Abstracts

Rationale: It has been established in several studies that interictal epileptic discharges (IEDs) recorded in temporal regions on scalp EEG are unlikely to originate from mesial temporal structures, in large part because of the distance between scalp and hippocampus. However, combined EEG-fMRI studies sometimes show mesial temporal activation at the time of scalp temporal IEDs. We make the following hypothesis for such a situation: the scalp IED results from propagation from mesial structures and the neocortical BOLD activation should be less than that of mesial structures, reflecting the fact that propagated activity makes less metabolic demand than the original discharge. Methods: Twelve patients with epilepsy who have BOLD response in mesial temporal structures were selected from our EEG-fMRI database, independently of the presence of other regions of activation. We searched the temporal lobe ipsilateral to IEDs and checked whether there is positive BOLD response in the neocortex, as well as the contralateral temporal lobe for comparison. EEG was recorded inside 3T MRI scanner with 25 electrodes placed on the scalp according to 10-20 system plus inferior temporal electrodes. For each patient, epileptic discharges similar to those recorded outside the scanner were marked as events. The timing of events was convolved with HRFs at multiple latencies. A statistical t-map was created showing voxels which significantly correlated with marked EEG events. Clusters showing corrected p < 0.05 for its spatial extent were considered significant. Results: We obtained 16 IED types from 12 patients. All types showed a BOLD response in the temporal neocortex ipsilateral to the mesial temporal BOLD response. When we compared mesial and neocortical clusters in each event, t-values were higher in mesial temporal structures than in neocortex in 14/16 cases. In 2 events, the necortex had a higher t-value than the mesial temporal lobe. Seven events also showed BOLD response in the contralateral temporal neocortex; t-values were always higher in the ipsilateral than in the contralateral neocortex. Conclusions: Hemodynamic changes were observed in the mesial temporal lobe at the time of IEDs recorded from the temporal region on the scalp. The smaller BOLD change in the ipsilateral neocortex is in agreement with our hypothesis that the neocortical IEDs result from propagation from mesial structures. The occasional contralateral BOLD increase is in agreement with an earlier study showing this occurrence even in the absence of visible IED contralaterally. These results fit the general interpretation of BOLD results, in which the highest activation corresponds to the source of the discharge and lower activations result from propagation. (This study was supported by CIHR grant MOP-38079.)