Background motion affects global shape perception

Abstract

Local-motion can induce an apparent position shift in objects. We studied how motion affectsthe shape perception of a global structure generated by a group of moving random dots. Globaldot-motion stimuli, which consisted of two overlaid groups of dots, were used in the reportedexperiments. One group of dots, the target, moved along an elliptical trajectory (1.21 deg sˇ1),while the other group, the background, was divided into quadrants with dots alternately havingexpanding and contracting motion (1.21 deg sˇ1). The apses of the elliptical trajectory lay eitheron the 458/2258axis, or on the 1358/3158axis. A one-interval forced-choice paradigm was used;the observers were asked to judge in which direction the apses of the ellipse lay. Perceived circular-ity was found to be distorted, such that an ellipse, with an aspect ratio of 0.85, appeared to bemore circular when its apses were located in quadrants with contracting motion. Similar resultswere found when the speeds of the dots were varied (from 0.25 deg sˇ1to 5.10 deg sˇ1), thougha direct relationship between the shape effect and speed was not found. In further experiments,we examined the relative contribution of polarity and disparity to this shape distortion, andfound that the distortion diminished if there were cues available that could effectively segmentthe figure from the background. We propose that veridical retinal positions are frequently not thesole information used by the visual system in shape perception.