A Moment of Science

What Is Saccade?

Look in the mirror... do you see your eyes move? Learn about saccade.

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Photo: code poet (flickr)

Get in the car and have someone drive you past a roadside fence. Without moving your head, glance quickly from front to back; you can make the fenceposts seem to freeze for an instant.

Have you ever noticed that you can’t see your eyes move when you look in a mirror?

Put your face close to the bathroom mirror and look yourself in the eye. Notice the appearance of your eyes. Now, without blinking, look down at your nose, but continue to notice your eyes. Your eyes look different. They’ve moved, but the movement itself was invisible to you.

An Interesting Movement

If you have someone else do the same thing while you watch, you’ll easily see the other person’s eyes move. To make the comparison as fair as possible, you should put your face right next to the other person’s and look at the other person’s nose in the mirror.

You’ve just witnessed a mysterious process that shuts off our vision, at least partly, whenever our eyeballs move rapidly.

Saccade

The kind of quick eye motion we’re talking about is called a saccade, from a French word meaning to twitch or jerk.

During the fraction of a second that a saccade takes, images sweep over our retinas at high speed. Yet we don’t get a feeling of motion, because our brain suppresses visual perception during saccades. Otherwise, the world might look to us like a bad home video where the photographer held the “record” button down while swinging the camera around the room.

An odd thing about this suppression is that it’s not complete. Get in the car and have someone drive you past a roadside fence. Without moving your head, glance quickly from front to back; you can make the fenceposts seem to freeze for an instant.

Why, in this case, is vision not suppressed?

A team of visual scientists published a study of this question in the journal Nature. They found that what gets suppressed during a saccade are large areas of light and dark.

Those are the perceptions that seem to contribute the most to a sense of motion. Finer details, like fenceposts, are not suppressed, maybe because there’s no need to suppress them rapid eye motions usually turn them into a blur anyway.

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