Please note that these demonstrations have moved to:

The visual system can construct complex 3-D shape representations from motion information, as this movie demonstrates. You may gain an impression of a rotating sphere in the middle of the display, but only when the pattern is moving (try pausing the movie using the controls along the bottom of the window). Depth is conveyed solely by the variation in velocity and direction of each dot. In which direction does the sphere appear to rotate? The perspective information (variation in dot path and velocity) defines anticlockwise rotation (viewed from above), but the display may appear ambiguous (see Mather, 1989).

This kind of effect was first described by Wallach and O'Connell (1953).