Distant effects of a focal epileptogenic process

Abstract

Epileptiform activity originating from a discrete epileptogenic cortical focus may give rise to concomitant 'projected discharges' in distant regions of the brain which are synaptically related to the primary epileptogenic area. In the field of clinical EEG, the occurrence of such discharges may represent an additional complicating factor in the interpretation of a tracing. In the experimental field, the phenomenon of projection of epileptiform discharges induced by topical application of strychnine has been utilized to detect interconnections between various cerebral structures ('strychnine neuronography', see ref. 24). It seems likely that such projected discharges may also disrupt function in 'normal' structures and thus contribute to the deficiencies in learning and memory observed in patients and in experimental animals with focal epileptogenic lesions 17,25,27,a4,a6. According to Morrell and his associates, cerebral structures under the effect of such repeated bombardment may become capable of independent epileptogenesis ~,37. Projection of paroxysmal discharge, probably involving callosal, commissural, and other subcortical pathways, is particularly evident in homotopic areas of cerebral cortex. This study was designed to analyze the inter-hemispheric projection of epileptiform discharges at the neuronal level. Using micro-electrode techniques, we have examined the effects of inter-ictal and ictal epileptiform activity originating within one area of the cortex upon spontaneous and evoked activities of individual neurons and of populations of neurons in the homologous contralateral cortex.