Illusions: Sliding Stripes; June/July 2008; Scientific American Mind; by Vilayanur S. Ramachandran and Diane Rogers-Ramachandran; 3 Page(s)

Early gestalt psychologists--including Stuart Anstis of the University of California, San Diego, and the late Hans Wallach¿were intrigued by what they referred to as the Barber Pole Illusion. A vertical cylinder with spiraling red and white stripes painted on its surface is made to spin on its long axis. Even though the stripes are actually moving horizontally, around the pole, they appear to move vertically (up or down the pole, depending on direction of spin).

The illusion is a powerful demonstration of the point we have made repeatedly in this column--perception does not mimic physics. It involves the brain¿s interpretation--derived from an image on the retina located at the back of the eye--to pass judgment on what is happening out there in the world. But what causes the illusion?