Human Systems Integration Division @ NASA Ames

We performed a series of experiments examining the effect of contrast on the perception of moving plaids to test the hypothesis that the human visual system determines the direction of a moving plaid in a two-staged process: decomposition into component motion followed by application of the intersection of constraints rule. Although there is recent evidence that the f'u'st tenet of the hypothesis is correct, i.e., that plaid motion is initially decomposed into the motion of the individual grating components, the nature of the second-stage combination rule has not yet been established. We found that when the gratings within the plaid are of different contrast the perceived direction is not predicted by the intersection of constraints rule. There is a strong (up to 20°) bias in the direction of the higher-contrast grating. A revised model, which incorporates a contrast-dependent weighting of perceived grating speed as has been observed for one-dimensional patterns, can quantitatively predict most of our results. We then discuss our results in the context of various models of human visual motion processing and of physiological responses of neurons in the primate visual system.