On the origin of stroboscopic induced motion

Abstract

Apparent motion can be induced either by (1) relative motion between a target and a moving frame or (2) a tendency to judge the median plane to be biased toward the center of an asymmetrically positioned frame (the “Roelofs effect”): target position is then judged relative to the misplaced median plane. The first theory requires real motion; the second does not. We tested the two theories by abruptly displacing both a continuously visible target and a frame, asking subjects which of five possible positions the target occupied after the displacement. Rapid motions of target and frame simulated sensory events during saccadic eye movements in a structured visual field. Results with the continuously visible target were compared with results from a second condition identical to the first except that the projected stimuli were blanked for 1.0 sec during the time of the jump. In a second experiment, the stimuli appeared in their offset positions with no transient. Subject behavior in the two experiments was identical, showing that the presence of a transient had no statistically significant effect. The Roelofs effect can account for our results, but relative motion cannot. The background frame offset perceptual judgments in these experiments, but would stabilize them under normal conditions.