A smooth blend between four frames, where two frames are the luminance inversions of the other two.

I think this is directly related to the "Four stroke apparent motion" illusion.

In two of the frames there is a black and a white "edge" at opposite sides of each dot. See the "Lucy in the Sky demonstration" for a simplified version of the effect where the motion rocks back and forth. This one continues the apparent motion in one direction using the inverse luminance for half of the full cycle.

The Shepard scale auditory illusion is somewhat analogous to this visual illusion, although the neural mechanism might be different.

Also see Squeel pool, which depends on the same effect.

The illusion of motion is surprisingly robust with respect to various parameters (frame rate, smoothness of the animation, width and other details of the surround's luminance, the circle's center luminance, etc.). Here's a page with many variations for comparison.

MATLAB


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