Abstracts

Rationale: Verbal memory decline is frequent after left anterior temporal lobectomy (L-ATL). We showed recently that language lateralization is more predictive of verbal memory decline than Wada memory asymmetry or hippocampal fMRI activation asymmetry (Binder et al., 2008; 2010). Here we assess whether hippocampal volume asymmetry contributes independent predictive value beyond fMRI language lateralization alone. Hypothesis: Larger left hippocampal volume and smaller right-left hippocampal asymmetry will be associated with more severe verbal memory decline following L-ATL. Methods: Participants: Participants were 53 left temporal lobe epilepsy patients treated with L-ATL. All patients underwent preoperative fMRI using a Semantic Decision - Tone Decision contrast to measure language lateralization (Binder et al., 2008). All patients had preoperative and 6-month postoperative neuropsychological testing, including the 6-trial Selective Reminding Test (SRT), a standardized measure of verbal episodic memory encoding. Volumetric Measures: FreeSurfer software was used to create automated parcellations of the hippocampus from high-resolution T1-weighted preoperative structural MRI scans (Pardoe et al., 2009; Shen et al., 2010). A hippocampal asymmetry index was calculated using the formula (right-left)/(right left), where left and right represent the left and right hippocampal volumes. Results: Average hippocampal volume was smaller on the left than the right (paired t-test, p <.00001), suggesting that the parcellation method is sensitive to hippocampal structural pathology. Both left hippocampal volume (r = -.267, p < .05) and hippocampal asymmetry (r = .252, p <.05) were modestly predictive of post-operative verbal memory change on the SRT, with larger left hippocampal volume and smaller asymmetry values correlating with worse postoperative memory outcomes. The fMRI language lateralization index was more strongly correlated with memory outcome (r = .440, p < .001). Neither left hippocampal volume or hippocampal asymmetry provided additional predictive value when included with fMRI language lateralization in a multivariate regression model. Conclusions: Preoperative hippocampal volume measures are sensitive to unilateral hippocampal pathology but are only modestly predictive of postoperative verbal memory outcomes. Verbal memory outcome is strongly correlated with language lateralization, supporting recent claims that lateralization of material-specific verbal memory functions is more closely tied to language lateralization than to hippocampal pathology.