All points in the time course of the evoked and self-sustained, DC-recorded potentials elicited by repetitive interhemispheric stimulation were found to remain negative by -1 mV and more with respect to the prestimulation DC level only when the coinciding, directly recorded, extracellular potassium concentration was 11 mM. A slow oscilloscope sweep displayed the -1 mV level as a virtual line limiting the initial positive waves of the interhemispheric response. This line, approximately parallel with the potassium concentration record of around 11 mM, probably expresses the potassium-dependent membrane depolarizations, which were not revealed at lower potassium concentrations. The evoked and self-sustained potentials coinciding with the latter were displayed in negative as well as positive values.

Institute of Physiology Czechoslovak Academy of Sciences Budejovická 1083 142 20 Prague 4 Czechoslovakia

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Epilepsy Research in the Institute of Physiology of the Czech Academy of Sciences in Prague