EEG source imaging of anterior temporal lobe spikes: validity and reliability

To assess the reliability and validity of EEG source localization of anterior temporal lobe spikes through direct comparison with simultaneously recorded intracranial spike fields.

We recently showed that classical anterior temporal spikes recorded in mesial temporal lobe epilepsy (MTLE) are non-propagated potentials generated in the anterolateral temporal neocortex (Wennberg et al., 2011). In this study EEG source imaging (ESI) was performed on 64 identical right anterior temporal spikes (and 48 homologous left anterior temporal spikes) in a patient with MTLE investigated with simultaneous depth and subdural intracranial EEG and 27 channel scalp EEG. The effects of different realistic forward models, low frequency filters (LFFs) and spike averaging were assessed in terms of the reliability and physiologic validity of the source solutions.

Dipole mapping and distributed source modeling solutions for the grand average of all spikes were accurately localized to the superficial anterolateral temporal neocortex within 1cm of the intracranially defined spike generator, irrespective of forward model or LFF. ESI of single spikes, however, showed poor reliability (i.e., dissimilar localization results for intracranially identical spikes). Even with an optimal combination of individualized volume conductor and 3Hz LFF more than one third of single spike source solutions were physiologically invalid. Spike averaging, especially of 8 or more spikes, significantly increased the proportion of valid source solutions.

ESI of individual anterior temporal spikes was limited by low reliability and a high likelihood of physiologically invalid source solutions. Spike averaging of 8 or more identical spikes prior to ESI, however, reliably produced accurate source solutions localized to the anterolateral temporal neocortex.

ESI performed on averages of identical spikes can provide highly accurate noninvasive source localization of the anterolateral temporal neocortical region responsible for generating classical anterior temporal lobe spikes. The reliability and validity of ESI performed on individual spikes, however, is relatively limited.