Catching what we can’t see: Contribution of epileptic mesial sources to EEG

Published in Neurophysiologie Clinique/Clinical Neurophysiology, 43(5-6):316, 2013; International audience; The possibility to observe electrical potentials from deep brain sources to surface EEG remains unclear and debated among the neuroscience community. This question is particularly crucial in the temporal lobe epilepsies investigations because they involve complex (mesial and/or lateral) epileptogenic networks. Seven patients undergoing pre-surgical evaluation of drug resistant temporal lobe epilepsy were selected from a prospective series of thirty patients in whom simultaneous EEG-SEEG recordings had been performed since 2009. Interictal intracerebral spikes (IIS) were selected on SEEG signals blinded to EEG signals. These IIS were triggered as temporally known (T0) brain sources, and then EEG signals were automatically averaged according to these T0 markers in order to obtain mean interictal surface spikes (ISS). In mean, 9 SEEG electrodes and 16 surface EEG electrodes were simultaneously used where 684Â ± 186 IIS were selected by patient (total number: 4,787). According to the anatomical distribution of the IIS, 21 foci were defined and classified according to three categories: mesial (9 foci), mesial and neocortical (M + NC, 5 foci) and neocortical part of the temporal lobe (NC, 7 foci). Negative mean interictal surface spikes (ISS) were mainly observed in anterior and basal temporal region. In mean, 8.7 mean EEG signals were validated as an ISS for each IIS network. Amplitude, duration and SNR of each IIS class were: 7.2 μV, 72 ms and 16.5 dB for M class; 36.1 μV, 78.1 ms and 22 dB for M + NC class; 10 μV, 87.1 ms and 17.7 dB for NC class. Mesial brain sources contribute to surface EEG. Contrary to several hypothesis, mesial temporal structures cannot be considered as closed electrical field structures or too depth to contribute. The main problem to observe signals from these deep structures concerns the low signal to noise ratio, which for instance required signal processing to see ISS originated from mesial structures.