Sensation & Perception, 4e

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Foveal Acuity


Position: Foveated
1 2 3 4 5 6
Sequential Random

Introduction

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.

Instructions

In this activity you will do a short experiment demonstrating the acuity advantage of central over peripheral areas of the retina. In each trial of the experiment, you will see a sequence of three letters somewhere in the rectangle above. Your task is to identify the middle letter of the sequence by clicking on the letter in the red box.

Start each trial by clicking the START button, then fixing your gaze on the small red circle. The letters will then appear for a quarter of a second (not long enough for you to move your eyes), and then you should make your guess by clicking that letter in the red box. After each trial, you will see the cumulative results of all the trials you’ve done so far on the left.

By default, the letters will be shown in the center of the screen (foveated) on the first trial, then move sequentially outward from there. (Whether the letters appear to the right or left of fixation is randomly determined on every trial.) Use the “Position” radio buttons on the lower-left side of the window to select other options:

About the Results

As you do the trials, you should notice that, as advertised, it is very easy to identify letters that fall on your fovea, but that your accuracy goes down as images move outward from that point. If you do enough trials (at least 20 per position), you should see such a pattern emerge in the crude bar graph (made from *s) of your results.

Why is foveal acuity so much better than peripheral acuity? The answer has two parts. First, as you learned in Chapter 3, 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:

Click the START button at right to start a new trial.

In the red rectangle at left, click on the MIDDLE of the three letters you just saw (e.g., if the display showed I E O you would click on the E in the red box)‚Äč.

 
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