Visual Perception Online Laboratory

Purdue University

Visual Acuity Experiment

Visual Acuity is our ability to distinguish details in the visual field. The highest visual acuity occurs using the foveal (central vision) region of the retina. The farther away from the fovea that a stimulus falls on the retina, the worse will be the visual acuity, or the ability to perceive the small details of the stimulus.

An everyday example of a visual acuity test is the familiar Snellen chart test, where a patient is asked by their optometrist or ophthalmologist to read off lines of letters, where the letters become smaller and smaller with each line.


1.) Start by clicking on the 'Start Experiment' button below. A new frame will pop up. (Note: if you are using Netscape on HP-UX, you should not resize the new frame. If you are using MS Internet Explorer or a PC version of Netscape, you may want to resize your new frame to be about as big as your monitor's screen - but not bigger. Also, if you are using either MS Internet Explorer or a PC version of Netscape, you will need to click your mouse somewhere inside the new frame to get things rolling for either of these 2 platforms.)

2.) When the new frame pops up, you will see a fixation cross on the left hand side of the frame. Close your right eye or cover it with a patch and fixate your left eye on the cross. Sit about arm's length from the monitor.

3.) You will run 50 trials, 10 trials X 5 conditions, where each condition represents a certain distance, the eccentricity, from the fixation cross to a stimulus in that condition.

4.) On each trial, you will be shown one of the stimuli at one of the 5 eccentricities. The stimulus will be 2 white dots in the same imaginary column on the screen. If you are able to see the 2 dots, hit the spacebar to save this trial and begin a new one. If you cannot see the 2 dots, adjust the distance between the two until you do see each one. Then hit return. Meanwhile, your left eye should be still fixating on the cross (stare right at it - don't let your eye wander).
Use the number '1' to increase the distance between the 2 dots and the number '0' to decrease the distance between the 2 dots. When satisfied with your response, having made sure the 2 dots are far enough way to be perceived but not too far away, hit the spacebar to store the results of this trial and go to the next trial.

5.) When all trials are finished, click on 'Plot' in the 'File' menu to plot your results. You should see that as eccentricity (distance between the pair of dots and the fixation cross) increases, the 2 dots have to be farther and farther apart for you to determine that there are really 2 dots displayed rather than 1.

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This page constructed and maintained by Michael R. Scheessele. Last Revised 01/06/98.
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