a

totally dark room. You have applied mechanical pressure, but that mechanical pressure

excited visual receptors in your eye; anything that excites those receptors is perceived as

light.



Role of Arousal and Attention in Visual Perception

It is well established that we continuously filter “the wheat from the chaff” of incoming

sensory information, selectively allocating attention to what is important to our well-being,

and suppressing distracting information. By continuously focusing and refocusing our

“attentional spotlight”, we prioritize what we process. Research over the past decade has

demonstrated that emotion is an important factor in focusing attention. Researches have

contributed to a growing body of evidence suggesting that, from early childhood onward,

emotionally arousing stimuli require less attention in initial stages of processing, and

subsequently capture and maintain more attentional resources for sustained processing, than

neutral stimuli. Recent research have also demonstrated that one’s emotional state can

modulate the extent of perceptual processing in the visual cortex, and that producing

emotional facial expressions literally reduces or increases what we take in from the world.

Attention is generally viewed as a limited resource that must be allocated just as physical

resources are allocated to facilitate an organism’s survival. Indeed, attentional and metabolic

resources may be linked, as information capturing attention and awareness should be of

sufficient biological importance to also harness metabolic resources. Such allocation of

resources requires integration of incoming information with explicit goals and the ongoing

requirements of bodily systems. Indeed, attentional filtering has often been discussed in terms

of either “top-down” or “bottom-up” processes. Top-down processes involve a prioritizing of

visual information that is shaped by expectations, effortful attentional processes, and explicit

goals. For example, in the context of a laboratory experiment, top-down processing might

involve holding the task rules in mind and, based on these rules, attending to one type of

stimulus while ignoring another. In daily life, top-down processing allows us attend to traffic

while driving, ignoring an otherwise interesting conversation or beautiful scenery. It allows

us to attend to a boring lecture for the sake of a grade or collegiality rather than more

immediately rewarding thoughts of lunch or the attractive person seated nearby.

Top-down and bottom-up processes typically interact with each other, so that strongly salient

stimuli can capture attentional resources at the expense of explicit goals, yet explicit goals

modulate the capture of attention. Indeed, evidence suggests that processing of even the most

basic qualities of a stimulus (orientation, colour, motion) is reduced during inattention, and

the most motivationally salient stimulus will require some degree of attention for processing.

Research suggests that the amygdala is a key hub for integration of topdown and bottom-up

processes – mediating the interface between the internal state of the organism and the

incoming stream of visual information from the world.

A body of research on “motivated attention” has investigated the hypothesis that emotionally

salient stimuli enjoy privileged perceptual processing – that emotionally arousing images are

processed more rapidly and more vividly than neutral images, and require less top-down

attention to reach awareness. The body of research has consistently demonstrated that

emotionally salient images elicit higher levels of activation in the visual cortices than neutral

stimuli. Functional magnetic resonance imaging (fMRI) and positron emission tomography

(PET) studies have found that, in several regions of visual cortex, positive and negative

emotionally arousing scenes elicit greater activation than neutral ones. Event-related potential

(ERP) studies suggest that the effect of motivational salience – the importance or relevance of

an image to one’s well-being – on visual processing is also rapid, occurring within 170 ms of

stimulus onset for faces, and between 200 and 300 ms for complex scenes.



The attentional blink studies established that emotional stimuli have privileged access to

awareness when attentional resources are stressed. Further, this advantage is dependent on the

amygdala. The fMRI studies provided evidence for an alternate route model of visual

processing, including a fast route for rapid transmission of salience-related visual information

to the amygdala, and showed that the amygdala is more sensitive to arousal/intensity than

valence. Also emotionally arousing images elicit greater visual cortex activation, enjoy

privileged access to awareness, and are associated with the subjective experience of sensory

vividness, there is also evidence of individual and developmental differences in biases

towards positive vs. negative stimuli.

The Structures and Functions of the Visual System

Animals have different solutions for the exact placement of the eyes in the head, in case of

human, having eyes in the front of the head provides superior depth perception that is

advantageous for hunting.

Protecting the Eye

A number of mechanisms are designed to support and protect the eye. Eyes are located in the

bony orbit of the skull, which can deflect many blows. In addition, the eye is cushioned by

fat. When people are starving, they show a characteristic hollow-eyed look due to the loss of

this important fat cushion.

A second line of defense is provided by the eyelids. The eyelids can be opened and closed

either voluntarily or involuntarily. Involuntary closure of the eyelids, or a blink, both protects

the eye from incoming objects and moistens and cleans the front of the eye. Under most

circumstances, we blink about once every four to six seconds.

Tears, another feature of the eyes’ protective system, are produced in the lacrimal gland at the

outer corner of each eye. The fluid is composed primarily of water and salt but also contains

proteins, glucose, and substances that kill bacteria. Tears flush away dust and debris and

moisten the eye so that the eyelids don’t scratch the surface during blinks. Tears that are shed in

response to emotional events contain about 24 percent more protein than tears responding to irritants,

such as onions, but the exact purposes of this difference remain unknown.

The Anatomy of the Eye

The human eye is roughly a sphere with a diameter of about 24 mm, just under one inch, and

individual variations are very small, no more than 1 or 2 mm. Newborns’ eyes are about 16–

17 mm in diameter (about 6/10 of an inch) and attain nearly their adult size by the age of 3

years. The “white” of the eye, or sclera, provides a tough outer covering that helps the fluid-

filled eyeball maintain its shape. The major anatomical features of the eye are illustrated in

Figure.



Light entering the eye first passes through the outer layer, or cornea. Because the cornea is

curved, it begins the process of bending or refracting light rays to form an image in the back

of the eye. The cornea is actually a clear, blood vessel–free extension of the sclera. Special

proteins on the surface of the cornea discourage the growth of blood vessels. The lack of a

blood supply and the orderly alignment of the cornea’s fiber structure make it transparent. As

living tissue, the cornea still requires nutrients, but it obtains them from the fluid in the

adjacent anterior chamber rather than from blood. This fluid is known as the aqueous

humor. The cornea has the dubious distinction of having a greater density of pain receptors

than nearly any other part of the body.

After light travels through the cornea and the aqueous humor of the anterior chamber, it next

enters the pupil. The pupil is actually an opening formed by the circular muscle of the iris,

which comes from the Greek word for rainbow. The iris adjusts the opening of the pupil in

response to the amount of light present in the environment. Pupil diameter is also affected by

your emotional state through the activity of the autonomic nervous system. The color of the

iris is influenced primarily by its amount of melanin pigment, which varies from brown to

black, in combination with the reflection and absorption of light by other elements in the iris

such as its blood supply and connective tissue. The irises of people with blue or gray eyes

contain relatively less melanin than the irises of people with brown eyes. Consequently, some wavelengths are reflected and scattered from the blue or gray iris in ways that are similar to

light in the atmosphere, which is also perceived as blue. Green eyes contain a moderate

amount of melanin, and brown or black eyes contain the greatest amounts. “Amber” eyes,

brown eyes with a golden look, contain an additional yellowish pigment.

Directly behind the iris is the lens. The lens helps focus light on the retina in the back of the

eye and functions very much like the lens of a camera. Like the cornea, the lens is transparent

due to its fiber organization and lack of blood supply. It, too, depends on the aqueous humor



for nutrients. Muscles attached to the lens allow us to adjust our focus as we look at objects

near to us or far away. This process is called accommodation.

The major interior chamber of the eye, known as the vitreous chamber, is filled with a

jellylike substance called vitreous humor. Unlike the aqueous humor, which circulates and is

constantly renewed, the vitreous humor we have today is the same vitreous humor with which

we were born. Under certain circumstances, we can see floaters, or debris, in the vitreous

humor, especially as we get older. Finally, light will reach the retina at the back of the eye.

The image that is projected on the retina is upside down and reversed relative to the actual

orientation of the object being viewed. We can duplicate this process by looking at our image

in both sides of a shiny spoon. In the convex, or outwardly curving side, we will see our

image normally. If we look at the concave side, we will see our image as our retina sees it.

The visual system has no difficulty decoding this image to give us a realistic perception of the

actual orientation of objects.

The word retina comes from the Latin word for “fisherman’s net.” As the name implies, the

retina is a thin but complex network containing special light-sensing cells known as

photoreceptors. The photoreceptors are located in the deepest layer of the retina. Before

light can reach the photoreceptors, it must pass through the vitreous humor, numerous blood

vessels, and a number of neural layers. We don’t normally see the blood vessels and neural

layers in our eyes due to an interesting feature of our visual system. Our visual system

responds to change and tunes out stimuli that remain constant. Because the blood vessels and

neural layers are always present, we don’t “see” them. The blood vessels serving the eye and

the axons forming the optic nerve exit the back of the eye in a place known as the optic disk.

This area does not contain any photoreceptors at all, which gives each eye a blind spot. Under

normal conditions, we don’t notice these blind spots. Toward the middle of the retina, there is

a yellowish area about 6 mm in diameter that is lacking large blood vessels. This area is

known as the macula, from the Latin word for “spot.” When we stare directly at an object,

the image of that object is projected by the cornea and lens to the center of the macula. As a

result, we say that the macula is responsible for central vision as opposed to peripheral

vision. Peripheral vision is our ability to see objects that are off to the side while looking

straight ahead.

In the very center of the macula, the retina becomes thin and forms a pit. The pit is known as

the fovea, which is about 1.8 mm in diameter. In humans, the fovea is particularly specialized

for detailed vision and contains only one type of photoreceptor, the cones, which permit

vision in bright light. Primates, including humans, are the only mammals whose foveas

contain only cones. Other mammals, such as cats, have retinal areas that are similar to a

fovea, but these contain both cones and the photoreceptors known as rods, which allow vision

in dim light.

The retina is embedded in a pigmented layer of cells called the epithelium. These cells

support the photoreceptors and absorb random light. Because of this absorption of random

light, the interior of the eye looks black when seen through the pupil. When a bright light

source, such as a camera flash, is pointed directly at the eye, we see the reflection of the true

red color of the retina that results from its rich blood supply. The shine we see reflected from

the eyes of some animals at night has a different origin. Although it is normally advantageous

to reduce reflection in the eye, the epithelium of some nocturnal animals, such as the cat,

contains a white compound that acts more like a mirror. By reflecting light through the eye a

second time, the odds of perceiving very dim lights at night are improved.



The Layered Organization of the Retina

Although it is only 0.3 mm thick, the retina contains several layers of neurons and their

connections. Three layers of cell bodies are separated by two layers of axons and dendrites.

The retina’s first layer is the ganglion cell layer. Each ganglion cell has a single axon, and

these axons form the optic nerve as it leaves the retina. In the inner plexiform layer(“inner” in

this case refers to layers toward the center of the eye), the dendrites of ganglion cells form

connections with the amacrine and bipolar cells. The cell bodies of the bipolar, amacrine, and

horizontal cells are located in the inner nuclear layer. In the outer plexiform layer, the bipolar

cells form connections with horizontal cells and the photoreceptors. The outer nuclear area

contains the cell bodies of the photoreceptors.

Photoreceptors

The photoreceptors are the cells that convert light information into electrical signals.The two

types of photoreceptors, rods and cones, are named according to the shape of their outer

segments. The rods are used for seeing in low light conditions but are not capable of

encoding color information. The cones, by contrast, are used for color vision but only work

when the light level is higher. Hence, we are not able to see in color at night.

Rods:

A photoreceptor that responds to low levels of light but not to color. The human eye contains

about 120 million rods. Rods are responsible for scotopic vision, or the ability to see in dim

light. Rods have a long, cylinder-shaped outer segment containing large numbers of disks,

like a large stack of pancakes. These disks contain a photopigment known as rhodopsin. The

disks of the rods store large amounts of photopigment, which allows the rods to be about

1,000 times more sensitive to light than cones are. Under ideal conditions, the human eye can

see a single photon, or the equivalent of the light from a candle flame 30 miles away. The

cost for this extraordinary sensitivity to light is in the clarity and color of the image provided

by the rods. Rods do not provide any information about color, and they do not produce sharp

images. At night under starlight, our vision is no better than 20/200. An object seen at night

from a distance of only 20 feet would have the same clarity as the object viewed from a

distance of 200 feet at high noon.

Cones:

A photoreceptor that operates in bright conditions and responds differentially to color. There

are only about 6 million cones in the human eye. Cones are responsible for photopic vision,

or vision in bright light. Photopic vision is sensitive to color and provides images with

excellent clarity. There are three different kinds of cone that respond to different ranges of

wavelengths of light. Roughly, these wavelengths correspond to red, green or blue light. The

outer segment of cones is shorter and more pointed than that of the rods. Cones store one of

three different photopigments in a folded membrane rather than in disks, as the rods do.

Because cones work best in bright light, we do not really see color at night. We might know

that we’re wearing a green sweater, and, in a sense, we may think it looks green as a result of

that memory, but we require fairly bright light and the action of our cones to truly see the

color.

As we move from the fovea to the outer margins of the primate retina, the

concentration of rods increases and the number of cones decreases. As a result, the center of

the retina is superior for seeing fine detail and color in the presence of bright light, whereas

the periphery is superior for detecting very dim light. Because of this uneven distribution of



rods and cones across the retina, we see better in dim light when we do not look directly at an

object.

Differences between Rods and Cones

Rods and cones respond to a wide range of wavelengths, but their photopigments each have

different peak sensitivities. There are three classes of cones. The so-called blue or short-

wavelength cones, which contain the photopigment cyanolabe, respond maximally to

wavelengths of 419 nm (violet). The green, or middle-wavelength cones, containing

chlorolabe, have peak responses to 531 nm (green), and the red or long-wavelength cones,

containing erythrolabe, peak at 558 nm (yellow). The rhodopsin in rods absorbs photons most

effectively at wavelengths of 502 nm (a bluish-green). Rods and cones need different

amounts of light to respond. Rhodopsin breaks apart when relatively little light has been

absorbed, which explains in part the rods’ great sensitivity to low levels of light. The cone

photopigments aremuch more resistant to breaking apart and will do so only in the presence

of bright light. This is one of the reasons that cones are active in daylight rather than during

the night.

Activity in the photoreceptors:

The way in which the rods and cones convert light into electrical signals is by way of

complex chemical processes. Rods contain a pigment called rhodopsin that becomes

bleached when exposed to light. This bleaching leads to a cascade of events that result in a

reduced release of glutamate (a neurotransmitter) by the rod cell. This is the signal for the

presence of light. Note that this signal is transmitted through inhibition rather than excitation.

The process in the cones is very similar, except that the opsins in each cone are different and

this probably accounts for the different wavelengths of light responded to. As well as being

activated by light, the rods and cones can activate each other. One way in which this can

occur is via activation of the horizontal cells. This system serves to reduce the responses of

neighboring cells exposed to diffuse stimulation and to heighten the responses of cells at

places where the light level changes.

Blind spot:

The ganglion cell axons band together to form the optic nerve (or optic tract), an axon bundle

that exits through the back of the eye. The point at which it leaves (which is also where the

major blood vessels leave) is called the blind spot. . As this region is densely packed with

neurons, there is no room for photoreceptors. Hence, this region of the retina is literally blind.

The brain interpolates the blind spot based on surrounding detail and information from the

other eye, so the blind spot is not normally perceived.

The first documented observation of the phenomenon was in the 1660s by Edme Mariotte in

France. At the time 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. The blind spot is located about 12–15° nasal and 1.5° below

the horizontal and is roughly 7.5° high and 5.5° wide.

Every person is therefore blind in part of each eye. You can demonstrate your own blind spot

by following the test.

Demonstration of the blind spot

R

L



Instructions: Close one eye and focus the other on the appropriate letter (R for right or L for left). Place your

eye a distance from the screen approximately equal to 3× the distance between the R and the L. Move your eye

towards or away from the screen until you notice the other letter disappear. For example, close your right eye,

look at the "L" with your left eye, and the "R" will disappear.

Some people have a much larger blind spot because glaucoma has destroyed parts of the optic

nerve. Generally, they do not notice it any more than you notice your smaller one.

The Fovea

Perhaps the most important structural area on the retina is the fovea. The fovea is the area on

the retina where we have the best spatial and color vision. When we look at, or fixate, an

object in our visual field, we move our head and eyes such that the image of the object falls

on the fovea. As you are reading this text, you are moving your eyes to make the various

words fall on your fovea as you read them. To illustrate how drastically spatial acuity falls off

as the stimulus moves away from the fovea, try to read preceding text in this paragraph while

fixating on the period at the end of this sentence. It is probably difficult, if not impossible, to

read text that is only a few lines away from the point of fixation. The fovea covers an area

that subtends about two degrees of visual angle in the central field of vision. To visualize two

degrees of visual angle, a general rule is that the width of your thumbnail, held at arm’s

length, is approximately one degree of visual angle.

Visual pathways:

The visual pathway starts at the eye with the photoreceptors. These send an input to cells

called retinal ganglion cells in the retina, at the back of the eye. The bipolar and ganglion

cells provide a direct pathway for information from the photoreceptors to the brain that is

modified by input from the horizontal and amacrine cells. The horizontal and amacrine cells

integrate information across the surface of the retina. You can think of the

photoreceptorbipolar- ganglion connections as running perpendicular to the back of the eye,

whereas the horizontal and amacrine connections run parallel to the back of the eye. From

here, the pathway goes to the lateral geniculate nucleus and then to the visual cortex.



The most important thing to note about the figure is the way in which light travels from the

two eyes to the visual cortex. If we trace light that comes in from the left of center (the left

visual field) when we are looking straight ahead, we see that the light hits the inside (nasal)

part of the left retina and the outside (lateral) part of the right retina. If we now follow the

nasal pathway, we see that it crosses the midline at the optic chiasma. From here, the

information travels to the right lateral geniculate nucleus and on to the right visual cortex.

The path of the nasal visual field is described as contralateral. If we now follow the lateral

pathway, we see that it does not cross the midline at the optic chiasma but continues on the

same side to the right lateral geniculate nucleus. From here, the information travels on to the

right visual cortex. This path is described as ipsilateral. The net result is that information

from the left of center when we are looking straight ahead (the left visual field), all goes only

to the right hemisphere, even though the light goes to both eyes. The same principle is true

for information from the right of center (the right visual field), only all of the information

goes to the left hemisphere. The exception to this is that information from objects we are

directly focusing on (called foveal vision) goes to both hemispheres. The following sessions

discuss these in detail.

Horizontal Cells

The horizontal cells are located in the inner nuclear layer. They receive input from the

photoreceptors and provide output to another type of cell in the inner nuclear layer, the

bipolar cell. The major task of horizontal cells is to integrate information from photoreceptors

that are close to one another. The spreading structure of the horizontal cell is well suited to

this task. Like the photoreceptors, horizontal cells communicate through the formation of

graded potentials rather than of action potentials. About half the bipolar cells in the human

retina are on-center (Cells that depolarize when light hits the center of their receptive field are

called on-center. Cells that hyperpolarize when light hits the center of their receptive field are

called off-center.), and the other half are off-center. This arrangement of the receptive fields



is referred to as an antagonistic centersurround organization. The response of a bipolar

cell depends on the amount of light falling on its center relative to the amount of light falling

on its surround. It is called antagonistic because light falling on the center of the receptive

field always has the opposite effect on the cell’s activity from light falling on the surround.

Photoreceptors and horizontal cells serving the center (doughnut hole) and surround (the

doughnut) compete with each other to activate the bipolar cell in a process known as lateral

inhibition. Lateral means that the process occurs across the surface of the retina. In lateral

inhibition, active photoreceptors and horizontal cells limit the activity of neighboring, less

active cells. This produces a sharpening, or exaggeration, of the bipolar cells’ responses to

differences in light falling on adjacent areas. Through lateral inhibition, bipolar cells begin to

identify the boundaries of a visual stimulus by making comparisons between light levels

falling in adjacent areas of the retina. The message sent by the bipolar cells is “I see an edge

or boundary.”

Amacrine Cells

Amacrine cells, also located in the inner nuclear layer, form connections with bipolar cells,

ganglion cells, and other amacrine cells. In addition to integrating visual messages, amacrine

cells process movement.

Ganglion Cells

Ganglion cells receive input from bipolar and amacrine cells. Unlike the interneurons and

photoreceptors discussed so far, ganglion cells form conventional action potentials. However,

ganglion cells are never completely silent. The presence of light simply changes the ganglion

cells’ spontaneous rate of signaling. The axons of ganglion cells leave the eye and form the

optic nerve, which travels to higher levels of the brain. The human eye has approximately 1

million ganglion cells, yet they must accurately communicate input from about 126 million

photoreceptors. The ganglion cells accomplish this editing task through the organization of

their receptive fields.

Ganglion Receptive Fields

Ganglion receptive fields show the same antagonistic center-surround organization that we

observed in the receptive fields of bipolar cells. Ganglion cells replicate the information

passed to them by the bipolar cells. On-center bipolar cells connect to on-center ganglion

cells, whereas off-center bipolar cells connect to off-center ganglion cells. Ganglion cell

receptive fields vary in size. Receptive fields vary from 0.01 mm in diameter in the macula to

0.5 mm (50 times larger) in the periphery. Cells with small receptive fields respond best to

fine detail.

The Three Types of Ganglion Cells

About 90 percent of human ganglion cells are P cells (P stands for parvocellular, or small

cells), 5 percent are M cells (M stands for magnocellular, or big cells), and the remaining 5

percent are K cells (K stands for koniocellular). M cells are larger than P cells and have

thicker, faster axons. M cells have larger receptive fields than P cells. M cells respond to

smaller differences in light between the center and surround, whereas P cells require a greater

difference. This implies that M cells respond to subtle differences of contrast such as when

viewing gray letters on a black background. P cells respond to larger differences in contrast

such as when viewing black letters on a white background. M cells, but not P cells, respond

to stimuli that are turned on and off rapidly, such as the flicker of a monitor or television

screen.

A final difference is that P cells respond only to lights of a particular color, whereas M cells

respond to light regardless of its color. K cells share most of the characteristics of P cells. M



cells are primarily responsible for providing information about large, lowcontrast, moving

objects, whereas P cells are responsible for information about smaller, high-contrast, colorful

objects. This distinction between magnocellular pathways and parvocellular pathways is

preserved up through some of the highest levels of cortical visual processing.

Optic Nerve Connections The ganglion cell axons exit the eye through the optic disk, forming an optic nerve leaving

each eye. The optic nerves preserve the organization of the retina. In other words, axons from

adjacent ganglion cells remain next to one another in the optic nerves. each human optic

nerve divides in half, with the outer half continuing to travel to the same side of the brain

(ipsilaterally) while the inner half crosses to the other side of the brain (contralaterally). This

partial crossing ensures that information from both eyes regarding the same part of the visual

field will be processed in the same places in the brain. If you hold your eyes steady by

looking at a focal point, information from the visual field to the left of the focal point will be

transmitted to the right hemisphere. Information from the visual field to the right of the focal

point will be transmitted to the left hemisphere. In humans, about 50 percent of the fibers

from each eye cross to the opposite hemisphere. In rabbits and other animals with eyes placed

on the side of the head, 100 percent of the fibers cross the midline to the opposite side.

Because each of a rabbit’s eyes sees a completely different part of the rabbit’s visual field,

there is no need for the rabbit to reorganize the input.

The optic nerves cross at the optic chiasm (named after its X shape, or chi in Greek). The

nerves continue past the optic chiasm as the optic tracts. Most of the axons in the optic tract

proceed to the thalamus, which in turn projects to the primary visual cortex of the brain.

However, a few axons leave the optic tract and synapse in the suprachiasmatic nucleus of the

hypothalamus, providing the light information used to regulate daily rhythms. About 10

percent of the axons in the optic tract project to the superior colliculus in the midbrain.

The Superior Colliculus

In many species, including frogs and fish, the superior colliculus is the primary brain

structure for processing visual information. Because humans have a cerebral cortex for this

purpose, they use the superior colliculus to guide movements of the eyes and head toward

newly detected objects in the visual field.

The Lateral Geniculate Nucleus of the Thalamus

Most of the remaining 90 percent of optic tract axons form synapses in the lateral geniculate

nucleus (LGN), located in the dorsal thalamus. the LGN is a layered structure that is bent in

the middle. In primates, including humans, the LGN has about the same area as a credit card,

but is about three times thicker. The LGN features six distinct stacked layers, numbered from

ventral to dorsal. Layers 1 and 2 (the most ventral layers) contain larger neurons than the

other four layers. These magnocellular layers receive input from the M cells in the retina.

The other four are referred to as parvocellular layers, which receive input from the P cells.

Between each of the six layers are very small neurons making up the koniocellular layers,

which receive input from the K cells. The LGN keeps information from the two eyes

completely separate.

Neurons in the LGN show the same antagonistic center-surround organization of receptive

fields that we observed in the retinal bipolar and ganglion cells. In LGN neurons, however,

the lateral inhibition between center and surround is much stronger than we observed among

retinal cells. This greater inhibition causes cells in the LGN to amplify or boost the contrast

between areas of light and dark. Surprisingly, the retina is not the major source of input to the

LGN. About 80 percent of the input to the LGN comes from the primary visual cortex,



located in the occipital lobe. The function of this input remains unclear. The LGN also

receives input from the brainstem reticular formation. This input allows an animal to modify

the flow of information to the cortex from the LGN based on its level of arousal and

alertness. The exact role of the LGN in visual processing is not well understood. The LGN

might modify input to the cortex based on arousal. The LGN might also organize or sort

information prior to sending it to the cortex.

Primary visual cortex Primary visual cortex is often referred to as striate cortex, due to its striped appearance.

Striate cortex, located in the occipital lobe, contains approximately 250 million neurons as

opposed to the 1 million neurons found in the LGN. The cortex in this area ranges from 1.5 to

2 mm in thickness, or about the height of the letter m on this page. Like other areas of cortex,

the striate cortex has six distinct layers. Compared with other areas of cortex, striate cortex is

relatively thicker in layers II and IV, which receive most of the input from other parts of the

brain. Layer IV receives input from the LGN. Striate cortex is thinner in layers III, V, and VI,

which contain output neurons that communicate with other parts of the brain.

Visual Analysis Beyond the Striate Cortex

The striate cortex begins, but by no means finishes, the task of processing visual input. At

least a dozen additional areas of the human cerebral cortex participate in visual processing.

Because these areas are not included in the striate cortex, they are often referred to as

extrastriate areas. These areas are also referred to as secondary visual cortex.

Next to the striate cortex is an area known as V2. If you stain V2 for cytochrome oxidase, a

pattern of stripes emerges. Alternating thick and thin stripes are separated by interstripe

regions. The thick stripes form part of the magnocellular pathway and project to a visual

pathway known as the dorsal stream. The dorsal stream travels from the primary visual

cortex toward the parietal lobe and then proceeds to the medial temporal lobe. The dorsal

stream, commonly referred to as the “where” pathway, specializes in the analysis of

movement, object locations, and the coordination of eyes and arms in grasping or reaching.

The thin stripes and interstripe regions of V2 project to another visual region known as V4,

continuing the parvocellular pathway. Area V4 participates in a second major pathway, the

ventral stream, which proceeds from the primary visual cortex to the inferior temporal lobe.

This second pathway, commonly referred to as the “what” pathway, specializes in object

recognition.

The Dorsal Stream Area MT

MT stands for the medial temporal lobe, appears to play an important role in the processing

of motion. Input to Area MT is primarily from the magnocellular pathways. Recall that the

cells in this pathway have large receptive fields and often show responses to rapidly changing

light conditions and direction of movement. Most of the cells in Area MT respond to

movement in a particular direction. Unlike previous motion detectors, however, Area MT

cells respond to movement across large regions of the visual field. Further processing of

motion occurs adjacent to Area MT in Area MST, which stands for the medial superior

temporal lobe. Tanaka and Saito (1989) found that Area MST neurons respond to stimulus

rotation, stimulus expansion, and stimulus contraction. These are very large, global types of

movement that do not produce consistent responses in other areas. Area MST helps us use

vision to guide our movements. Melvyn Goodale and his colleagues suggested that the dorsal

stream would be more accurately characterized as a “how” stream than as a “where” stream.

According to this view, not only does the dorsal stream tell us an object’s location, but it also

provides information about how to interact with an object. Patients with damage to the dorsal



stream can judge the orientation of an object, such as lining up a card with a slot, but are

unable to combine orientation and action to push the card through the slot.

The Ventral Stream

As the information from the primary cortex and Area V2 travels ventrally toward the

temporal lobe, we come to Area V4. The cells in this area have large receptive fields and

respond to both shape and color. Cells in Area V4 project to the inferior temporal lobe, or

Area IT. Cells in Area IT respond to many shapes and colors. In humans and monkeys, a

small section of Area IT known as the fusiform face area (FFA).

Visual acuity Visual acuity (VA) is acuteness or clearness of vision. It depends on optical and neural

factors, i.e., (i) the sharpness of the retinal focus within the eye, (ii) the intactness and

functioning of the retina, and (iii) the sensitivity of the interpretative faculty of the brain. Snellen chart is frequently used for visual acuity testing.

A common cause of low visual acuity is refractive error (ametropia), or errors in how the

light is refracted in the eyeball. Causes of refractive errors include aberrations in the shape of

the eyeball, the shape of the cornea, and reduced flexibility of the lens. In the case

of pseudomyopia, the aberrations are caused by muscle spasms. Too high or too low

refractive error (in relation to the length of the eyeball) is the cause of nearsightedness

(myopia) or farsightedness (hyperopia) (normal refractive status is referred to

as emmetropia). Other optical causes are astigmatism or more complex corneal irregularities.

These anomalies can mostly be corrected by optical means (such as eyeglasses, contact

lenses, laser surgery, etc.).

Neural factors that limit acuity are located in the retina or the brain (or the pathway leading

there). Examples for the first are a detached retina and macular degeneration, to name just

two. A common impairment amblyopia caused by incorrect nerve pathway function

connecting eye with brain is involved. In some cases, low visual acuity is caused by brain

damage, such as from traumatic brain injury or stroke. When optical factors are corrected for,

acuity can be considered a measure of neural well-functioning.

Visual acuity is typically measured while fixating, i.e. as a measure of central (or foveal)

vision, for the reason that it is highest there. However, acuity in peripheral vision can be of

equal (or sometimes higher) importance in everyday life. Acuity declines towards the

periphery in an inverse-linear (i.e. hyperbolic) fashion.

Normal visual acuity is commonly referred to as 20/20 vision (even though acuity in

normally sighted people is generally higher), the metric equivalent of which is 6/6 vision. At

20 feet or 6 meters, a human eye with nominal performance is able to separate contours that

are approximately 1.75 mm apart. A vision of 20/40 corresponds to lower

than nominal performance, a vision of 20/10 to better performance.

20/20 is normal (daylight) vision. In low light (i.e., scotopic) vision, spatial resolution is

much lower. This is due to spatial summation of rods, i.e. a number of rods merge into a

bipolar cell, in turn connecting to a ganglion cell, and the resulting unit for resolution is large

(and acuity small). Visual acuity is much better in bright light than dim light, the former

reaching 2 with a bright center and surrounding, the latter perhaps being 0.4 (25 arc minutes).

In this case, the stimulus is 1.7 inches (4.3 cm) seen at a distance of 20 feet (6.1 m).

The maximum angular resolution of the human eye at a distance of 1 km is typically 30 to

60 cm. This gives an angular resolution of between 0.02 to 0.03 degrees, which is roughly 1.2

- 1.8 arc minutes per line pair, which implies a pixel spacing of 0.6-0.9 arc minutes. 20/20



vision is defined as the ability to resolve two points of light separated by a visual angle of one

minute of arc, or about 320-286 pixels per inch for a display on a device held 10 to 12 inches

from the eye (since by the Nyquist theorem two points are needed to resolve a space).

Thus, visual acuity, or resolving power (in daylight, central vision), 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 themacula having the highest density of cone photoreceptor cells (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.

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 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.

Any relatively sudden decrease in visual acuity is always a cause for concern. Common

causes of decreases in visual acuity are cataracts(Clouding of the lens) and scarredcorneas,

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.

Color Vision Color vision is the ability of an organism or machine to distinguish objects based on

the wavelengths (or frequencies) of the light they reflect, emit, or transmit. Colors can be

measured and quantified in various ways; indeed, a person's perception of colors is a

subjective process whereby the brain responds to the stimuli that are produced when

incoming light reacts with the several types of cone cells in the eye. In essence, different

people see the same illuminated object or light source in different ways. In the human visual

system, the shortest visible wavelengths, about 350 nm, are perceived as violet; progressively

longer wavelengths are perceived as blue, green, yellow, orange, and red, near 700 nm. The

“visible” wavelengths vary depending on a species’ receptors. For example, birds’ receptors

enable them to see shorter wavelengths than humans can. That is, wavelengths that we

describe as “ultraviolet” are simply violet to birds. (Of course, we don’t know what their

experience looks like.) In general, small songbirds see further into the ultraviolet range than

do predatory birds such as hawks and falcons. Many songbird species have taken advantage

of that tendency by evolving feathers that strongly reflect very short wavelengths, which can

be seen more easily by their own species than by predators.



Discrimination among colors poses a special coding problem for the nervous system. A cell

in the visual system, like any other neuron, can vary only its frequency of action potentials or,

in a cell with graded potentials, its membrane polarization. If the cell’s response indicates

brightness, then it cannot simultaneously signal color. Conversely, if each response indicates

a different color, the cell cannot signal brightness. The inevitable conclusion is that no single

neuron can simultaneously indicate brightness and color; our perceptions must depend on a

combination of responses by different neurons.

Coding for color:

The three types of cones clearly serve color vision. Since there is only one type of rod, the

rod system is incapable of color vision. Coding for color not only occurs through the

differential responding of the three types of cone and by the opponent processes in the lateral

geniculate nucleus. It also occurs in the visual cortex. In layers 2 and 3 of area V1 there are

cells called blobs and interblobs that code for color rather than for orientation. They receive

their input from the parvocellular layers of the lateral geniculate nucleus. Blobs are also

opponent-color cells that are either red–green or blue–yellow. Even so, color perception is a

global perception of the whole visual scene. The responses of these wavelengthspecific cells

do not give us the perceived color. This global perception occurs in area V4. Here, cells only

respond to one narrow band of wavelength. Indeed, damage to V4 can totally eliminate the

ability to perceive color. From V4, information is sent to the temporal lobe for further color

processing. Presumably this is the start of the integration of color information with other

types of information – memory, language, and so on. Scientists of the 1800s proposed two

major interpretations of color vision: the trichromatic theory and the opponent-process

theory.

Wavelength and hue detection

Isaac Newton discovered that white light splits into its component colors when passed

through a dispersive prism. Newton also found that he could recombine these colors by

passing them through a different prism to make white light.

The characteristic colors are, from long to short wavelengths (and, correspondingly, from low

to high frequency), red, orange, yellow, green, cyan, blue, and violet. Sufficient differences in

wavelength cause a difference in the 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 longer red and shorter blue wavelengths. Although the human 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.

In very low light levels, vision is scotopic: light is detected by rod cells of the retina. Rods are

maximally sensitive to wavelengths near 500 nm, and play little, if any, role in color vision.

In brighter light, such as daylight, vision is photopiv: light is detected by cone cells which are

responsible for color vision. Cones are sensitive to a range of wavelengths, but are most

sensitive to wavelengths near 555 nm. Between these regions, mesopic vision comes into

play and both rods and cones provide signals to the retinal ganglion cells. The shift in color

perception from dim light to daylight 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 by mixing just a pair of

complementary colors such as blue and yellow.

The Trichromatic (Young-Helmholtz) Theory



People can distinguish red, green, yellow, blue, orange, pink, purple, greenish-blue, and so

forth. If we don’t have a separate receptor for every distinguishable color, how many types do

we have? The first person to approach this question fruitfully was an amazingly productive

man named Thomas Young (1773–1829). Young was the first to decipher the Rosetta stone,

although his version was incomplete. He also founded the modern wave theory of light,

defined energy in its modern form, founded the calculation of annuities, introduced the

coefficient of elasticity, discovered much about the anatomy of the eye, and made major

contributions to many other fields. Previous scientists thought they could explain color by

understanding the physics of light. Young was among the first to recognize that color

required a biological explanation. He proposed that we perceive color by comparing the

responses across a few types of receptors, each of which was sensitive to a different range of

wavelengths.

This theory, later modified by Hermann von Helmholtz, is now known as the trichromatic

theory of color vision, or the Young-Helmholtz theory. According to this theory, we

perceive color through the relative rates of response by three kinds of cones, each kind

maximally sensitive to a different set of wavelengths (Trichromatic means “three colors.”).

For deciding the number three, he collected psychophysical observations, reports by

observers concerning their perceptions of various stimuli. He found that people could match

any color by mixing appropriate amounts of just three wavelengths. Therefore, he concluded

that three kinds of receptors—we now call them cones—are sufficient to account for human

color vision.

According to the trichromatic theory, we discriminate among wavelengths by the ratio of

activity across the three types of cones. For example, light at 550 nm excites the medium-

wavelength and long wavelength receptors about equally and the short-wavelength receptor

almost not at all. This ratio of responses among the three cones determines a perception of

yellow. More intense light increases the activity of all three cones without much change in

their ratio of responses. As a result, the color appears brighter but still yellow. When all three

types of cones are equally active, we see white or gray. Note that the response of any one

cone is ambiguous. For example, a low response rate by a middle wavelength cone might

indicate low-intensity 540-nm light or brighter 500-nm light or still brighter 460- nm light. A

high response rate could indicate either bright light at 540 nm or bright white light, which

includes 540 nm. The nervous system can determine the color and brightness of the light only

by comparing the responses of the three types of cones.

Given the desirability of seeing all colors in all locations, we might suppose that the three

kinds of cones would be equally abundant and evenly distributed. In fact, they are not. Long-

and medium-wavelength cones are far more abundant than short-wavelength (blue) cones,

and consequently, it is easier to see tiny red, yellow, or green dots than blue dots.

Furthermore, the three kinds of cones are distributed randomly within the retina. The

trichromatic (three-receptor) nature of color vision was not in doubt, but the idea of three

images being transmitted to the brain is both inefficient and fails to explain several visually

observed phenomena.

The Opponent-Process Theory

The trichromatic theory correctly predicted the discovery of three kinds of cones, but it is

incomplete as a theory of color vision. Ewald Hering, a 19th-century physiologist, proposed

the opponent-process theory: We perceive color in terms of paired opposites: red versus



green, yellow versus blue, and white versus black. That is, there is no such thing as reddish

green, greenish red, or yellowish blue. The brain has some mechanism that perceives color on

a continuum from red to green and another from yellow to blue. Hering also observed that

there was a distinct pattern to the color of the after images we see. For example if one looks

at a unique red patch for about a minute and then switches the gaze to a homogeneous white

area they will see a greenish patch in the white area.

Hering hypothesized that trichromatic signals from the cones fed into subsequent neural

stages and exhibited two major opponent classes of processing. 1. Spectrally opponent

processes which were red vs. green and yellow vs. blue. 2. Spectrally non-opponent processes

which was black vs. white. This opponent process model lay relatively dormant for many

years until a pair of visual scientists working at Eastman Kodak at the time, conceived of a

method for quantitatively measuring the opponent processes responses. Leo Hurvich and

Dorothea Jameson invented the hue cancellation method to psychophysically evaluate the

opponent processing nature of color vision.

Due in large measure to the efforts of Hurvich and Jameson the opponent processes theory

attained a central position shared with the the trichromatic theory. One very fortuitous

scientific event to that also took place in the 1950s was the discovery of electrophysiological

responses that emulated opponent processing. Consequently, with the quantitative data

provided by the psychophysics and direct neurophysiological responses provided by

electrophysiology opponent processing is no longer questioned

The Retinex Theory

The trichromatic theory and the opponent-process theory have limitations, especially in

explaining color constancy. Color constancy is the ability to recognize the color of an object

despite changes in lighting. If you put on green-tinted glasses or replace your white light bulb

with a green one, you will notice the tint, but you still identify bananas as yellow, paper as

white, walls as brown (or whatever), and so forth. You do so by comparing the color of one

object with. The color of another, in effect subtracting a fixed amount of green from each.

Color constancy requires you to compare a variety of objects.

To account for color and brightness constancy, Edwin Land proposed the retinex theory (a

combination of the words retina and cortex): The cortex compares information from various

parts of the retina to determine the brightness and color for each area. For example, if the

cortex notes a constant amount of green throughout a scene, it subtracts some green from

each object to determine its true color.

Dale Purves and colleagues have expressed a similar idea in more general terms: Whenever

we see anything, we make an inference or construction. For example, when we look at the

objects, we ask our self, “On occasions when I have seen something that looked like this,

what did it turn out to be?” We go through the same process for perceiving shapes, motion, or

anything else: and calculate what objects probably produced the pattern of stimulation we just

had. That is, visual perception requires a kind of reasoning process, not just retinal

stimulation.

Colorblindness

Occasional errors occur in the chromosomes that carry the genes that encode the cone

photopigments. As a result, individuals with these genes show several kinds of atypical

responses to color, known as colorblindness.

Dichromacy (having two cone photopigments) is the most common type of abnormality and

results from a missing or abnormal cone pigment. Because genes for the red and green

photopigments appear on the X chromosome, this type of dichromacy is sex-linked. Men are



about ten times more likely to be colorblind than women. There are very rare cases in which

the blue photopigment is missing. The gene for the blue photopigment is located on

Chromosome 7, so these cases are not sex-linked and appear equally in males and females.

Rarer still are cases of monochromacy. This condition occurs when a person has only one

type of cone or a complete absence of cones. In either case, the person can’t see color at all. Some individuals have three cone pigments, but their peak response occurs at slightly

different wavelengths than is typical. This leads to a condition known as anomalous

trichromacy. These individuals match colors in a slightly different way than most people,

but they might not even know that they are unusual. As many as 50 percent of all women

may be tetrachromats, or people with four different color pigments. These individuals match

colors in a manner that would be predicted by their having four color pigments rather than

three.

Review Questions

1. -----------determines the perceived color of objects

a) Frequency

b) Wavelength

c) Amplitude

d) Refraction

2. ---------fills the space between the lens and the retina of the eyeball

a) Vitreous humour

b) Aqueous humour

c) Ciliary body

d) Optic nerve

3. ----- helps to see in dim light

a) Cons

b) Rods

c) Brightness

d) Wavelength

4. Fovea contains ------

a) More rods

b) More cones

c) No cones

d) Less cones

5. Retinex theory is proposed by

a) Thomas Young

b) Edwin Land

c) Ewald Hering

d) Hurvich and Jameson

6. Briefly explain the visual pathway.

7. Differentiate the roles of rods and cons.

8. Give a short note on blind spot

9. Write two theories of color vision.

Bibliography

Freberg, L. (2010). Discovering Biological Psychology (2nd

edition). USA: Wadsworth.

Kalat, J.W. (2007). Biological Psychology. Canada: Thomson Wadsworth.

Rogers, K. (2011). The Brain and the Nervous System. NY: Britannica Educational

Publishing.



Silber, K. (2005). The physiological basis of behavior: Neural and hormonal processes.

London: Routledge.

Wagner, H., & Silber, K. (2004). Instant notes: Physiological Psychology. London: BIOS

Scientific Publishers.

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