Abstracts

Rationale: Cognitive-induced high frequency EEG activities (HFAs, 50-150Hz) recorded in patients with epilepsy undergoing intracerebral EEG (icEEG) monitoring provide a complementary approach to that offered by functional Magnetic Resonance Imaging (fMRI) to study brain function. There is an increasing interest to combine these two techniques in humans (Lachaux et al., Human Brain Mapping 2007), as previously done in animals (Logothetis et al., Nature 2001). So far, a few patients underwent simultaneous icEEG-fMRI with the view to record epileptic spikes and their fMRI counterpart, further suggesting the feasibility and safety of these combined acquisitions (Vulliemoz et al., Neuroimage 2011; Cunningham et al., Epilepsia 2012) . Investigate HFAs is more challenging due to the overlap between their frequency band and that of MR acquisition artifacts. We specifically tackled this issue by recording HFAs induced by a reading task performed by three patients in both icEEG-only and icEEG-fMRI. We looked at whether or not HFAs identical to that recorded in icEEG-only could be recorded during icEEG-fMRI.Methods: Three patients undergoing icEEG as part of pre-surgical evaluation performed a reading task, known to induce HFA, during an icEEG-only and an icEEG-fMRI sessions. Between 53 and 57 depth electrode contacts were recorded in each patient during both experimental conditions. icEEG-only acquisitions were performed with the Micromed video-EEG monitoring system used in clinical routine. icEEG-fMRI recordings were acquired with BrainProducts MR Plus amplifiers under a 1.5T MRI scanner (Siemens). BrainVision analyzer software was then used to remove fMRI gradients artifacts from this dataset. Data processing was performed with Matlab and a built-in software. Written informed consent was obtained from each patient. This study was reviewed and approved by the local ethical committee.Results: When looking at averaged HFAs across trials, patients displayed between one and 11 recording leads showing significant reading-induced HFA (sustained for at least 200ms) during the icEEG-only modality. With one exception, the same or immediately adjacent recording leads also showed significant averaged HFAs during icEEG-fMRI. At a single trial level, both methods of acquisition demonstrated comparable findings, though icEEG-fMRI was associated with a greater number of significant reading-induced HFAs than icEEG-only (p=0.015).Conclusions: These results show that even subtle icEEG signal components such as cognitive-induced HFAs can be recovered during icEEG-fMRI acquisition. This paves the way for the use of icEEG-fMRI to address various fundamental and clinical issues. Notably, this finding supports the possibility of detecting high-frequency markers of the epileptogenic zone, such as ripples, during icEEG-fMRI. This research was funded by the ERA-Net NEURON JTC2012 programme, n°12-NEUR-0006-01, “SEMAINE"" project