Sensitivity of Neurones in Aplysia to Temporal Pattern of Arriving Impulses.

A complete description of the sequence of spikes in a neuron discharge consists of an ordered series of intervals tlt t2, ..., tn, extending over the entire observation time. Neurophysiologists commonly discuss ongoing activity in terms of mean frequency of spikes thus ignoring variability and sequence of intervals. Histograms of intervals between two consecutive spikes or histograms of time between three, four, . . . , or n consecutive spikes (Gerstein & Kiang, i960; Rodieck, Kiang& Gerstein, 1962), while giving information about distribution, also ignore the precise succession of events. Two-dimensional histograms showing the relative occurrence of all ordered pairs of inervals between three successive spikes or, more generally, a display of the frequency of all ordered sequences of n intervals between n +1 successive spikes can reveal a tendency for patterns of spikes. Each method must be evaluated according to two criteria: first, the accuracy and extent with which it conveys information about the discharge; secondly, the degree in which it summarizes features that are functionally significant. From the latter viewpoint, the analysis of mean spike frequency has contributed much to our understanding of neuronal systems. It is necessary to pose the additional query of whether more sophisticated statistical descriptions are of value also. The present study was designed to investigate whether physiologically significant differences in output occur in neurons submitted to an input that has constant mean frequency but whose higher order statistics, namely interval histogram and pattern, are systematically varied. The only previous study of this question appears to be that of Wiersma and collaborators in neuromuscular preparations of Crustacea, where changes of timing of the nerve stimulation led to remarkable changes in contractions (Ripley & Wiersma, 1953; Wiersma & Adams, 1950).