THE SIGNIFICANCE OF PRECISELY REPLICATING PATTERNS IN MAMMALIAN CNS SPIKE TRAINS

Neuronal spike trains from both single and multi-unit recordings often contain patterns such as doublets and triplets of spikes that precise ly replicate themselves at a later time. The presence of such precisel y replicating patterns can still be detected when the tolerance on int erval replication is shortened to a fraction of a millisecond. In this context we examine here data taken from various parts of the central nervous systems of anesthetized rats, cats and monkeys. The relative a bundance of replicating triplets varies from centre to centre, and is nearly always significantly greater than obtained in Monte-Carlo simul ations of either a Poisson-like process or a renewal process having th e same interspike interval distribution as the neuronal data. However, a remarkable exception is found in the activity of retinal ganglion c ells. Significant deviations were found in the primary visual cortex a nd, even more so, in the lateral geniculate body and the mitral cells of the olfactory bulb. Using a fixed tolerance for the replication of intervals (0.5 ms) it is usually observed that replicating patterns ar e produced in excess (with respect to renewal process models) mostly i n low firing rate episodes (less than or equal to 100 Hz). However, us ing a tolerance that varies in direct proportion to the mean interval (i.e. as the reciprocal of the firing rate), one generally observes th at replicating triplets occur with higher than expected frequency in c omparable proportions at all firing rates. This observation suggests t he existence of a scale invariance principle in these phenomena with r espect to certain neuronal codes. In order to decrease the influence o f the estimated neuronal firing rate on the results of the comparisons , we computed also the ratio NT2/ND3, of the number of replicating tri plets to the number of doublets replicating three times [Lestienne R. (1994) Proc. Sec. Neurosci 20, 22; Lestienne R. (1996) Biol. Cybern. 7 4, 55-61], using both a fixed or a variable tolerance. In spike trains obeying a Poisson process, NT2/ND3 ratios should be nearly independen t of the frequency, especially when using a variable tolerance. These studies supported previous results : significant deviations from the m odels are found in all the spike trains examined, except in the case o f retinal ganglion cells, and the most significant deviations are foun d in recordings from the lateral geniculate nucleus and the mitral cel ls of the olfactory bulb. Removing spikes that belong to bursts having large ''Poisson surprise'' values [Legendy C. R. and Salcman M. (1985 ) J. Neurophysiol. 53, 926-939] (except the very first spike of the bu rst) significantly decreases NT2/ND3 ratios in the record from the lat eral geniculate nucleus, suggesting that in this case bursty episodes greatly contribute to the production of replicating patterns, but such a removal does not affect results from the piriform record. Finally, in both the lateral geniculate nucleus and in the mitral cells of the olfactory bulb records, perturbing the timing of spikes by applying to interspike intervals small jitters of uniform probability density wit h amplitude up to 3 ms, very significantly decrease NT2/ND3 ratios in these centres, but does not change much the NT2/ND3 ratios in other ne uronal recordings. Implications of these findings for a possible role of precisely replicating patterns in temporal coding of neuronal infor mation is discussed, as well as possible mechanisms for their producti on. (C) 1997 IBRO. Published by Elsevier Science Ltd.