Catching the Invisible: Mesial Temporal Source Contribution to Simultaneous EEG and SEEG Recordings

Abstract

Mesial temporal sources are presumed to escape detection in scalp electroencephalographic recordings. This is attributed to the deep localization and infolded geometry of mesial temporal structures that leads to a cancellation of electrical potentials, and to the blurring effect of the superimposed neocortical background activity. In this study, we analyzed simultaneous scalp and intracerebral electroencephalographic recordings to delineate the contribution of mesial temporal sources to scalp electroencephalogram. Interictal intracerebral spike networks were classified in three distinct categories: solely mesial, mesial as well as neocortical, and solely neocortical. The highest and earliest intracerebral spikes generated by the leader source of each network were marked and the corresponding simultaneous intracerebral and scalp electroencephalograms were averaged and then characterized both in terms of amplitude and spatial distribution. In seven drug-resistant epileptic patients, 21 interictal intracerebral networks were identified: nine mesial, five mesial plus neocortical and seven neocortical. Averaged scalp spikes arising respectively from mesial, mesial plus neocortical and neocortical networks had a 7.1 (n = 1,949), 36.1 (n = 628) and 10 (n = 1,471) 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 these different scalp electroencephalogram maps and hierarchical clustering clearly differentiated them. In our study, we have thus shown for the first time that mesial temporal sources (1) cannot be spontaneously visible (mean signal-to-noise ratio -2.1 dB) on the scalp at the single trial level and (2) contribute to scalp electroencephalogram despite their curved geometry and deep localization.

Yield of MRI, high-density electric source imaging (HD-ESI), SPECT and PET in epilepsy surgery candidates

“…This is relatively high, given that intracranial EEG alone is probably an imperfect ''gold standard'' due to potential spatial undersampling and an interelectrode distance of already 10 mm. These results as well of studies with simultaneous intra-and extracranial EEG recordings also suggest that ESI is capable to detect deeper sources (Koessler et al, 2015;Ramantani et al, 2014).…”

Correlation of invasive EEG and scalp EEG

“…It should be noted that, so far, this key issue has been analyzed in invasive EEG studies performed mainly with foramen ovale or subdural electrodes [27,28,[34][35][36][37][38] and only rarely with depth electrodes [13,27,29]. In the study of Koessler et al [8], it has been confirmed that epileptic discharges arising from mesial temporal sources are not spontaneously visible in scalp EEG. However, mesial temporal sources significantly contribute to scalp EEG, as can be demonstrated by extraction from the respective background activity (Fig.…”

“…The contribution of electrical potentials arising from deep sources to scalp EEG, a crucial issue in the presurgical workup of temporal lobe epilepsy [7,25,26], has recently been addressed in a study of simultaneous scalp and intracerebral EEG [8]. Based on the routine visual analysis of scalp EEG in simultaneous scalp and invasive recordings, it has been previously postulated that interictal [27,28] as well as ictal discharges [29] confined to mesial temporal structures escape detection in scalp EEG.…”

“…Subdural recordings offer extensive cortical coverage, but are prone to sampling limitations for deep sources, such as sulcal sources [5][6][7]. Depth electrode recordings provide information for selected deep structures [7][8][9], but are plagued from sampling limitations due to incomplete and irregular cortical coverage. The rapid developments in computational studies, including simulation as well as electrical source localization (ESL) methods, address the urgent need to compensate for the Ever since the implementation of invasive EEG recordings in the clinical setting, it has been perceived that a considerable proportion of epileptic discharges present at a cortical level are missed by routine scalp EEG recordings.…”

“…shortcomings of both subdural and depth recordings and, most of all, to clarify their correlation with scalp EEG recordings [5,8,10,11].…”