CJ Shuster
Lab Addenum
Physiology of Vision
1
PHYSIOLOGY OF VISION
(Adapted from Johnson, Weipz and Savage Lab Book)
Introduction
The sense of vision is a very complex sense involving several processes that occur
simultaneously to allow us to perceive the world around us. Special light-sensitive
photoreceptors (some able to distinguish differences in the wavelength or color of light) are
present in the retina of the eye. Other structures such as the lens and cornea gather the light
entering the eye and bend or refract that light to form a clear image so that we can distinguish
size, shape, and form. The iris adjusts the intensity of light by adjusting pupil size so that the
image is neither too dark nor too bright. Extrinsic eye muscles work so that both eyes are
directed to the same point. Finally, the information from the photoreceptors must be directed to
the brain where this information can be processed. All these processes must be occurring
correctly in order to experience normal vision. Disruptions or abnormalities of any of the
structures of the eye can cause visual impairment.
Ophthalmoscopy.
The ophthalmoscope is a device that allows the retina of the eye to be examined by peering
through the pupil. A concentrated beam of light is directed by the ophthalmoscope to allow you
to see inside the posterior cavity of the eye. Examining the retina can give you information
about the fitness of the eye itself, and it can also give indication of other disorders such as
atherosclerosis and diabetes.
The ophthalmoscope is not an easy instrument to use successfully. They are available in the
lab for your use if you wish. Instructions on the use are with the ophthalmoscopes, and your
instructor will assist you if you wish to use one.
CJ Shuster
Lab Addenum
Physiology of Vision
2
Lab Exercises
Near Point Determination.
If you have needed corrective lenses for reading since you were a youngster (i.e., teens or
before), you should wear them for this test. If you did not need corrective lenses for reading as
a youngster, but do now, you should remove them for this test.
Obtain the near point determination ruler that is calibrated to make this measurement easier.
Test each eye individually while covering or closing the opposite eye. Place the "zero" end of
the rule directly under the eye being tested with the rule resting against your face. Move the
sliding piece that holds the lettered card as close to your face as you can without causing the
letters to be blurred. This is the closest distance at which you can bring your eye to focus. This
can be read from the calibrated rule as a distance (in cm or inches) or as an age equivalent.
The 4th scale is a diopter scale which refers to the degree of curvature and is used to
determine proper corrective lenses.
As you can see from the rule, the near point moves father out with age; particularly after age
35-40. This is due to a loss of elasticity of the lens that occurs with aging and is the reason why
older people frequently need corrective lenses for reading or close work. Record your results
on the Data/Analysis Sheet.
Visual Acuity.
If you wear corrective lenses, you must remove them for this test in order to determine your true
visual acuity.
Within you eye, most refraction happens in the anterior chamber(due to the aqueous humor)
and the lens.
*where these lines intersect is the FOCAL POINT.
Acuity depends on the ability of the lens to get the focal point on the macula lutea.
*NOTE: anything that does not hit the macula lutea will not be in focus! Lack of acuity!
<-------this is blurry vision
CJ Shuster
Lab Addenum
Physiology of Vision
3
SO: the ability to focus depends on the lens’s ability to refract properly. The ability of any lens
to refract properly depend on 2 factors:
1. Distance of the object from the lens.
*in this example, moving the image
closer will put the focal point on the
macula lutea.
2. Shape of the lens. Rounder shortens the
focal distance; flatter lengthens it. So, I
could also move the focal point to the
macula lutea by flattening the lens!
* You eye changes the focal length by changing the shape of the lens ... keeping focal
length constant is called ACCOMMODATION. The suspensory ligaments do this. Under
normal conditions (“EMMETROPIA”), a person focuses on a line that is about 5" long
from 20 feet away. To test: ability to distinguish arabic letters. If the patient can see at
20 feet what a “normal” person can see, they have “20/20". Maybe they can see the
same letters at a farther distance (say, they can see the letters at 30 feet = better than
normal = “30/20"). If their vision is less than normal, they have to stand closer (say, for
example, they have to stand 10 feet away to see the line that a normal person can see
from 20 feet away = 10/20).
Abnormal conditions:
1. M yop ia. Elongated eye. Focal point
within poste rior ca vity. Person s elf-co rrects
the condition by moving the object closer
(“near-sightedness ”).
2. Hyp eropia. Narrowed eye. Focal point
at imaginary point behind retina. Person
self-c orrects b y kee ping the objec t at a
greater distance (“far-sightedness ”).
3. Presbyop ia. Special case of hyperopia,
wh ere lens loses elastic ity.
4. Astigmatism. Due to sm all
impe rfections in cornea/lens (facets), there
are multiple focal points.
5. Legally Blind. Visual acuity falls below
20/200.
CJ Shuster
Lab Addenum
Physiology of Vision
4
The Snellen eye chart and a variety of other eye charts are routinely used to determine visual
acuity, or the sharpness of vision. In expressing visual acuity, the
eye being tested is always rated in comparison to a "normal" eye.
The charts are made so that the "normal" eye should be able to
resolve the characters on the line marked 20 feet from the actual
distance of 20 feet. If the eye being tested can resolve smaller
characters, the eye has better visual acuity than normal. If the
eye being tested can only resolve larger characters, the eye has
visual acuity poorer than normal.
Visual acuity is expressed as a ratio (i.e., 20/20). The first
number is always 20 and represents where you were standing
when you were able to correctly read all the letters of a particular
line on the eye chart. The second number represents the
distance at which the normal eye would be able to correctly read
all the letters of that same line.
If the second number of the ratio is smaller than 20, visual acuity is better than normal (i.e., you
can resolve the same characters from farther away than the normal eye).
If the second number of the ratio is greater than 20, visual acuity is diminished (i.e., the normal
eye can resolve the same characters from farther away than you can).
To test your visual acuity stand on the 20 foot line and look at the eye chart. Test each eye
independently by closing or covering the opposite eye. Read the chart while a lab partner
checks your responses. Stop as soon as you make an error. Your visual acuity should be
expressed as 20/the number of the last line you resolved entirely correctly. Record your
results on the Data/Analysis Sheet.
Pupil Accommodation to Distance.
As a general rule, objects closer to the eye reflect a greater intensity of light than do
objects further from the eye. This being the case, you should be able to see a change
in pupil size as a subject changes focus from a far object to a close one.
Have a lab partner stand and focus on an object across the room. Then have him/her
raise a piece of paper in front of his/her face and focus on the words on the paper.
Note if there is any change in pupil size as your lab partner switches between far and
near focusing. (The change is more observable if you use a lab partner with a light
colored iris.)
CJ Shuster
Lab Addenum
Physiology of Vision
5
Pupil Accommodation to Light Intensity.
The eye responds to increases in light intensity by constricting the pupil to limit the light
entering the eye. This test looks to see if shining a light in one eye only causes the
opposite eye to react as well as the stimulated eye.
Obtain a small penlight flashlight. Hold it directly in front of a lab partner's face about
2-3 inches from one eye and briefly turn the light on while you watch the opposite eye.
Note if the opposite pupil constricts and dilates as you turn the light on and off. (The
change is more observable if you use a lab partner with a light colored iris.)
The result of this test is explained by examining the pathway that nerve impulses
originating in the retina follow as they are carried to the visual cortex on the occipital
lobe of the cerebrum of the brain (see Figure #1 on the following page). After leaving
each eye the two optic nerves extend toward the midline of the body and cross over at
a point just anterior to the pituitary gland. This cross-over point is called the optic
chiasma. At the optic chiasma the nerve impulses originating from photoreceptors on
the inside portion of each retina cross over and are transmitted to the opposite side of
the brain. However, nerve impulses originating from photoreceptors on the outside
portion of each retina do NOT cross over and are transmitted to the same side of the
brain. As a result the retina of each eye transmits nerve impulses to both halves of the
visual cortex. This allows the input from the two eyes to be processed together rather
than separately. One big advantage of processing information in this way is the
enhancement of depth perception.
Record your observed results of this test on the Data/Analysis Sheet and answer the
questions there.
CJ Shuster
Lab Addenum
Physiology of Vision
Figure #1 — Afferent pathway for visual impulses.
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CJ Shuster
Lab Addenum
Physiology of Vision
7
Astigmatism Test.
An astigmatism is an uneven curvature of the lens and/or
cornea caused by imperfections called facets. This
causes different light rays to be bent at different angles
depending on where they pass through the lens or
cornea, and this prevents a clear image from being
formed in all parts of the visual field at the same time.
To test for astigmatism, stare at the center of the
pattern printed below. Test each eye independently by
closing or covering the opposite eye. If in looking at
the pattern some of the lines appear clear while others
are a little fuzzy or if some appear dark while others
appear lighter, an astigmatism is indicated. Record
your results on the Data/Analysis Sheet.
Nearsightedness (Myopia).
Nearsightedness or myopia is a condition where the
individual can focus clearly on objects that are close, but is unable to focus clearly on objects
that are far away. This can be due to an eye that is abnormally long from front to back or a lens
that is too strong in terms of the refraction or bending of light rays. In both cases the light is
brought to focus an image too early, before the light has reached the retina. Since light coming
from close objects requires a greater degree of refraction, clear images of close objects can be
formed.
Nearsightedness can be corrected by use of an external lens (glasses or contacts) that delays
the focusing of the light rays. Such a lens is called a divergent or concave lens and is thinner in
the center and thicker on the edges. Answer the questions about myopia on the Data/Analysis
Sheet.
Farsightedness (Hypermetropia).
Farsightedness or hypermetropia is a condition where the individual can focus clearly on
objects that are far away, but is unable to focus clearly on objects that are close. This can be
due to an eye that is abnormally short from front to back or a lens that is too weak in terms of
the refraction or bending of light rays. In both cases the light is not yet focused when it reaches
the retina. Since light coming from objects far away requires a lesser degree of refraction,
clear images of far away objects can be formed.
CJ Shuster
Lab Addenum
Physiology of Vision
8
Farsightedness can be corrected by use of an external lens (glasses or contacts) that aids the
eye in the focusing of the light rays. Such a lens is called a convergent or convex lens and is
thicker in the center and thinner on the edges. Answer the questions about hypermetropia on
the Data/Analysis Sheet.
Field of Vision.
Obtain the disk that allows you to test your field of vision. To use this disk, place it against your
forehead and stare intently, straight ahead, at the target on the disk. Have your partner move
the mobile target, first to one side, then to the other while you continue to stare straight ahead.
Read the exact angle at which the mobile target completely disappears from your field of vision.
Record your results on the Data/Analysis Sheet.
Depth Perception.
Locate the apparatus for testing depth perception. The person being tested should be seated
in front of the test apparatus several feet away from it, and close or cover one eye. Their
partner should take the upright standard which is not attached to the string and place it at a
position of the partner's choosing along the test apparatus millimeter scale. The person being
tested then uses the string to move the second upright standard back and forth until they feel
the two arrows are aligned exactly. When the person being tested is satisfied that the arrows
are aligned exactly, their partner should look at the millimeter scale to see if there is any
difference in alignment. Record any error factor (i.e., distance in millimeters between the two
uprights) on the Data/Analysis Sheet.
Color Blindness.
There are three kinds of cones, each "tuned" to absorb light from a portion of the spectrum of
visible light
a. cones that absorb long-wavelength light (red)
b. cones that absorb middle-wavelength light (green)
c. cones that absorb short-wavelength light (blue)
In color blindness, one or more classes of cones are non-functional. Genes for red & green
cones are on the X chromosome; therefor it is a “sex-linked” trait, and males get it more than
females.
- males only have one “X” chromosome, so a mistake on this chromosome is always
exhibited. Females have 2 “X” chromosomes, so a defect on one is made up for by the
other chromosome. In order for the female to get the disorder, she must have 2 “bad”
chromosomes, which is MUCH rarer.
CJ Shuster
Lab Addenum
Physiology of Vision
9
-NOTICE: the test uses “points” of color; the brain will “fill them in” with the nearest
color, just as it did with the blind spot. Or, it will see the “other colors present”, and they
will see something completely different (perhaps another number).
A set of charts that test for color blindness is available in the lab for your examination. These
test for the different forms and degrees of color blindness.
Blind Spot Test.
As mentioned on the first page, there are no photoreceptors on the retina at the point where the
optic nerve forms and passes through the wall of the eyeball. SEE IMAGE ON FIRST PAGE
OF THIS HANDOUT.
Although the eye cannot "sense" the light that falls on this part of the retina, the brain "fills in"
this area for you by assuming that the light falling there is similar to the light falling on
photoreceptors adjacent to the blind spot. A simple exercise can trick the brain into "filling in"
the wrong information and thus can show you that the blind spot does exist.
Hold the lab sheet about 18 inches from your face. To test your right eye, close or cover the
left eye and stare at the plus sign printed below. Slowly move the paper close to your face
while you continue to stare at the plus sign. At some point you should notice that the black dot
seems to disappear as the light from it falls on your right eye's blind spot. Use a ruler to
measure the distance between your eye and the paper when you experience this phenomenon.
Record the distances on the Data/Analysis Sheet.
Test the left eye, also. To do this you must stare at the dot and watch for the plus sign to
disappear. Record your results on the Data/Analysis Sheet.
CJ Shuster
Lab Addenum
Physiology of Vision
10
Data/Analysis Sheet.
Right Eye
1.
blind spot (inches)
2.
near point (inches and age)
3.
visual acuity (without lenses)
4.
visual acuity (with lenses)
5.
astigmatism (yes or no)
6.
field of vision (angle)
7.
depth perception (mm)
(error factor)
8.
light intensity and pupil size
a.
9.
Left Eye
When you directed a light into one eye, did the opposite eye also respond by
constriction of the pupil? Use your knowledge of the visual pathway to the brain to
speculate as to why this occurs.
accommodation reflex
a.
What change in pupil size do you notice as a person changes from looking at a far
object to looking at a closer object?
b.
Speculate as to why this change occurs.
CJ Shuster
10.
Lab Addenum
Physiology of Vision
11
Myopia and Hypermetropia
The illustrations below represent these two common abnormalities of refraction within the
eye. Indicate below each illustration the name for each condition and the abnormality of
eye shape or lens strength that could cause that condition. Also, sketch in front of the
eyeball at the right the proper corrective lens for the condition with the light rays passing
through the lens into the eye and focusing on the retina.
condition
caused by:
abnormal eye shape
lens abnormality
condition
caused by:
abnormal eye shape
lens abnormality
11. Define:
Glaucoma
Astigmatism
Cataracts
Legally blind
20/20 vision