Enhanced slow waves at the periphery of human epileptic foci

Abstract

Objective: Experimental epilepsy foci are surrounded by an enhanced inhibition zone. We looked for evidence of peripheral inhibition in human epilepsy foci by analyzing the waveforms of discharges. The sharp-wave of an epileptic discharge is thought to reflect EPSP synchronization, and the subsequent slow-wave to reflect inhibition. Ratios of amplitudes of the sharp- and slow-waves in human EEGs may show how excitatory and inhibitory processes relate to discharge spread implicating peripheral inhibition in human epilepsy, too.

Method: In electrocorticography from 10 adult patients we compared amplitudes of sharp-waves and of slow-waves and their ratios in each electrodes as a function of their distance from the highest sharp-wave electrode.

Results: Sharp-wave amplitude decreases as a function of electrode distance from the highest sharp-wave electrode, but the slow-wave voltage exhibits a slight increase. The ratio slow-wave/sharp-wave increases several-fold within 2-3 cm from the highest sharp-wave electrode.

Conclusion: In human cortex epileptic discharges at the periphery of a focus exhibit a prevalent slow-wave consistent with a possible local enhanced inhibition.

Significance: Waveform analysis of electrocorticography epileptic discharges suggests the presence in human neocortex of surround inhibition, a basic mechanism limiting the spread of epileptic activity, long studied in experimental models.

Keywords: Epileptic foci; Peripheral inhibition; Spike-and-slow waves.