Peripheral Visual Acuity Experiment
USD Internet Sensation & Perception Laboratory


Visual acuity, in its most basic form, refers to our ability to resolve fine spatial detail.  A person with average visual acuity can discriminate a gap in a spatial pattern that subtends 1 minarc (i.e., minute of arc).  A coin the size of a quarter subtends 1 minarc at a distance of 90 yards (82 meters).  A number of factors can influence our visual acuity.  For example, acuity improves with increases in stimulus luminance or contrast.  However, visual acuity declines rapidly as the location of the retinal image of the stimulus moves away from the fovea.   This relationship between visual acuity and the retinal eccentricity of the target is the focus of the current experiment.

When you fixate a stimulus, the corresponding retinal image of that stimulus falls directly upon the fovea.  However, when a stimulus target is presented at a location other than the fixation point its corresponding retinal image falls outside of the fovea.   Since the fovea contains the highest density of cones and is subserved by a disproportionately greater volume of cortical tissue than any other area of the retina (i.e., the so-called cortical magnification effect) it should not be surprising to learn that optimal visual acuity is observed for targets falling directly upon the fovea.  Acuity for non-foveal targets is clearly suboptimal as can be seen in the special eye chart presented below:

acuity.GIF (38444 bytes)

Figure 1. Special chart prepared to demonstrate how visual acuity
decreases rapidly with target distance from the fovea.  According to
Anstis (1974), when the center of the chart is fixated at approximately
normal reading distance, all the letters should be equally legible, since
increasing target distance from the fovea is offset by a corresponding
increase in letter size. [Anstis, S. (1974). A chart demonstrating vari-
ation in acuity with retinal position, Vision Research, 14 , 589-592.


The purpose of this experiment is to determine how visual acuity varies as a function of stimulus eccentricity.


1) Sit about arm's length from the computer's display monitor.  Try to maintain this viewing distance throughout the experiment.

2) Carefully fixate the button with the cross (+) that will be presented on the left side of the screen.

3) Click the fixation (+) button.  Soon after clicking this button, you will be shown a brief (150 msec) presentation of a single stimulus letter. The letter will be presented at one of three possible distances from the fixation button (i.e., the near, mid or far eccentricity levels).  The size of the letter will vary from trial to trial.   Your task is to recognize the letter that was briefly presented by the computer on each trial.

There is a natural tendency to quickly shift your gaze to the right after clicking the button.
It is your responsibility to resist this tendency by concentrating and fixing your gaze.

It is essential that you maintain your gaze on the fixation (+) button.

4) After the stimulus has been presented, you can release you gaze from the fixation (+) button and enter your recognition response by clicking the appropriately labeled button.  These 8 response buttons will be arrayed across the bottom of the display and will be labeled A, E, N, O, R, S, T, Z - all possible values of the briefly presented stimulus targets.  If you're not sure about your response -- enter your best guess.  You cannot initiate the next stimulus trial until you enter a response for the previous trial.

5) Repeat this process until all 144 experimental trials have been completed.



Immediately following the completion of the last stimulus trial, a summary of your results will be displayed in a new page on your web browser.  Use the Print command under the File menu to print a copy of your results.  The printout will contain a listing of the proportion of the letters that you correctly identified as a function of the stimulus size for each of the three levels of eccentricity examined in this experiment.  You will need this printout for your lab report and the analyses specified below.


Lab Report

1.  What are the two independent variables?
2.  What is the dependent variable?
3.  What psychophysical method was used?
4.  Plot the psychophysical function relating percent recognition performance to stimulus size for each of the three levels of eccentricity examined in this experiment.
5.  Graphically interpolate the 75% recognition threshold (i.e., acuity) for each of the three psychophysical functions specified above.
6.  OPTIONAL:  Express your three recognition threshold letter sizes in minarc units.
     (Hint: 1 point = 1/72 inch)
7.  Describe the relationship observed between measured visual acuity and stimulus eccentricity.  Discuss the mechanism(s) that mediate this relationship.