NEURAL CODING OF VISUAL STIMULI IN THE AWAKE MONKEY

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We are recording from single neurons in the visual system of monkeys in the attempt to uncover the neuronal mechanisms underlying visual perception and visual recognition. One set of experiments carried out of neurons of inferior temporal cortex has shown that changing the attentional or cognitive demand of visual tasks changes the neuronal responses to a visual stimulus. For example, when the task was made more difficult so that the monkey committed more errors, the neuronal responses to the stimulus were increased. Also, neuronal responses to the stimulus differed depending on whether the animal was discriminating it from another on the basis of its texture or shape. In a second set of experiments, involving recording from single neurons of both inferior temporal and striate cortex, different visual stimuli were found to yield different temporal sequences of action potentials. We have developed quantitative techniques to identify which features of this temporal modulation are significant in carrying messages about stimuli. As a result, we have now been able to analyze the neuron as a communication channel carrying encoded information in the sequence of action potentials. The analysis showed that with a code that makes use of temporal modulation the neurons transmits twice as much information as when the code uses only the number of action potentials. The number of action potentials is not correlated with the information transmitted.