Regulation of Cortical Excitability in Patients with Epilepsy and its Measurement by Means of Slow Cortical Potentials

Abstract

Surface negative slow cortical potentials (SPs), representing the depolarisation of apical dendrites of the cortical pyramidal cells, can be considered as indicating excitability of cortical neuronal networks (Elbert and Rockstroh, 1987; Elbert, this volume), this excitability being regulated according to the constantly varying environmental and metabolic demands. However, it has to be assumed that the thresholds of neuronal firing must be regulated within certain limits: Extreme excitability of cortical tissue, for instance due to a transient failure in down-regulating mechanisms, would allow an explosive chain reaction of excitation among cell assemblies. Such overexcitation constitutes an epileptic attack. Extreme excitability, meaning depolarisation in large dendritic networks, will give rise to extraneous negative SPs (DC shifts), and thus indicates a high risk for the emergence of seizures. Conversely, we may argue that epileptic patients, who suffer from impaired or transiently failing regulation of neuronal excitability, should exhibit altered SP regulation and pronounced SPs prior to epileptic attacks. In the present chapter these contentions will be discussed on the basis of data obtained from two experimental approaches: the recording of DC shifts during hyperventilation (a method known to provoke epileptic activity) and SP self-regulation within a biofeedback paradigm. Epileptic patients may be expected to experience more difficulty in controlling SPs upon command than healthy control subjects. If this is the case, are they able to acquire control over their SPs, and to what extent can they do so?