Abstracts

Rationale: Functional MRI (fMRI) and lesion-deficit studies have provided evidence for a functional dissociation between the anterior and posterior left and right medial temporal lobe (MTL) mediating verbal and visual memory encoding. The aim of this study was to use functional MRI and compare blocked and event-related designs in order to investigate material specific memory functions in patients with temporal lobe epilepsy (TLE).Methods: We scanned 65 patients with unilateral TLE (37 with left hippocampal sclerosis (HS), 28 with right HS) and 20 healthy controls on a 3T GE-MRI scanner. All subjects performed an fMRI memory paradigm which examined the encoding of words, pictures and faces, allowing verbal and visual memory encoding to be tested in a single scanning session. Two ways of analysis, a blocked and event-related analysis were performed on the same data set. Results: Event-related fMRI analysis revealed significantly less activation in the left anterior medial temporal lobe (MTL) in left TLE patients and significantly less right anterior MTL activation in right TLE patients compared to controls. Results of the blocked analysis showed a similar lateralization pattern, located in more posterior MTL regions. Secondly, event-related analysis showed that greater left than right anterior MTL activation on encoding words was correlated with better verbal memory performance on standard neuropsychological tests in left TLE; in right TLE, greater right than left anterior MTL activation on encoding faces was correlated with better visual memory performance. Moreover, relatively greater ipsilateral than contralateral posterior MTL activation predicted better verbal memory outcome in left TLE and better visual memory outcome in right TLE following ATLR. The blocked analysis did not reveal any significant correlations in anterior or posterior MTL regions with out of scanner memory performance. Conclusions: We demonstrate the advantages of event-related designs compared to blocked designs for evaluation of verbal and visual memory function. Although less powerful than blocked designs event-related designs allow detection of activations that arise specifically during successful encoding. Our results from the event-related analysis suggest that subsequent memory effects are effectively mediated by anterior MTL structures. Preoperative reorganization of memory function to posterior MTL regions was associated with better postoperative memory outcome, while more posterior activity as detected with a blocked analysis was related to deficiencies of blocked designs in the analysis of memory functions most likely caused by additional cognitive processes other than memory encoding.