Limits to the temporal fidelity of cortical spike rate signals

The cerebral cortex processes information primarily through changes in the spike rates of neurons within local ensembles. To evaluate how reliably the average spike rate of a group of cortical neurons can represent a time-varying signal, we simulated an ensemble with realistic spike discharge behavior. We found that weak interneuronal correlation, or synchrony, allows the variability in spike rates of individual neurons to compromise the ensemble representation of time-varying signals. Brief cycles of sinusoidal modulation at frequencies above 115 Hz could not be represented by an ensemble of hundreds of neurons whose interneuronal correlation mimics that of the visual cortex. The spike variability and correlation assumed in our simulations are likely to apply to many areas of cortex and therefore may constrain the fidelity of neural computations underlying higher brain function.