Noninvasive Localization of Epileptic Sites from Stable Phase Synchronization Patterns on Different Days Derived from Short Duration Interictal Scalp dEEG

The stochastic behavior of the phase synchronization index (SI) on different days during a hospital stay of epileptic patients was studied for noninvasive localization of the epileptogenic areas from high density (256 channel) scalp EEG recordings. The study was performed on three subjects with interictal EEG data on different days. The seizure areas were localized with subdural recordings with an 8 x 8 grid electrode array. The study was performed in low gamma (30-50 Hz) band with short duration (0-180 s), seizure-free and spike-free scalp EEG data. A detrended fluctuation analysis was used to find the averaged stochastic fluctuations in the SI. The phase synchronization was computed after taking Hilbert transform of the EEG data. Contour plots were constructed with 20 s time-frames using a montage of the layout of 256 electrode positions. It was found that the stochastic behavior of the SI was higher in epileptogenic areas on different days for each subject. Also, a stable higher pattern of SI emerged after 60-100 s in the epileptogenic areas. These findings suggest that it is possible to localize the epileptogenic areas from the short duration (60-100 s), seizure-free and spike-free high density scalp EEG recordings.