Abstract
When a toothed cogwheel — a circle with 12 inward pointing radial teeth — is moved rapidly along a clockwise circular path without rotation (like the circling sponge that remains upright in the hand of a window cleaner), it appears to rotate counterclockwise. We measured how this illusion is affected by motion amplitude, tooth length, and Michelson contrast. At low contrast the pattern appears to reverse and rotate clockwise. Conclusion: The trajectories of the inner tips of the moving teeth stick out into the empty middle of the circle. They are more perceptually salient than the rest of the figure, and they actually move counterclockwise. The motion of these salient tips propagates to the rest of the figure. At low contrast the T junctions, where the teeth join the circle, become more salient than the tips of the teeth.