Abstracts

Mesial temporal lobe (MTL) structures are important for episodic memory. Lesional studies have suggested a dissociation in function between left hippocampus subserving verbal memory, and right hippocampus subserving non-verbal memory.
Lateralization and localisation of both verbal and non-verbal memory function would be useful for the prediction of memory deficits following anterior temporal lobe resection. We describe here the findings from a memory encoding experiment in patients with both left and right mesial temporal lobe epilepsy (MTLE) due to unilateral hippocampal sclerosis, compared to controls. We studied 10 right-handed healthy volunteers, 7 patients with left MTLE and 7 patients with right MTLE using a novel event-related memory encoding paradigm.
A total of 210 stimuli of three different types (pictures (P), words (W) and faces(F)) were presented during a single scanning session. Subjects were instructed to decide whether they found each stimulus pleasant or unpleasant.
Recognition tests were performed 60 min after scanning for each of the stimulus types. 210 stimuli presented during scanning were randomly mixed with 105 foils. Subjects were instructed to indicate whether they could remember seeing each word during scanning (R response) or whether it was new (N response).
Data was analysed using SPM2. An event-related design was employed and contrasts of parameter estimates were calculated to produce 3 contrast images for each subject (PR-minus-PN, WR-minus-WN and FR-minus-WN). A second level random effects analysis was used to look at the group effects (p[lt]0.001 uncorrected unless stated otherwise). The control group showed activations in the left MTL associated with memory for words, bilateral MTL for pictures, and right MTL for faces.
Patients with left MTLE showed decreased left sided MTL activation associated with memory for words compared to the control group and right MTLE patients(p[lt]0.01 uncorrected). Compared to controls, left MTLE patients showed increased activation in the right MTL (p[lt]0.05 uncorrected) .
Patients with right mTLE showed decreased activation in the right MTL in association with memory for faces compared to left mTLE patients (p[lt]0.01 uncorrected). Compared to controls, patients with right mTLE showed reduced activations in the right fusiform gyrus associated with memory for faces (p[lt]0.01 uncorrected). We demonstrated that the normal pattern of material-specific lateralization of encoding activation in the MTL is affected by unilateral hippocampal pathology with reorganisation of encoding processes to the contralateral MTL.
We hypothesize that the lateralized activation patterns demonstrated here could be used in the prediction of memory deficits seen following anterior temporal lobe resection. (Supported by Wellcome Trust, National Society for Epilepsy)