Abstracts

Rationale: Removal of the anterior temporal lobe (ATL) is an effective treatment for intractable temporal lobe epilepsy, but carries a risk of memory decline from damage to the hippocampal memory system. The level of risk is related to functional status of the ipsilateral hippocampus as measured indirectly with hippocampal volumetry (Trenerry et al., Mag. Reson. Imaging, 1995). Emerging literature also suggests that material specificity of the hippocampus for verbal memory reflects ipsilateral language lateralization (Binder et al., Epilepsia, 2010). Here, we investigate whether preoperative hippocampal volume is correlated with decline on a rote word list learning task, which loads exclusively on verbal memory, and a story memory task, which is thought to more extensively engage bilateral conceptual processing.Decline on the word list memory task following left ATL surgery depends mainly on language lateralization and therefore will not be predicted by ipsilateral hippocampal volume. Conversely, decline on the story memory task is independent of language lateralization and will be associated with larger ipsilateral hippocampal volume. Methods: Participants were 140 left temporal lobe epilepsy patients treated with surgical removal of the hippocampus (126 standard temporal lobectomy, 14 hippocampal laser ablation). All patients had preoperative and 6-month postoperative neuropsychological testing, including the 6-trial Selective Reminding Test (SRT, a standardized measure of rote word list learning) and the logical memory subtest of the Wechsler Memory Scale, a task that requires patients to recall meaningful stories.FreeSurfer software was used to create automated parcellations of the hippocampus from high-resolution T1-weighted preoperative structural MRI scans (Pardoe et al., Epilepsia, 2009). Automated volumes were then manually edited using visual landmarks. Editing was done by a single researcher (P.B.) blinded to brain hemisphere and repeated on 10 individuals to ensure intra-rater reliability (left hippocampus, r = 0.970; right hippocampus, r = 0.966). Results: Mean percent change on the story memory task was -5.5% (SD 15.2) and on the SRT was -10.3% (SD 14.8). Preoperative left and total hippocampal volumes were both significantly correlated with change scores on the story memory task (r = -0.302, p < 0.001 and r = -0.276, p = 0.001) but were not correlated with performance on the SRT (r = -0.123, p = 0.15 and r = -0.074, p = 0.39). Conclusions: Preoperative hippocampal volume is not predictive of rote verbal memory decline, but is predictive of story memory decline. These data support the theory that material-specific verbal memory (as measured by rote word list encoding) strongly co-lateralizes with language, thus risk to this type of episodic memory depends on underlying language lateralization independent of hippocampal functional status. Narrative stories, in contrast, are processed in bilateral verbal and non-verbal semantic systems (Binder et al., Neuroimage, 2011) and presumably encoded bilaterally into episodic memory. Decline on this type of memory task is therefore related to overall change in hippocampal volume. Funding: NINDS R01 NS35929