In vivo neuronal firing patterns during human epileptiform discharges replicated by electrical stimulation

OBJECTIVE: To describe neuronal firing patterns observed during human spontaneous interictal epileptiform discharges (IEDs) and responses to single pulse electrical stimulation (SPES). METHODS: Activity of single neurons was recorded during IEDs and after SPES in 11 consecutive patients assessed with depth EEG electrodes and attached microelectrodes. RESULTS: A total of 66 neurons were recorded during IEDs and 151 during SPES. We have found essentially similar patterns of neuronal firing during IEDs and after SPES, namely: (a) a burst of high frequency firing lasting less than 100 ms (in 39% and 25% of local neurons, respectively for IED and SPES); (b) a period of suppression in firing lasting around 100-1300 ms (in 19% and 14%, respectively); (c) a burst followed by suppression (in 10% and 12%, respectively); (d) no-change (in 32% and 50%, respectively). CONCLUSIONS: The similarities in neuronal firing patterns associated with IEDs and SPES suggest that, although both phenomena are initiated differently, they result in the activation of a common cortical mechanism, probably initiated by brief synchronised burst firing in some cells followed by long inhibition. SIGNIFICANCE: The findings provide direct in vivo human evidence to further comprehend the pathophysiology of human focal epilepsy.