Looking inside the brain: What EEG can detect or see

Contribution of deep brain sources on scalp EEG is still under debate. It is particularly true for mesial temporal sources which are deep with an infolded geometry. We analyzed simultaneous multi-scale EEG recordings (scalp and intracerebral) to delineate their contribution to scalp EEG. Interictal intracerebral spike networks were classified in 3 distinct categories: solely mesial, mesial as well as neocortical, and solely neocortical. The highest and earliest intracerebral spikes were marked and the corresponding simultaneous intracerebral and scalp electroencephalograms were averaged and characterized. In 7 drug-resistant epileptic patients, 21 intracerebral networks (4048 spikes) were identified. Averaged scalp spikes arising respectively from mesial, mesial plus neocortical and neocortical networks had a 7.1, 36.1 and 10 μV average amplitude. Their scalp electroencephalogram electrical field presented a negativity in the ipsilateral anterior and basal temporal electrodes in all networks and a significant positivity in the fronto-centro-parietal electrodes solely in the mesial plus neocortical and neocortical networks. Topographic consistency test proved the consistency of scalp electroencephalogram maps and hierarchical clustering clearly differentiated them. We have thus shown for the first time that mesial temporal sources (i) contribute to scalp electroencephalogram but (ii) cannot be spontaneously visible (mean SNR: −2.1 dB) on scalp at the single trial level.