Abstracts

Rationale: Functional magnetic resonance imaging (fMRI) activation of the mesial temporal lobe is important for epilepsy surgical planning and helps to minimize potential post-operative memory impairments. A key guiding principle is the material-specificity model of memory, which posits that the left hippocampus is primarily involved in verbal memory whereas the right hippocampus is predominately involved in visual memory. Further, prior work has suggested additional functional subdivision of the hippocampus along its longitudinal axis, with verbal memory associated with anterior activation compared to pictorial stimuli associated with posterior activation. Most research provides evidence for these models in verbal memory, but findings in visual memory are limited. To assess visual memory hippocampal functional organization, we examined neural correlates of visual learning and recognition in adults with epilepsy and healthy controls.

Methods: A total of 21 adults with focal epilepsy (13 left focal [LFE], 8 right focal [RFE]) and 19 healthy controls participated in a fMRI block design paired association learning task (abstract visual stimuli) with learning and recognition blocks. Imaging processing and statistical analyses were conducted in SPM12. We extracted beta values from anterior and posterior sections of the hippocampus, based on Anatomical Atlas Library in Wake Forest PickAtlas, and calculated lateralization indices (LI). Hippocampal laterality (LI) and activation (betas) were investigated using repeat measures analysis of variance to assess differences between side (right versus left) and location on the longitudinal axis (anterior versus posterior) among patients and controls. We also examined relationships between hippocampal activation and memory ability via a composite performance score (derived from WMS-IV subtests: Faces and Verbal Paired Associates) using Pearson’s correlations.

Results: The task activated the hippocampus both at individual and group (small-volume corrected) levels for the majority of subjects. During both learning and recognition trials, we found increased posterior hippocampal activation across all patients and controls (p< 0.001). For LFE, hippocampal recruitment was more right-lateralized anteriorly and more bilateral posteriorly (p=0.01). These anterior and posterior differences were not observed in controls nor RFE (p > 0.21), with both groups showing bilateral LI across the longitudinal axis. We found increased right posterior activation was associated with stronger performance on memory composite scores for all subjects during learning (p=0.03) and recognition (p< 0.001).