Basic demo

When looking at the central red dot, most observers perceive a slow rotation of the ring, interrupted by very fast jumps once a second. The fast jumps are an illusion. The slow rotation is perceived because on every frame of the animation, the same pattern is presented, but rotated by a small amount as compared to the previous frame. During what is perceived as the fast jumps, on the other hand, a series of three completely random, uncorrelated patterns is presented (lasting about 50-100 ms, depending on the speed of your computer). There is no known reason to perceive this random sequence as motion, even less so as extremely fast motion. The fast jumps are usually seen in the direction opposite to the slow rotation: this can be verified by reversing the rotation direction.

We call this motion illusion "false phi". These pages demonstrate false phi for rotational, but it also works for other kinds of motion: translation, expansion, contraction, or shear.