STEREOKINETIC PHENOMENA: DEMONSTRATIONS AND A POSSIBLE EXPLANATION

Stereokinetic phenomena first investigated by Musatti (1924, 1955) are three-dimensional objects produced by 2-D figures in slow rotatory motion, that appear solid and real. A circle with an eccentric dot inside appears as a cone with well defined dimensions. An ellipse of uniform colour appears as a rigid disc or as an ellipsoid with its major axe slanted in depth. A uniform rectangle with semi-circular minor sides appears as a cylinder. A bar of constant length appears slanted in depth and 1.5 longer than when it is stationary on the frontal plain. Three aspects of these objects are theoretically interesting: 1) they do not appear to rotate but describe a circular translation, 2) they appear as 3-D and solid, 3) they appear of a well defined length in depth. These three aspects were explained by Musatti and others as due to different perceptual mechanisms. The first as due to a tendency to a constant orientation in space or to the limited aperture of peripheral movement receptor (misperception). The second as due to a constancy of form (Musatti) similar to the rigidity hypothesis (Ullman, 1979,1983). The third aspect, the apparent length in depth, cannot be explained by either of these hypotheses