Learning to see stereokinetic effects.

The Saturn illusion is a stereokinetic effect that occurs when a flat pattern composed of a full ellipse with two symmetrical semirings is rotated slowly in the frontoparallel plane. Subjects report seeing an egg-shaped object inserted into a circular ring, and the two objects move solidly into 3-D space as a single rigid body. Inexperienced observers show a conspicuous delay before reaching this percept. Two experiments are reported in which it is shown that this incubation time progressively decreases with repeated exposures to the stimulus pattern. A certain amount of time (14 s on average) is, however, required to obtain the effect, even after six successive exposures. It is argued that this time, which is independent of the speed of rotation and is not further reducible, is a fixed entity and is needed to compute the most rigid 3-D solution from deformations in the 2-D image. The results are discussed in relation to current theories of perception of structure from motion.