The anatomy of an illusion, and what it tells us about the visual system

Take a look at this amazing illusion created by Arthur Shapiro



You're looking at two donut-shaped figures whose "holes" are gradually changing color from black to white and back again. It appears that the holes are changing in an opposite pattern -- when one is light, the other is dark, and so on. But if you click to remove the surrounding donuts, you'll see that the two holes are actually changing together.

If you're still not convinced, get a friend to help. One of you looks at the light donut and the other looks at the dark donut. Then each of you says "light" when your donut hole turns light. You'll soon be saying "light" simultaneously!

Shapiro calls this the Contrast Asynchrony illusion, and he argues that it can tell us a lot about how the visual system works. Below is an interactive version of the illusion. You can manipulate all sorts of variables to change the way the illusion appears. There's even more than one way to eliminate the appearance of the illusion entirely.
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