Apparent Movement Illusion

This illusion is experienced when a series of motionless pictures is projected one after another at an appropriate rate, referred to as the "phi-phenomenon". We are influenced by exactly this kind of illusion when we watch moving pictures in a cinema. A succession of flickering electrical lights can also generate this illusion. This phenomenon can be studied experimentally with the help of an instrument that presents two or more lights in succession; Wertheimer reported that experiencing this illusion depends on an appropriate level of brightness, size, spatial gap, and temporal contiguity among the different lights being presented. In the absence of these conditions, the light points will not appear to be moving at all -- they will instead appear either as a single point, or as separate points appearing one after another, with no accompanying experience of motion between them. …