NEURAL CODING OF VISUAL STIMULI IN THE AWAKE MONKEY

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Single neurons were recorded from both the first and last cortical stations of the visual system of monkeys to study mechanisms underlying visual perception and memory. Neurons in both these regions, striate cortex and inferior temporal cortex, showed different temporal sequences of action potentials in response to different visual stimulus patterns. Analysis of the responses of these neurons as if they were information carriers in communications channels revealed that different temporal patterns of neuronal activity conveyed different information about stimulus features. In both these cortical regions, several (3-6) simultaneous, independent temporal patterns were needed to represent the modulation of the stimulus driven neuronal activity. A code made up of three simultaneous temporal patterns conveyed twice as much information about the stimulus as a more traditional measure of the response, the number of action potentials. This suggests a new hypothesis about visual processing, the multiplex-filter hypothesis: visual system neurons act as if they transmit several simultaneous or multiplexed messages describing the stimulus. In a test of this hypothesis, the responses of complex cells from striate cortex were characterized using a small set (16) of one-dimensional basic stimuli. A computer simulation of neuronal responsiveness based on the multiplex-filter hypothesis successfully predicted the actual temporally modulated responses of these neurons to 44 other one-dimensional patterns.