Eye Dominance:
Place a coin on the floor. Carefully roll this paper into a tube and look through the tube with both eyes in
order to observe the penny. Now close one eye at a time while still looking through the tube. With which
eye could you see the penny?
This is your dominant eye!
Corresponding points:
All retinal sites in the binocular (overlapping) visual field are corresponding points, so you see only one
image even though both eyes are stimulated. The object that you are focusing on is brought to focus on
the fovea centralis of each retina. If light rays fall on non-corresponding points outside the binocular
visual field, you see two images. In your peripheral vision, for example, you perceive different images
with each eye. To demonstrate this, look straight ahead and raise your right thumb up with your arm fully
extended in front of your nose.
Diagram how this image appears in your field of view. The shaded areas of your R and L visual fields
below represent areas of overlap -binocular vision.
L
R
F
F
Notice that the blind spot is not on corresponding points of each retina. Your brain “fills in” the missing
piece from the other side. The fovea centralis (F) is on corresponding points.
Now, looking straight ahead and keeping your arm fully extended, move your thumb slowly to the side of
your head at eye level until it is just visible. Close your left eye. Do you see your thumb?
Close your right eye? Do you see your thumb?
Diagram the location of your thumb in your field of
view.
Look at the diagram above and gently push on the lateral (temporal) side of your left eyeball. How many
images do you see? Why?
Binocular Vision and Depth Perception
Depth perception results from the fact that the visual fields of the two eyes is slightly different and is
interpreted by the brain to give a 3-dimensional view of the world. Vision with one eye is 2-D. To test
this, have your lab partner hold a test tube about 2 feet in front of you. Quickly insert your pen into the
tube. Close one eye and repeat. What happens? Why?
To test your depth perception quantitatively, use the depth perception test kit (slide the moveable arrow
on the rope until you think that it is lined up with the stationary arrow; record the distance between the
two) with each eye and then both eyes. Was your dominant eye better?
R eye:
L eye:
Both eyes:
Near Point of Accomodation:
Close your right eye. Hold your pen at arm’s length and slowly move it inward toward your nose. (notice
what happens to your partner’s eyes when she does this. Measure the distance at which you can no longer
see the pen tip clearly. Repeat for your left eye. Repeat with both eyes open.
R eye:
L eye:
Both eyes:
Describe the 3 reflexes that occur in order to allow you to focus on an object closer than 20 feet away.
Why do you see 2 pens when you move the pens closed than your near point?
What effect does aging have on near point? Why?
Mapping The Blind Spot
Mark an X in the center of the back of this paper. Close your right eye and stare at the X with your left
eye from a distance of about 30 cm. Have your partner slowly move a coin from the left edge of the paper
towards the X. Tell your partner when you no longer see the leading edge of the coin and have her mark
the spot on the paper. Have her keep moving the coin slowly towards the X until the leading edge
reappears; mark that spot. Repeat for the other eye.
Would you expect the blind spot to be the same size in each eye? Why?
Would you expect the blind spot to be in the same location in each eye? Why?
What is the actual diameter of the blind spot on your retina? Using the image below, if we estimate the
focal length of your eye (distance from center of lens to fovea centralis) to be 17 mm, you can use the
formula below to solve for X:
Diameter of blind spot on paper = distance from paper to eye (300mm)
-------------------------------------------------------------------------------X
focal length of eye (17 mm)
X= __________ mm = actual diameter of your blind spot / optic nerve !!!
Distribution of Photoreceptors in the Peripheral Retina
The fovea centralis, the area of maximum visual acuity, is packed with cones. Toward the periphery of
the retina, cones become less abundant and rods become more abundant. The different cones are not
distributed uniformly along the retina.
Use the Field of View set-up described below ( or hold a protractor, with a 2 foot length of string tied
securely to the midpoint of the straight side, securely against your forehead just above your eyebrows).
Stare straight ahead while your partner slowly moves a colored disk (red, green, blue) from the 180
degree mark around toward the center, into your field of view. Your partner should read the angle at
which you first see the disk and the angle at which you can correctly identify the color.
*Repeat with each colored disk. *Repeat with the disks in a different order.
*Repeat for the other eye. (You may alternately use the Field of Vision Disk)
Disc
Color
Angle of Initial Sighting
Right
Left
Trial 1 Trial 2
Trial 1 Trial 2
Angle of Accurate Color Identification
Right
Left
Trial 1 Trial 2
Trial 1 Trial 2
Red
Green
Blue
Which types of receptors are responsible for the initial sighting?
Describe the distribution of rods and red, green and blue cones on the retina:
Afterimages
Afterimages are virtual sensations experienced after the light stimulus is no longer present.
Positive afterimages may result from activity persisting at some level of the visual pathway. Face a
bright lamp with your eyes closed and covered by your hands for 30 seconds. Uncover your eyes and
stare at the light source for 5 seconds. Close and cover your eyes again.
What do you perceive?
How long does it persist?
Why is it called a positive afterimage?
Negative afterimages result from fatigue of one type of cone and an overlap in sensitivity of that
wavelength by another cone. Stare at a green (or red) square in the center of a white piece of paper in a
well lit area for 20 seconds. Close your eyes for a second and then stare at the plain white paper.
What do you perceive?
How long does it exist?
Why does it occur and why is it called a negative afterimage?
Can you draw the US flag so that its afterimage will be the correct color?
Can you draw a road sign so that its afterimage is correctly colored?
Colorblindness
1. Ichihara Color Plates
Were you able to see all of the numbers?
If not, which colors were problematic?
2. Holmgren Test – Match the colored swatches of yarn as quickly as possible. Time how long it takes to
do so.
___________ sec.
__________ # wrong matches
What is the possible cause of any type of color “blindness”?
Read the article that follows and explain how it is possible that different people may perceive the color of
the same object differently.