The Stereokinetic Phenomenon from Two or Three Points

Abstract

The stereokinetic phenomenon occurs when certain 2-D patterns are rotated in the frontoparallel plane. Zanforlin explained this phenomenon by the hypothesis of the minimisation of velocity differences. Our visual system minimises relative velocity differences between the various points of the pattern, which determines the apparent height of the stereokinetic cone. Zanforlin and Vallortigara applied the hypothesis to a rotating straight line or the two end-points of a straight line. They showed that the apparent length of the line does not depend on the absolute physical velocities of the end-points but rather on the relative velocities, and the absolute physical velocities merely affect the apparent position of the line with respect to the plane of the disk. In the present experiments with two points, the apparent length indeed depended on the relative velocities, but the absolute velocities did not affect the apparent position. The apparent position of the line with respect to the plane of the disk depended on whether or not the centre of rotation was between the two end-points. Adding one more point gave a triangle that appeared slanted into 3-D space. The shape composed of three points was held constant, even when rotated and slanted. The height of the apparent triangle also depended on the relative velocities.