Abstracts

Rationale: Clinical neuropsychological studies of memory have shown that patients with left temporal lobe epilepsy (TLE) are impaired in retention of verbal material whereas right temporal lobe focus impairs encoding but not retrieval of nonverbal information. While such studies suggest both material-specific and processing differences in hemispheric specialization, they do not address the extent of the specific contribution of different regions of the medial temporal lobes to the deficits observed in these patients. Here we report the effect of reductions in hippocampal volume on encoding and retention of different types of stimuli. Methods: We studied 51 pre-surgical patients with unilateral pharmacoresistant TLE. Right handed patients with IQ of 75 or above were classified into four groups, based on side of EEG focus in the context of normal hippocampal volume (NV) or hippocampal atrophy (HA; i.e. greater than 2 standard deviations from the mean of healthy control subjects). All patients were administered two matched tasks: the Abstract Word List and Abstract Design List learning tests. Groups were compared on learning across four repeated trials and delayed recall after 24 hours. Results: During encoding, all groups improved similarly across trials regardless of the type of material to be learned. A clear effect of hemisphere and volume was observed during retention of verbal material: left HA patients showed greater forgetting of words in comparison with patients with right HA or TLE with normal hippocampal volumes. In contrast, delayed recall of nonverbal material was comparable in all groups. When examined separately, the performance profiles of left- or right-sided patients were different: right TLE resulted in overall slower learning of designs in comparison with words but no significant losses of either type of material on delayed recall; left TLE patients showed poor learning of designs compared with words but also forgetting specific to verbal material after delay. Conclusions: The present findings extend earlier work that showed impaired retention of verbal material in left TLE by demonstrating that the deficit is related to the presence of hippocampal atrophy in this group. The finding that left HA did not affect verbal encoding suggests that the role of the left hippocampus may be selective to remembering information already learned. Right HA did not aggravate learning or retention in either modality in comparison with other groups, suggesting that slower learning of nonverbal information in the right TLE profile cannot be attributed to atrophy in this structure. Together, the results expand our previous findings of functional asymmetries between the hemispheres and point to processing differences within the hippocampus in left and right temporal regions. In addition, the findings suggest that volumetric changes limited to the hippocampus cannot account for the whole of the memory deficits in TLE and stress the importance of examining the contribution of other structures within the mesial temporal region. (Funding: NSERC)