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. We know that neuronal responses simultaneously carry several different messages. The time the neurons start firing when a pattern is presented, the latency, is a code indicating how easily the pattern can be seen, and the intensity of the firing, the response strength, is a code indicating what the pattern is. Each member of a neuronal pair recorded from simultaneously with a single electrode in a given cortical visual area has responses that are independent of the other. A theoretical analysis shows that these neurons have the properties that would be expected if the cortex developed so that local groups of neurons had developed to minimize the loss of information during visual processing. Based on all of these findings taken together, we suggest that the visual system is sending messages that describe stimuli as complex, integrated objects from the earliest cortical stages, and that each neuron provides an independent description of the stimulus being viewed. We now know that the responses of inferior temporal neurons encode information about how well a stimulus fits a preconceived template of stimulus type, even when the stimulus is obscured by visual noise. We suggest that the traditional view of hierarchical processing should be modified to emphasize that features are combined specifically in the early parts of the processing sequence, and generalized by the later parts of the processing sequence.