NEURAL CODING OF VISUAL STIMULI IN THE AWAKE MONKEY

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To study the mechanisms underlying visual perception we recorded the activity of ganglion cell fibers, the fibers that send visual information from the retina to the brain, and compared it with the recordings of single neurons in the lateral geniculate nucleus and primary visual cortex, the first two extraretinal stages of visual processing, and inferior temporal cortex, the last visual processing station in the cortex. The three sets of neuronal recordings all showed different temporal response patterns to different visual stimulus patterns. When 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 this code was found to contain twice as much information as that conveyed by the response strength, the usual measure of neuronal response. However, the ganglion cells fibers, which carry as much information as the neurons in subsequent stages, have substantially less information in the temporal modulation code than in the response strength code. Thus, temporal modulation carries a significantly greater proportion of visual information outside of the retina. 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 neuronal responses of primary visual cortex led to the discovery that information about each of these parameters is carried separately in the response. A geometrical analysis of the data shows a potential structure for a neural code. When a 3-dimensional space is used to represent the responses, the responses to a single pattern appear to lie in a single plane regardless of luminance or duration, with the planes for different patterns frequently being separable. The equations for these planes describe a neural code.