Responses to opposed directions of motion: continuum or independent mechanisms?

Opponency between opposite directions of motion is a characteristic of many models of movement detection and is commonly invoked in explanations of the motion after-effect. If detection of opposite directions is mediated by a single mechanism, then a single, smooth psychometric function for the discrimination of global direction in random-dot kinematograms should be found as a function of the percent of directional coherence of dots in the display, ranging from 100% coherence leftwards through 0% coherence to 100% coherence rightwards. Moreover, after rightward motion adaptation, a single psychometric function should still be observed if adaptation affects the perceptual system prior to opponent interactions. If, however, leftward and rightward detectors operate independently, then the slopes of the leftward and rightward halves of the function may differ, particularly after unidirectional adaptation. We measured the probability of a "rightward" direction response for nine values of motion coherence for five observers with and without prior rightward motion adaptation. Although a smooth psychometric function was found without motion adaptation, after adaptation, the rightward half of the psychometric function was flattened whereas the leftward half remained unchanged. Such results indicate that movement direction analysers operate in a non-opponent manner.