Cone signal interactions in direction-selective neurons in the middle temporal area

Many experimental measurements support the hypothesis that the middle temporal (MT) area of the rhesus monkey has a central role in processing visual motion. Most 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 the MT area, 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,Mcone signals shows that for a significant minority of cells, these S-cone signals are carried to the MT area 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-cone-initiated gratings. The sensitivity of single MT neurons and 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 the MT area in processing chromatic motion.