The mechanism of isoluminant chromatic motion perception

An isoluminant chromatic display is a color display in which the component colors have been so carefully equated in luminance that they stimulate only color-sensitive perceptual mechanisms and not luminance-sensitive mechanis ms. The nature of the mechanism by which isoluminant chromatic motion is pe rceived is an important issue because color and motion processing historica lly have been associated with different neural pathways. Here we show that isoluminant chromatic motion (i) fails a pedestal test, (ii) has a temporal tuning function that declines to half-amplitude. at 3-6 Hz, and (iii) is p erceived equally well when the entire motion sequence is presented monocula rly (entire motion sequence to one eye) versus interocularly (the frames of motion sequence alternate between eyes so that neither eye individually co uld perceive motion). These three characteristics indicate that chromatic m otion is detected by the third-order motion system. Based on this theory, i t was possible to take a moving isoluminant red-green grating and, by simpl y increasing the chromatic contrast of the green component, to generate the full gamut of motion percepts, from compelling smooth motion to motion sta ndstill. The perception of motion standstill when the third order mechanism is nullified indicates that there is no other motion computation available for purely chromatic motion. It follows that isoluminant chromatic motion is not computed by specialized chromatic motion mechanisms within a color p athway but by the third-order motion system at a brain level where binocula r inputs of form, color, depth, and texture are simultaneously available an d where selective attention can exert a major influence.