Irregularity in neocortical spike trains: influence of measurement factors and another method of estimation

Irregularity in the interspike interval is a common phenomenon especially in the neocortex. A measure of this random variation in the spacing between neuronal spikes is usually obtained with the coefficient of variation CV (standard deviation/mean interspike interval). In excitable cells, the standard deviation in the interspike interval can be large and the mean firing rate often fluctuates. As a result, there can be substantial variability in the value of the CV computed for the same spike train using only slightly different samples as we show. Moreover, these CV values can be comparatively meaningless unless certain conditions are met. In doing so some researchers have selectively sampled data over a stable mean while others have used a wide range of trial times or subsets thereof (capture window) to compute the CV. This has made interpretation of the raw CV cumbersome. We demonstrate that the CV has a triple sensitivity, namely, for the size of the capture window, the spike count and the refractory period. We assuage these difficulties by introducing a modified term, the coefficient of variation proportion of maximum (CVpm) that offers transportability across different experimental conditions by compensating for the triplet.