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 found that the response patterns could be represented as the sum of several (3-6) simultaneous, time-varying patterns of activity, showing that the responses of single neurons can be regarded as carrying several independent messages. These neurons signal figure-ground separation. We found that the time the neurons start firing when a pattern is presented, i.e. the response latency, is a code indicating how easily the pattern can be seen, whereas the intensity of the firing, i.e 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 minimized 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.