Origin of firing varibility of the integrate-and-fire model

It has been reported that neurones in the visual cortex fire with a high CV (standard deviation/mean) of interspike interval, greater than 0.5. In terms of the integrate-and-fire model with and without reversal potentials, we elaborate the underlying mechanism of producing spike trains with CV greater than 0.5. When the attractor of deterministic part of models is above the threshold of the membrane potential, the firing is mainly due to deterministic forces and its output CV is usually lower than 0.5; whereas if the attractor is below the threshold, the generation of spikes results from random oscillations and the CV is usually greater than 0.5. The critical value of the number of active inhibitory synapses at which CV is greater or smaller than 0.5 is determined, which gives a clear picture of how neurones adjust their synaptic inputs to elicit irregular spike trains. (C) 1999 Elsevier Science B.V. All rights reserved.