INFORMATION IN THE NEURONAL REPRESENTATION OF INDIVIDUAL STIMULI IN THE PRIMATE TEMPORAL VISUAL-CORTEX

To analyze the information provided about individual visual stimuli in the responses of single neurons in the primate temporal lobe visual c ortex, neuronal responses to a set of 65 visual stimuli were recorded in macaques performing a visual fixation task and analyzed using infor mation theoretical measures. The population of neurons analyzed respon ded primarily to faces. The stimuli included 23 faces and 42 nonface i mages of real-world scenes, so that the function of this brain region could be analyzed when it was processing relatively natural scenes. It was found that for the majority of the neurons significant amounts of information were reflected about which of several of the 23 faces had been seen. Thus the representation was not local, for in a local repr esentation almost all the information available can be obtained when t he single stimulus to which the neuron responds best is shown. It is s hown that the information available about any one stimulus depended on how different (for example, how many standard deviations) the respons e to that stimulus was from the average response to all stimuli. This was the case for responses below the average response as well as above . It is shown that the fraction of information carried by the low firi ng Fates of a cell was large-much larger than that carried by the high firing rates. Part of the reason for this is that the probability dis tribution of different firing rates is biased toward low values (thoug h with fewer very low values than would be predicted by an exponential distribution). Another factor is that the variability of the response is large at intermediate and high firing rates. Another finding is th at at short sampling intervals (such as 20 ms) the neurons code inform ation efficiently, by effectively acting as binary variables and behav ing less noisily than would be expected of a Poisson process.