Correlated firing in macaque visual area MT: Time scales and relationship to behavior

We studied the simultaneous activity of pairs of neurons recorded with a si ngle electrode in visual cortical area MT while monkeys performed a directi on discrimination task. Previously, we reported the strength of interneuron al correlation of spike count on the time scale of the behavioral epoch (2 sec) and noted its potential impact on signal pooling (Zohary et al., 1994) . We have now examined correlation at longer and shorter time scales and fo und that pair-wise cross-correlation was predominantly short term (10-100 m sec). Narrow, central peaks in the spike train cross-correlograms were larg ely responsible for correlated spike counts on the time scale of the behavi oral epoch. Longer-term (many seconds to minutes) changes in the responsive ness of single neurons were observed in auto-correlations; however, these s low changes in time were on average uncorrelated between neurons. Knowledge of the limited time scale of correlation allowed the derivation of a more efficient metric for spike count correlation based on spike timing informat ion, and it also revealed a potential relative advantage of larger neuronal pools for shorter integration times. Finally, correlation did not depend o n the presence of the visual stimulus or the behavioral choice of the anima l. It varied little with stimulus condition but was stronger between neuron s with similar direction tuning curves. Taken together, our results strengt hen the view that common input, common stimulus selectivity, and common noi se are tightly linked in functioning cortical circuits.