Structure and Function of the Eye: Study Questions 1 ... - Mrs Stovel

Biology 200
Structure and Function of the Eye: Study Questions
1) What stimuli cause the eye to blink reflexively?
2) What is the purpose of: regular blinking?
tears?
3) What causes ambylopia?
4) Which muscles would cause your right eye to look to the
left?
5) What allows the eyeball to retain i ts shape?
6) What causes glaucoma?
7) Describe the layers of the eyeball and their function.
8) Describe the function of the muscles in the iris df the
eye,
9) Describe the muscle control of the lens of the eye.
10) In what orientation are images projected on the retina
of the eye and why do we not perceive them in this way?
11) What are the 3 reflex adjustments that are known as
accomodation?
12) What are the 2 types of receptor cells found in the
retina and what is their function?
13) Describe the adaptation in the cells of the retina that
serves to sharpen the visual image and fr rove
contrast.
14) What is visual acuity?
15) Describe what the fovea centralis is and its
implications for vision.
16) What does the term 20/20 vision refer to?
17) In terms of the Snellen test, describe what hyperopic
and myopic conditions are.
18) Why is it that the rod cells function best in low light
conditions?
19) Explain how the cone cells in the retina perceive
different colors of light,

20) What is the cell ular basis for color blindness? What
part of the population does it affect the most?
21) Briefly describe the following vision disorders:
a) presbyopia
b) cataracts
c) myopia
d) hyperopia
e) astigmatism
22) What is binocular vision?

Structure and Function of the
External Structure
The human eyeball is a slightly battened sphere
about 25 mm (I inch) in diameter. The eyes are
protected from behind by the cone-shaped `b ny
eye sockets (orbits) which are padded by layers of
fatty cushions. Protection of the exposed surface
(cornea) of the eye is provided by moveable
eyelids, fringed with protective hairs (eyelashes]
and moistened by tears secreted by the lacrimal
and conjunctival ;lands (Fig. 1).
The eyelids shut re flexively in respoi se t_
-a )ioi v :sporoadhzng objects and, certatu
stimuli such as sudden bright ilghts. Ever-, ew
seconds throughout the waking hours, the eyelids
blink shut for a fraction of a second to clear debris
from the cornea and to spread the tears evenly over
it. Tears flow at all times, washing away foreign
particles, lubricating the eyelids. and keeping the.
transparent cornea moist. Excess tears drain into
the nasal cavity through the nasolocri a l duct.
The flow of tears is reflexively increased by irritation
of the eyes. emotional stress, and physical
pain. Tears stream from under the lower lids while
others drain into the nasal cavity, producing the
familiar "sniffles;" that accompany crying.
Eye Muscles
Obiique
Figure 2 Extrins'c eye muac€ea, Side one too dims of the a e
and its muscular attachments ure shown n the upper and cwsr
Iiustretions, respectsv fy. ^ontracaon ,t the na i! ra, *,us and
relaxation of the iaterai rectus cause the eye to ro!Pze rtnwarti
(toward the nose). 5im:ler actions of tt^e other wpoosingg p3r3 Qt
muscles allow the eye to look upward. do°wn rard, or laiefaity,
Six extrinsic (extraocular) muscles cause the eve to
move about in its orbit. This makes it possible to
fix the gaze on stationary or moving objects, and to
coo. dinate the two eyes so they ordinarily look at
the same thing at the same time (Fig. 2). These
muscles originate on the bones of the orbit and
insert on the eyeball itself. The control and coerduration
of these muscles, which represent some
of the quickest and most precise movements the
body can make, appears to be a learned process
which develops with the maturation of v?suci
areas in the brain. Malfunctions cause a variety of
defects in visual alignment. including "crosseves"
and amblvopia, which is wandering of the
eye from the intended point of focus.
Three pairs of striated extrinsic muscles are responsible
for each eye's tall range of movement.
Four of the muscles are attached to the superior,
inferior, medial, and lateral surfaces of the eyeball
and, are named accordingly: superior rectus, to
ter c::r medial rectus, and lateral r,-,ctus.
T hey cause the eyeball to turn up, down, medially
(inward], and laterally (outward), respectively
(F g. 3The two re raining muscles are tart=
sta er c,r am fi `e:;:: r ohiiques which, acting alone,
rotate the eyeball.
The Eyeball
Eac:h eye consists of a three-layered membranous
sac and its',ontents. theciqueous humci, vitrcut s

Inferior reCtUS Superior oblique inferior oblique
Figure 3 Eye movements caused by the extrinsic eye muscles.
The eye at the top is focused straight ahead.
body, and lens (Fig. 4). Although the eye is basically
a saclike structure. its shape is maintained as
a fairly rigid sphere by the presence of sern.i-fluid
material within distinct anterior and posterior
compartments. The anterior chamber, located between
the cornea and the lens, is filled with semiliquid
aqueous humor produced by the ciliary
Jody. The aqueous humor drains into tiny canals,
the canals of Schlemm, near the junction of the
cornea and iris, at a rate sufficient to maintain the
shape of the eyeball. Insufficient drainage of aqueous
humor may result in an elevated intrancular
pressure, a painful condition (glaucoma) that can
seriously damage the eye.
The large posterior cavity of the eye, bordered
by the retina, ciliary body, suspensory ligaments,
and lens, is called the vitreous body. It is filled
with a transparent jelly which, like the aqueous
humor, maintains the spherical shape of the
eyeball.
The outermost layer is a tough, fibrous, graywhite
membrane in the posterior part of the eye
called the sclera or sclerotic coat, This layer is
continuous with the transparent cornea at the
front of the eye and appears at each margin of the
cornea as the white of the eye.
The middle laver of the eve wall is the choroid
layer which contains many blood vessels that
bring oxygen and nourishment to the cells of this
structure and to the other coats as well. Many
?,igment granules are found in the chcroid laver,
,`ausing it to have a deep reddish-purple color.
This prevents te:lection of light inside the eye in
much the sane way as a coating of black paint
inside .3 c_ur era el,minates stray rays of light irsat
might otherwise ruin the film.
Around the edge of the cornea the choroid
coat forms the ciliary, body, a thickened structure
containing smooth muscle. Circling the anterior
margin of the ciliary body is a thin muscular diaphragm
with an opening in the center called the
iris. The iris may be blue, brown, green, gray, or
black depending on the amount of pigment (c elanin)
contained in its tissue. If no pigment is present,
the iris appears to be blue. Small amounts of
pigment cause the eve to have a gray or green color
and, as the amount of pigment increases, the depth
of color approaches black. Eye color is a hereditary
trait, meaning that the distribution of melanin
pigment in the eye is controlled by genes.
Two sets of smooth muscles are found Within
the iris of the eye. The inner set is circular and
serves to constrict the inner edge of the iris. The
other set is perpendicular to the circular muscles.
When this set of muscles contracts, the inner edge
of the...iri is dt w-n open. Contraction of the iris
muscles is regulated by the autonomic nervous
system through the oculomotor (third cranial)
nerve.
The opening in the center of the iris is the
pupil, through which light enters the eye. The
diameter of the pupil varies continually and is
regulated by the amount of contraction of the two
sets of iris muscles. Thus, the amount of light
allowed to enter the eye is determined by the size
of the pupil, If a bright light is flashed into the eye,
the circular muscles contract very rapidly and the
pupil becomes smaller. In dim light the other set of
iris muscles contracts and the pupil becomes
larger.
The Lens
Directly behind the pupil is the lens, a transparent,
semi-elastic structure shaped somewhat like a
biconvex disk. As show in Figure 5. the lens is
held in place by suspensory ligaments attached to
the ciliary body of the choroid laver. The suspensory'
ligaments control the shape of the lens by
exerting varying amounts of tension on it. An increase
in tension causes the lens to flatten slightly:,
while a decrease in tension allows the lens to
return to a thicker configuration by virtue of its
elasticity. The amount of tension imposed on the
6

Oistart
okjoct
.'7'uSCiSrr3t3F`;F
tiyga?t9i't
Latera l recfu3 _
Figure pole of th lens it acorn nodatic n fordatant arts n r
vision. 7co- Raiaxation of the c:ilarv musi.ls causes f' sac:: r:
sory ligam ents todraw the iens.nto a flattened configurati or: so
provides ::ha minimum amount of focusing power for cistart
vision. Bcc rr: Contraction of the ciliary muscle rsi Uva,3 rtj.
tension on the suspensory ligamen ts and allays the era to
thicken ' r greater focusing power i, near vision.
refroc: on, 4"Vh ' a parallel light rains pass 1 ough
the biconvex lens of'.:ie eye, the y are bent so as to
converge bt a single point to form a bright pot on
theretin ;:., The point at which a clear focus c:c: nt
is the `+, 3F point or principal rocus and its -, i -
ance from the Jens is the focal length (Fig. 61. he
focal l ength decreases as the curvature o :cr s
becomess more pronounced and increases as the
Flguru 4 Top: Horizontal cross section through the right eye
lens becomes flatter. Thusv when distant obji is
showing interior structures. The bloo d vessels run a,ong the hack
of the eye between the retina and the vitreous body. Bottom: are viewed. the lens is pulled into a slighs_iy flatter
Vert #cat cross section of the antarior portion of the eye showing shape by increased tensiat:t in the suspensor,, hg
the lens ;n relation to the suspensory ligaments e,td chary muscle.
aments, A decrease in tension, accomplished by
contraction of the ciliary muscle, allows t`e ien. to
thicken slightly, which provides additional headli
%ng of lig h t rays for near vsitr:°s.
t r^ lens by the the suspensor ligaments light is re ulated bysource is an abject instead of
the ciliary muscle, a smooth -muscle in the viii .rv point. many light rays from. each part of the object
body.
come into focus at a partic ular distance from the
ens. That Light rays, way, which normally an travel inverter straight image of the obh tct is
lines, slow down when passing through transparent
materials such as water, glass, or the '.ens of the prltted '-jy the brain, it is flipped over and app4, 3As
projected onto the retina. When the image is i`:ter--
eye. "'he path of the light beam, i s bent as it slows so right side= flp.
it travels at a different angle. an event caked Since light rays are bent whenever the:, ; : s

-'.._"'-.....^ Point Source
of light
- C
Figure 6 Refrectwon (bending) of 4fight rays by the lens sv9tc' of -,e eye. i',e kiyht soi.°r

erabie degree before it reaches the Ae o-lying .onto
and cones, so several photoreceptors may be
stimulated by a tiny point of light. If ail of the
receptors responded, you would perceive only
fuzzy blob of light surrounded by a halo. However.
nerve impulses that arise from a light-stimulated
retinal receptor inhibit the adjacent receptors so
they fail to respond to the illumination. This inhibition
sharpens the visual image and improves its
contrast. The mutual inhibition of retinal cells is
demonstrated in Figure B.
When an evenly illuminated surface is
3
:'tO%'=' t;d. all of 11-e retinal rece ptors arc partly infiic)
ited: as brightness increases, so does inhibition.
with the result that the visual field does not seem
as bright as it otherwise would. Therefore, a
brightly sunlit day appears only a few times brighter
than a cloudy one. although the difference in
illumination may be as great as a hundredfold.
'visual Acuity
x tte amp c at at detail the eye can distinguish ;s
known as vi sual acuity and depends upon. the
retina a s well as the refractive system of the eve.
For two distant objects to be perceived as separate
objects, the light rays from those objects must be
focused on separate retinal receptors with at least
are unstimulated receptor in between. in the
center of the retina is a tiny depression called the
fovea centralis. This area contains only cones.
which are packed together more densely than in
any other portion of the retina. Therefore, the fovea
centralis is the region of greatest visual acuity.
When one looks directly at an object, most of the
light rays from that object fall upon the fovea.
The angle formed by light rays entering the
eye from two separate points determines the ability
to distinguish the points as definite o bjects.
Ught
Figure; Dlagrarnmatic cross section of the human retina. incoming light must pass through the fusers and Cella of
the various retinal layers before reaching the Sensitive tips of the rocs and cones. Nerve mpulses from the rods and
cones travel' toward the brain via the optic nerve fibers.
9

Figure S The grillwork Wusion is an exa.-mpie of optical inriibitior, Either set of diagonal bars !!eft and center) can by
viewed as a pattern of alternating black-and-white stripes. However, when the patterns are superimposed to form a
gridwork, as in the right hand illustration, nonexistent gray spots seem to appear at the intersections of the white
Stripes.
n
Visual acuity can be measured with the aid of a
Snellen eye chart (Fig. 0 and 10).
At a distance of 20 feet from the chart, a normal
eye can read the lines down through the
2{}-foot ILne, but none smaller. The visual acuity of
this eye is said to be 200/20 because at 20 feet the
individual can read the 20-foot line. This is normal
vision, which also requires a normal refractive
system because visual acuity is reduced by defective
refraction. A farsighted (hyperopic) person
may be able to read the 15-foot line and have 20115
vision, meaning that he can distinguish letters at
20 feet that a person with normal vision would be
able to read only at 15 feet. Conversely, if the
smallest line that a nearsighted (myopic) individual
can read is that for 100 feet, his visual acuity
is 201100, considerably less than normal according
to the Snellen test.
Chemistry of Vision
Rhodopsin, a chemical compound found in the
rod cells, and a series of closely related chemicals
located in the cones are the substances ultimately
responsible for the ability to see. These chemicals
are broken down by the action of light, stimulating
the photoreceptor to discharge a small electrical
impulse that is transmitted to other neurons in the
retina and, by way of tho optic nerve, to the brain.
Rhodopsin and the other visual chemicals are then
resynthesized by the photoreceptors.
Rhodopsin consists of a form of vitamin A
10
combined with a large protein molecule cailc&J.
opsin, Strong daylight or artificial 'tight causes
rhodopsin to be broken clown to retiner.e and
scotopsin. Since this breakdown occurs faster than
the rhodops n can be resy°nt.hesized (Fig. 11), the
rod cells do not function ore] under strong illumination,
althhough they participate to some decree In
daylight vision. The rods function mainly under
conditions of low light intensity, providing us
with the ability to see at night (night vision). Vision
in dim light has poor detail and little or no color.
Objects can be seen more clearly in dim light if
viewed from the side of the eye, because rods are
lacking in the central portion of the posterior retinal
lining: (fovea centralis) where the majority of
the incoming light is normally focused.
Lack of dietary vitamin A leads to rhodop,-;ire
deficiency, which results in night blindness
(rnyctalopia). Long-term vitamin A deficiency may
lead to permanent loss of rod cells which, like
other types of nerve tissues. are incapable of .regeneration.
Color Vision
Visible light can be separated into six distinct c^ for
components-violet, blue, green, yellow, orange,
and red-by passing the light through a prism (Fig.
12). The wavelengths of these light rays go from
00 0 A (violet) up to 7000 A (red), the limits of
4
F 'S Y
h1rnan colot reception.
Any visible color can be produced by corr^biu

I-1 N
D F 9^I
1-0 T
T F
0 F N P T H
P H U N T D Z
Figure 10 At the prescribed viewing distance. ail ie*lters -„- i
visual acuity chart subtend the same visual angle and for ni rettr
images of the same silo.
It is a common experience to look directly fit L
bright light for an instant and Find thati a similar
ima ge o the glowing bulb remains in ihe
f.e:id for some time afterward. The phenomi::no n
railed a positive afterimage because t r u C:f
the image are both about the same col Dr. `yc
prolonged exposure of the eye to the same bri
light may result in an afterimage called a negat;ve
afterimage. Viewing the light for a few se o^:_
through a piece of red cellophane will produce
negative afterimage that appears to be the complementary
color, green. Repeating the experimeu`
with green cellophane will yield a red negative.
afterimage.
N P X 'r z F, H
1
Figure 9 The standard Snatlen visual acuity chart. The numbers
along the right border indicate the distance from whicn a person
with "normal" visual acuity can perceive the lertara in each line.
The numbers along the left border give the visual acuity index for
each line of print when the chart is viewed from a distance of 2C
Net,
Rhodopsin
Lmi-rhodpsin
i
ing lights of three optical primary coicrs-red,
blue, and green-in varying proportions (Fig. 13).
The retina contains three types of cones, each sensitive
to a wavelength of light that corresponds to
one of the primary colors. The sensitivities of the
retinal cone cells are not exactly to red, blue, and
green light, so considerable overlap exists (Fig. 14).
Any of the hundreds of visible colors can be
achieved by stimulating the various cones in the
proper proportions, just as a painter can obtain any
color by mixing certain amounts of the three primary
hues. Equal stimulation of the three types of
cones is perceived as white light.
Ffgu;e #t A schematic summary of the chemical changes .n t=a
rhodopsin cycle during exposure to light and dark cwe^oitior^..
Rhodopsin is converted to scotopsin and retinene in the pr, w.
ence of strong light. iesynthesis of rhodopsin, which orc.::: r.-
in deiknes$, reguirss a certain amount of vitamin A.
I
,,Jal r
Scotopsin
Retinerle
4 t
Vitamin A

A
Figure 12 A Prism seParates white !igttt into The colors of the spectru
wavelengths of visible light are shown in Angstrom nos.
The occurrence of visual afterimages is
thought to be dependent upon the way incoming
light rays are processed by the retina. As explained
earlier, light reaching the eye is focused by the lens
to form an image on the reti na. -Brief exposure to an
ntense light source apparently stimulates the
cone cells to convey signals to the visual cortex in
the brain for a few moments after the eye has been
directed away from the light, resulting in a posit; ve
afterimage. Prolonged exposure to the light
stimulus causes the cones responding to those particular
wavelengths to become temporarily
fatigued or aadapted. The negative afterimage, produced
by continued activity of cones that are less
intensely stimulated., is darker than the bright light
that caused it: The appearance of afterimages in
complementary colors (red following green:. o„
example) can be explained in the same way.
Afteric-oages are Illusions, serving to remind
us that the senses are sometimes imperfect
mediators between the external world and our
perception of it. Other types of optical illusions
include those involving the iaterprotatiori of spatial
relationships (Fig. 16),
1
,figure 13 V arious =ntermediate colors are produced when the
?rrmary co lors of light are superimposed. Wrritu light results
when equal amounts of the three primary coicrs (red. uue :in,d
aree„' are mixed.
Color Deficiency
inadequate function of one or more types of cones
results in co'or deficiency, the inability to perceive
correctly one or more of the primary colors. This
term is synonymous with "color blindness,'
which is no longer used because of the negative
feelings generated by the word "blindness." The
degree of deficiency is variable, ranging fro °rl
minor confusion of color shades to a total lack of
color inerce.ption. The :nest common fn m of color
deficiency is red-green color deficiency, which is
due to s l ro e:m with tie red receptors. Lorafu u.u
12

4000 bow
An"
wfY^'en'h IA)
Figure to Sensitivity ranges of the three retinal pigments,
between certain shades of red and green occurs,
since light rays of these wavelengths stimulate the
same combination of cones. This defect is a sex -
linked genetic trait which affects 8 to 9 percent of
the male population.
Figure 15 Demonstration of negat.ve
afterimages. Stare fixedly at the center
of the cross for 20 to 30 seconds, then
close your eyes or look at a •,srh €te scar..
face. You was see a cross with the col
ors reversed (i is., the vertical limb will=.
be red and the hor €zontal limb green) if
the image 3 s produced on a white surface.
i ts size will depend upon the d„-
tance of the surface from the eyes
fl
I
Figure 1fi Optical illusions. Left: Two identical ractanrjies super imposed on a photograph of receding railroad
tracks. Although the rectangles are exactly the same size, the uoDer one seems to be longer. We know that the distant
railroad ties are as large as those in the foreground, but any object lying between the rails in the middle distance fin
this case. the top rectangle) is unc snsoiousty enlarged. -enter An i;ptiCal rIiusion in which the vertical lines in bath
figures are exactly the same length, although the one on ins +e`t appears to be apprec,abiy longer Extending the
diagonal lines outward in the left illustration ano inward in the rigryt ilustration causes the viewer to meite an incorrect
comparison of the lengths of the two vertical lines. Rrght An ambiguous "gore redrawn from an illustration by
cartoonist W. E- HO demonstrates the importance of interpretation in the process of vi,ual perception. Eithera young
g€ri or an old woman can be seen, depending upon the viewer s perspective. The Chln at the young girl s also the nose
of they old woman.

Common Disorders of Vision
Presbyopia
Thickening of the lens for near vision is permitted
by contraction of the ciliary muscle, which lessens
the tension imposed upon the lens by the suspensory
, ,gaments. However, it is the elasUeftye#
lens thatactually causes the thickening because
the lens automatically assumes a fairly round
shape as external tension is removed. An estimation
of the degree of lens elasticity can be obtained
by measuring the near point, which is the shortest
distance from the eye that an object can be viewed
clearly.
One of the normal consequences of aging is a
gradual loss of lens elasticity, which begins at an
early age but proceeds more rapidly during later
years. As the elasticity of the lens decreases, the
near point recedes and accommodation becomes
diminished to the point where reading becomes
difficult (Fig. 17). This lack of near vision, known
as presbyopia, is often corrected with convex
lenses or bifocals (a convex insert in regular eyeglasses).
100
90a
7
Cataract
The lens of the eye occasionally becomes opaque
and light rays are prevented from reaching the
tw 20 30 i0 50 00 70
Age to Y"ra
Figure 17 The near point recedes with advancing ag:e.oecauss
the lens of the eye gradually loses its ability to accomr: ;date `,,r
near vision. The most dramatic changes are usually no,eo in
humans beyond 40 years of age.
Figure 18 Left: A normal lens transrnits it -°n ng iinh' so it fFilis on toe r t;na `Ovesa.. Ighr: C^pactty of ire lens, a
condition known as cataract, prevents the incoming Sight rays from oass?ng through to the retina.
14

ens
Norm,
eye
MacuO .- _^^._ # tit
r_ _.-
lutes L
Astigmat"u.'n
Figure 1 S ton: Nesrsigfltecf ess. or myos ,a, s a c onc;itioa in n?sr. ;ncc^rnang ri:fit t r2ys oc spa to focus in ira t 2" °ne
retina. The ur biem is Cnrrecre by placing a biconcave lens in !r:nt of the eye, i'enrer: The focal coint for,ncoro:rr,g
light rays is behind the retina lr, the :ar dition known as farsighteortess or hyperopia. A biconvex lens is used to correct
this problem Bottom: Astigmatism its condition in which uneven focus^nq of fns visual image results from distort,Q",
of Me curvature of the lens or Cornea. The problem is corrected with an optical lens that has the same degree u
astigmatic n: but at right angles 'o the otarie of astigmatisrn In ti^e eye.
retinal photoreceptors. This condition. .:own as
cataract, is treated by S irgirc l re.Tr va of th e lens
and strong eyegleisses to conip risate or its absence
(Fig. 18). A new treatment, which involves
repiacement of the clouded lens with a plastic one
F
lacking the power of accommodation, is stil l
in the
experimental stage.
Nearsightedness
Nearsightedness or myopia occurs rshert Nle
eyeball is abnormally long. This causes the visual
imeg€ in come i.ito focus .',?7 front of the -tiny (A '
t 91.
myopic 'individual Can See i ell at slhoct
.%xr F:t`sa, but tf e lens cannot flatten -n, g i E
focus distant obiects on the retina. The, problem IS
ct . cectesd with a conc .ve lens, which causes light
m vs from a distant obiect to diverse li gfitly as t .€;i
ente.1 the eye o the focal point falls or, ti e: retina.
Farsightedness
TN opposite of nears'gtitfrlness is t € t dies:,
or iIvperr^i^irz, wahich occurs ^vh n the y==ball i,,
kJ

abnormally short. This Ca^nses the visua
to be focused behind the retina (Fig. %91. The eye
inctions normally when distant objects are
sewed, but the refractive system, lacks adequate
strength for accommodation in near vision. The
condition is treated by placing a convex lens in
front of the eye so light rays from near objects
converge slightly before they enter the eye, This
causes the visual image to, be focused properly on
the retina.
Astigmatism
unfortunately, the elements of the refractive systern
are not always perfectly symmetrical. imperfections
in the shape of the cornea or lens cause
incoming light rays to converge unevenly so they
do not focus at a specific point on the retina. This
condition is called astigmatism (Fig. 1`'i and results
in blurred or distorted visual images. Using
an astigmatism testing chart (Fig. 20), even minor
amounts of astigmatism can be detected. To an
individual with an asymmetrical cornea, lines in
one plane appear sharp and clear while those at
right angles are fuzzy and poorly focused. The
tuation is corrected by a lens with the same de-
,;#ee of astigmatism. but at right angles to that of the
cornea so the effects cancel each other,
Figure 20 An asi^gmatism testing chart. € he radiating lines we
appear to be darker aro bolder in t he planes of the eye which are
astigmatic. Since Eight rays are bent to a greater degree where the
cornea has the most pronounced curvature, light passing
through that area wilt come to a tacos in front of the retina,
resulting in a blur. The eye tends to focus light from Me flattened
(astigmatic) area; that from She normally curved surfaces will be
refracted too much.
16

Visual Fields and Binocular Vision
We have seen how rays of light are focused by the
lens to fall in a concentrated pattern on the sensitive
photoreceptors of the retina. thus stimulating
the formation of visual images. The portion of the
external world t be seen. Brit t mmoving,
by one eye is called the visual field. The extent of
the visual field is limited chiefly by the bony structure
of the eye socket and by the bridge of the nose,
in humans the two eyes are only a few centimeters
apart so they register similar views of the external
world. In fact. the visual fields actually overlap,
which makes binocular vision possible. Binocular
vision is the reception of slightly different visual
images by each of the two eyes. Light rays from
different parts of the visual field come to a focus at
a particular point on each retina. The light rays
from the object of focus (i,e.. the object that is being
looked at) fail upon the fovea centralis. while
those from surrounding areas come into focus at
nearby retinal sites.
When a single object such as a pencil tip is
viewed with both eyes, the light rays from it fall
upon the fovea of each retina, yet the visual image
formed is of a single object. The retinal sites stimulated
in the two eyes are said to be corresponding
points. All sites in the binocular field of vision
have corresponding points in the two retinas, and
when these points are stimulated, a single visual
image is seen. If the extrinsic eye muscles do not
move one of the eyes properly, the visual image
may not fall on corresponding points and, as a
result, two images are perceived. This condition of
double vision is called diplopia, The individual
often learns to ignore the aberrant image and, if the
situation is not corrected, the ignored eye may
eventually become functionally blind.
--------- Visual F.eid of Rignt Eye ----------+.
po'l,o r of v'Sclal
fisid pro €actao onto w...__
lets Yasuai cortex
T*W "W-01 We"
xl
Depth Perception
Depth perception is the ability to judge the relative
distances between far and near objects. This ability
is required in many situations, such as judging the
speed of oncoming cars to avoid a collision. The
basis of meaningful depth perception is the fact
that the two eyes do not see exactly the same visual
field. With only one eye the field of view appears
to be two-dimensional; a person with only one eye
must rely upon learned cues to judge distances,
Flgurn 21 The visual fields overlap to permit birtocu'ar vi ,
Ugt't reflected from a single obtect causes nerve it puaos to
transmitted from woth retinas. but they travel to only orte cere,)F ai
ham.sphere, Nerve impulses from the left sides of both ret:ras a,'a
Conducted via the optic nerve to the optic Chlasrn, 'cvhc e t^,a
fibers from the medial portion of the right eye cross over to ;ci7
the eft optic tract. This crossover occurs in the opposite direction
rt the case of optic ne-ve tibers from the medial s e E ` t:=
eft retina and the lateral portion of the right retina. After syrap•s,
ing the 'lateral geniculate bodies, the nerve imt+ul :as proceec
to the r•,mary visual areas rn the ocC;pltaf lobes of the person-n

such as the fact that objects appear to become
smaller as they move away. Two eyes result in
greater perception, because the two eyes see
slightly different views of the same objects. These
images are interpreted in the visual centers of the
brain as one three-dimensional image.
Central Visual Pathways
he translation of light rays into detailed visual
images is a complicated process involving the
brain as well as the eye (Fig. 21). Light rays from
objects in the left portion of the visual field fall on
the right side of the retina, and those from objects
in the right portion of the visual field come into
focus on the left side of the retina. Nerve fibers
from all of the retinal receptors come together to
form the optic nerve, which conveys impulses
from the light-stimulated receptors to the brain,
Some of the nerve fibers cross over at the optic
chiasm, so all fibers arising from the nasal (medial)
half of each retina -,ross to the opposite side. As a
result the left optic tract carries fibers from the left
half of each retina, and this represents the right
half of the visual field for both eyes. The optic tract
continues to the lateral geniculate body, a part of
the thalamus, and onward to the visual cortex in
the occipital lobes of the cerebrum. The primary
visual receiving area is localized mainly in the
medial portion of the occipital lobes, which is in
the hind part of the cerebrum. Each occipital lobe
receives visual stimuli from one half of each retina
(Fig. 21).
Anterior to the visual : e.eiving area are the
visual association areas where integration, interpretation,
and storage of visual information
occur. This is the final point in the complex
neurochemical pathway along which light rays
reflected from objects in our external world are
perceived in terms of form, color, and detail.
18

Suggested Activities
Visual Acuity
Visual acuity, the degree of detail the eye can
distinguish. is measured with a Snellen visual
acuity chart (Fig. 9). The chart should be attached
to a wall or door so it can be viewed from,* distanceof
exactly 20 feet.
The subject should perform the test initially
without eyeglasses or contact lenses. The test can
then be repeated with corrective eyewear. Standing
20 feet from the Snelle:n chart, the subject
covers one eye with a slip of paper and reads the
successively smaller lines of print until the letters
can no longer be distinguished correctly. The last
line that can be read without mistakes identifies
the limit of visual acuity for that eye. The test is
then repeated for the other eye.
If the smallest letters that can be read are on
the 20-foot line (marked on the border of the Snelten
chart), the subject has 20120 vision in that eye.
This is considered to be normal visual acuity. If the
letters below the 100-foot line cannot be distinguished,
the subject's visual acuity is 20/100, considerably
less than normal. On the other band,
visual acuity of 20115 is better than average according
to the Sneilen test, but a farsighted (hyperopic)
individual who achieves this score may not be able
to focus well on closer objects.
Astig matism
Astigmatism is blurring or distortion of the visual
image by improper convergence of light rays as
they enter the eye through imperfections in the
cornea. Many people have varying degrees of astigmatism
without being aware of the condition.
The test for astigmatism should be performed
without corrective lenses because astigmatic compensation
is usually included in eyeglass prescriptions.
The astigmatism test chart (Fig. 20) resembles
a wheel with the spokes radiating outward from
the center. When viewed from a distance of a to iC
feet, the lines in certain planes will seem exceptionally
thick and dark to the astigmatic eye. The
chart should be evenly lighted and viewed
monocularly (one eye at a time) to test for astigmatism
in each eye individually. Repeat the test
with corrective lenses, if usually worn, to determine
whether total astigmatic compensation is
provided.
Visual Fields
The portion of the external world that can be
viewed by one eye while the attention is focused
on a stationary point is the visual field of that eye _
\.isu t fields are measured with a perimeter,
which is a device shaped like a large half circle
marked with a white fixation point in the center of
the arc, The subject is positioned behind the
perimeter so the curvature of the arc comes toward
his or her head in the horizontal plane. WVith one
eye occluded, the subject directs the focus of the
other ;ye upon the fixation point. The person conducting
the test places a movable marker, similar
to the fixation point, beside the fixation point and
slowly moves the marker around the arc t: ware
the side of the subject's head until the subject c n
no longer see it.
t is important for the subject to refrain fm m
following the marker with his or her eye. T. he angle
marked on the perimeter at the point whexe the
moving target is no longer visible defines the lateral
limit of the visual field horizontally. The same
procedure is followed to establish the limit in the
opposite direction (toward the nose), These points
are recorded on a perimetey record chart. The process
is repeated with the perimeter set venicady
and at 45' angles in both directions. The other eye
is then tested in the same manner.
Color Vision
Deficiencies in color perception are ascertained by
a variety of testing methods. One of the best is the
Ichikawa color vision test. This consists of a set of
printed forms containing numbers which cane
recognized only by individuals whose perception
of certain colors is normal. The subject examines
all plates in the booklet and records the number
that appears in each- Correct interpretations of the
various colored plates are included with the testing
materials.
The i-Loirngren color vision test is another c-...-
cellent method for testing color perception in thou
classroom. A set of wool strands, each dyed a
specific color, is mounted in a folder. The subject
attempts to match each strand with an identic.ai
strip of yarn. Mismatching or hesitation in placing
the strands correctly is probable evidence of color
perception deficienccy.
5

Near Point and Accommodation
Near point and accommodation exercises are performed
by teams of two students. The near point is
determined by holding a ruler near the outer (lateral)
edge of the subject's eye while the subject
moves a pencil or similar object toward the eye
until the visual image becomes unclear. Be careful
to avoid actually touching the eye. The near point
is the distance of the object from the eye (read
directly from the ruler). Record the near point and
repeat the test for the other eve, Compare the two
readings to see if the near point is the same for both
eyes. Correlate these values with Figure 17 to determine
whether the near point is normal for the
subject's age.
Make a pinhole in a slip of paper with the tip
of a pencil. Repeat the near point test while the
subject views the object through the pinhole. The
pinhole causes incoming light rays to be directed
through the central portion of the lens so a clear
visual image is formed, even at distances shorter
than the near point. The pinhole in this case serves
the same purpose as pupillary constriction in accommodation
for near vision, although the iris of
the eye cannot produce such a small aperture.
There are three simultaneous reflex adjustrents
in accommodation: bulging of the lens, constriction
of the pupils, and convergence of the
eyeballs. Of the three, only the latter two can be
demonstrated in the classroom. Hold a pencil
about 60 cm (2 ft) in front of the subject's eyes and
move it slowly toward his or her face. Watch for
convergence and pupillary constriction as the object
approaches the near point. List the muscles
involved in the process of accommodation,
face through the pinhole card used in the last
exercise. Close the other eye, and note the apparent
increase in size of the illuminated field. Reopening
the eve causes the field to become smaller. The
changes in size of the illuminated field are the
result of alternate dilation and constriction of the
pupil of the eye that remained open. although the
light and dark stimuli were presented to the other
eye.
Afterimages
Flash the beam of a penlight into one eve for an
instant an then close your eyes. T he light bulb
will be seen for a short time as a positive afterimage
(the same color as the light). Repeat the experiment
two more times with sheets of transparent red and
green plastic in front of your eye. What color the
afterimage produced by the red plastic? The green
plastic? if the flash of light is brief (one second or
less), the afterimage will be positive (red for the red
plastic and green for the other).
Shine the penlight into your eve for about 20
seconds and quickly look at the piece of white
paper. Blinking your eye will make the afterimage
more apparent. Repeat the procedure using the red
and green plastic sheets. The afterimages produced
are opposite the colors of the light stimuli
(dark for the yellow light, red for green light, an 6
green for red light). These are negative afterimages
caused by the bleaching or fatiguing of certain
visual pigments by prolonged contact with colored
light. Saturation of the pigments responsive to red
light gives a negative afterimage in the complementary
color (green) and vice versa.
Consensual Light Reflex
The amount of light entering the eye is determined
by the size of the pupil. Direct the beam of a penlight
into one eye of the subject and observe what
happens to the pupils of both eyes. W,Vhen a bright
light is flashed into one eye, the circular iris muscles
ofboth eyes contract. constricting both pupils,
This phenomenon is the consensual light reflex.
Darken one eye while observing the pupil of the
other eve. Do the pupils dilate in both eyes when
oniv one eve is darkened?
The consensual light reflex can be observed in
yet another way. Look at a swell-illuminated sur-
The Blind Spot
We are ordinarily uneware of the blind spot in the
visual field produced by the optic disc. its existence
can he demonstrated by closing the left eye
while focusing the right eye on the cross in Figure
22. hold the page about 50 cm {20 inches) fro dm
20

your race with the figure directly in front of your
right eye. At that location. the cross and the dot
should both be visible, While keeping your left eye
closed. slowly move the page toward your face
until the dot seems to disappear. At this point, the
image of the dot fails an the optic disc or blind
spot, which contains neither rods nor cones.
Measure the distance at which the dot disappears
and compare this value with results obtained by
other members of the class. Plot these data on a
graph with distance as the X-axis and the number
of students on the Y-axis.
The size of the blind field can be determined
quite easily. Hang a sheet of plain white paper an a
wall and mark a dark "X" on the left hand side,
The subject should close the left eye and stand 3C+
cm (12 inches) from the wall, staring intently at the
"X" with the right eye. It is easy to maintain the
30-cm distance by placing one end of a ruler
against the wall and the other end against the
subject's forehead. The subject's partner moves a
pencil, covered with white paper except for the
tip, slowly across the sheet of paper. When the
pencil point seems to disappear, a mark is made on
the paper. When it reappears, another mark is
made. This procedure is repeated in various directions
until an area is mapped on the paper which
cannot be seen while staring at the "X." Repeat the
optic di sc i s located on the med°iai portion of .,. .
retina.
The optics of this experiment are shown diagraammatically
in Figure 23. From the results of
your experiments and the following formula, ccl
culate the diameter of your own retinal blind s pot:
Ala CD
A • Dimenian on paper ifmm experiment)
s = Dimension on retina (unknown)
C = Distance from paper to eye (300 mm)
D = Focal length of eye (17 mm)
if the diameter of the blind field an ' ; € r==
is -40 :mn. the retinal blind spot can he eterm.zr
by applying; the formula as follows:
40 .m = „ Gi,
a 17mm
300 B aao rpm
a 2.,3 mm (diameter)
The distance from the center of th e bii= ;,f s,-J,.-
to the. fixation point in the center of the tos rs r a
be determined by the same formula, usi r: ft c
distance from the center of the blind field to t
"X" as value A.
Dominant Eye Determination,
Most experiment individuals for the left eye, using a sheet of paper do not make equal use of e
with an "X" on the right hand side. The area mapped
for each eye is the blind field. which is located than the other, making it the dominant ey,. T_
eyes. One eye is usually relied upon more heed
an the lateral side of the visual field because the determine which eye is dominan t, roll a sheer
300 T
Targ4<

paper into a tube about 4 cm (1'° inches) in diameter.
Using both eyes simultaneously, view some
object across the room through the tube. Holding
the tube steady, close and reopen first one eye and
then the other. The eye that is most closely aligned
with the object being viewed through the tube is
the dominant eye.
Retinal Blood Vessels
The layers of the human retina (Fig. 7) are arranged
so incoming light rays must pass through the entire
retina, including the vascular bed, before reaching
the rods and cones. The blood vessels, therefore,
cast a continuous shadow on the visual field. We
are not usually aware of these shadows, because
receptor adaptation and fatigue make it impossible
Further Reading
Favreau, O.E. and Corballis, ,4.C., Negative aftereffects
in visual perception, Scientific American,
1976, 235(6), 42-48.
Guyton, A.C., Basic Human Physiology: Normal
Function and Mechanisms of Disease, W.B.
Saunders Co., Philadelphia, 1977.
)acob. S.;'., Francone, C.A., and Lassow, L't',).,
Structure and Function in Man,
Saunders Co., Philadelphia, 1978.
Landau, B,R., Essential Human Anatomy and FhvsioloW,f.
Scott, Foresman and Co., Glenvie^^ ,
ill., 1976,
Luciano. '7.S.. Vander, Al,, .and Sherman. J.H,,
Human Function and Structure, McGraw-
Hill, Inc., New York, 1973.
artin, to view indefinitely R., What any image fixed it means in position on to be color deficicn"',
the retinal surface, The retinal blood vessels become
apparent, however, when we make their )ulesz, B., Cooperative phenomena in binocular
innov io s in Sight, May, 1979.
shadows move.
depth perception, American Scientist, 19"4.
Using the pinhole card made previously to 62, 32-43.
determine near point and accommodation. look at McClintic, 1.R., Physiology of the Human Body,
a brightly illuminated white surface while moving John Wiley and Sons, New York, 1978.
the card rapidly from side-to-side, Within a short Michael, --,R.. ,Retinal processing of visual images,
time you should become aware of a delicate, lacy Scientific American, 1969, 220(5), 104-114.
network with an open space in the center. The lacy Morrison, T.F., Cornett. F.]., Tether, l.E., and Gratz.
filaments represent the shadows of the retinal P., Human Physiology, Holt, Rinehart and
capillaries and the clear opening is the fovea centrails,
from which most of these vessels are lack-
Soloman, E.P, and Davis, P:W., Understanding
Winston, New York, 1977.
ing.
Human Anatomy and Physiology, McGraw-
Hill, New York, 1978.
22