seeing, hearing, and smelling the world

8/4/2019 Seeing, Hearing, And Smelling the World

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Seeing, Hearing, andSmelling the World


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The Brain and Love

A Day in the Life of the Brain

How the Brain Grows

Inside Your Brain

Seeing, Hearing, and Smelling the World


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Seeing, Hearing, andSmelling the World

Carl Y. Saab

SERIES EDITOREric H. Chudler, Ph.D.


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This book is dedicated to the animals sacrificed for laboratory research.

The author is indebted to Rafa for her editorial contribution

and to Samuel Owolabi, M.D., for his review.

Seeing, Hearing, and Smelling the World

Copyright 2007 by Infobase Publishing

All rights reserved. No part of this book may be reproduced or utilized in any form or by any means,

electronic or mechanical, including photocopying, recording, or by any information storage or

retrieval systems, without permission in writing from the publisher. For information contact:

Chelsea House

An imprint of Infobase Publishing

132 West 31st Street

New York NY 10001

ISBN-10: 0-7910-8945-2

ISBN-13: 978-0-7910-8945-3

Library of Congress Cataloging-in-Publication Data

Saab, Carl Y.

Seeing, hearing, and smelling the world / Carl Y. Saab.

p. cm. (Brain works)

Includes bibliographical references and index.

ISBN 0-7910-8945-2 (hardcover)

1. Senses and sensationJuvenile literature. I. Title. II. Series

QP434.S22 2006

612.8dc22 2006024117

Chelsea House books are available at special discounts when purchased in bulk quantities for

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All links and Web addresses were checked and verified to be correct at the time of publication.

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1 Neurons and Nerves 7

2 Hearing 18

3 The Ear 25

4 Vision 32

5 The Eye 43

6 Visual Abnormalities 62

7 Smell and Taste 68

8 Synesthesia 76

Glossary 88

Bibliography 92

Further Reading 93

Index 96

Table of Contents


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7

Neuronsand Nerves

1

Sensation is a long journey that begins when different stimuli

(light for colors and gases for odors) come in contact with their

proper receptor organs (eyes for light and nose for gases). This

journey of light or gas ends when the stimulus is transformed into

messages that are created by connections between cells in the ner-

vous system. The messages are finally transmitted to the brain for

perception (light as color and gas as odor). This book takes you

on this journey from the outside world of lights, sounds, and odors

into your own brain and the deepest memories of your mind.

LIVING AND NONLIVING

Any discussion of the senses should begin with the basics of biol-

ogy. All living creatures possess one fundamental feature: the cell.


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Seeing, Hearing, and Smelling the World8

The most basic forms of life are made up of only one cell and

are referred to as unicellular organisms. An example of a uni-

cellular organism is a bacterium (plural, bacteria). Unicellular

organisms are usually so small that they can be seen only by

using a microscope (Figure 1.1). More complicated, multicel-

lular forms of life require multiple cell types and use sophisti-

cated systems of communication between cells to sustain the

life of the organism. Any animal big enough to be seen with

the naked eye, including a small insect such as an ant, requires

Figure 1.1 A common example of a unicellular organism is the

amoeba, photographed above. An amoeba is a type of protozoa, a

single-celled organism that has a nucleus and characteristics similar

to those of animals, such as mobility. Amoebae are most commonly

found in freshwater.


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their location within the body (in the periphery, such as in

the hand or centrally, such as in the brain), their shape (small

neurons, such as those in the brain or neurons that are more

than 1 or 2 feet long, such as those in the legs), and their cel-

lular content (neurons are made out of internal parts, known

as organelles). Another general characteristic of neurons is

that, with few exceptions, each neuron is either connected to

another neuron or to a muscle (exceptions include those that

connect to a gland or othervisceral organs).

When first discoveredand until about 10 years agoglia

were thought to play a supporting role by gluing neurons

together. In fact, scientists are just starting to recognize

other important roles that glia play in maintaining neuronal

Figure 1.2 The shape and length of a neuron determines the role it

will serve in the nervous system. Pictured above are three different

types of neurons.


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Neurons and Nerves 11

homeostasis. Glia protect the nervous system against invad-

ing microbes and repair the system after damage (such as

after a severe car accident or a neurological disease).

If one had to describe the most basic function of the ner-vous system in one word, it would be communication. Think

about it: If a person wishes to move an arm, the brain has to

command the arm. If a person places his or her hand over

a stove accidentally, pain is produced by the activation of

specific brain areas. In both cases, a message has to be trans-

ferred from the brain to the arm (for movement) or from the

hand to the brain (for pain). The message also has to be sent

quickly in order to produce an action without too much delay

(Figure 1.3). These messages are relayed from one neuron to

another, either between two neurons or among thousands! In

cases in which only two neurons are involved, the neurons

can be remarkably long compared to the dimensions of the

human body. For pain messages relayed from the arm or the

leg, the first sensory neuron to signal the pain message could

be as long as 2 or 3 feet (.6.9 meters). Certain neurons arein fact the largest cells in the body.

Neurons communicate through synapses, the tiny gaps

where two neurons meet. Two neurons typically share one

synapse at their meeting point; however, it is not uncommon

to find two neurons with multiple synapses. As a result, the

message transmitted from one neuron to another can vary,

depending on the synapses used to transmit that message.

Even for a single synapse between two neurons, the mes-

sage transmitted across the synapse can be subject to change

(more accurately referred to as modulation) with time or

depending on changes in the neuronal environment.

There is a complicated process behind how we react to

a stimulus, and how our perception of that same stimulus


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changes over time according to varying circumstances. But

nevertheless, this process is reflected in the ability of the

nervous system to change or adapt. One adaptive charac-

teristic of the nervous system is memoryin other words,

neurons can learn. Neurons grow and synapses are formed

or broken down constantly in our brains and elsewhere in the

nervous system from birth and until death. Living organisms

are not robots. They constantly evolve due to the ability of

Figure 1.3 Electrical signals travel along the axon of a neu-

ron, also known as a nerve fiber. The speed of the signals

can vary, depending on the type of nerve fiber. Speeds of

different nerve fibers are compared in the graph above.


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the nervous system and synapses to adapt, learn, change, and

bounce back.

NEURON = CELL BODY + DENDRITE + AXON

Unlike other cells in the body, some neurons have long exten-

sions, which help them communicate over long distances.

The arrangement of neurons is somewhat like the network

of telephone wires that connects the homes and businesses

in a city. In a telephone network, short wires carry a signal

a short distance, while long wires can carry a signal much

further. The same type of relationship is at work in the bodys

nervous system.

Neurons are equipped with two types of extensions at the

head or the tail end (Figure 1.4). The cell body of the neuron

contains the nucleus and the rest of the cellular machinery

necessary to make proteins, generate energy, and sustain the

life of the neuron. Out of this cell body emerges dendrites

(head) and an axon (tail end). Neurons generally receivemessages through their dendritic synapses and send mes-

sages down their axons to synapses on one or many neurons.

A message is relayed from one neuron to another, and the

flow of communication is secured. These simple rules and

those highlighted in the previous paragraph are essential to

understanding more complicatedneuroscience facts.

Sometimes the function of a neuron can be predicted

based on the structure of the dendrites or the axon. Neurons

with long axons transmit messages that need to travel to

faraway destinations, such as sensory neurons in the hand

relaying information to the brain about objects touching

the skin. It is important that these messages are transported

faster than other messages in the body (such as hormones


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transported by the blood). Sensory neurons communicate

information that is vital to protect the skin and other body

parts, such as warning about very hot surfaces. Information

Figure 1.4 A neuron consists of a cell body, axon, and den-

drites. The cell body contains the nucleus, which is the con-

trol center of the neuron. Axons carry nerve impulses away

from the cell body. They are often wrapped in myelin, which

helps increase the speed of transmission of the impulse.

Dendrites receive nerve impulses from adjacent neurons.


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about very high temperatures needs to get to the nervous

system centers responsible for withdrawal of the hand as

quickly as possible in order to prevent or minimize injury.

Other types of neurons with long axons convey messages tothe muscles for movement. Imagine how fast the brain needs

to communicate with leg muscles to yield a smooth pattern

of movement.

Some neurons may have multiple dendrites, often referred

to as a dendritic tree. Such neurons receive multiple inputs

from many neurons (and thus from many axons) and could be

recruited to coordinate or integrate multiple messages.

THE NERVOUS SYSTEM

Two structures, the skull and the vertebral column, separate

the nervous system into two main compartments (Figure

1.5). The brain rests inside the skull, and the spinal cord is

found inside the vertebral column. The brain and spinal cord

make up the central nervous system (CNS), and all neuronslocated outside of this central compartment are contained in

the peripheral nervous system (PNS). The thick bones of the

skull and the vertebral column shield the CNS against physi-

cal injuries. Other tools also help ensure the best protection

of the CNS for a good reason: The majority of neurons in the

CNS, if damaged, cannot regenerate. Paralysis after spinal

cord injury is largely a result of the bodys inability to repair

damaged CNS neurons. One example of CNS damage caused

by a disease is multiple sclerosis, a condition in which differ-

ent areas of the CNS degenerate, causing irreversible paral-

ysis and other problems. Research into neuronal regeneration

andstem cells may result in new therapies to cure paralysis

and CNS degenerative diseases.


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The PNS, in contrast, is able to bounce back from injury

a bit better. A typical example of PNS tissue is a peripheral

nerve. A nerve is a collection of axons generally longer

than those found in the brain; examples include sensory or

motor neurons. A peripheral nerve, however, may contain

only sensory or motor neurons or a collection of both. The

Figure 1.5 The brain and spinal cord make up the central

nervous system. The nerves that extend from the spinal cord

to the distant parts of the body make up the peripheral ner-

vous system.


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most prominent nerve in humans is the sciatic nerve, which

transmits sensory messages (such as gentle touch or pain-

ful pinprick) and conveys motor commands to muscles in

the entire leg. The sciatic nerve, like many other nerves,branches out into different smaller nerves as it travels away

from the spinal cord.


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18

Hearing

2

Sound, such as music and speech, is physical energy

perceived by an organ in the body designed especially for this

task. There is no best way to define a sound that has not been

heard before. For example, it is impossible to accurately describe

such a sound to someone who is deaf, especially if that person has

been deaf since birth. To better illustrate this example, imagine

describing a color to someone who is blind or a smell to someone

who has anosmia (inability to perceive odors). In fact, sharing

all feelings that result from sensory perceptiontouch, sound, or

smellis never exact; in the end, all of our experiences remain

deeply personal. Even close friends or family members differ in

their interpretation of the same event or phenomenon.

Music produced by a string instrument such as a violin is one

pleasant example of sound (if well performed!). The violin is a


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19Hearing

delicately built instrument with a common basic feature:

strings attached at both ends. When a bow is brushed against

these strings, the friction that results from this mechanical

interaction causes the string to vibratethat is, to move

quickly with a speed referred to as frequency. This high-

speed vibration causes a similar vibration in the air near the

part of the string where the bow strikes (Figure 2.1).

Air is formed of many molecules (mostly nitrogen and oxy-

gen). As a result, when air molecules are pushed to vibrate

by the moving string, the resulting energy is transmitted to

neighboring air molecules (similar to how billiard or pool

balls bounce off of each other). How does all this vibration

Figure 2.1 A man plays a violin and appeals to the auditory senses.

The violin, which is a string instrument, is used in many different

types of music.


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and energy reach us as music? Before we answer this ques-

tion, it is necessary to understand how sound is created.

SOUND IS . . .

It is difficult to imagine how the motion of air molecules,

so small as to be invisible to the naked eye, can result in

sound. In order to visualize how sound is created, think of the

smooth and calm surface of a lake and the disturbance caused

when a pebble is thrown in the water (Figure 2.2). The reac-

tion typically takes the form of many rapidly expanding

circles, with the point where the pebble hit the surface at the

center. Although the pebble may be the size of a fingernail,

the waves that spread across the lake are infinitely larger.

Like ever-expanding circles of waves at the surface of the

water, air molecules travel by forming waves of compressed

(packed together) and decompressed (spread apart) gas

molecules. This movement mimics the vibration of a string.

Mechanical friction of the bow causes vibration of the stringin a violin and ultimately the sound that spreads throughout

an infinitely larger space.

Sound is detected when vibrating air molecules reach the

human ear. Many conditions need to be met before sound is

heard:

Intensity: Faint and loud sounds reflect the strength

(loudness) of vibration. Very weak vibrations may

reach the ear but may be too weak to be perceived.

Attention: Although many sounds reach our ears, we

do not perceive all of them. This is especially true of

weaker sounds. For example, although thousands of

people may be screaming at a concert, we can only


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21Hearing

perceive a conversation within the crowd if our atten-

tion is shifted to these specific sounds. Another exam-

ple is sleeping through an alarm bell, even though itis loud enough to wake another person equally distant

from the bell.

Normal hearing biology: The ear is a complex biologi-

cal organ with elegant morphology (shape) and design

connected to the brain by neurons. In the end, the

brain is the organ capable of decoding and unlocking

the secrets of sensory messages that bombard us con-

stantly in a busy environment. These sounds that we

experience would go unnoticed in this universe without

the brain.

Small defects in ear biology, connections to the brain, or the

brain itself may lead to hearing abnormalities ranging from

Figure 2.2 This birds-eye view of a pebble tossed into a

pond depicts the way in which sound waves travel throughair. The pebble causes ripples moving outward from the

point of impact, which mimics vibrating air molecules.


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loss of hearing, to hearing abnormal sounds (even imaginary

sounds), to complete deafness.

CONDITIONS FOR NORMAL HEARING

Many conditions are necessary for a person to perceive sound.

First, the sound has to travel through an environment, called

a medium, that allows the transmission of vibration. Air is

the medium in which humans live. It is made of molecules

that bounce against each other and is capable of shrinking

and expanding, and thus can create a wave-like effect. In con-

trast, imagine talking to someone underwater. What is heard

underwater is mostly mumbled sounds that are much softer

than those produced in air. This is largely due to the fact that

water molecules are less free to vibrate than air molecules

(forces of cohesion between water molecules are stronger

than those between air molecules). Sound travels much faster

underwater than in air, however, because water molecules are

closer together than air molecules are. In fact, the speed ofsound in sea water is approximately 1,530 meters per sec-

ond (3,423 miles per hour), or roughly more than four times

faster than the speed of sound in air (343 meters per second;

767 miles per hour).

Another necessary condition is related to the physical

property of the sound itself. When air molecules vibrate, they

travel in waves (Figure 2.3). Imagine surfing at the beach and

waiting for waves. The time spent trying to catch a wave is

directly related to the length or distance that separates one

wave from another. As a result, longer delays are related to

waves being farther apart (longer wavelength). Therefore,

even if sound travels through a medium such as air, we may

not be able to hear it if it falls outside of the certain wave-


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23Hearing

lengths that the human ear can detect. One example is the

sound of a special whistle used to call dogs, which the human

ear cannot detect.

WHEN VIBRATING AIR MOLECULESREACH THE EAR

Vibrating air molecules spread in all directions, just as the

smell of dinner cooking on the stove can reach upstairs

to a bedroom, out the front door, and into the basement at

nearly the same time. This vibration (or waves of molecules

Figure 2.3 Waves can be described by their three proper-

ties: wavelength, frequency, and amplitude. The wavelength

is the distance from the top of one wave, also known as the

crest, to the next. Frequency is the number of waves per

second. The amplitude measures the height of the wave.


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compressing and decompressing) then undergoes two major

transformations to make sound:

1. Transformation by specialized organs in the ear frommechanical energy (vibration) to electrical and chemi-

cal energy:

mechanical energy specific receptor

electrical energy

2. Transmission of nerve signals to the brain, ultimately

transforming mechanical air vibration into sound per-

ception such as music, speech, or even random noise:

electrical energy specific pathway within the

nervous system brain sensory perception

These two major pathways are discussed in detail in the fol-

lowing chapters, which will also highlight similarities and

differences among hearing, vision, smell, and other sensory

perceptions.


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25

3

The Ear

The human ear is not just the part that sticks out from the

head (outer ear). Another major part of the ear is hidden inside

the head and connects to the brain (inner ear). Although the outer

ear (pinna) looks complex, it is a simpler biological structure than

the inner ear. The main function of the outer ear is to maximize

the amount of sound that reaches the ear, almost like a funnel for

sound. After being guided through the pinna, vibrating air mol-

ecules hit the eardrum (tympanic membrane). The eardrum can

be compared to the surface of a real drum that turns tapping or

striking into louder sounds. When the surface of a drum is struck,

it vibrates and causes air molecules inside the drum to vibrate and

escape out of the other end of the drum as loud drumbeats. In


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the case of the ear, vibrating air molecules gently tap on the

eardrum, which then vibrates as well. Any matter lodged in

the outer ear (wax or water left over from a shower or swim-

ming) may obstruct airflow to the eardrum or may mechani-cally prevent the eardrum from freely moving and vibrating

with air molecules. As a result, the affected ear will be less

sensitive to sounds.

THE MIDDLE EAR

The outer ear (eardrum, ear canal) is connected to the middle

ear by three small bones (the ossicles). These bonescalled

the malleus, the incus, and the stapes are connected to

each other and stretch from the eardrum to the inner ear

(Figure 3.1).

The main function of the ossicles is to relay the mechani-

cal vibration toward the nervous system. The mechanical

properties of these bones are unique in terms of amplifying

eardrum vibrations and transmitting them to the inner earwith extreme accuracy. The point of touch between the bones

of the middle ear and the inner ear is a thin oval sheet called

the oval window. Physical damage to the bones of the middle

ear may result in bumping them out of place or even breaking

them, which will cause severe hearing loss.

Medical intervention can successfully restore hearing loss

that results from damage to the external or middle parts of the

ear. Hearing loss caused by nerve damage within the internal

ear is more difficult to restore and often is permanent. This

is true for other sensory perceptions as well: Damage to the

optic (visual) nerve results in permanent visual deficits

including blindness.


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27

THE INNER EAR

The inner ears oval window is connected to the bone in the

middle ear on one side. On the other side (closer to the brain)

it is connected to a thin, spiral-shaped covering within a bony

The Ear

Figure 3.1 The external part of the ear receives sound

vibrations, which travel down the auditory canal toward themiddle ear. In the middle ear, the auditory ossicles (mal-

leus, incus, and stapes) connect to form a chain of bones

that is responsible for transmitting sound vibrations from

the eardrum (tympanic membrane) to the inner ear. The

sound vibrations are converted to an electrical impulse that

travels along the auditory nerve to the brain.


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structure called the cochlea, which resembles a snail. The

cochlea forms a closed compartment filled with fluid. Small

hair cells are immersed inside it. Although referred to as hair

cells, they are not biologically similar to typical hairs found

on the skin or on the head (Figure 3.2). Instead, these hair

cells are tiny extensions with roots attached to a membrane

(known as the basilar membrane). The extensions float freely

within the fluid space of the cochlea.

Figure 3.2 Inner (bottom row)and outer (three upper rows)hair

cells within the inner ear are shown in this colored scanning electron

micrograph. When sound enters the ear, waves form in the surround-

ing cell fluid called endolymph. The waves cause the hair cells to

move, which generates a nerve impulse that is passed to the brain.


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29

Air molecules tap on the eardrum, which then vibrates

the bones of the middle ear. The middle ear in turn vibrates

the oval window, which causes the fluid inside the cochlea

to vibrate. The slightest fluid motion is sensed by the float-ing hair cells, which begin to swing similar to the way

algae or corals sway in shallow ocean waters when moved

by gentle waves.

When the fluid inside the cochlea vibrates, the roots of the

hair cells are gently pulled and stretched as they sway in the

fluid medium. The roots of the hair cells are directly attached

to neuronal terminals. Therefore, as hair cells sway, their

roots wake up the neurons. This interaction between hair

cells and neurons is directly related: the stronger the initial

vibration that is transmitted to the cochlea, the stronger the

hair cells sway, the stronger the excitation of the neurons,

and thus the louder the sound is perceived to be.

THE DIRECTION OF SOUNDWhen a person hears a sound, he or she turns to look for the

source of the sound. This immediate attempt to locate the

sound source is not random but rather is specific and well

executed. How do we correctly guess the direction of sound?

This question is especially intriguing when sound happens

without any visual cuesthat is, when sound or noise is not

clearly associated with a visual event. The answer to this

question lies in how our system of hearing is set up.

Consider, for example, a sound coming from the left side

of the body. This sound will reach the left ear slightly before

it reaches the right ear simply because the left ear is closer

to the sound source than the right ear is. This is called a time

The Ear


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delay, and it alerts the brain to the source of the sound and

may prompt an immediate rotation of the head in the direc-

tion of the sound. Another hint that the brain uses to correctly

guess the source of the sound is the difference in the sound

intensity that reaches the ears. In the previous example,

not only will the left ear receive the sound first, it will also

receive a sound that is just a little bit louder than what the

right ear receives. What happens if the sound comes from

neither left nor right, but from straight ahead? In this case,

Figure 3.3 The cerebral cortex consists of four different areas known

as lobes. Regions of each lobe are responsible for different functions,

such as hearing, smell, and vision.


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31

sound will reach both ears at the same time (and with the

same intensity). If the visual event that goes with the sound

is not obvious at first (for example, a mosquito too small to

be easily observed), the brain will command the eyes to lookstraight ahead until the source of the sound is located.

FROM THE EAR TO THE BRAIN

Neurons in the inner ear gather to form the auditory nerve.

The auditory nerve transmits signals to many brain areas,

including deep brain structures and the cerebral cortex

(Figure 3.3). The journey of air molecules vibrating because

of drumbeats, clapping, or singing ends in the brain, where

sound is ultimately perceived. Mysterious as the human

sensory experience is, the way neuronal signals in the brain

cause a sensory phenomenon is still being studied. This

limitation in understanding brain function and its relation to

human consciousness is not limited to hearing. Scientists are

still figuring out the exact processes involved in touch, pain,vision, smell, taste, and higher brain functions.

The Ear


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32

4

Vision

Sight is what we perceive when our eyes are open and there

is enough light in the environment. Humans cannot see in com-

plete darkness, and therefore any helpful discussion of vision

must include an explanation of the physical properties of light

and the reflection of light on objects to produce colors. The sight

of lit objects, including still images (photographs, trees) or mov-

ing ones (a flying bird, a falling star), is our perception of light

that is bright enough to stimulate visual neurons when our eyes

are open (the term visual neurons here refers to neurons in our

eyes that are sensitive to light). Sight is similar to hearing in that

they are both the perception of a physical event in our environ-

ment that a specialized organ (the ear for hearing and the eyes

for sight) changes into an electrical signal that is then sent to the


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33

brain for processing. The whole process of perceiving light

is calledvision.

To better understand the process of vision, consider the

questions below before getting into the details. Keep the fol-lowing sequence in mind:

electrical energy (light) specific pathway

within the nervous system (eye and connections

to the brain) sensory perception (vision)

1. What are the conditions necessary to see an object?

Can you see in total darkness or with your eyelids

closed?

Hint: You see light reflected off of objects.

2. When you see, do you always see clearly? Do you wear

glasses or contact lenses?

Hint: You focus for clear vision.

3. What happens if your eyes do not focus together on the

same object?

Hint: Your ability to focus is limited.4. Why does the sound of a flying airplane usually seem

to come after the sight of it?

Hint: The speed of light is faster than the speed of

sound.

5. Why do you perceive objects directly in front of you

better than those slightly to the sides but within your

vision?

Hint: Your field of vision has limits.

These questions form the basis of how vision works. They

may be obvious to some people, but nevertheless, further

analysis is necessary for normal and abnormal vision to be

understood.

Vision


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SIGHT IS . . .

With our eyes open, we see objects of different colors lit by

an illuminating sourcea lamp, a candle, a cars headlight,

or natural sunlight. For humans, the ultimate source of light

is the Sun. Without sunlight, we can see only by using artifi-

cial light sources. Why is light necessary for vision? Imagine

standing in a closed room with only one light source. Objects

in that room can be perceived only when the light is on. When

Figure 4.1 The composition of an object affects how it reflects

light. In this computer illustration, the sphere at right is opaque and

reflects very little light. The sphere in the middle has a mirrored sur-

face that reflects all light that strikes it. The sphere at left is trans-lucent. Light that strikes a translucent surface is both reflected and

refracted (bent).


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35

the light is off, objects disappear from our sight. Switching

a light from on to off does not cause objects to mysteriously

vanish. It is more logical to assume that the objects remain

where they are but that the light gives them appearance,or brings them to life. Light is physical energy (similar

to sound) that travels in space. When light encounters an

object, it will hit the object and reflect off of it, just like a ball

bounces off of a wall. In this case, the object is referred to

as opaque (Figure 4.1). If the object is too thin, however,

light may penetrate the object, and the object is said to be

transparent.

LIGHT IS . . .

Light is similar to sound in a way. Just like sound, it can

be described by its wavelength. Sound is a physical event

(movement or vibration of air molecules) that obeys physi-

cal laws (travels at a specific speed, in all directions). Light

is also a physical event (movement or vibration ofphotons)that obeys the same physical laws. The following concept is

hard to grasp at first: Light is not infinitely fast. When a light

bulb is switched on, it may appear as though light is gener-

ated instantaneously, but this is not the case. In fact, light

travels at a defined and measurable speed, just like sound

although light travels much faster than sound. Because of

its extreme speed, light reaches us in almost no time for

relatively close illuminated objects. For example, light from

a source one mile away reaches us in approximately five mil-

lionths of a second. The speed of light is approximately 300

million meters per second (approximately 671 million miles

per hour). Compare this speed to the typical speed of a car

on the highway (27 meters per second; 60 miles per hour),

Vision


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In the early seventeenth century, many scientists believed that

there was no such thing as the speed of light. They thought

that light could travel any distance in no time at all. Galileo

disagreed, and he came up with an experiment to measure

lights speed. He and his assistant each took a shuttered lantern,

and they stood on hilltops one mile apart. Galileo flashed his

lantern, and the assistant was supposed to open the shutter to

his own lantern as soon as he saw Galileos light. Galileo timed

how long it took before he saw the light from the other hilltop.

He did not find significant delay because it takes light less than

10 millionths of a second to travel a mile, which was too fast

to be measured at the time. The speed of light was more or less

accurately measured half a century later by two French scientists

(Armand Fizeau and Leon Foucault). Each used a slightly

different technique but Fizeau relied on a lantern, a mirror, and

a fast-rotating toothed wheel. The wheel was placed between the

lantern and the mirror so that, as the wheel rotated with a known

speed, the light flickered through the gaps in the wheel and

hit the mirror. If the wheel rotated at a certain speed, the light

would not return to its source because it hit the teeth instead

of the gaps in the wheel. Taking into consideration the speed of

the wheel, the distance between the mirror and the wheel, and

the distance between two teeth of the wheel, the speed of light

was measured with an acceptable accuracy. The speed of light

was further refined at the turn of the twentieth century by AlbertAbraham Michelson to be 186,355 miles per second. In 1983,

the value for the speed of light was defined as 299,792,458 m/s

(186,282 miles/s).

Measuring the Speed of Light


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37

or that of a bullet fired from a gun (1,000 meters per second;

2,237 miles per hour).

When illuminated objects are far away from us, although

light eventually reaches us, it does so with considerabledelay. It is thought that some stars we observe shining at

night may not exist at the time we see them. This is because

some stars are so far away that light from these shining stars

takes months or even years to reach us. Imagine someone

running toward you from a starting point a few feet away

and another person starting a mile away. Who would reach

you first if they both run at the same speed? Similarly, light

escapes from a star and sets out on a long journey through

the vast emptiness of the universe to get to you; that star

could have exploded and disappeared before its light reached

you. The result of this is that the glittering stars we enjoy on

a summer night may not be the real picture of what is in the

universe at the time we are gazing up at the night sky. What

we see is relative to how far the object is from us.

LIGHT PARTICLES TRAVELIN WAVES THAT EVOKE COLORS

What is the physical object that we call light that is

capable of traveling from distant places? Light is made up

of particles called photons. Photons are so small that we can-

not see them with the naked eye or any type of microscope.

Light is created whenever an event frees enough energy to

move, or animate, photons. Animated photons vibrate at a

certain speed and with certain, specific characteristics that

determine the intensity of the light (brightness) and color

(reddish for weaker light and bluish for more intense light).

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In fact, ordinary artificial light (electric lamp) or natural light

(sunlight) is perceived as white to yellowish in color, which

is actually a combination of all colors. White light is whatyou get when all colors of light combine (including blue and

green). White color can in turn be separated into individual

colors as it travels through space and encounters certain

objects. For example, when light hits water vapor in the air

during a light rainfall (or just after a heavier rain), a rainbow

may appear. That is because the sunlight is bouncing against

the tiny drops of rainwater still hanging in the air. This splits

sunlight into all the colors of the rainbowor the spectrum

of light (Figure 4.2).

Another way to demonstrate the nature of light is by look-

ing at a natural phenomenon that happens every day: sunset.

At noon on a clear day, the Sun is very bright and white. This

is the time of day when the Sun is directly vertical to the

Figure 4.2 Light that passes through a prism is split into the full

spectrum of light. Each color of light has a specific wavelength.


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39Vision

Figure 4.3 A picture of a sunset reflected on a lake at

Superstition Mountain Country Club in Arizona. Sunsets

occur at different times each day and are noted for the soft

shades of red, orange, and yellow light that cover the sky.


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surface of Earth. The closer the Sun is to Earth (at noon), the

more intense the light will be and the brighter the sunlight.

At sunset, however, Earths surface (at the point at which we

stand to observe the sunset) rotates away from the Sun. Aswe move away from the sun, the distance between the Sun

and us increases. This makes it more difficult for light (made

of photons) to reach us because the light has to travel a larger

distance at the same speed it travels any other time. When

the light finally reaches us, it is less bright than at noon. In

addition to the less intense light, sunsets are characterized by

the smooth and rather pleasant transition from bright yellow-

ish light to a softer orange, and finally a reddish color as

the sun sinks behind the horizon (Figure 4.3). Partly cloudy

skies at sunset may appear completely red minutes before

dark. When the Sun quietly disappears, it does not actually

dim the way a light switch in your home might dim. In other

words, the light intensity of the Sun is still the same, but it

loses more energy as it reaches us, which results in a change

in color.

AT FIRST SIGHT

With an opaque object, light is reflected off of it and trav-

els in many directions, including toward our eyes. If asked

to identify what we see, we usually start by describing the

objects shape and color. The shape is determined by the

different reflections of the light off of the different parts of

the objects. If the object is a simple box, we describe it as

such because light that hits the different corners of the box is

reflected in such a way that the front edge of the box emits

reflected light a bit more strongly than does the back edge

(giving the impression of perspective). This results in light


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41

that reaches our eyes at different intensities. Stated differ-

ently, light reflected off of objects can be compared to a mold

or cast that, when hit with your fist, will retain the shape of

the fist. In a similar way, light that hits an object will retainthe exact form of that object and reflect it in many directions.

Once reflected light reaches our eyes, we perceive the shape

and color of the object.

LIFE IN COLORS

Shape, color, and dimension are what we see in any object.

But though you might say, My house is brown, or My

shirt is red, objects do not have colors. They simply

reflect light. So what exactly is color, then?

First of all, objects must be opaque (not see-through) in

order for them to have color. Once you know that, though,

you might wonder why certain apples look red and not blue

or green, when all they do is reflect light. Where does the

quality of redness come from? Remember that all light iswhite, but that it can be split into the colors of the rainbow.

When light hits a red delicious apple, we see the apple as red.

This is because the apple absorbs all the colors in the light

spectrum except red, which is reflected back and absorbed

by our eyes.

Transparent objects, however, do not react this way to

light. For example, glass looks clear because light passes

right through and does not break up. Why, then, even though

air is also transparent, does the sky on a clear day appear

blue? And why, if the ocean is made of water, does it look

blue, green, or gray?

Lets start with the first question: The sky looks blue

because of a layer of air in the sky that reflects the light as

Vision


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blue. Molecules of air in that layer split up the white light

from the sun, which you see as the color blue. That bright

blue sky is part of what causes ocean water to look blue, too.

Ocean life (plant and animals) and sand are opaque objectsthat mix with the water to give it an opaque surface. The sur-

face of the ocean reflects the blue sky almost like a mirror.

A clean, healthy ocean looks blue on a sunny day because

it reflects the color of the sky. On a cloudy day, that same

ocean will appear pale or whiter than usual because white

light from the clouds is reflected in the water.


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5

The Eye

It may sound bizarre to compare an eye to an ear, but these

two sensory organs have many features in common. A discus-

sion of the similarities may help clarify how the nervous system

transforms light into vision and sound into hearing. This will also

answer the following questions and explain the links between

other types of sensory perception and corresponding organs:

1. Does the eye contain specialized neurons that sense light?

2. Are the light-sensing neurons in the eye connected to a

nerve that transmits information about light to the brain?


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3. Can the human eye detect all visual stimuli detected by

other animals such as cats or bats?

In contrast to the ear, the eye is connected to a set of mus-cles that allows it to achieve a wide range of motion without a

person having to rotate his or her head to locate a visual cue.

This range, however, is not complete, and rotating the head is

often necessary to follow a moving visual target.

EYE MOVEMENTS

Unlike the human ear, the human eye can move, and it is

protected by an eyelid that closes regularly (blinks). The eye

is controlled by muscles that contract to move the eye in all

directions except backward. These are the extraocular mus-

cles. A set of two muscles connected to either side of the eye

permits left or right gaze. However, the eye actually rotates

away from the side of the contracting muscle and toward

the muscle that is simultaneously relaxing. Eye movementsare mostly under conscious control and therefore obey brain

commands. Accordingly, muscles connected to the eye are

themselves connected to the brain by nerves and respond to

brain commands for eye movements.

Most often eyes move at the same time, and in the same

speed and direction. Not only are both eyes under conscious

brain control, but they also receive similar commands from

neuronal pathways. This descending brain control first

comes from both sides of the brain (the left side of the brain

controls the right eye and vice versa) and then meets on a

specific nucleus in the brain stem before separating again

into left and right muscle command pathways. This meeting

in the brain stem ensures that the eyes move together.


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45

ANATOMY OF THE EYE

But what is the eye made up of? The answer to this question

lies in the anatomy of the visual pathway, which starts from

the cornea and the optic nerve in the eye and goes to the back

of the brain (Figure 5.1).

The Eye

Figure 5.1 The eye converts light into electrical signals that are

passed on to the brain by the optic nerve.


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The Cornea

The front part of the eye is covered by a transparent sheet, or

membrane, that can easily be seen in the mirror. This mem-

brane is the cornea, a clear surface that covers the iris and

pupil (discussed in the next sections). The eye is a fragile and

important organ that is exposed to the outside environment.

The cornea provides protection against physical damage and

foreign objects (such as insects and germs) because it is as

strong and durable as plastic. It is also as transparent and clear

as glass to allow as much light as possible to enter the eye. In

addition, the cornea resembles a special glass that functionslike the eyes outermost lens. This focuses the light onto the

retina. Another role for the cornea is protection against dam-

aging ultraviolet (UV) radiation from natural sunlight, which

can be harmful to neurons in the retina. Unlike most tissues

in the body, the cornea does not receive a blood supply (per-

haps to remain as transparent as possibleblood vessels may

interfere with light) and therefore relies on tears for nourish-

ment. The cornea is also filled with many neurons that aresensitive to painful events such as rubbing or scratching the

surface of the eye.

The Iris

The iris is the colored part of the eye. It is in fact a muscle

that cannot be consciously controlled. In contrast to the

transparent pupil that it surrounds, the iris is opaque. Pigment

in the iris gives the external color, such as blue, green, or,

more commonly, brown (or a combination of these colors)

to the human eye. Pigmentation may change slightly during

the first year or two after birth, but eye color almost always

remains permanent afterward. Surrounding the iris is another


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47

opaque surface of white color called the sclera. Although

normally white, the sclera has many small-diameter blood

vessels that cross just underneath to supply oxygen and

nutrients to neurons inside the eye. These blood vessels maydilate, becoming more visible, when a person hasnt slept or

is exposed to prolonged high winds (driving with the win-

dows down) or chlorinated water (swimming in a pool). Such

conditions give the sclera a reddish color (red eye).

The Pupil

In the outermost part of the eye, light first penetrates the

cornea and then enters through a transparent hole or win-

dow called the pupil, which is surrounded by the iris.

The pupil of the human eye is dark. Dark objects absorb

all natural light, while white objects reflect all of the wave-

lengths of light. Think of it this way: On a hot summer day, if

you stand outdoors exposed to the sunlight, you feel warmer

wearing dark clothes than you do in paler colors. Dark fab-rics absorb more light, trapping more energy in the form

of heat. (That is a helpful tip for your next trip to the beach:

remember not to wear a black T-shirt!) Likewise, having a

dark pupil allows the human eye to absorb as much light as

possible.

Whereas the diameter of the iris does not change, the diam-

eter of the pupil does change. In fact, the pupil is a hole that

varies in size depending on the intensity of the light. Small

muscles in the iris adjust the diameter of the pupil. When

these muscles contract, the pupils diameter enlarges, thus

widening the pupils size to allow more light into the eye;

this process is referred to as pupillary dilation (Figure 5.2). In

contrast, pupillary contraction occurs when the iris muscles

The Eye


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relax, thus decreasing pupillary diameter. Therefore, in bright

light, iris muscles contract and decrease pupil diameter. The

size of the pupil changes rapidly in response to light; this is

easily observed when bright light is directed into someones

eye. In fact, pupils in both eyes will diminish in size even if

only one eye is subjected to direct light.

Humans cannot consciously adjust the size of their pupils;

therefore, it is referred to as a reflex. Similar to quickly and

unconsciously withdrawing an arm in response to a hot stove,

Figure 5.2 This photograph, taken in dim light, shows the

human eye with a dilated pupil. Pupillary dilation occurs

when an iris needs more light, if a person is aroused, or it

can be induced by drugs. Extreme dilation is also known asmydriasis.


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49

the amount of light allowed to enter the pupil is an automatic

action that occurs unconsciously. As is the case of an arm

jerking away from a hot stove, specialized parts of the spinal

cord control the reflex (Figure 5.3). Pupillary reflex, however,

is controlled by neuronal structures in the base of the brain

The Eye

Figure 5.3 A reflex is an action that is performed without

conscious effort. The knee-jerk reflex is controlled by neu-

rons within the spinal cord. When the kneecap is tapped

with a mallet, sensory neurons transmit a signal to the

spinal cord. The signal is then relayed to the quadricepsmuscle, which contracts and causes the leg to kick up. The

papillary reflex (increasing or decreasing the size of the

pupils) works in a similar way. Unlike the knee-jerk reflex,

however, the papillary reflex is controlled by neurons in the

brain stem.


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(the brain stem). The pupils in both eyes contract or relax

to the same extent. When body movements are well coordi-

nated, almost in an automatic or robotic way, such as in eye

movements, walking or jumping, it is generally a result of thesynchronized control of neurons in the spinal cord (for walk-

ing and jumping) or the brain (for eye movements). Neurons

in the brain may also contribute to the control of walking and

jumping in a different way: The brain sends a general order

to a neuronal command center in the spinal cord, which then

takes care of the second-by-second control of the relevant

group of muscles in the legs to work together. The brain also

sends another command to speed up or stop.

So, if bright light is directed at only one eye, the pupillary

reflex will be evoked in both eyes (even if the other eye is

in the dark). Medical doctors and emergency health profes-

sionals use this valuable information to test for brain damage

after events such as a car accident or a fall. They do this by

directing a light into the eye of a traumatized, unconscious

person. An absence of the pupillary reflex indicates severebrain damage that involves the brain stem.

The Lens

After passing through the pupil, light reaches the lens of

the eye. Without a lens, light would spread throughout the

internal surface of the eye. Light rays travel straight if unin-

terrupted, but they may either reflect back on their original

source if they encounter an opaque object (for example, a

mirror) or continue to travel through a transparent object but

change from a straight path (Figure 5.4). As a result, trans-

parent objects bend (refract) light differently, so that each

transparent object has a different refractive index.


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51

For example, a diamond has a very high refractive index

compared to glass or water; therefore, light is highly

refracted by diamond, resulting in a diamonds shiny glitter.

As photons (tiny particles that make up light) cross from one

medium to another (such as from air to glass or water), their

speed is slowed by the atoms within the medium. This loss

of speed is evident by the transfer of energy from photons to

The Eye

Figure 5.4 Light is refracted when it passes through glass

or the lens of the eye. The angle at which the light strikes

the surface is known as the angle of incidence (1). The

angle of the light that is refracted is known as the angle of

refraction (2). The amount of refraction is equal to the dif-ference between the angle of incidence (1) and the angle

of refraction (2). The greater the difference, the more the

light has been refracted.


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the atoms that make up the transparent object, mainly in the

form of heat (objects penetrated by light heat up).

The lens not only refracts light that enters the human eye,

but also focuses it on a particularly sensitive area in the backof the eye for the detection of photons. Light rays enter the

eye at different degrees, however, depending on the location

of the light source. The lens then adapts to different light

angles to refract different light rays properly and bends

them to strike the sensitive area in the back of the eye. The

refractive index or power of the lens to adapt is a process

calledaccommodation.

The eyes lens is connected to muscles located behind the

iris within a structure called the ciliary body. In people who

have normal vision, the ciliary body flattens the lens in order

to bring objects into focus at a distance of 20 feet (6 meters)

or more. To see closer objects, this muscle contracts to

thicken the lens (Figure 5.5). Young children can see objects

at very close range, whereas many older people have to hold

objects farther and farther away to see them clearly. This isbecause the lens becomes less elastic as people age. Just like

a camera lens, the eyes lens focuses light to form sharp,

clear images. It is important to note that distant objects tend

to emit light in a nearly parallel trajectory, thus requiring

minimal refraction by the eye for proper accommodation.

Light emitted by closer objects reaches the eye along a more

diverging path, thus requiring stronger accommodation

to converge them back into the photosensitive area in the

back of the internal eye. In this case, the ciliary body con-

tracts and thickens the lens, which refracts light more.

Because we are constantly bombarded by light sources at

close range, such as computers and televisions, it is recom-

mended that we relax our ciliary bodies by taking a break


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53

and looking at faraway objects, such as a landscape in natural

sunlight, for at least a few minutes daily.

The Retina

Light that enters the eye eventually hits the photosensitive

area in the back of the inner eye called the retina. The retina

contains a layer of cells sensitive to light known as photo-

receptors. All of the structures in the eye serve three main

The Eye

Figure 5.5 Ciliary muscles relax and the lens flattens to

focus on distant objects (top). To focus on close objects,

the ciliary muscles contract and the lens becomes more

round (bottom).


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purposes: (1) to protect the eye from foreign objects such as

insects and microbes, (2) to capture light most efficiently, and

(3) to focus light on photoreceptors in the retina. The photo-

receptors are connected to neurons that transfer light-relatedinformation to the brain so we can see objects. Without the

retina, normal vision is impossible.

When a stimulus in the environment comes in contact with

our skin, we perceive the touch sensation as a gentle stroke,

tingle, pressure, or pinch depending on the properties of the

stimulus. In addition, every object has a temperature that is

detected by specialized receptors on our skin. Neurons then

transform these physical phenomena (touch and temperature)

into sensory experiences that are often memorable if either

pleasant (such as a kiss) or unpleasant (such as a burn). This

principle of transforming physical energy from the external

environment into codes that the brain can decode as sensa-

tion also applies to vision. Light focused on the retina excites

photoreceptors that create electrical activity that in turn

excites neurons connected to these receptors.There are two types of photoreceptors in the human eye,

which are named according to their shape (Figure 5.6). These

photoreceptors resemble a cone or a rod, and thus they are

referred to as cones or rods. They differ not only in size and

shape, but also in how they work. Rods are more numerous

(roughly 120 million per eye) and are more sensitive to bright-

ness or light intensity than cones. Cones are more sensitive to

color, however, so the 6 to 7 million cones provide the eye

with color sensitivity. Cones are also tuned to certain colors

of the light spectrum and are better adapted for vision during

the day and in bright light. In contrast, rods are better adapted

for dark vision or vision in dim light. Cones also detect details

in a visual stimulus such as small-type on a page or the fine


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55

texture of an object. In contrast, rods tend to be less sensitive

to details and rely mostly on the general features of an object

such as its outline or rough dimensions such as height.

The Eye

Figure 5.6 Above, a cross-section of the human retina, a thin mem-

brane that lines the back of the eyeball, containing photoreceptor

cells known as rods and cones. The rod cells (white) are responsible

for distinguishing between light and dark, while the cone cells (yellow)

are responsible for color vision and acuity.


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The Macula, the Fovea, and the Foveola

Photoreceptors and neurons within the retina are not spread

out on the retinal surface equally. These cells are densely

packed within one area of the retina called the macula. In

addition, at the center of the macula, there is a smaller area

where only cones, and no rods, are found. This area in the

macula is called the fovea centralis (or simply, the fovea).

Within the fovea, an even smaller area called the foveola is

more densely packed with cones.

Cones allow humans to have sharp vision. Loss of cones

or damage to the retina at the macula causes legal blindness,which is blindness as defined by law. A legally blind person

can detect some light, rough shadows, and shapes, but not

letters or signs. The eye moves constantly to keep the source

of light reflected from the object of interest falling on the

fovea, where cones are found in the highest density.

Cones also provide the eyes color sensitivity. Each group

of cones responds in different ways to different colors. In

fact, each group of cones may be sensitive to different wave-lengths of light. It is estimated that millions of conesmore

than half of the cone populationcan be classified as red

cones. Green and blue cones make up the rest of the

population. The green and red cones are concentrated in

the fovea centralis. The blue cones have the highest sensi-

tivity and are mostly found outside the fovea, leading to some

distinctions in the eyes perception of the color blue.

Natural light is the combination of all colors. Daylight,

therefore, would be expected to stimulate all types of cones

in the fovea, whereas light reflected from a red apple will

stimulate red cones much more than the green or blue

cones. The fovea is the point of sharpest vision because of the

high density of cones.


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Light waves from an object, such as a tree, enter the eye first

through the cornea, which is the clear dome at the front of the

eye. The light then progresses through the pupil, the circular

opening in the center of the colored iris. Next, the light passes

through the crystalline lens, which is located immediately behind

the iris and the pupil.

Initially, the light waves are bent by the cornea and then

further by the crystalline lens, to a nodal point (N) located

immediately behind the back surface of the lens. At that point,

the image becomes reversed (turned backward) and inverted

(turned upside down).

The World Upside Down

Images formed on the retina are reversed and upside down. When

the image is processed by the brain, it is restored to its correct

orientation.

The Eye


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Because the fovea is located roughly in the center of the

macula (itself located in the center of the retina in the back

of the eye), the ability to see details and colors is dependent

on light hitting the fovea. In other words, a person must lookstraight at the object in question by coordinating eye move-

ments and head rotation to bring that object into his or her

central field of vision.

Rod Photoreceptors

In spite of the contribution of cones to color vision and sharp-

ness of vision, cones are in fact less sensitive to light than

rods are. Rods are incredibly efficient photoreceptors, about a

thousand times more sensitive to light and much more numer-

ous than cones. Being less sensitive to light, cones respond

better to strong light, whereas rods, being more sensitive,

respond to both weak and strong light. It turns out that rods

require more time (a few seconds to as much as 10 to 20

minutes) than cones to adapt when suddenly exposed to light.Daylight vision (cone vision) adapts much more rapidly to

changing bright light levels. Cones can adjust to rapid color

and intensity changes in less than a few seconds.

Differences in daylight vision and night vision can be

demonstrated easily. For example, a person needs a few

minutes to adjust fully when stepping from bright daylight

into a dark room, or when driving in the open on a clear day

and suddenly entering a tunnel. This type of vision is medi-

ated by cones. Night vision is not affected by colored lights

because rods are not sensitive to color. This is partly why

cars are equipped with red taillights, which do not disturb

night vision as much as white light, such as that produced

by a cars headlights.


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HOW DO CONES AND RODS WORK?

From its source, light (natural sunlight or artificial electri-

cal light) bounces off objects and toward the eye, where

it enters through the cornea and the pupil. The light trav-

els all the way to the retina, where photoreceptors (cones

and rods) await the light after its long journey in space

and within the eye. The cones and rods contain proteins

that are deformed by light photons and initiate a chemi-

cal reaction that results in an electrical current. In fact,

cones and rods are neurons. Like all other neurons, they

generate electrical signals in response to proper stimuli orrelay the message from other neurons.

Cones and rods contain different light-sensitive proteins.

In rods, the protein is called rhodopsin. Rhodopsin breaks

down into two different molecules called opsin andretinal

when it is exposed to even one photon. Interestingly, retinal

is a derivative of vitamin A. Because carrots provide a natural

nutrient source for vitamin A, it is commonly believed that

eating carrots aids vision. Although there is some truth to thisbelief, it is misleading to think that carrots can treat serious

visual problems such as astigmatism or cataracts.

Light causes electrical activity in rods and cones that are

connected to other neurons that are, in turn, connected to

other neurons. In this visual pathway, the message is carried

from one neuron (starting in the rods and cones in the retina

of the eye) to the next in the pathway until it reaches specific

brain areas. Like all neurons, cones produce an electrical

impulse that travels along the nerve fiber and then must reset

to fire again. The light adaptation is thought to occur through

adjustment of this reset time, which simply takes longer in

cones. Brain areas that contain neurons that finally receive

and process this neuronal electrical signal are located mainly

The Eye


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in the back of the brain. All the light information that has

been converted in the retina to an electrical signal is sent

outside of the retina through the optic nerve in each eye.

Tracing the visual pathway from neurons in each eye to the

visual centers in the back of the brain is complicated, because

Figure 5.7 Visual signals crossover to the opposite side of

the brain for processing. The left side of the brain is respon-

sible for processing the right visual field (red area in front

of eyes), while the right side of the brain processes the left

visual field (blue area in front of the eyes).


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61

some neurons send their axons along a twisted path to many

brain centers, including in the brain stem. There are two por-

tions of the optic nerve in each eye that meet in the middle of

the brain, forming the optic chiasm (Figure 5.7).Beyond the optic chiasm, the nerves carrying visual infor-

mation toward the back of the brain are referred to as optic

tracts (instead of optic nerves). This complex migration of

nerves from the eye ensures that the left side of the brain is

responsible for vision of objects viewed on the right side of

the body (or in the right visual field) and vice versa. This

is not surprising if one compares the visual system to other

sensory pathways, such as that for touch, whereby the right

side of the body is felt by the left side of the brain and vice

versa. This setup is not limited to sensory systems, but is also

a property of the motor system: Willful orders to move the

right side of the body are initiated on the left side of the brain

and vice versa.

The Eye


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62

The cones and rods are nourished by many blood vessels that

lie just beneath the surface of the retina, forming a layer known as

the choroid. Cones and rods in the retina are highly active (almost

all the time your eyelids are open), requiring maintenance and

nutrients from the choroid layer. The photoreceptors also generate

waste chemicals as by-products of their high activity level. The

outermost surface of the retina creates a critical passageway for

nutrients from the choroid to the retina and helps remove waste

products from the retina to the choroids. This layer is called the

retinal pigment epithelium (RPE).

VisualAbnormalities

6


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COMMON VISUAL DEFECTS

The RPE normally deteriorates with age and can lose its pig-

ment and become thin. As a result, the waste-removing and

nutritional functions between the retina and the choroid can

gradually deteriorate. Lacking nutrients, the light-sensitive

cells of the macula become damaged. The damaged cells can

no longer send normal signals through the optic nerve to the

brain, and vision may become blurry. This is often the first

symptom of the condition known as macular degeneration.

Other visual abnormalities include astigmatism, nearsight-

edness, farsightedness, strabismus, cataracts, and color blind-ness. The first three of these conditions relate to one common

mechanism in the eye called accommodation. Recall how

light enters the eye through the cornea first and then through

the pupil and the lens and travels all the way to the back of

the eye, where the retina contains photoreceptors. The main

function of the lens is to focus the entering beam of light onto

the retina.

If the muscles that control the accommodation power ofthe lens are weak, light from far-away objects will be focused

behind the retina and therefore the image of the object will be

out of focus. Similarly, nearby objects could be out of focus

with the image focused in front of the retina. Both of these

conditions can be corrected by wearing eyeglasses or contact

lenses. These lenses help bring images within the retina in

focus.

Astigmatism is a visual defect that causes blurred

vision as a result of an abnormal curve of the cornea.

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Wearing eyeglasses or contact lenses can generallyimprove vision.

Nearsightedness (myopia or shortsightedness) occurs

when the lens of the eye focuses light in front of the

retina instead of directly on it (Figure 6.1). This causes

the image of an object to form slightly in front of the

retina, making it blurry. People with myopia do not see

well far away but can see close objects clearly. This

condition tends to become gradually worse with age.

Laser eye surgery is a treatment option that changes the

shape of the cornea.

Farsightedness (hyperopia) is a condition in which the

incoming image is focused behind the retina, result-

ing in a blurred image of close objects (distant objects

Figure 6.1 Nearsightedness and farsightedness are cor-

rected by using a lens to move the focal point to the correct

location on the retina. The path of light without correction

is marked by black lines in the illustration above.


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are still seen clearly). Corrective lenses can help this

defect, but the condition may get worse with age.

Cataracts are a cloudiness of the eyes lens that pre-

vents light from reaching the retina (Figure 6.2).

Having cataracts has been described as looking through

a dirty window. Corrective lenses are not a good option

for people with cataracts. Rather, surgery to remove the

cataracts is common. It is advisable to treat cataracts at

a young age to prevent permanent visual defects.

Color blindness is the inability to detect or perceive

one color, some colors, or all colors. It is generally

Figure 6.2 The grey, opaque mass obscuring the pupil of

the eye is a mature cataract. Aging, steroid use, and dis-

eases such as diabetes can all lead to the onset of cata-

racts. Although cataracts never cause complete blindness,

a persons sight becomes limited and vision progressively

worsens if not corrected by surgery.



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caused by the absence of certain cones in the retina

that are responsible for detecting colors. People with

color blindness usually do not have other visual

defects. The most frequent type of color blindness isthe inability to distinguish red and green pigments.

This condition is mostly genetic. Unfortunately, it can-

not be corrected.

Complete blindness is complete insensitivity to light.

This severe condition may be temporary or permanent.

Damage to any of the structures of the eye, by disease

(such as diabetes), accident, or old age may lead to

blindness. Permanent blindness, for example, can be

caused by an object penetrating the eye and severely

damaging the retina (where photosensitive neurons

transform light into electrical signals). It could also be

caused by a tumor growing outside the eye and pressing

against the optic nerve, interrupting the flow of infor-

mation from the eye to the brain.

Conjunctivitis is inflammation of the external eye. Itis associated with redness around the pupil and some-

times pain. One type of conjunctivitis is called pinkeye.

Some forms of conjunctivitis result from allergies or

a scratch on the surface of the eye and can be easily

treated with medicine in the form of eyedrops.

Strabismus is a condition in which the eyes look

crossed. Strabismus results from a muscle coordination

defect that may later lead to a visual defect because

images formed on the retina may not match in both

eyes. Corrective lenses cannot solve the problem of

strabismus. Instead, surgery, especially early in child-

hood, can be used to prevent permanent visual defects.


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Untreated visual defects may also cause severe headaches.

Proper eye care may prevent damage that causes visual

defects. Such care includes periodic eye exams by an eye

specialist.



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Smell and taste are chemical senses that provide us with

valuable information to explore our environment. Thanks to smell,

we are constantly testing the quality of the air as we breathe.

Aside from being used to smell perfumes and food, the sense of

smell can save lives. For example, people often detect the smell of

smoke from a hazardous fire before they hear an alarm.

Even newborn babies make faces that indicate their dislike of

fishy or rotten odors; however, the sense of smell declines with

age. Older people gradually lose their sense of smell to the point

of being anosmic (unable to smell a certain odor or several odors,

probably because of the loss of neurons). With the loss of the

sense of smell comes the loss of the sense of taste. As is most

obvious in conditions of nose blocks caused by a common cold or

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a runny nose, the sense of smell contributes to the sense of

taste (this is why food tastes different when you are sick and

stuffed up). Smell and taste are both essential for animals

to explore their environment. Some scientists estimate thathumans can distinguish among as many as 10,000 different

smells.

Gases mixing with objects or evaporating from them carry

certain molecules characteristic of these objects. These mol-

ecules may reach the nose and dissolve in the mucus, helping

to generate particular odors. These molecules are referred

to as odorants. To be able to reach us, odorants need to be

volatile (dispersed in air).

Our sensitivity to smell depends on more than just the

strength of our senses. For instance, we are less sensitive

to odorants in the cold for two reasons. One reason for this

is that gases evaporate less in cold weather than in warm

weather; therefore, spring and summer are the best times to

smell. Another reason is that warm weather makes odorants

more soluble and so they mix better with our mucus. Thisenhances our sense of smell.

Odors are distinctiveso distinctive that certain animals,

including common pets, use them to identify other animals or

humans. Odorants can have a powerful influence over mat-

ing behavior, whereby secreted molecules may prepare an

animal for pairing. In addition, strong odorants secreted in a

dogs urine are used to mark territory. In these cases, odor-

ants are more accurately called pheromones (Figure 7.1).

Although the sense of smell is highly developed in humans,

pheromones are thought to influence human behavior less

than animal behavior.

The sensitivity of a dogs sense of smell allows it to deter-

mine the direction or trail of a human by odor. Apparently,

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each one of us has a unique smell or body odor. In fact, even

certain types of twins have distinct body odors. Identical

twins, however, who share similar genes, also share similar

odors; therefore, dogs cannot distinguish between identical

twins based on odor.

THE NOSE

Odorants, when inhaled, enter the nose through the nostrils.

In the roof of each nostril is a region called the nasal mucosa.

Figure 7.1 Pheromones are chemicals produced to send messages to

other members of the same species. In the above photograph, a hon-

eybee fans pheromones from its Nasanoff gland, a common form of

communication with other bees.


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This region contains the mucus-covered olfactory epithe-

lium that in turn contains the sensory receptors or neurons.

Humans have approximately 10 million olfactory receptors;

other animals such as rats and cats have more. The sensory

sheath at the roof of the nostril also contains glands that pro-

duce mucus that bathes the surface of the receptors. This is

where odorant gas molecules dissolve.

Inside the nose, air travels toward odorant-sensitive cells

(neurons) that lie close to the bony structure at the top of the

nasal cavity (Figure 7.2). These cells have extensively branched

dendrites with receptors for different gaseous odorants.

Figure 7.2 Smell receptors are located in the nasal cavity. Once a

receptor is stimulated, an electrical signal is created and passed on to

the olfactory bulb, which then relays the information to the brain.

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When activated by an odorant, these neurons generate an

electrical current that is relayed to another neuron above the

nasal cavity. These two types of neurons are separated by a

bony structure with small holes that allow communicationof electrical signals. Above the nasal cavity, a collection of

axons forms a swelling called a bulb (one per nostril). The

axons then carry odorant information to the brain through

distinct pathways. The brain processes this information as

a perception of smell. Some odorants are strongly linked to

powerful memories and therefore are processed by multiple

brain areas.

ANOSMIA

Severe head injury may damage communication among neu-

rons in the smell pathway. This may cause a medical condi-

tion known as anosmia. Anosmia can be either the complete

absence of smell or loss of the ability to smell particular

odorants. Anosmia may also be temporary, caused by lessserious conditions such as a common cold with a running

or stuffy nose. In this case, the nasal cavity and membrane

may be inflamed and neurons do not process odorants as

well if the nasal cavity is very wet. This common type of

anosmia is obviously reversible, whereas anosmia caused

by brain injury is usually permanent. Other conditions may

trigger anosmia, including allergy to certain odorants or

thick smoke.

The sense of smell also undergoes adaptation. Notice how

we become accustomed to an odorant after we are exposed

to it for a long time: We lose our awareness of the smell. A

typical example of adaptation to smell is reduced sensitivity


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to ones own perfume or natural smell after a short period

of time.

TASTE

The last sensory experience to be discussed in this book is

taste. This is not because taste is the least important of the

senses, but it is the least understood

seeing, hearing, and smelling the world

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