Abstracts

Rationale: Epilepsy has adverse effects on memory functioning. However, studies investigating functional remodeling of memory in epilepsy are scarce. The goals of this study were to investigate the effects of seizure onset on memory lateralization and to determine the effects of memory lateralization on memory performance. We expected memory to be lateralized to the unaffected hemisphere. We also expected L-lateralization of fMRI activation to be associated better verbal memory functioning and R-lateralization to be associated with better visuospatial memory. Methods: 56 epilepsy (17F; M age=38 years) and 29 healthy (13F; M age=32 years) subjects were enrolled. The fMRI task involved scene encoding using a balanced mixture of color indoor and outdoor scenes (50/50) which can be coded both verbally and nonverbally. Subjects were asked to decide if pictures were indoor or outdoor using button box responses. Within 10 minutes post-MRI, subjects were administered recognition tests for the images presented during fMRI. FMRI data were processed using CCHIPS including motion correction and spatial normalization. For each subject, voxels activated in the encoding task were identified using GLM; random-effects analysis was performed to determine significant group activations (corrected p<0.05). The group activation map for the healthy controls was used to generate a functional ROI (FROI) within the medial temporal structures. Two anatomically-defined ROIs were also created (hippocampal HROI; parahippocampal PHROI). Activation in each ROI was used to calculate a laterality index (LI). Epilepsy patients were administered measures of memory functioning: the Wechsler Memory Scale, The Benton Visual Memory Retention Test, and the Warrington Recognition Memory Tests for Faces. One-way analysis of variance was used to investigate group differences in LI and memory performance. In order to investigate the relationship between LI and behavioral performance, Spearman s rank correlation analysis was conducted.Results: Healthy controls demonstrated bilateral activation (M:-0.00, SD:0.17) whereas RHE patients (M:0.01, SD:0.26) demonstrated slight L-lateralization and LHE patients demonstrated slight R-lateralization (M:-0.08, SD:0.18). RHE patients performed significantly better than LHE patients on list learning, p<0.01. As R-lateralization increased among LHE so did performance on visuospatial recognition memory, p<0.05. As RHE patients demonstrated increased L-lateralization, performance on list learning increased whereas memories for stories and visual retention decreased, p<0.05. Conclusions: Among epilepsy patients, memory was lateralized to the hemisphere contralateral to seizure focus. In addition, increased R-lateralized activation was associated with improved visuospatial memory functioning, whereas increased L-lateralization was associated both with improved verbal memory functioning and diminished visuospatial memory functioning. These findings support the notion that intractable epilepsy is associated with negative effects of memory performance and leads to cortical plasticity of memory functions.