What happens if it changes color when it moves?: psychophysical experiments on the nature of chromatic input to motion detectors.

Several lines of evidence indicate that the processing of motion by the primate visual system continues even when a moving stimulus differs from its surroundings by color alone. To illuminate the mechanisms by which our visual system uses color as a token for motion correspondence, we have developed an "apparent motion" paradigm in which red/green sine-wave gratings undergo reversal of chromatic contrast sign each time they are displaced in a particular direction. Under such conditions, correspondence based upon conservation of chromatic sign conflicts with correspondence based upon chromatically-defined borders. When these heterochromatic stimuli also possess luminance modulation, motion is always perceived in the direction in which the sign of luminance contrast is preserved. At isoluminance, however, two very different chromatic influences on motion detection are revealed. First, when stimuli undergo small spatial displacements, motion is perceived in the direction of the nearest chromatically-defined border even when the sign of chromatic contrast at that border alternates over time. Under these conditions, motion detectors apparently exploit information about image borders defined by color while sacrificing information about the colors that make up those borders. By contrast, when spatial displacement is large, motion is more apt to be perceived in the direction for which sign of chromatic contrast is preserved. In this instance, information about the polarity of chromatic contrast facilitates motion detection. These results suggest that chromatic signals contributing to motion detection are of two distinct types. This conclusion has implications for the degree of crosstalk between magnocellular and parvocellular processing streams in the primate visual system and it reinforces our understanding of how image features affect the way we see things move.