Irregular dynamics in purely inhibitory (a-c) and inhibitory and excitatory (d-f) coupled random networks of identical leaky integrate-and-fire neurons (N = 400). Starting with a network where all connections are inhibitory, we consecutively replace inhibitory connections by excitatory ones. We reduce the external current to keep the average input to a single cell constant. Whereas the spiking activity is similar and highly irregular in both regimes, we demonstrate that the first one is stable while the second one is chaotic. (a, d) The upper panel displays the spiking times (blue lines) of a subset of 40 neurons. The lower panel displays the membrane potential trajectory of one single neuron. (b, e) Histogram of mean firing rates, vi. (c, f) Histogram of the coefficients of variation, CVi, averaged over time.