Mw. Oram et al., Excess synchrony in motor cortical neurons provides redundant direction information with that from coarse temporal measures, J NEUROPHYS, 86(4), 2001, pp. 1700-1716
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.