CHROMATIC PROPERTIES OF NEURONS IN MACAQUE MT

We have studied the responses of MT neurons to moving gratings, spatia lly modulated in luminance and chromaticity. Most MT neurons responded briskly and with high contrast sensitivity to targets whose luminance was modulated, with or without added chromatic contrast. When luminan ce modulation was removed and only chromatic stimulation was used, the responses of air MT neurons were attenuated. Most were completely unr esponsive to stimulation with targets whose modulation fell within a ' 'null'' plane in color space; these null planes varied from neuron to neuron, but all lay close to the plane of constant photometric luminan ce. For about a third of the neurons, there was no color direction in which responses were completely abolished; almost all of these neurons had a definite minimum response for chromatic modulation near the iso luminant plane. MT neurons that responded to isoluminant targets did s o inconsistently and with poor contrast sensitivity, so that only inte nsely modulated targets were effective. Whereas the best thresholds of MT neurons for luminance targets are close to behavioral contrast thr eshold, the thresholds for isoluminant targets lie considerably above behavioral contrast threshold. Therefore, although some MT neurons do give responses to isoluminant targets, they are unlikely to be the sou rce of the chromatic motion signals revealed behaviorally.