Abstracts

Rationale: Functional magnetic resonance imaging (fMRI) allows noninvasive visualization of regional brain activation, and has been used for noninvasive presurgical lateralization of language and memory functions. Studies of memory lateralization have primarily focused on the mesial temporal lobe, but extratemporal regions are also involved in memory encoding and retrieval. In this study, we examined the utility of extratemporal activation asymmetries for predicting seizure lateralization and memory outcomes in a small cohort of controls and patients with left- or right-sided mesial temporal lobe epilepsy (mTLE). We specifically examined activation in frontal and visual cortices expected to be engaged by our complex visual scene-encoding task.Methods: Whole-brain activation was acquired using BOLD fMRI while subjects performed a complex visual scene-encoding task designed to engage basic episodic memory encoding function. Activation differences in frontal and visual cortex regions of interest activation were assessed. FMRI data were processed using statistical parametric mapping and suprathreshold activation values in five extratemporal regions of interest were extracted from each hemisphere. Activation asymmetry ratios were calculated and correlated with seizure lateralization and the results from memory specific changes in neuropsychological testing conducted in patients before and after surgery. Results: Patients with left mTLE demonstrated increased task-correlated activation in the right frontal and right visual areas, as compared to controls and right mTLE patients, who experienced a more bilateral activation pattern. When correlated with change in memory processing after surgery, contralateral activation in frontal and visual areas was predictive of better performance in left mTLE patients and less so, in right mTLE patients. The findings suggest that a compensatory pre-surgical network for memory processing in left mTLE patients may exist and could mediate post-surgical preservation of memory function in this group.Conclusions: This study demonstrated pre-surgical activation profiles in extratemporal brain areas, which correlate with measures of post-surgical memory outcome depending on the side of epilepsy. A differing activation pattern and memory outcome of left mTLE patients as compared to controls and right mTLE patients has also been demonstrated in a prior study. While further validation of these findings is needed, the predictive value of presurgical fMRI of memory in mTLE could be improved by considering both mesial temporal and extratemporal activation.