STIMULUS-DEPENDENT SYNCHRONIZATION OF NEURONAL RESPONSES IN THE VISUAL-CORTEX OF THE AWAKE MACAQUE MONKEY

In visual areas of the cerebral cortex, most neurons exhibit preferenc es for particular features of visual stimuli, but in general, the tuni ng is broad. Thus, even simple stimuli evoke responses in numerous neu rons with differing but overlapping feature preferences, and it is com monly held that a particular feature is encoded in the pattern of grad ed responses of the activated population rather than in the optimal re sponses of individual cells. To decipher this population code, respons es evoked by a particular stimulus need to be identified and bound tog ether for further joint processing and must not be confounded with res ponses to other, nearby stimuli. Such selection of related responses c ould be achieved by synchronizing the respective discharges at a time scale of milliseconds, as this would selectively and jointly enhance t heir saliency. This hypothesis predicts that a given set of neurons sh ould exhibit synchronized discharges more often when responding to a s ingle stimulus than when activated by different but simultaneously pre sented stimuli. To test this prediction, recordings were performed wit h two electrodes from spatially segregated cells in the middle tempora l area (MT) of the awake behaving macaque monkey. It was found that ce lls with overlapping receptive fields, but different preferences for d irections of motion, can engage in synchronous activity if they are st imulated with a single moving bar. In contrast, if the same cells are activated with two different bars, each moving in the direction prefer red by the cells at the two respective sites, responses show no or muc h fewer synchronous epochs. Control experiments exclude that this effe ct is attributable to changes in response amplitude, the mere presence of two stimuli, or the specific orientation of the bars. The critical variable determining the strength of correlation is the extent to whi ch both sites are activated by a common stimulus or by two different s timuli with different directions of motion.