Footsteps Illusion in Binocular Interaction

Abstract

The footsteps illusion (FI) is a phenomenon where the perceived speed of a moving stimulus changes depending on its background (Anstis, 2001). FI provides a unique tool for studying motion and positional integration in binocular vision because it allows independent control over motion and position signals. Here, we used FI stimuli to investigate how the visual system integrates motion and positional information from binocular inputs, and whether the binocular differences contribute to the depth perception. Three experimental conditions were tested. In FI condition, the physical speed of the moving stimulus was constant while the perceived speed fluctuated due to the FI effect. In constant FI condition, the physical speed followed a square wave patten, while the perceived speed remained nearly constant because of the FI effect. In control condition, the physical speed followed a square wave pattern without FI effect. Motion phase differences between the two eyes ranged from 0° to 360°. At 180° phase, the perceived or physical speed of the stimulus in one eye was maximumly different from that in the other eye. In Experiment 1, participants observed these stimuli in their peripheral vision and reported whether the stimulus appeared to move with constant velocity. Experiment 2 repeated the same conditions, but the stimulus was viewed in the fovea. In Experiment 3, participants observed the same set of stimuli in the fovea and reported whether they perceived motion in depth. The results showed that, although FI stimuli generate strong FI effects in monocular vision and produce significant interocular velocity differences (IOVD), they fail to contribute to depth perception. In contrast, under constant FI conditions, depth was perceived in the fovea despite the absence of detectable IOVD. These findings suggest that differences in position and motion uncertainty between the fovea and periphery play distinct roles in binocular perception.