European Conference on Visual Perception edition:30 location:Arezzo date:27-31 August 2007
The classical two-element apparent motion stimuli originally studied by Wertheimer can give rise to a host of qualitatively different motion percepts. A particularly interesting one is that of pure phi, which refers to the percept of the two stimulus elements being stationary, while something else appears to move in front of them. This percept (also referred to as 'omega', 'shadow' or 'afterimage' motion in the literature) has occasionally been observed when the two stimulus elements are presented in a strictly alternating sequence (zero interstimulus interval, ISI), and occurs at stimulus onset asynchronies (SOAs) briefer than those necessary for optimal (beta) motion. We investigated how the occurrence of pure phi depends on SOA and ISI. SOAs between 35 and 120 ms were found to be most effective in producing pure phi. At all SOA levels where pure phi was observed, it occurred mainly at negative interstimulus intervals, ie in cases where the two stimulus elements were simultaneously visible for some time during the animation cycle.