Neuronal interactions improve cortical population coding of movement direction

Interactions among groups of neurons in primary motor cortex (MI) may conve y information about motor behavior. We investigated the information carried by interactions in MI of macaque monkeys using a novel multielectrode arra y to record simultaneously from 12-16 neurons during an arm-reaching task. Pairs of simultaneously recorded cells revealed significant correlations in their trial-to-trial firing rate variation when estimated over broad (600 msec) time intervals. This covariation was only weakly related to the prefe rred directions of the individual MI neurons estimated from the firing rate and did not vary significantly with interelectrode distance. Most signific antly, in a portion of cell pairs, correlation strength varied with the dir ection of the arm movement. We evaluated to what extent correlated activity provided additional information about movement direction beyond that avail able in single neuron firing rate. A multivariate statistical model success fully classified direction from single trials of neural data. However, clas sification was consistently better when correlations were incorporated into the model as compared to one in which neurons were treated as independent encoders. Information-theoretic analysis demonstrated that interactions cau sed by correlated activity carry additional information about movement dire ction beyond that based on the firing rates of independently acting neurons . These results also show that cortical representations incorporating highe r order features of population activity would be richer than codes based so lely on firing rate, if such information can exploited by the nervous syste m.