Excess synchrony in motor cortical neurons provides redundant direction information with that from coarse temporal measures

Previous studies have shown that measures of fine temporal correlation, suc h as synchronous spikes, across responses of motor cortical neurons carries more directional information than that predicted from statistically indepe ndent neurons. It is also known, however, that the coarse temporal measures of responses, such as spike count, are not independent. We therefore exami ned whether the information carried by coincident firing was related to tha t of coarsely defined spike counts and their correlation. Synchronous spike s were counted in the responses from 94 pairs of simultaneously recorded ne urons in primary motor cortex (MI) while monkeys performed arm movement tas ks. Direct measurement of the movement-related information indicated that t he coincident spikes (1- to 5-ms; precision) carry similar to 10% of the in formation carried by a code of the two spike counts. Inclusion of the numbe rs of synchronous spikes did not add information to that available from the spike counts and their coarse temporal correlation. To assess the signific ance of the numbers of coincident spikes, we extended the stochastic spike count matched (SCM) model to include correlations between spike counts of t he individual neural responses and slow temporal dependencies within neural responses (similar to 30 Hz bandwidth). The extended SCM model underestima ted the numbers of synchronous spikes. Therefore as with previous studies, we found that there were more synchronous spikes in the neural data than co uld be accounted for by this stochastic model. However, the SCM model accou nts for most (R-2 = 0.93 +/-0.05, mean SE) of the differences in the observ ed number of synchronous spikes to different directions of arm movement, in dicating that synchronous spiking is directly related to spike counts and t heir broad correlation. Further, this model supports the information theore tic analysis that the synchronous spikes do not provide directional informa tion beyond that available from the firing rates of the same pool of direct ionally tuned MI neurons. These results show that detection of precisely ti med spike patterns above chance levels does not imply that those spike patt erns carry information unavailable from coarser population codes but leaves open the possibility that excess synchrony carries other forms of informat ion or serves other roles in cortical information processing not studied he re.