STOCHASTIC VERSUS DETERMINISTIC VARIABILITY IN SIMPLE NEURONAL CIRCUITS .2. HIPPOCAMPAL SLICE

Long time series of Schaffer collateral to CA1 pyramidal cell presynap tic volleys (stratum radiatum) and population spikes (stratum pyramida le) were evoked (driven) in rat hippocampal slices. From the driven CA 1 region in normal [K+] perfusate, both population spike amplitude and an input-output function consisting of population spike amplitude div ided by the presynaptic volley amplitude were analyzed. Raising [K+] i n the perfusion medium to 8.5 mM, slices were induced to spontaneously burst fire in CA3 and long time series of inter-burst intervals were recorded. Three tests for determinism were applied to these series: a discrete adaptation of a local flow approach, a local dispersion appro ach, and nonlinear prediction. Surrogate data were generated to serve as mathematical and statistical controls. All of the population spike (6/6) and input-output (6/6) time series from the normal [K+] driven c ircuitry were stochastic by all three methods. Although most of the ti me series (5/6) from the autonomously bursting high [K+] state failed to demonstrate evidence of determinism, one (1/6) of these time series did demonstrate significant determinism. This single instance of pred ictability could not be accounted for by the linear correlation in the se data.