TEMPORAL DYNAMICS OF CHROMATIC TUNING IN MACAQUE PRIMARY VISUAL-CORTEX

The ability to distinguish colour from intensity variations is a diffi cult computational problem for the visual system because each of the t hree cone photoreceptor types absorb all wavelengths of light, althoug h their peak sensitivities are at relatively short (S cones), medium ( M cones), or long (L cones) wavelengths. The first stage in colour pro cessing is the comparison of the outputs of different cone types by sp ectrally opponent neurons in the retina and upstream in the lateral ge niculate nucleus(1-3). Some neurons receive opponent inputs from L and M cones, whereas others receive input from S cones opposed by combine d signals from L and M cones. Here we report how the outputs of the L/ M- and S-opponent geniculate cell types are combined in time at the ne xt stage of colour processing, in the macaque primary visual cortex (V 1). Some V1 neurons respond to a single chromatic region, with either a short (68-95 ms) or a longer (96-135 ms) latency, whereas others res pond to two chromatic regions with a difference in latency of 20-30 ms . Across all types, short latency responses are mostly evoked by L/M-o pponent inputs whereas longer latency responses are evoked mostly by S -opponent inputs. Furthermore, neurons with late S-cone inputs exhibit dynamic changes in the sharpness of their chromatic tuning over time. We propose that the sparse, S-opponent signal in the lateral genicula te nucleus is amplified in area V1, possibly through recurrent excitat ory networks. This results in a delayed, sluggish cortical S-cone sign al which is then integrated with L/M-opponent signals to rotate the la teral geniculate nucleus chromatic axes(4-5).