SYNCHRONIZATION OF NEURONAL-ACTIVITY DURING STIMULUS EXPECTATION IN ADIRECTION DISCRIMINATION TASK

The dorsal pathway of the primate brain, especially the middle tempora l area (MT or V5) and the superior middle temporal area (MST or V5a), is strongly involved in motion detection. The relation between neural firing rates and psychophysical performance has led to the assumption that the neural code used by these areas consists of the relative disc harge rates of neuronal populations. As an additional neural code, tem poral correlation of neural activity has been suggested. Our study add resses the involvement of such a code in awake monkeys performing a mo tion discrimination task. We found significant temporal correlations b etween simultaneously recorded pairs of units in areas MT and MST and other extrastriate cortical areas. Units recorded from the same electr ode were more frequently synchronized than units recorded from differe nt electrodes placed within the same or different cortical areas. Acti vity synchronization was present in the expectation period before stim ulus presentation and could not be induced de novo by the stimulus. Ra ther, we found a contrast-dependent reduction of correlation strength on stimulus onset. Correlation strength did not vary systematically wi th stimulus directions. We conclude that under the conditions of this study, temporal decorrelation of MT and MST neurons could be used to d etect the stimulus, but synchronization does not convey specific infor mation about its direction of motion and therefore is unlikely to cont ribute to performance in our direction discrimination task. Activity s ynchronization in the period before stimulus onset could be related to attentive expectation.