Activity (Alternative) 3.2 Foveal Acuity

The fovea, located directly behind the pupil and lens, is the area of the retina that provides the highest degree of visual acuity. In other words, you see an object most clearly when its image falls on the fovea. Furthermore, the farther an object’s image moves away from the fovea and into the periphery, the less clear it will be.

You usually don’t notice this falloff in acuity because whenever you need to identify an object, you look directly at it so its image falls on the fovea. In this activity, you will be forced to identify images in both the fovea and the periphery, allowing you to observe the advantage of foveal over peripheral vision.

Description of Activity
In this activity the user performs a short experiment demonstrating the acuity advantage of central over peripheral areas of the retina. There is a large, horizontal rectangle with a small dot in its center. In each trial of the experiment, the user sees a sequence of three letters somewhere in the horizontal rectangle. The user’s task is to identify the middle letter of the sequence by clicking on one of eight letters listed below in the response area.

The user starts each trial by clicking the START button, then fixing their gaze on the small red circle. The letters then appear for a quarter of a second (not long enough for an eye movement), and then the user makes a guess about what letter the middle letter was by clicking that letter below. Sometimes the letters appear close to the fixation dot and sometimes they appear far off to the left or the right of the fixation dot. The middle letter is much easier to see when the stimulus is presented near the fixation dot. After each trial, the user sees the cumulative results of all the trials they’ve done so far in a table. Typical results are described in the next section.

About the Results
As the user does the trials, they tend to notice that it is very easy to identify letters that fall on their fovea (i.e., near the fixation dot), but that their accuracy goes down as images are presented more peripherally from that point. If the user does enough trials (at least 20 per position), they typically see such a pattern emerge in the results. The results show the percentage of letters correctly identified when they are located at position 1, in the center where the fixation dot is, and also in five other positions to either side of the fixation dot. Each number represents a location farther away from the central fixation dot. The percentage of correct trials is typically high for positions 1-3 and then get lower for positions 4-6.

Why is foveal acuity so much better than peripheral acuity? The answer has two parts. First, as you learned in Chapter 2, the fovea is packed with cones that each have exclusive access to a single retinal ganglion cell, a situation that is “wired” for good acuity (see the activity on Acuity versus Sensitivity). Second, a very large chunk of your striate cortex is dedicated to processing information from the fovea—the fovea is cortically magnified relative to other areas of the retina.

Here are some questions to consider as you play with this activity:

Would you expect accuracy to go up or down if you move your head closer or farther from the monitor? Try it and find out. (Answer: Accuracy decreases as you move your head closer to the monitor because the letter stimuli appear farther in the user’s peripheral vision.)

At which position does your accuracy rate drop to about 50%? The user is invited to recruit a friend or two to try the experiment. Do they drop to 50% at the same position? (Answer: Different people will have different 50% points on their results, depending on how good their vision is. If both people have normal or corrected-to-normal vision, they should have roughly the same results. If one person has poorer vision, their 50% point should be closer to the central dot than the person with better visual acuity.)