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.
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 'Display
Summary Report'
command in the 'File' menu. A new window containing your results will
appear (This new window is named "Mapping the Blind Spot". Warning: This
window may be hidden behind your stimulus screen). You will see a rectangular grid of dots, but some of the dots will be
missing from this grid. The displayed dots represent the stimuli that you saw
during the experiment, while the missing ones represent the positions of the
stimuli that you didn't see - thus mapping the blind spot of your eye. (You can
also print your results in a somewhat different format using the 'Dump Results
to Printer' menu command).
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
The easiest way to analyze your results is to go to the Commands Menu and
select 'Plot Results to Screen'.
Typical results for the Right Eye are depicted in Figure 1 below:
Figure 1.
Blind Spot Mapping Results (Right Eye).
While the graphical representation of your blind spot map is displayed on your computer monitor, use a ruler to measure the maximum width of your blind spot (e.g., 13 cm in Fig. 1) as well as the distance of your blind spot (i.e., eccentricity) from the fixation point (e.g., 13.5 cm in Fig. 1). Next, with a help of a lab partner, use a tape measure to determine your viewing distance from the screen (A typical value is around 60-80 cm). With these three measurements, you should compute the width and eccentricity of your blind spot in DEGREES OF VISUAL ANGLE.
In order to convert your measurements of width and eccentricity into degrees of visual angle you need to use 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 might be somewhere in the vicinity of 80 cm. 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 80 cm and a width of 13 cm, the angular diameter of the blind spot depicted in Figure 1 would be calculated as follows: Tangent Visual Angle (deg) = 13 cm / 80 cm = 0.1625. Taking the inverse tangent of 0.1625 yields an estimated blind spot angular diameter of 9.3 degrees [Note: The easiest way to calculate the inverse tangent is to enter 0.1625 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 using Window's built-in Calculator app]. 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. Annotate the print-out of your Blind Spot Map with the width and
eccentricity (expressed in degrees of visual angle).
8. Compare the maps generated for your right versus left
eye. How do they differ? Explain this difference.