Abstracts

Rationale: fMRI memory paradigms may be useful in assessing functionality of tissue prior to resection for intractable temporal lobe epilepsy (TLE). To date, verbal memory fMRI tasks have been somewhat useful in demonstrating function of the left hippocampus and nearby structures, but attempts at investigating function of the right medial temporal structures with nonverbal tests have had less encouraging results. Furthermore, recent research suggests that the left and right hippocampi may have different roles based on how novel the encoded stimuli are, e.g., the left hippocampus appears more sensitive to never-before-seen stimuli. We identified aspects of verbal and nonverbal memory tests that resulted in increased activation in the left or right hippocampi, respectively, in healthy individuals. The present study compares activation in patients with TLE to that of healthy subjects on these tasks and investigates whether individual activation profiles can be useful in presurgical assessment. Methods: We used two matched tests, each comprising initial encoding, initial recognition, second encoding, second recognition and delayed recognition. Targets were presented in a block design with appropriate baseline stimuli. Stimuli were designed to maximize specificity: verbal items were pronounceable pseudowords (difficult to associate with an image) and nonverbal items were abstract designs (not easily named). We used 3T MRI to collect high-resolution images. Subjects were patients with unilateral TLE and age and education matched healthy volunteers. After spatial preprocessing, data were analyzed on the group level. Contrasts in activation were assessed within a masked area including the hippocampus, amygdala and parahippocampal region. Percent signal change in the hippocampi and parahippocampal gyri were calculated and used to assess results on an individual basis. Results: Healthy subjects showed isolated hemispheric MTL activation in particular material specific conditions: For verbal information, the left hippocampus was more active during initial compared to second encoding; for nonverbal information, the right hippocampus was more active during second retrieval compared with first. Patients showed reversals in the regions activated: Left TLE patients showed increased right activation during encoding of never-before-seen verbal information but performed worse on this test than controls, and patients with right TLE showed increased left activation on the second attempt at recognizing nonverbal information with poorer (although not significantly so) task performance. Conclusions: Our findings suggest a complex interaction between material specificity, familiarity and memory process in left and right MTL function. Using contrasts reflecting this interaction, we demonstrated apparent reversals in lateralization in patients with unilateral TLE compared to healthy individuals, without preservation of normal function. This approach may be useful in identifying the eloquency of tissue in individuals during preoperative assessment.