Ghost phi
Click on:
Polarity inversion:

A different kind of motion illusion can be perceived by clicking on the "Launch ghost" buttons above. Although one's first impression may be a flash, many (but not all) observers perceive an extremely fast, but very strange rotation in the direction opposite to the ring's slow rotation. (If your computer is slow, you may not see anything at all.) The rotation seems "ghostly", because nothing seems to be rotating—yet there's a very fast rotation. This "ghost phi" effect may be clearer if you compare the two directions, by reversing the direction of the slow rotation.

What's actually happening is very similar to "false phi", but with the transient being immediately undone. In polarity inversion, there are two inversions immediately following one another, so that the original texture is recovered. In the randomization version, the texture is randomized but then the old texture is immediately brought back.

Each of the "false phi" versions is one-half of the "ghost phi" stimulus, with the inversion or randomization not being undone. In false phi, the impression is that of a very fast jump, but one that seems to "land" somewhere on the new pattern. (Though this landing spot is not based on any actual correspondence between the stimuli.) In ghost phi, on the other hand, the visual system actually has a correspondence or landing spot—zero rotation—and the resulting motion, though released, seems to be unbound.