NEURAL CODING OF VISUAL STIMULI

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We recorded the activity of single neurons in the primary visual cortex (the first cortical stage of visual processing) and inferior temporal cortex (the last cortical stage), to study the mechanisms underlying visual perception. When the responses were analyzed for the information they contained, we found that the response patterns could be represented as the sum of several (3-6) simultaneous, independent patterns of activity. These activity patterns were analyzed as a temporal code, and this code was found to contain more information than that conveyed by the response strength, the usual measure of neuronal response. Conceptualizing the neuron as having several independent activity patterns allowed us to use the responses elicited by one set of visual patterns to predict those elicited by other patterns. We also found that neurons in inferior temporal cortex recorded from while monkeys perform a delayed matching-to-sample task carry stimulus-specific information differentiating the pattern stimuli, and reflecting whether a given stimulus is the sample, the match, or the nonmatch. The change in the neuronal response across conditions for a few stimuli could be used to predict the change in neuronal response to other stimuli across those same conditions. Inferior temporal neurons also carry information about the stimulus being searched for (i.e. the sample) when the stimulus is either the match or nonmatch. Using a different model we were able to show how the responses to the stimulus being viewed were modified by the responses to the preceding stimulus. Each member of a neuronal pair recorded from simultaneously with a single electrode in a given cortical visual area is independent of the other, i.e., their information adds. This suggests that local processing is carried out by adding independent analyses of the stimulus being viewed.