The Saturn illusion: a new stereokinetic effect.

When a 2-D pattern composed of a solid ellipse with two symmetrical semi-rings (corresponding to the visible parts of a contour ellipse whose major axis is perpendicular to that of the solid ellipse) is slowly rotated about an axis coincident with the line of sight, a compelling 3-dimensional impression occurs. Subjects report seeing an egg-shaped object which is inserted into a circular ring: the two objects move solidly into 3-D space and a moving visual phantom is generated so that the ring appears completed by an illusory curved segment in the region nearer to the observer during rotation. A sequence of nonrigid and rigid percepts (both 2-D and 3-D) precedes this Saturn-like configuration.