Periodic pacing an in vitro epileptic focus

1. The effects of frequency of orthodromic and antidromic stimulation of CA3 on formation of electrographic seizurelike discharges in CA1 was studied in the high potassium hippocampal slice. 2. Within the range of stimulation of CA3 used, 0.1-10.0 Hz, a narrow range was identified between 1.0 and 1.3 Hz, where the tonic phase of seizure generation in CA1 was suppressed. This suppression was observed for both Schaffer collateral and mossy fiber stimulation.

Controlling chaos in the brain

In a spontaneously bursting neuronal network in vitro, chaos can be demonstrated by the presence of unstable fixed-point behaviour. Chaos control techniques can increase the periodicity of such neuronal population bursting behaviour. Periodic pacing is also effective in entraining such systems, although in a qualitatively different fashion. Using a strategy of anticontrol such systems can be made less periodic. These techniques may be applicable to in vivo epileptic foci.

Stochastic versus deterministic variability in simple neuronal circuits: II. Hippocampal slice

Long time series of Schaffer collateral to CA1 pyramidal cell presynaptic volleys (stratum radiatum) and population spikes (stratum pyramidale) were evoked (driven) in rat hippocampal slices. From the driven CA1 region in normal [K+] perfusate, both population spike amplitude and an input-output function consisting of population spike amplitude divided by the presynaptic volley amplitude were analyzed. Raising [K+] in 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 approach, 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 circuitry were stochastic by all three methods. Although most of the time 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 predictability could not be accounted for by the linear correlation in these data.

P50 suppression is not affected by attentional manipulations

Auditory evoked potentials (EP) to high or moderate intensity, single or paired clicks were recorded from normal young adult subjects. A choice-reaction-time paradigm had two sets of instructions, for intensity discrimination and for number (single versus paired stimulus) discrimination. For intensity discrimination, the second click had no informative value and its N100 amplitude was markedly reduced relative to the first click. For number discrimination, the presence or absence of the second click provided the salient information, and N100 amplitude was actually slightly larger for the second compared to the first click. In contrast, the attentional manipulation had no effect on P50 amplitude, which showed over 50% suppression from the first to the second click for both tasks. Thus, suppression of P50 amplitude to the second of a pair of clicks is insensitive to attentional manipulations that have major effects on N100 amplitude. These findings suggest that abnormalities of schizophrenic P50 suppression reflect neuronal rather than psychological phenomena.