Spatial coherence patterns of fast ictal activities coincide with those of slow interictal oscillations in the human brain

In this study, we analyzed intracranial EEG (iEEG) recorded via subdural grid electrodes from patients with extratemporal lobe epilepsy, measuring interelectrode wavelet phase coherence (WPC), between neighbouring and distant cortical areas, during interictal and ictal activity. As several studies have explored phase coherence in the epileptic brain, in relation to lower rhythms (<; 80 Hz), we have applied WPC analysis to both slow and fast neuronal rhythms-in the 1-400 Hz frequency range-to study their respective temporal and spatial coherence patterns. In analyzing the WPC profiles generated from all possible electrode pairings, two prominent patterns of phase coherence were observed in select electrode pairings on the implanted patient grids: during interictal activity, strong coherence was observed in the 8-11 Hz frequency range, while increased coherence was observed in activity > 80 Hz during seizure episodes. Furthermore, electrodes on the grid with elevated coherence in faster rhythms during seizures showed overlap with areas possessing elevated low-frequency coherence during interictal activity. As several studies have identified ictal fast activities as possible seizure biomarkers, the interictal coherence patterns observed here may also help in defining seizure-related regions of interest in the epileptic brain.