Microelectrode Studies in Chronic Epileptic Foci

INTRODUCTION In the context of this Symposium our own microelectrode studies in chronic epileptic foci may serve to emphasize some of the points brought out by previous speakers and also may raise some pertinent issues for discussion. Since the work of Li, McLennan and Jasper (8) there has been general agreement that there is no one-to-one correlation between the unit activity recorded with microelectrodes and the slow potentials of surface or depth derived with larger electrodes. These authors indicated that such findings cast doubt upon the hypothesis that the “brain waves” of the cortical tracing represented an envelop of unit spikes. One may reasonably conclude that the extremely productive work of the ensuing years has unequivocally laid to rest the “envelop theory” of the origin of brain waves at least as applied to all-or-none unit discharge. Yet the terms “synchronization” and “desynchronization” are still universal laboratory parlance. Experiments of Li (7) and of Enomoto and Ajmone Marsan ( 4 ) have shown “synchronization” of discharge in adjacent units under the influence of “activating” agents spch as strychnine and penicillin or with the “burst potentials” of the “cerveau isol6” of the cat. Epileptiform discharges in particular are generally viewed as “hypersynchronous” implying the simultaneous activation of thousands of “units”. Current sophistication allows us the freedom to assume that the units of such synchrony involve graded or synaptic potentials of the cell rather than the all-or-none spike. Yet even this assumption awaits conclusive demonstration, although an exciting beginning has been made in this day’s report by Choh-Luh Li (9). The point at issue here is in large measure a reflection of the tremendous sampling error of the microelectrode method. Burns (2) has clearly pointed out the difficulty of inferring the behavior of a neuronal population from a sampling of a single constituent unit. Listening in on the private life of a single cell (to paraphrase Burns, 2) does not afford a picture of the role of that unit in the integrated behavior of a population.