Classification of apparent motion percepts based on temporal factors.

As pointed out by M. Wertheimer (1912), a number of qualitatively different motion impressions, such as "optimal motion," "part motion," and "pure phi," may be evoked by manipulating the temporal parameters of two-element apparent motion sequences. We investigated how the transitions between the different percepts depend on temporal variables over a large range of interstimulus intervals and stimulus onset asynchronies. On the basis of these data, we present a hierarchical classification scheme describing the critical temporal conditions for alternative percepts. A particularly interesting finding is that the distinction between pure phi, on the one hand, and part and optimal motion, on the other, depends on the temporal duty cycle of the stimulus elements. It is suggested that this temporal variable may be used as a cue to resolve occlusion-related ambiguities in classical motion stimuli.