CORTICAL EXTINCTION IN CONVULSIONS

Abstract

The relations between liminal, supraliminal and subliminal stimulus values, the character and extent of "spontaneous" and electrically-induced seizures and temporal factors have been quantitatively detd. in 10 human subjects by tapping in on the electrical potentials of the cortex exposed in the course of operations for "epilepsy." The data demonstrate that the stimulus value required to produce a seizure at any given time by excitation of an "epileptogenous focus" is a function of the previous activity of the cortex. Following the subsidence of a seizure, the cortical potentials become isoelectric for a period averaging 17 mins. During the early part of this period, supraliminal values are ineffective for the production of a new seizure. As the isoelectric state proceeds, supralaminal stimuli may excite larger and larger fragments of the characteristic convulsion. For a brief period after the subsidence of the isoelectric period, the cortex is evidently in an exalted state of excitability and at this time a more violent seizure than that first demonstrated may be fired by a liminal or even a subliminal stimulus. The period of isoelectricity does not necessarily correspond to the period of unconsciousness or post-convulsive stupor. It is, moreover, not present to a uniform degree at all cortical regions. The data permit a partial account of the phenomenon clinically known as status epilepticus.