Abstract. We consider how the output of the perfect integrate-and-®re (I&F) model of a single neuron is aected by the properties of the input, ®rst of all by the distribution of aerent excitatory and inhibitory postsynaptic potential (EPSP, IPSP) inter-arrival times, discriminating particularly between short-and longtailed forms, and by the degree of balance between excitation and inhibition (as measured by the ratio, r, between the numbers of inhibitory and excitatory inputs). We ®nd that the coecient of variation (CV; standard deviation divided by mean) of eerent interspike interval (ISI) is an increasing function of the length of the tail of the distribution of EPSP inter-arrival times and the ratio r. There is a range of values of r in which the CV of output ISIs is between 0.5 and 1. Too tight a balance between EPSPs and IPSPs will cause the model to produce a CV outside the interval considered to correspond to the physiological range. Going to the extreme, an exact balance between EPSPs and IPSPs as considered in [24] ensures a long-tailed ISI output distribution for which the moments such as mean and variance cannot be de®ned. In this case it is meaningless to consider quantities like output jitter, CV, etc. of the eerent ISIs. The longer the tail of the input inter-arrival time distribution, the less is the requirement for balance between EPSPs and IPSPs in order to evoke output spike trains with a CV between 0.5 and 1. For a given short-tailed input distribution, the range of values of r in which the CV of eerent ISIs is between 0.5 and 1 is almost completely inside the range in which output jitter (standard deviation of eerent ISI) is greater than input jitter. Only when the CV is smaller than 0.5 or the input distribution is a long-tailed one is output less than input jitter [21]. The I&F model tends to enlarge low input jitter and reduce high input jitter. We also provide a novel theoretical framework, based upon extreme value theory in statistics, for estimating output jitter, CV and mean ®ring time.
IntroductionThe integrate and ®re (I&F) model is a rudimentary model that underpins the idea of a neuron being a device which accumulates inputs until it reaches a threshold, whereupon it ®res. The model has recently been examined by several authors [1,24,25,28]. Softky and Koch [25] compared the behaviour of the model with data obtained in the visual cortex of the monkey, via theoretical calculations and numerical simulations. They found, with exclusively excitatory postsynaptic potential (EPSP) inputs, that the output of the I&F model is very regular, with the coecient of variation (CV) of the inter-spike interval (ISI, the standard deviation or so-called output jitter divided by the mean) converging to zero at a rate 1ax th p , as x th becomes large, where x th is the number of EPSPs needed to trigger a spike. Therefore, they concluded that the model cannot account for the phenomena observed in neurones in the visual cortex with a CV between 0X5 and 1X Their results cast doubt on the rate coding assu...