Perpetual rotation of real and illusory shapes

The field of perceptual psychology has long been interested in determining how the perception of global shape arises from the interplay between local elements. We investigate this issue by combining the Kanizsa square illusion (an illusory square that emerges from the placement of four Pacman-like shapes) with edge motion illusions (the illusory continuous motion perceived when the luminance of a stationary edge and neighboring fields is modulated in time). Our basic demonstration builds on cascading levels of motion signals: illusory motion at a point is known to be ambiguous as to direction (the aperture problem); illusory motion of edges in isolation (here, the local elements) is perceived as either vertical or horizontal; illusory motion of edges when combined at right angles is perceived as diagonal; and illusory motion of the Kanizsa square (the global percept) is perceived as rotational. The perception of a rotating square suggests that the interpretation of the local motion signals depends on the global context and is, perhaps, informed by a tendency to perceive shapes as rigid. We created variants of the original figure to identify and assess critical factors of this phenomenon, including changes in temporal phase and amodal completion of motion. Our investigations with the basic figure and variants – examples of multistable motion illusions – demonstrate that observers can easily switch their attention between perceptual modes constructed separately from the local and global motion information.