Abstracts

Rationale: Whereas the scalp EEG during wakefulness in healthy individuals is fully defined, the accumulated knowledge on physiological intracranial EEG activity is surprisingly sparse. The lack of knowledge on normative EEG data of brain activity is mainly explained by i) the relatively rare placement of electrodes in healthy brain tissue and the challenge in identifying healthy brain regions, and ii) the difficulty of standardization of the electrode placement compared to scalp EEG resulting in problems performing inter-individual comparisons of EEG activity. This multicenter study aimed to provide an atlas of normal intracranial EEG. Methods: Intracranial EEG recordings with channels displaying presumably normal physiological activity were selected from three tertiary epilepsy centers. Channels with normal activity were defined as channels localized in normal tissue, located outside the seizure onset zone, having no interictal epileptic discharges, and no overt slow wave anomaly. All contacts were localized in a common stereotactic space enabling to perform direct comparisons of EEG activity across subjects and to assess the cerebral structure to which each contact belongs. Sixty-second artifact-free sections during wakefulness (eyes closed) were selected. Power spectral density plots were calculated for all investigated brain regions. A comparison with the background, which was defined as set of channels with no peaks in the spectrum, was performed to identify significant peaks in the different regions. Results: A total of 1785 channels (1520 from S-EEG electrodes, 265 from cortical grids/strips) with normal physiological brain activity from 106 patients were identified (Figure 1). The power spectral density distribution was similar between channels recorded with S-EEG electrodes and cortical grids/strips. Significant differences in power spectral density were identified for the different regions. The power spectra of the combined regions of the major brain lobes are given in Figure 2: There was a significant peak in beta and gamma in the frontal lobe, significant peaks in alpha and beta in the parietal lobe, peaks in alpha and delta in the temporal lobe, and a clear peak in alpha in the occipital lobe. Clear peaks present in more than 60% of channels were found in the precentral gyrus (64% of 123 channels; range, 20-24 Hz), medial segment of the precentral gyrus (72% of 18 channels; range, 24-30 Hz), opercular part of the inferior frontal gyrus (72% of 39 channels; range, 20-24 Hz), cuneus (68% of 19 channels; range, 7.75–8.75 Hz), and hippocampus (72% of 38 channels; range, 0.75–1.25 Hz). Conclusions: This is the first atlas of normal intracranial EEG activity in a common stereotactic space which enables to perform direct comparisons of EEG activity across subjects using quantative analysis. This atlas can serve to enhance the interpretation of the abnormal intracranial EEG on the basis of a solid knowledge of the normal EEG, as has always been done for scalp EEG. Funding: This study was supported by the Savoy Epilepsy Foundation (grant to B.F.), the Botterell Powell’s Foundation (grant to B.F.), and the Canadian Institute of Health Research (grant FDN-143208 to J.G.).