| 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 cue for motion correspondence, we have developed an "apparent motion" paradigm in which red/green sine-wave gratings undergo contrast-reversal each time they are displaced in a particular direction. Under such conditions, motion correspondence based upon conservation of color identity conflicts with motion correspondence based upon color-defined borders. We have employed these stimuli in human psychophysical experiments in order to determine which color cue dominates our perceptual experience of motion. In parallel, we have conducted neurophysiological studies of response properties in the middle temporal visual area (MT) of awake macaque monkeys as a means of investigating neural correlates of the perceptual effects.;The results from both our psychophysical and neurophysiological experiments reveal the existence of a hybrid color correspondence mechanism: one in which both color-defined contours and the colors themselves enter into the motion pathway. The respective conditions under which the two types of color signals dictate motion correspondence vary as a function of spatial displacement, luminance contrast, chromatic contrast and temporal frequency. The results of these various manipulations shed light on the neural mechanisms involved, and permit some degree of speculation about the relative contributions of magnocellular and parvocellular pathways to color-facilitated motion detection in the primate visual system. |
