Cone Signal Interactions in Direction-Selective Neurons in Area MT

Many experimental measurements support the hypothesis that area MT of the rhesus monkey has a central role in processing visual motion. The great majority of these studies were performed using luminance stimuli, leaving open the question of how color information is used during motion processing. We investigated the specific question of how S-cone signals, an important source of color information, interact with (L+M)-cone signals, the dominant source of luminance information. In MT, S-cone-initiated signals combine synergistically with (L+M)-cone (luminance) signals over most of the stimulus range, regardless of whether the stimuli are added or subtracted. A quantitative analysis of the responses to the combination of Sand (L+M)-cone signals shows that for a significant minority of cells, these S-cone signals are carried to MT by a color-opponent ("blue-yellow") pathway, such that in certain limited contrast ranges, a small amount of Sand (L+M)-cone cancellation is observed. Both Sand (L+M)-cone responses are direction-selective, suggesting that MT processes a wide range of motion signals, including those carried by luminance and color. To investigate this possibility further, we measured MT responses while monkeys discriminated the direction of motion of luminance and S-coneinitiated gratings. The sensitivity of single MT neurons, as well as the correlation between trial-to-trial variations in single neuron firing and perception, are similar for Sand (L+M)-cone stimuli, further supporting a role for MT in processing chromatic motion.