Mind & Body

You Go Blind Thousands of Times a Day Thanks to Saccadic Masking

Go stand in front of a mirror. Hold still, and look around, moving only your eyes. Look left, right, up, down. Look at one eye, then the other. Their reflection is right there in front of you, but no matter how hard you try, you'll never see your eyes move. That's because when your eyes are in motion, you briefly go blind — and you can't even tell. This phenomenon is called "saccadic masking."

Your Eye's Achilles Heel

Frankly, motion and human vision don't mix so well. Objects in motion — like trains or the legs of racehorses — look like blurs. When you're moving quickly, the world around you starts to blur, too. So theoretically, the world around you should blur every time your eyes move. Practically, however, this would be a mess. You'd be dizzy and motion sick all the time.

So the human brain has evolved to prevent constant blurring. The brain shuts off visual processing while the eyes are in motion, and restarts it once they're still again. In the "saccade," the brief window of eye motion — which each last about 50 milliseconds — we can miss even major visual events, like a flash of light. And though less than a second of blindness doesn't sound so serious, keep in mind that these tiny bursts of blindness happen thousands of times a day. Cumulatively, saccadic masking means we're blind for about 40 minutes a day. We've avoided chronic motion sickness, but at a price.

Why Don't We Know We're Semi-Blind?

When someone says "I saw it with my own eyes," they mean "it definitely happened," not "I could be wrong, vision is complicated!" We trust our eyes a lot. Sure, they occasionally let us down when it comes to optical illusions, but it's hard to believe that our day-to-day vision, which we perceive to be high-resolution and continuous, is secretly full of holes.

Keep in mind, though, that there's a constant gap between what you perceive yourself to "see" and the raw visual data that enters your eyeballs. Before you "see" anything, your brain integrates data from both eyes into one picture, compensating as best they can for your natural blind spots and the fact you see far better in the center of your vision, or the fovea, than you do at the sides.

In saccadic masking, your brain doesn't stop controlling what you see; it just takes a little more control of it than usual. In addition to editing your visual perception, it edits your perception of time, too. Your internal clock is malleable enough that when your visual wiring reopens after a saccade, your brain can tweak your sense of time, essentially editing out the break in your vision. This is why your sense of sight feels seamless and cohesive when it's more like a doily.

This is all beneficial when it comes to keeping your vision clear. But it can be dangerous, especially these days when we're driving at high speeds. On a highway, danger can appear in the blink of an eye — or in the masked blink of a saccade. That's not to say you should live in constant fear of your flawed vision, but it's worth remembering that you can miss things right in front of your nose through no fault of your own.

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Discover more mysteries of your eyes in "A Natural History of Seeing: The Art and Science of Vision" by Simon Ings. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Correction 8/16/2018: A previous version of this article stated that saccades last 50 microseconds. Saccades last 50 milliseconds, or thousandths of a second. The article has been updated to reflect this.

The Amazing Truth Behind the Tricks of Optical Illusions

Written by Mae Rice August 14, 2018

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