The contribution of color to motion processing in macaque middle temporal area

The chromatic properties of an image yield strong cues for object boundaries and thus hold the potential to facilitate the detection of object motion. The extent to which cortical motion detectors exploit chromatic information, however, remains a matter of debate. To address this further, we quantified the strength of chromatic input to directionally selective neurons in the middle temporal area (MT) of macaque cerebral cortex using an equivalent luminance contrast (EqLC) paradigm. This paradigm, in which two sinusoidal gratings, one heterochromatic and the other achromatic, are superimposed and moved in opposite directions, allows the sensitivity of motion detectors to heterochromatic stimuli to be quantified and expressed relative to the benchmark of sensitivity for a luminance-defined stimulus. The results of these experiments demonstrate that the chromatic contrast in a moving red-green heterochromatic grating strongly influences directional responses in MT when the luminance contrast in that same grating is relatively low; for such stimuli, EqLC is at least 5%. When luminance contrast is added to the heterochromatic grating, however, EqLC wanes sharply and becomes negative (-4%) when luminance contrast is sufficiently high (>17-23%). Thus, the chromatic properties of an object appear to confer little or no benefit to motion processing by MT neurons when sufficient luminance contrast concurrently exists. These data support a simple model in which chromatic motion processing in MT is almost exclusively determined by magnocellular input. Additionally, a comparison of neuronal and psychophysical data suggests that MT may not be the sole contributor to the perceptual experience elicited by motion of heterochromatic patterns, or that only a subset of MT neurons serve this function.