Abstracts

Rationale: To determine if similar focal patterns of phase synchronization are present on separate days in the interictal, high density scalp EEG, and if these patterns correspond to the proven epileptogenic zone. Methods: We studied one patient with refractory epilepsy who underwent intracranial EEG to establish the localization of seizure onset. Prior to invasive EEG studies the subject underwent dense array 256 channel EEG (dEEG) recordings. Three minutes of interictal dEEG data from first day and from the second day was selected for analysis. The selected segment in each day was at least two hours temporally from an epilepic seizure and, based on visual analysis, free of interictal epileptiform patterns. Excessively noisy channels were removed and replaced with averages of surrounding electrodes. Data were imported into MATLAB for analysis. The EEG data was filtered in the low gamma (30-50 Hz) band. The phase synchronization index (SI) was computed after taking Hilbert transform of the EEG data. The SI between a pair of channel was inferred from a statistical tendency to maintain a nearly constant phase difference over a given period of time even though the analytic phase of each channel may change markedly during that time frame. The SI for each electrode was averaged over with the nearby six electrodes. A detrended fluctuation analysis (DFA) was used to find the stochastic behavior of the SI. Contour plots with 10 sec intervals were constructed using a montage of the layout of 256 electrode positions. Results: Contour plots displayed over the scalp show that the stochastic behavior of the SI becomes stronger with time in the proven epileptogenic area while in other areas it becomes fragmented and scattered. For this subject, we were able to identify stable patterns of stochastic behavior in the epileptogenic area within 60 seconds of analysis. The same findings were observed in interictal scalp EEG on two separate days. Conclusions: Examination of 60 sec of stochastic behavior of phase synchronization during successive days, derived from interictal scalp dEEG that is free of epileptiform discharges, reveals stable patterns and may have the potential to assist in localizing epileptic sites in subjects with proven epilepsy.