Single unit action potential recordings in humans : implications for epilepsy

Abstract

Spike-sorting algorithms have been used to identify the firing patterns of isolated neurons (‘single units’) from implanted electrode recordings in patients undergoing assessment for epilepsy surgery, but we do not know their potential for providing helpful clinical information. It is important to characterize both the stability of these recordings and also their context. I therefore analysed microelectrode array recordings from four patients undergoing videotelemetry monitoring for surgical evaluation of focal neocortical epilepsies. I investigated whether units could be followed reliably over prolonged periods during the initial days postimplantation in humans. Unit specific features showed stability over at least 48 hours, including across multiple seizures. A critical consideration regarding the clinical information in these recordings is where the units are located with respect to the focus of pathology. Recent analyses have demonstrated the importance of considering seizure activity in terms of two distinct territories: the ictal core and penumbra. The pathological information in these two areas is likely very different. I isolated unit recordings from several hundred neurons across the four patients in the peri-ictal period, with reference to these regions. A key finding was that in the penumbra, spike stereotypy was maintained even during the seizure, with little alteration to population firing rate. In contrast, within ictal core territories, regions characterized by intense hypersynchronous firing, spike sorting failed as the units were incorporated into the seizure activity. Recovery of spike shape was rapid following seizure termination, and the mean firing rate returned to pre-ictal levels in the first few minutes. However, the spatiotemporal activity of units in the ictal core displayed significant alterations to firing patterns, lasting at least 30 minutes after seizure termination. These observations lay the foundation for future investigations of how these recordings may inform clinical practice.