Many animals perceive object motion rather than specific objects. For the humans, too, the ability to react effectively to motion is often a matter of survival. It is not surprising then that the human visual mechanism is sometimes deluded into seeing motion where there is none.

One particular instance is presented by the applet. Focus at the center of the design and make it rotate fast clockwise. You are likely to see two spirals: the dark one winding inward towards the center and a light one winding out away from the center. The white divergent spiral is a dominant feature of the experiment that is hard to miss. The variety is introduced by a number of setup parameters. When the design rotates counterclockwise, both spirals seem to converge to the center.

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