Illusory motion in visual displays.

The apparent motion of a change in the structure of a random check pattern is studied by spatially masking it with another noise pattern and it is compared with phi motion. A fundamental difference with phi motion is the insensitivity of second order correlators (Reichardt mechanisms) to this apparent motion. The following experimental characteristics distinguish this motion from phi motion: it induces no motion after-effect, it is not transparent to another simultaneous motion, it is strongly influenced by spatial masking and it does not evoke optokinetic nystagmus. A fourth order detector is introduced which is sensitive to this illusory motion as well as to phi motion. Simulation experiments with this detector together with the subjective reports of the observers lead us to the conclusion that human subjects inadvertently treat the coarsest spatial structures as signal and the finest as the disturbing noise.