Abstract
The stochastic behavior of the phase synchronization index (SI) and cross-frequency couplings on different days during a hospital stay of three epileptic patients was studied for non-invasive localization of the epileptogenic areas from high density, 256-channel, scalp EEG (dEEG) recordings. The study was performed with short-duration (0–180 s), seizure-free, epileptiform-free, and spike-free interictal dEEG data on different days of three subjects. The seizure areas were localized with subdural recordings with an 8 × 8 macro-electrode grid array and strip electrodes. The study was performed in theta (3–7 Hz), alpha (7–12 Hz), beta (12–30 Hz), and low gamma (30–50 Hz) bands. A detrended fluctuation analysis was used to find the long range temporal correlations in the SI that reveals the stochastic behavior of the SI in a given time period. 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 and in nearby areas on different days for each subject. The low gamma band was found to be the best to localize the epileptic sites. Also, a stable higher pattern of SI emerged after 60–120 s in the epileptogenic areas. The cross-frequency couplings of SI in theta–gamma, beta–gamma, and alpha–gamma bands were decreased and spatial patterns were fragmented in epileptogenic areas. Combinations of an increase in the stochastic behavior of the SI and decrease in cross-frequency couplings are potential markers to assist in localizing epileptogenic areas. These findings suggest that it is possible to localize the epileptogenic areas non-invasively from a short-duration (∼180 s), seizure-free and spike-free interictal scalp dEEG recordings.
Highlights
In previous studies (Ramon and Holmes, 2012, 2013), we have demonstrated that the stochastic behavior of the phase synchronization index (SI) derived from the high density (256-channel) scalp EEG data has a potential to localize the epileptic sites in human subjects
The present work is an extension and a comprehensive summary of the previous work on multiple days in four EEG bands, viz., theta (3–7 Hz), alpha (7–12 Hz), beta (12–30 Hz), and low gamma (30–50 Hz) and it confirms our previous results that the stochastic behavior of the SI in the low gamma band was best suited to localize the epileptic sites derived from a short-duration (∼180 s) interictal scalp dEEG data
An increase in the phase synchronization has been observed in epileptogenic zones (Schevon et al, 2007; Warren et al, 2010) and, the connectivity patterns are different in epileptogenic zones as compared with other cortical zones not involved in epileptic activities (Varotto et al, 2012)
Summary
In previous studies (Ramon and Holmes, 2012, 2013), we have demonstrated that the stochastic behavior of the phase synchronization index (SI) derived from the high density (256-channel) scalp EEG data has a potential to localize the epileptic sites in human subjects. From invasive recordings it has been observed and suggested that there is an increase in the phase synchronization activity in the epileptogenic regions of the brain and these regions are functionally isolated from the surrounding regions of the brain (Warren et al, 2010). It has been observed with magnetoencephalogram (MEG) recordings in epileptic patients that there were fluctuations in synchrony between neighboring cortical networks (Dominguez et al, 2005). The stochastic behavior of these random fluctuations can be studied with detrended fluctuation analysis (DFA) which is, often, used www.frontiersin.org
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