On the principle of minimal relative motion--the bar, the circle with a dot, and the ellipse.

Beghi, Xausa, & Zanforlin (1991a) and Beghi, Xausa, De Biasio, & Zanforlin (1991b) have presented visual stereokinetic phenomena. When a bar is rotated in the image plane, it appears to be slanted in depth. Likewise, when a circle with an off-centered dot is rotated, a three-dimensional (3-D) cone is perceived. Finally, when an ellipse is rotated in the image plane, an ellipsoid is perceived that is tilted in depth. To explain these phenomena, Beghi et al. (1991a,b) offer an analytic model that assumes that the visual system nullifies the speed differences between all stimulus points. I critique this analytic model, and show that it cannot explain the perceptual phenomena.