The optic disk is where axons of a retina's cells come together to form the optic
nerve. The optic nerves of each eye meet at the optic chiasm. For each optic
nerve, temporal-side axons continue along on the same side of the brain to the Lateral
Geniculate Nucleus (LGN), while nasal-side axons cross over at the optic chiasm to the
other side of the brain, on their way to the LGN on that side of the brain. From both
halves of the LGN, the connections continue on to visual cortex.
Because there are no photoreceptors (rods or cones) where the optic disk is
located, we have a blind spot in each of our eyes. This experiment will help you to
map your blind spot for your right eye.
1) Sit about arm's length from the monitor screen.
2) Begin by clicking on the 'Assess Right Eye' button below. A new "frame"
labeled Blind Spot Experiment will pop up. (Note: You may want to resize the new frame to
be about as big as your computer monitor's screen - but not bigger).
3) When the new frame pops up, you will see fixation cross on the left side
of the frame.
3) There will be 200 trials presented during test. On each trial a white dot will be
displayed. Your task is simply to press the number '1' on the keyboard if you see the
white dot. If you don't see the white dot, press the number '0'. Pressing '1' or '0'
records your response for the trial and brings up the next trial automatically. It
is very important that you try to maintain fixation upon the cross during all of these
trials.
4) When finished with the assessment of your right eye, click the 'Plot/Print Results'
command in the 'File' menu. You will see a rectangular grid of white dots, but some of the
white dots will be missing from this grid. The white dots displayed represent the white
dots you saw during the experiment, while the missing ones represent the white dots you
didn't see - thus mapping the blind spot of your eye. A copy of these results will also be
sent to the default printer.
5) Now, click the 'Assess Left Eye' button and repeat the experiment using your left eye instead of your right eye.
Click Button to Start the Experiment
Typical results from the blind spot assessment procedure for the right eye are presented in Figure 1 below.
Figure 1.
Sample printout from blind Spot Assessment experiment.
You will need to measure the width and eccentricity of the blind spot and express these measurements in "degrees" of visual angle. The following sample calculations are based upon some assumptions about the presumed size of your computer display monitor. The current displays in the USD Undergraduate Psychology Laboratory are Gateway LCD panels with a nominal diagonal screen size of 17 inches. Given the default screen size of the Blind Spot Experiment viewing window, the distance between the fixation cross and the closest stimulus in the 10 row by 20 column stimulus matrix is approximately 63 mm. The interstimulus spacing across the columns is approximately 11 mm. Hence, the eccentricity for the blind spot given by the sample data in Figure 1 above would be approximately 184 mm. This value was obtained by counting the number of stimulus columns to the closest edge of the blind spot (11 in this case) and multiplying this count by the interstimulus distance of 11 mm. This result was then added to the distance between the fixation cross and the closest edge of the stimulus matrix (63 mm). Thus, (11 x 11 = 121) + 63 = 184 mm = eccentricity = the distance between the fixation cross and the near edge of the blind spot. The width of the blind spot can be estimated in a similar fashion. The sample blind spot in Figure 1 appears to be about 7 stimulus columns wide. Given an interstimulus spacing of 11 mm, this translates into a width of 77 mm (7 columns x 11 mm per column).
In order to convert your measurements of width and eccentricity into degrees of visual angle you need to know the viewing distance to the screen along with a little high school trigonometry (You remember that stuff, don't you?). If you observed the instructions and sat about "arm's length" from the screen, your viewing distance should be somewhere in the vicinity of 500 mm (20 inches). Once you know the viewing distance to the screen, you can convert your measurements of blind spot width or eccentricity into degrees of visual angle using the following formula:
| Tangent Visual Angle (deg) = X / Viewing Distance (where X = eccentricity or width) |
For example, given a viewing distance of 500 mm and a width of 77 mm, the angular diameter of the blind spot depicted in Figure 1 would be calculated as follows: Tangent Visual Angle (deg) = 77 mm / 500 mm = 0.154. Taking the inverse tangent of 0.154 yields an estimated blind spot angular diameter of 8.75 degrees [Note: The easiest way to calculate the inverse tangent is to enter 0.154 on your calculator then press the 2nd key followed by the TAN key. Make sure that your calculator is configured to give trigonometric results in "degrees" rather than "radians". Consult your lab TA if you need help]. The angular eccentricity of the blind spot can be converted to degrees using the same procedure.
Alternative Step-by-Step Data Analysis Instructions:
1. Display results on screen using
Commands|Plot Results on Screen menu.
2. Measure width of the blind spot for your right eye.
3. Measure eccentricity between the fixation point and the blind spot of your
right eye.
4. Measure the viewing distance between your eye and the
computer's display monitor.
5. Calculate the angular diameter of your blind spot as follows: TAN-1
(width/viewing distance)
6. Calculate the angular eccentricity of your blind spot as follows: TAN-1
(eccentricity/viewing distance)
7. Compare the raw sensitivity maps generated for your right versus left
eye. How do they differ? Explain.