Single-Unit Activities and Local Field Potentials during Spontaneous Seizures in the Human Hippocampus

KOCHEN SILVIA; MARIA BELÉN GORI; MAURO GRANADO; BLENKMANN ALEJANDRO

Washington DC

Encuentro; SFN 2015 annual meeting; 2015

Society for Neuroscience

We studied the spatiotemporal scale of focal epilepsy, the dynamics of its interaction, and the spread of epileptogenesis. We used wide-bandwidth electrophysiological intracranial recordings using clinical macro- and research microelectrodes in patients with epilepsy. All seizures were identified by an epileptologist from the macroelectrode recordings. We analyzed 23 spontaneous seizures of two patients with insular epilepsy (IE) and frontal epilepsy (FE). For further analysis, we only included seizures (n=11) that presented a local field potential (LFP) with single unit activity during ictal period and at least within 15 minutes before seizure onset. LFP and single units were recorded across multiple days using Ad Tech (40 um width) microwires. Single units were classified by an automated cluster identification program (wave_clus, MATLAB). When a given unit was identified as a single cell across multiple seizures and multiple days, results were manually grouped based on similarities in waveform, spike widths, spike density, and interspike interval histogram. The firing rate (FR) during basal, preictal, ictal, and postictal periods was calculated as the total number of action potentials per unit of time. In the IE case, microelectrodes were localized within the epileptogenic zone (EZ) in the posterior ínsula. However, seizures recorded with macroelectrodes were not simultaneously observed with adjacent microelectrodes in all cases. LFP with single unit recording during seizures showed unstructured activity different to basal period, but did not present epileptiform discharges. The FR either remained constant or decreased. In the FE case, microwires were localized in the hippocampus, outside the EZ. When seizures spread to the hippocampal area, the macroelectrode recorded epileptiform discharges that were simultaneously observed on LFP, showing sharp waves higher than 2 Hz. In this case, a marked increase in FR was observed in all seizures. The areas involves directly in the seizure or its propagation that showed hypersynchronous discharges on LFP, presented an increased in FR. While the LFP did not show epileptiform discharges in spite of being part of the EZ, the FR only show low-level, and unstructured activity. These findings could have important implications for how we localize seizure activity and how we map its propagation.