Abstract

Plaids, the sum of sinusoidal gratings of different orientations, have been useful in determining how the human visual system determines the motion of an object as a whole from information about the motion of oriented components within that object. The intersection of the constraints imposed by the motion of the two oriented components within a plaid (the constraints rule) provides a reasonable qualitative estimate of psychophysical results¹ but has yet to be tested quantitatively. We made quantitative measurements of the perceived direction of motion of plaids whose component gratings were of different contrast. Human subjects viewed moving plaids composed of two achromatic luminance gratings of equal spatial frequency and fixed orientation. The perceived direction varied substantially and systematically with the contrast ratio of the components. Up to 20° biases were commonly observed. One possible explanation for our results is a scheme which uses orientation-tuned sensors that subject the estimated speed of the oriented components to a contrast-dependent distortion before applying the constraints rule. The observed significant deviation from the constraints rule represents an important restriction on future models of motion processing.

© 1988 Optical Society of America