Cyclical Spike Driving in Chronically Isolated Cortex

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Abstract
SUMMARY: Chronic cortical isolations were prepared in 12 cats and the effects of direct cortical stimulation on a penicillin epileptogenic focus in each slab were evaluated. Trains of low frequency stimuli (0.5–2.0/sec) could trigger spikes in all preparations. In 9 experiments when such stimulation was continued over many minutes it induced “cyclical spike driving” in the focus, i.e. trains of DCR‐spike complexes alternated with trains of simple DCR's. Cycles were remarkably regular and of long duration. This pattern of periodic activity responded in a predictable fashion to small changes in stimulus intensity, frequency and regularity. Characteristic latency changes between stimulus and triggered spike were seen in all prepartions with cyclical spike driving. The variations in SP level which occur during the cycles appeared to result from the spike activity and did not lead to the changes in spike triggering. Evidence is presented which suggests that the spike itself initiates a process with a long time constant which in turn acts to limit spike generation. These data may be relevant to the general problem of the limitation and control of cortical seizure discharges.RÉASUMÉA: Des cortex isoleAs chroniques ont eAteA preApareAs chez 12 chats et les effets d'une stimulation corticale directe sur un foyer eApileptique aG la peAnicilline ont eAteA eAvalueAs pour chaque lambeau. Des trains de stimuli aG basse freAquence (0.5—2.0/sec.) pouvaient deAclencher des pointes dans toutes les preAparations. Dans 9 expeAriences, lorsqu'une telle stimulation a eAteA continueAe au cours de nombreuses minutes, elle a provoqueA un “deAclenchement cyclique de pointes” dans le foyer, c'est aG dire des trains de complexes de pointes “DCR” alterneAs avec des complexes de simples “DCR's”. Les cycles eAtaient remarquablement reAguliers et de longue dureAe. Ce “pattern” d'activiteA reApondait d'une manieGre preAvisible aG de petites modifications de l'intensiteA, de la freAquence et de la reAgulariteA du stimulus. Des changements de latence caracteAristiques entre le stimulus et la pointe deAclencheAe ont eAteA observeAs dans toutes les preAparations avec deAclenchement cyclique des pointes. Les variations du niveau “SP” qui se produisent pendant les cycles apparaissent eCtre le reAsultat de l'activiteA de pointes. Ces constatations suggeGrent que la pointe elle‐meCme engendre un processus avec une longue constante de temps qui, aG son tour, agit pour limiter l'engendrement de la pointe. Ces donneAes peuvent se rapporter au probleGme geAneAral de la limitation et du controCle des deAcharges critiques corticales.