A New Bistable Motion Illusion Based upon ‘Kinetic Optical Occlusion’

A display was devised for the purpose of studying the information afforded by kinetic optical occlusion (the progressive erasure and replacement of static elements within a display). A microcomputer generated a series of equally spaced light bars on a dark background. The first bar on the left was suddenly blanked and, after a pause of variable duration (an interblank interval, or IBI), was replaced. As the first bar was replaced, the second bar in the series was blanked, and so on, until each bar in the pattern had been blanked and replaced. Depending upon the duration of the IBI, this display gave rise to one of two alternative percepts: the observer either saw movement of a dark shadow ‘in front of’ the pattern of bars (with IBIs ≲ 50 ms) or he saw right-to-left stroboscopic movement of successive bars (with IBIs ≳ 85 ms). At some intermediate IBI (the transition IBI) the display was bistable. A two-bar variant of the original display was also studied and found to be bistable under appropriate conditions. In a series of experiments it was found that the transition IBI for the original display did not depend upon whether the observer was tracking the sequence of events or fixating a stationary point in the display. The transition IBI was an increasing function of spatial frequency above about 3 cycles deg⁻¹, and depended upon whether the display was focused or optically blurred. Empirically determined transition IBIs correspond well to estimates of the integration times of visual mechanisms studied in other paradigms.