Correlation between neural spike trains increases with firing rate

Populations of neurons in the retina, olfactory system, visualand somatosensory6 thalamus, and several cortical regions show temporal correlation between their spike trains. Correlated firing has been linked to stimulus encoding, attention, stimulus discrimination, and motor behavior. Nevertheless, the mechanisms underlying correlated spiking are poorly understood, and its coding implications are still debated. It is not clear, for instance, whether correlations between the discharges of two neurons are determined solely by the correlation between their afferent currents, or whether they also depend on the mean and variance of the input. We addressed this question by computing the spike train correlation coefficient of unconnected pairs of in vitro cortical neurons receiving correlated inputs. Remarkably, even when input correlation remained fixed, the spike train output correlation increased with the firing rate, but was largely independent of spike train variability. With a combination of analytical techniques and numerical simulations we show that this relation between correlation and rate is quite general, being observed in heterogeneous populations with a wide range of input statistics.

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