Mind & Body

The Extinction Illusion Makes Dots Disappear Before Your Eyes

We don't think of dots as particularly hard to see. Polka-dotted shirts, dalmatians, stars — visually, none of these things are hard to grasp. But when we look at a grid with dots at its intersection points, something odd happens.

The Extinction Effect

No, the extinction effect isn't why the dinosaurs died. (That was a meteor.) This is the reason the dots on the left side of this image are so hard to see. There's actually an equal number of white dots on the left and right side of this image — they just tend to blend in when they're placed at the intersection of several lines.

It's not that a single dot is hard to see. If you focus on any given dot, it's totally normal: a white dot, outlined in black. When you look at the image as a whole, though, the white dots at the intersections appear in erratic, blinky clusters. You can only see a smattering of them; the rest disappear into the gray of the lines. This is the soul of the extinction effect — you can see individual trees, but not the whole forest. Of dots.

The photonegative of the image above also produces the extinction effect — the important thing is that the lines in the grid are gray, and the dots and background are opposite colors. The extinction effect also occurs when the dots are squares, or diamonds, of similar dimensions; size matters, but shape doesn't.

What Makes Us Dot-Blind?

It's not totally clear why this illusion works, but the authors of this paper argue that it has to do with grayscale. You see in extremely high resolution, but only at the very center of your eye — an area called the fovea. So maybe the extinction effect occurs because you can only make granular grayscale distinctions there. In the outer reaches of our vision, we can still tell black from white, but maybe we can't tell a dot of either color from gray lines. Our eyes are too busy differentiating the gray lines from the background.

Of course, this illusion can't be exclusively about grayscale distinctions. The size of the dots also matters — they have to be close to the width of the lines — and so does the pattern of the grid. The extinction effect is strongest on a triangular grid, because six lines radiate out from any given intersection, creating plenty of places for dots to disappear. On a square grid, only four lines radiate out from an intersection — so the area of the intersection is smaller, and the effect is weaker overall.

Though no one's certain why this image fools our eyes the way it does, it's no surprise that the human eye can be fooled. Not only is our peripheral vision a fuzzy mess, but we also have permanent blind spots in each eye. (You can find yours here.) On top of that, we go very briefly blind whenever our eyes move — so, thousands of times a day. Our eyes aren't as infallible as we'd like to think. Of course they can be fooled by the occasional funky grid, and countless other optical illusions, too.

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Check out even more illusions in "The Ultimate Book of Optical Illusions" by Al Seckel. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Mae Rice October 31, 2018

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