Abstracts

Rationale: Presurgical investigation of patients with pharmacoresistant focal epilepsy mainly consists in localizing the focus where epileptic discharges (EDs) are generated. However, functional connectivity (FC) studies of slow hemodynamic fluctuations using resting state fMRI suggested rather distributed network reorganizations (Pittau, Epilepsia 2012, 53:1013–23, Dansereau, Front Neurosci 2014, 8:419). Resting state FC might enhance our understanding of the disease pattern, regardless of EDs, and might also be used as a promising tool to tailor surgery or to predict surgical outcome (Englot, BRAIN 2015, 138:2249–62). Here, we considered Magnetoencephalography (MEG) to investigate differences in FC patterns within the alpha band for the presumed focus and the contralateral homologous region. Methods: All interictal EDs from MEG recordings of 4 pharmacoresistant focal epilepsy patients were marked by an epileptologist (GP) and source localization was done using coherent Maximum Entropy on the Mean (cMEM) (Chowdhury, NIMG 2016, 143:175-95) at the peak of the averaged EDs. Presumed focus region of interest (ROIFoc), based on cMEM results, and a contralateral homologous region (ROICon) of the same size were drawn along the cortical surface. We selected 20 epochs, of 2s each, without any EDs or artifacts and concatenated them before filtering in alpha band (8-13 Hz). Wavelet based MEM (wMEM), offering a framework to perform source localization of oscillatory activity in the time-frequency domain, was used to reconstruct fluctuations of alpha band activity along the cortical surface (Lina, IEEE TBME 2014, 61(8):2350-64). Band limited amplitude envelope correlations is a MEG metric providing similar results to fMRI resting state FC (Brookes, PNAS 2011, 108(40):16783-8). Envelope correlations were estimated between each vertex of the selected ROI and all other vertices of the cortical surface (~8000), averaged and thresholded at 0.2, resulting in a single seed-based FC map for each ROI. Results: We observed distinct FC patterns for ROIFoc in comparison to ROICon. For 3 out of 4 patients, FC map with the presumed focus exhibited correlated slow fluctuations of the amplitude of alpha band activity mainly limited to the same hemisphere and nearby regions, suggesting more local networks. On the other hand, FC maps with the ROICon showed both short and long distance connections, exhibiting correlations with the contralateral hemisphere notably (Figs 1 and 2).In patient 1 (P1), we found higher FC between ROICon and the contralateral temporal pole and visual cortex in comparison to the FC map involving the focus (Fig1). For P2 contralateral inferio-parietal/angular and for P3 contralateral temporal lobe connections were higher when considering FC maps involving ROICon (Fig2). FC maps for P4 did not exhibit the same pattern, since ROIFoc map was showing bilateral involvement. Conclusions: These preliminary findings suggest decreased long distance connections (measured as alpha band amplitude modulation) with the presumed epileptic focus, when compared to FC maps involving the contralateral ROI. Similar reorganizations of resting state FC patterns were also observed using fMRI (Negishi, Epilepsia 2011, 52(9):1733-40). Whether this decrease in FC is a protection mechanism to isolate the epileptic focus or a result of recurring seizures is currently unknown. However, MEG FC analysis might provide valuable additional information to further characterize these reorganizations of ongoing resting state neuronal activity, occurring even in the absence of epileptic discharges. Funding: Savoy foundation and FRQS postdoctoral fellowships, CIHR MOP 133619 and 93614, NSERC Discovery grant and FRQNT Research team grant.