Terminator Position Affects the Perceived Speed of Line Segments Tilted with Respect to Direction of Motion

Abstract

We investigated factors producing bias in the perceived speed of tilted lines in horizontal translation. The effects of grouping, collinearity, eccentricity, terminator proximity, and stimulus uncertainty on perceived speed were studied. The matched speed of vertical line compared to an inclined line was estimated with the use of a double random interleaved staircase for speed discrimination with a two-alternative forced choice. The speed of the tilted stimulus was held constant while the speed of the vertical stimulus was modified by the subject's response. Stimuli were single lines and groups of lines. The groups of short lines were arranged either in a collinear or in a randomly scattered fashion. The length of the line stimuli ranged from 0.33 deg to 7.0 deg of visual angle. Speed estimates were obtained for angles of tilt ranging from 0° to 90°. For line segments, collinearity was found to be the critical factor in determining perceived speed. Collinear segments showed a similar bias in perceived speed to single lines of the same overall length. However, randomly scattered segments were not subject to a bias in perceived speed. Random perturbation of the length or vertical position of a single line abolished the bias in perceived speed of a tilted line compared to a vertical line. Current models of the integration of motion measurements should be changed to account for the effects of topological arrangement and terminator position on the perceived speed of inclined lines.