Chronic epileptic foci in monkeys: correlation between seizure frequency and proportion of pacemaker epileptic neurons.

A total of 1,802 neurons from 15 alert, undrugged Macaca mulatta monkeys were studied. Thirteen monkeys had chronic epilepsy induced by subpial alumina injections in precentral cortex. Precentral neurons were judged epileptic by the magnitude and variability of the percentage of interspike intervals less than 5 msec during periods when the monkeys were awake. This method of quantifying epileptic single neuron activity appears highly reliable in distinguishing epileptic neurons from precentral neurons in either normal cortex, cortex contralateral to, or within the focus. For the 13 epileptic monkeys, the relative proportion of strongly epileptic neurons found within foci was logarithmically correlated with the mean number of daily seizures. Because of the similarity between the physiology of the alumina focus in monkeys and epileptic foci in humans, these data imply that the severity of focal human epilepsy is a function of epileptic neuronal mass.