How the Brain Tames the Eyes' Wild Motions Our eyes dart around constantly when taking in a scene. Yet the scene appears stable to us. Scientists have spent years trying to figure out how the brain accomplishes this feat. Now they think they know. The key is a brain circuit that monitors every eye movement and tells the brain how to compensate for it.
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How the Brain Tames the Eyes' Wild Motions

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How the Brain Tames the Eyes' Wild Motions

How the Brain Tames the Eyes' Wild Motions

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MELISSA BLOCK, host:

From NPR News this is ALL THINGS CONSIDERED. I'm Melissa Block.

MICHELE NORRIS, host:

And I'm Michele Norris.

In the time it takes to listen to this sentence, your eyes will make at least a dozen quick jerky movements. But the world doesn't appear jittery. Somehow, your brain is filtering out the eyes' sudden motions. Precisely how this happens has been a mystery until now.

Here's NPR's Jon Hamilton.

JON HAMILTON: Life would look a lot more exciting if we actually saw everything our eyes see. Bob Wurtz is a senior investigator at the National Eye Institute. He describes the human's eye view like this.

Mr. BOB WURTZ (National Eye Institute): It's as if I had a movie camera mounted on a bronco. It's jumping up and down. The camera's jumping up and down. What's photographed would be jumping all around enough to make you sick.

HAMILTON: But that's not how things look to us, at least not usually. That's because the part of the brain that assembles the image we perceive gets a heads up from the eye is about to make a sudden shift. It's a sort of warning signal.

Mr. WURTZ: This tells the visual system that the eyes are going to move. So the visual system knows beforehand that the eyes are going to move and in some way it compensates for this.

HAMILTON: Wurtz says the warning system is possible because the brain treats certain messages a bit like e-mail.

Mr. WURTZ: So if I've sent e-mail to you, I keep a copy for myself. It lets me know what I've done.

HAMILTON: He says if the part of the brain that tells the eye what to look at sends an e-mail saying move. At the same time, a copy of that e-mail goes to the brain cells that put together our image of the world. Until now, it hasn't been clear how that copy gets from A to B.

Wurtz and Marc Sommer at the University of Pittsburgh thought they knew where to find the circuit. To test their idea, the scientists used monkeys. They monitored specific cells in the two brain areas while the monkeys looked at moving targets. Then, Wurtz says, they injected a substance into the part of the brain they believed was involved in storing copies of the messages sent to the eye.

Mr. WURTZ: We were blocking the pathway for saving the e-mail on your hard disk.

HAMILTON: Wurtz predicted that the messages telling the eyes to move should still get through, but copies of those messages shouldn't. And, that's what happened. The monkey's eyes move normally but their brains lost much of their ability to compensate for sudden eye movements.

Michael Goldberg is professor of neurology at Columbia University. He says this study pretty much completes our understanding of how we keep a steady picture of the world in our heads. Goldberg says this ability is pretty useful and not just for avoiding motion sickness. He says imagine that a tiger appears in the corner of your eye. The brain says -

Dr. MICHAEL GOLDBERG (Columbia University): Okay. You got to make an eye movement.

HAMILTON: Presumably to look directly at a tiger. The brain holds the image steady so we know that it's our eye that has moved, not the tiger.

Dr. GOLDBERG: And then we can correlate accurately where the tiger is in space and you won't get eaten.

HAMILTON: Goldberg says this isn't just of academic interest. Some people who have a stroke actually lose this ability to keep track of things.

Dr. GOLDBERG: There's a major artery called the middle cerebral artery and if you had a stroke in the right side of the middle cerebral artery, you're deficit is as if it's a spatial deficit.

HAMILTON: Goldberg says the new research may offer clues about how to help these patients. The study appears in the journal Nature.

Jon Hamilton, NPR News.

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