The ganglion cells that collect information from the rods and cones converge upon a region of the retina called the optic disk. The optic nerve originates at this point and conveys ascending visual information to the lateral geniculate nucleus (LGN) of the thalamus. The optic disk is located nasally and just inferior to the fovea. An interesting characteristic of this region of the retina (i.e., optic disk) is that it is devoid of both rods and cones. As a result, this area of the retina is completely insensitive to light and has become known as the blind spot.
This experiment will enable you to construct a "functional map" of the blind spot in your right eye. At the completion of this exercise, you should be able to precisely specify the size and the location of your blind spot (relative to the fovea).
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 Assessment 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 assessment. 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 Results
entry in the Commands 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. Don't
forget to print a hard copy of the results of each assessment for your lab
notebook. A printed copy of the results can be sent to the default printer via the Print
Results item in the Commands menu at the top of
the stimulus pop-up frame..
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 EV500's with a nominal diagonal screen size of 14 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 50 mm. The interstimulus spacing across the columns is approximately 7 mm. Hence, the eccentricity for the blind spot given by the sample data in Figure 1 above would be approximately 127 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 7 mm. This result was then added to the distance between the fixation cross and the closest edge of the stimulus matrix (50 mm). Thus, (11 x 7 = 77) + 50 = 127 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 7 mm, this translates into a width of 49 mm (7 columns x 7 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 tot he 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 and eccentricity into degrees of visual angle using the following formula:
| Tangent Visual Angle (deg) = Size / Viewing Distance |
For example, given a viewing distance of 500 mm and a size of 49 mm, the width of the blond spot depicted in Figure 1 would be calculated as follows: Tangent Visual Angle (deg) = 49 mm / 500 mm = 0.098. Taking the inverse tangent of 0.098 yields an estimated blind spot width of 5.59 degrees [Note: The easiest way to calculate the inverse tangent is to enter 0.098 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 eccentricity of the blind spot can be converted to degrees using the same procedure.