Flash-lag in depth is induced by stereomotion but not looming: Distorted size andposition perception

Abstract

The apparent spatial offset of a flash behind a moving object, called the flash-lag effect(eg Nijhawan, 1994Nature370256 ^ 257), has been extensively investigated. Here we compare theflash-lag effect from motion-in-depth simulated by stereomotion and looming. Motion-in-depthevoked by looming can be nulled by stereomotion, which suggests that both cues converge ontoa common motion-in-depth stage (Regan and Beverley, 1979Vision Research191331 ^ 1342). Weprovide evidence that the flash-lag-in-depth effect occurs before any common stage. The stimuluswas a stereogram containing a frontoparallel square (4.63 deg64.63 deg) defined by randomlyplaced dots. The perception of motion-in-depth (towards or away from the observer) was generatedby the opposed motion of the stereoscopic images (stereomotion), by the radial motion of thedots (looming), or by both cues in combination. A Gaussian blob (SDà22 min of arc) wasflashed (1 frame) in a hole in the centre of the square half-way through each motion sequence.Observers adjusted the disparity of the blob until its perceived position in depth matched that ofthe square. All observers perceived a strong apparent offset in depth (0.22 cm at 12.54 cm sˇ1)of the flashed blob only when stereomotion was present. The flash-lag effect was speed-dependent, and was accompanied by an apparent change in the blob size depending on the direction of motion.This size illusion was quantified (3% at 12.54 cm sˇ1) by requiring observers to adjust the sizeof the blob in one direction of motion until its perceived size matched that of another blob inthe opposite direction. Looming had negligible influence on both the position and size settings of theblob. Our results suggest that the mechanisms responsible for stereomotion and looming are bothindependent and have qualitatively different outputs.