PRIMATE STRIATE AND PRESTRIATE CORTICAL-NEURONS DURING DISCRIMINATION.2. SEPARABLE TEMPORAL CODES FOR COLOR AND PATTERN

1. In the previous paper we reported our analysis of the responses of neurons in cortical areas V1, V2, and V4 to a set of stimuli that cons isted of all 36 combinations of six colors and six patterns. Neurons i n all three cortical areas simultaneously encoded information about bo th the color and pattern of the stimulus in the number and temporal di stribution of spikes in their responses. To account for this ability, we propose that a neuron's response consists of separable temporal cod es representing the color and pattern of the stimulus that are multipl exed together. 2. We used nonlinear regression to fit the model parame ters to the data. We used the responses to 30 of the 36 stimuli as a t raining set to estimate the parameters of the model and the responses to the remaining 6 stimuli as a test set. After training, the model fi tted the responses to stimuli in the training sets very well and predi cted the responses to stimuli in the test sets. Thus neuronal response s to colored patterns contain separate temporal codes representing col or and pattern. 3. After establishing the model parameters, we obtaine d the waveforms that represented each neuron's temporal codes for the six colors and six patterns of our stimulus set. We then proceeded wit h a series of analyses to determine whether these waveforms were viabl e candidates for neuronal codes. Cluster analysis revealed that there were only a few different classes of waveforms representing each color and pattern, and there were many neurons in each class. Further, neur ons that used similar waveforms to represent one color or pattern also tended to use similar waveforms to represent other colors or patterns . The waveforms representing five of the six colors and three of the s ix patterns were similar in the two monkeys used in this study. 4. We compared the shapes of the code waveforms across cortical areas and fo und no differences among areas in the shapes of the waveforms represen ting four of the six colors. In contrast, we found that there were dif ferences among areas in the shapes of the waveforms representing all s ix patterns. These results suggest that messages about color are encod ed at an early level and are then propagated upward, but that messages about pattern are altered in each successive cortical area. 5. Our re sults offer a neurophysiological explanation for the psychophysical ev idence that color and form are processed by different channels. We pro pose that the psychophysical channels for color and pattern arise from the separability of the temporal codes for color and pattern in the r esponses of single neurons. This hypothesis implies that psychophysica l channels correspond to classes of temporal codes rather than to clas ses of neurons.