Figure 2. Responses of two model neurons to four input correlation patterns.

Histograms on the left show average cross-correlations, like those in FIG. 1, between pairs of excitatory inputs (EE), between pairs of inhibitory inputs (II), and between excitatory–inhibitory pairs (EI). The y axes in the correlograms extend from 0.7 to 1.4. Red and green traces correspond to responses of balanced and unbalanced neurons, respectively, always driven by 160 excitatory and 40 inhibitory inputs. The rate of inhibitory inputs was always 1.7 times the excitatory rate. In the middle traces, all excitatory inputs fired at 42 spikes s⁻¹. In the plots on the right, the mean firing rate of the excitatory inputs varies along the x axes, and the y axes correspond to the output firing rates of the two postsynaptic model neurons. All responses were obtained using leaky integrate-and-fire models (see BOX 1). a| All input spike trains were independent. In the middle traces, both postsynaptic neurons are shown to fire at about 30 spikes s^{− 1}. b| Excitatory inputs were synchronous, with 10% shared inputs, as in FIG. 1a. Balanced and unbalanced neurons fired at 67 and 45 spikes s^{− 1}, respectively. c| Inhibitory inputs oscillated with an amplitude equal to 50% of the mean rate, as in FIG. 1e. Balanced and unbalanced neurons fired at 59 and 30 spikes s^{− 1}, respectively. d| All inputs were synchronous, with 10% shared inputs. Balanced and unbalanced neurons fired at 31 and 41 spikes s^{− 1}, respectively. For comparison, broken lines in the input–output rate plots (b–d) are the curves obtained with independent inputs (a). The balanced neuron is much more sensitive to correlations than the unbalanced one.