Autonomous theta-gamma oscillations in networks with mixed GABAA kinetics. (a) Raster plots for a network containing 50 GABAA,fast (Bottom) and 50 GABAA,slow (Top) neurons. CVI = the coefficient of variation (ratio of standard deviation to mean) of the level of dc current drive given to the population of cells. Increasing CVI implies increasing heterogeneity in drive. (b and c) Population histograms for (b) GABAA,slow and (c) GABAA,fast cells, generated from the spike trains in a. (d) Magnitude spectra of the histograms of the GABAA,fast cells. The theta (4–12 Hz) and gamma (30–80 Hz) frequency bands are marked. Model specifications: mean values of applied current = 5 μA/cm2 for GABAA,fast cells and 3 μA/cm2 for GABAA,slow cells. In addition to randomly distributed mean currents, time-dependent current noise (normally distributed, σ = 5 μA/cm2) was added to each cell independently. Cells were all-to-all coupled with coupling conductances (in mS/cm2): Gfast,fast = 0.04, Gfast,slow = 0, Gslow,slow = 0.08, Gslow,fast = 0.04.