Figure 7.
Varying noise amplitude results in trade-off between synchrony and periodicity. a, The degree of output correlation increases with the amplitude of the input noise for correlated (Cin = 1.0, closed circles) but not uncorrelated (Cin = 0, open circles) filtered white noise inputs. Here, we plot the degree of input noise in terms of the SD of the membrane potential rather than of the input current to compensate for differences in membrane properties across neurons. Simulations (open and closed diamonds) show similar effects of noise amplitude when the magnitude of uncorrelated noise is ∼20% of the magnitude of the correlated noise. b, c, Experimental data (b) and simulations (c) and show the effect of the amplitude of correlated noise on the coherence of spike trains. For smaller noise amplitude (10–30 pA fluctuations) in b, the cross-power spectrum of neuronal output is not altered by the noisy correlated inputs, which shift only the relative timing of their action potentials. Therefore, the cross-power spectrum has a clear peak near the average firing rate of the neurons. In other words, the neurons filter the correlations on their intrinsic time scale. For larger noise amplitudes (40–60 pA in b), the periodicity of the neural firing is reduced and, as a consequence, the coherence peak broadens. Regardless of noise amplitude, the coherence rapidly decreases >40 Hz, which indicates that the neurons only take advantage of the input correlations that have a time scale is similar to or less than their intrinsic firing rate. c, A similar trade-off was seen in simulations. Values of input noise are as described in Materials and Methods.