The importance of perceptual illusion research

My column for SEEDMAGAZINE.COM today covers the fascinating research on perceptual illusions. While these illusions are often amazing in their own right, what's more important is what they tell us about the visual system, and how common they really are:

Are you sitting in a swivel office chair as you read this article? Would you like to see a remarkable visual illusion? Just push yourself back from your desk and spin around four or five times from right to left with your eyes open. Then look back at this screen. You'll probably notice that now the onscreen text appears to be moving from left to right.

Of course, few adults are very impressed by this illusion because we've all experienced it as children. But here's an illusion you may have not seen:


Stare at the red dot in the center of the figure for a minute or two. Before long, the green ring will disappear--it simply seems to fade into the white background. There are no tricks: This is a simple, static image file. The effect has been known for more than two centuries and is named for its discoverer, Ignaz Paul Vital Troxler (1780-1866), a Swiss physician and philosopher. "Troxler fading" is actually related to what you experience when you get "dizzy": You become so habituated to a phenomenon (spinning in a circle or seeing a green ring in your peripheral vision) that you stop noticing it's there. Or, rather, you don't realize that your perceptual system has begun actively ignoring it. It's only when your circumstances change that you see what the phenomenon has done to your perceptual system. When you stop spinning, the world seems to continue, in reverse. When you look away from the green ring, you see a red ring in the same part of your visual field.

For more on illusions, read the whole column. For more on the Troxler Effect see this post.

Also, in case you missed it, here's my list of Editor's Selections for this week for psychology and neuroscience from

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I don't always see the whole circle disappear. Most of the time, only one half vanishes - almost always the right hand side. Anyone else get this? Is this an effect of a dominant eye? Or is one of my eyes twitching uncontrollably....

I think you're seeing the other half because you may be taking one eye off the red dot instead of just letting it disappear in your periphery. It's hard to keep it completely vanished.

By Mike Podevin (not verified) on 16 Sep 2009 #permalink

I wonder if it works if one is red/green colour blind? ... hmmmm

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