Abstracts

Rationale: Our objective was to determine if measures of long-range temporal correlation (LRTC) in interictal high-density, 256 channel, scalp EEG (dEEG) data can be useful for localizing the ictal onsets by comparing these measures to the intracranially-verified locations of seizure onset. Methods: Data from 5 subjects were used. The localization of epileptic seizures was established on the basis of intracranial EEG recordings. Prior to invasive studies, the subjects underwent 256 channel dEEG recordings. One minutes of interictal dEEG was selected for analysis. The selected segment was at least two hours from an electrographic seizure and, based on visual analysis, was 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 of one minute duration was filtered in the theta (3-7 Hz), alpha (7-12 Hz), beta (12-30 Hz) and gamma (30-50 Hz) band. A detrended fluctuation analysis (DFA) was used to find LRTC of the scalp EEG data. Contour plots were constructed using a montage of the layout of 256 electrode positions. The synchronization index (SI) was computed after taking Hilbert transform of the EEG data. The SI between a pair of channel was estimated 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. LRTC of the SI was computed and spatial plots were made. The results were plotted on color intensity plots constructed from a 2-D layout of the electrode positions. Results: The plots spatial revealed regions of maximal LRTC of scalp SI in the beta and gamma band that overlaid regions where epileptic seizures were proven to originate. In the theta band, changes are less pronounced, but did include the region of epileptic seizure. The LRTC of scalp SI in the alpha band are widespread, and less specific. The LRTC of EEG alone did not show a strong correlation with the seizure area as determined from the invasive recordings. Conclusions: The long-range temporal correlation of phase synchronization, particularly in beta and gamma frequncies, may be useful in assisting in localizing epileptogenic sites from interictal, high density scalp EEG recordings.