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How to interpret a poke in the eye

Close your eyes and gently rub the outside corner of one eye. Press a little harder. What do you see?

If you're like many of the approximately 50 participants at a talk last week by Jeremy Wolfe, visiting associate professor in brain and cognitive sciences, you saw a small white spot, or a black spot with a white rim. But where does the spot come from? What causes it?

According to Professor Wolfe, whose talk was titled "Ten Things You Can Learn by Poking Yourself in the Eye," "what you're seeing is the result of mechanically stimulating tissue that is ordinarily stimulated by light." In other words, the signals sent to the brain from the rubbing were interpreted as light, "even though you know [the signals] were caused by your finger."

As Professor Wolfe tried the technique again, he noted that what he was "seeing" also appeared to be a little bluish. "That's due to stimulating rods [one group of light-sensitive bodies], which are around the corners of the eye," he said. (Cones, the other group of light-sensitive bodies, are located more in the center of the eye.)

And so began a series of demonstrations in eye-poking that taught more than 10 things about the eye and visual phenomena. Some of these are summarized above and below.

In one demonstration-an example of advanced eye-poking-Professor Wolfe asked the audience to "take a couple of fists and push straight back on your eyes. Notice the patterns that appear."

One person described seeing black and white checkers. Another expanded on that by noting that the squares of the checkerboard appeared to grow larger with distance from the center.

Professor Wolfe acknowledged that "we don't actually know what [that checkerboard] is, but it looks like you could be looking at your own visual cortex," or the part of the brain that handles vision.

Further, he continued, the growth of the checkerboard squares could correspond to the organization of the visual cortex. The small "squares" in the center represent cells that can make out fine details. These cells don't extend across the whole visual field because the human brain isn't big enough to process all of the information that would come in if they did. So with distance from the center, cells get progressively larger and less capable of making out details.

In a brief aside, Professor Wolfe noted that some people see the same sort of "checkerboard" effect during migraine headaches. Here, he said, "you're almost definitely seeing your visual cortex."

Another demonstration involved slowly pressing on an eyelid, with the eyes open, while focusing on a certain object-in this case Professor Wolfe's nose. "Notice that I appear to move from side to side," he told the audience. "But where [is your brain] getting the signal for motion? With slow pushes, your eye is not moving." It turns out, he said, that by pressing on the eye "you're stretching your eye muscles." And these muscles have receptors in them that are telling the brain that the eye is actually in a different position. "So the brain says, `the eye must have moved.'"

This happens, Professor Wolfe said, because "the brain is continuously trying to make the best of a bad deal in terms of what you're seeing."

Throughout his talk Professor Wolfe encouraged questions and explained what members of the audience said they were seeing. He was stumped, however, after one demonstration when a participant described "lots of color, then swirls, then a butterfly."

A version of this article appeared in the January 27, 1993 issue of MIT Tech Talk (Volume 37, Number 20).

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