Trial to trial variability in either stimulus or action causes apparent correlation and synchrony in neuronal activity

In this report we show that the observed inter-neuronal correlation reflect s a superposition of correlations associated with the intrinsic correlation between neurons, and correlations associated with variability in the stimu li presented to, or the actions performed by, the subject. We argue that th e effects of either stimulus or action variability on the observed correlat ion, though generally ignored, can be substantial. Specifically, we demonst rate how observed correlations are effected by trial to trial variability i n either stimulus or action. In addition, assuming that all relevant stimul i and actions are known, we outline a method for eliminating their effects on the observed correlation. It is also shown that tuning of correlations t o a stimulus or an action might be a direct consequence of variability in t hat stimulus or action, even in the absence of any modulation of direct int er-neuronal interaction. The effects of stimulus and action variability sho uld therefore be carefully considered when designing and interpreting exper iments involving multi-neuronal recordings. (C) 2001 Elsevier Science B.V. All rights reserved.