What happens if it changes color when it moves?: the nature of chromatic input to macaque visual area MT

Neurons in the middle temporal visual area (MT) of macaque cerebral cortex are highly selective for the direction of motion but not the color of a moving stimulus. Recent experiments have shown, however, that the directional selectivity of many MT neurons persists even when a moving stimulus is defined solely by chromatic variation (Charles and Logothetis, 1989; Saito et al., 1989; Dobkins and Albright, 1991 a, b; Movshon et al., 1991; Gegenfurtner et al., 1994). To illuminate the mechanisms by which area MT uses color as a cue for motion correspondence, we recorded from MT neurons while rhesus monkeys viewed an "apparent motion" stimulus in which red/green sine wave gratings underwent contrast reversal each time they were displaced in a particular direction. Under such conditions, correspondence based upon chromatically defined borders conflicts with correspondence based upon conservation of chromatic sign. When our heterochromatic stimuli possessed sufficient luminance modulation, MT neurons responded best to motion in the direction for which the sign of luminance (and chromatic) contrast was preserved. At isoluminance, however, two different chromatic influences were revealed. First, when stimuli underwent small spatial displacements, directional selectivity was elicited by movement of the stimulus in the direction of the nearest chromatically defined border, even though the sign of chromatic contrast at that border alternated over time. Under these conditions, MT neurons apparently exploited information about image borders defined by chromatic contrast while sacrificing information about the colors that make up those borders. By contrast, when chromatically defined borders provided only ambiguous information about direction of motion, MT neurons were capable of using information about the sign of chromatic contrast to detect direction of motion. The results from these experiments suggest the existence of a hybrid mechanism, one in which both signed and unsigned chromatic signals contribute to motion processing in visual area MT.