Epileptiform EEG spikes and their functional significance.

Field potentials detected in the space surrounding cellular elements of the nervous system are essential in the diagnosis of epileptic seizures. This article describes the elementary mechanisms underlying the generation of field potentials and the special functional conditions leading to epileptiform field potentials. Primary transmembranous currents generate secondary ional currents along the cell membranes in intra- and extracellular compartments. The portion of these currents that flows through the brain tissue to the cortical surface can be detected as field potentials. A high synchronization of these field potentials is needed to induce brain signals. Field potentials recorded during epileptic activity are based on alterations in neuronal membrane potentials. Paroxysmal depolarization shift has proved to be characteristic in the epileptiform activity of individual neurons. Epileptiform field potentials are generated in functionally different structures with different elementary mechanisms. In focal convulsive activity limited to the cortex, the surface potential does not necessarily reflect the bioelectrical events in deeper cortical laminae and can be interrupted in different ways. The discrepancy between superficial EEG potentials and neocortical output may be the basis for dissociation between EEG signals and clinical signs.