Pavel's Blog

Like a lot of people, I've always been amused and intrigued by optical illusions, but a recent posting on Boing Boing really got to me. If you click on the small image to the right, a larger copy will appear in a separate window. Focus on the small plus-sign in the middle of the picture, paying attention to the fuzzy dots around it, but not looking directly at them. What do you notice?

Most folks almost immediately see that there's a fuzzy green dot moving around and around, even though there's actually no green in the animation at all. Okay, that's pretty weird, sure, but not too earth-shattering.

Much weirder is what most folks see next, or rather what they stop seeing: after about five seconds of staring at the little plus-sign, the fuzzy purple dots disappear completely, leaving just the grey background and the (nonexistent) green dot going around and around, all by itself. This completely did me in. I instantly wandered all over the nearby offices, looking for people to come confirm my experience.

On one site (Boing Boing's source for the image) this is claimed to be an example of a phenomenon called motion-induced blindness, which is certainly a cool and strange thing in its own right. But Michael Bach’s page on this one attributes it more believably to something much simpler, called Troxler fading. Whatever the explanation, it's certainly pretty cool. Or freaky.

By the way, if you have any interest in this sort of thing, you owe it to yourself to spend some time on Michael Bach’s really excellent Optical Illusions & Visual Phenomena site. This is a great large collection of visual illusions, each with detailed explanations reflecting the best current understanding of the human visual system. In many cases, Michael also provides a little Flash applet to let you play around with various parameters of the illusion. It's great fun, and the explanations are remarkably accessible to the layman.