Abstract

When a plaid made by superimposing two orthogonal sine wave gratings is moved, the apparent direction of motion is dependent on the contrasts and the relative spatial frequencies of the component gratings. We measured the apparent direction of plaids with 1 and 1.5-cycle/deg components. Because these patterns partially slide (i.e., are not completely coherent) the task was to respond to the main direction of motion. When scaled for equal multiples of threshold (equal visibility), the lower-spatial-frequency component dominates the direction of motion, indicating a disproportionally larger influence of low spatial frequencies on the motion system. The directional shift can be eliminated or even reversed by increasing the contrast of the high-spatial-frequency component relative to the low. However, the addition of contrast to the high-spatial-frequency component has less effect when the average contrast of the plaid is increased, suggesting a compressive contrast nonlinearity in the motion system. The movement characteristics of a red-green color plaid are similar to those of a low- contrast luminance plaid. The motion of the color plaid may be determined primarily by the small luminance distortions that are inevitably present due to chromatic aberrations.

© 1988 Optical Society of America