NEURAL CODING OF VISUAL STIMULI IN THE AWAKE MONKEY

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Single neurons were recorded from the lateral geniculate nucleus and primary visual cortex, the first two extraretinal stages of visual processing, and inferior temporal cortex, the last visual cortical processing station, to study the mechanisms underlying visual perception. Neurons in all three regions showed different temporal response patterns to different visual stimulus patterns. When these neurons were analyzed as communication channels carrying information about visual stimuli in their responses, the response patterns seen could only be represented as the sum of several (3-6) simultaneous, independent patterns of activity. Three of these activity patterns were analyzed as a temporal code, and the information contained in them was compared to the information conveyed by the number of action potentials, the usual measure of neuronal response; there was twice as much information available in the temporal code. This showed that each response can be considered the sum of activity from several independent channels, with each channel acting as a spatial-to- temporal filter. Traditionally, it has been thought that information about multiple stimulus parameters, such as luminance, pattern, and duration of presentation, must be confounded in the neuronal responses. However, based on this multiplex-filter hypothesis, a new analysis of the primary visual cortex responses led to the discovery that information about each of these parameters is carried separately in the response. This strongly suggests that a consistent neural code exists. A geometrical analysis of these data shows a potential structure for this code. When a 3-dimensional space is used to represent the responses, the responses to a single pattern appear to lie in a plane regardless of luminance or duration. For a single neuron's response, the planes for different patterns are frequently separable. The equations for these planes describe a neural code.