Near Poisson-type firing produced by concurrent excitation and inhibition

The effect of inhibition on the firing variability is examined in this pape r using the biologically-inspired temporal noisy-leaky integrator (TNLI) ne uron model. The TNLI incorporates hyperpolarising inhibition with negative current pulses of controlled shapes and it also separates dendritic from so matic integration. The firing variability is observed by looking at the coe fficient of "variation (C-V) (standard deviation/mean interspike interval) as a function of the mean interspike interval of firing (Deltat(M)) and by comparing the results with the theoretical curve for random spike trains, a s well as looking at the interspike interval (ISI) histogram distributions. The results show that with 80% inhibition, firing at high rates (up to 200 Hz) is nearly consistent with a Poisson-type variability, which complies w ith the analysis of cortical neuron firing recordings by Softky and Koch [1 993, J. Neurosci. 13(1) 334-530]. We also demonstrate that the mechanism by which inhibition increases the C-V values is by introducing more short int ervals in the firing pattern as indicated by a small initial hump at the be ginning of the ISI histogram distribution. The use of stochastic inputs and the separation of the dendritic and somatic integration which we model in TNLI, also affect the high firing, near Poisson-type (explained in the pape r) variability produced. We have also found that partial dendritic reset in creases slightly the firing variability especially at short ISIs. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.