Abstracts

Rationale: The prefrontal cortex (PFC) is important for efficient memory encoding and recognition. In healthy individuals, functional MRI (fMRI) studies demonstrated material specific PFC activations with verbal and non-verbal memory tasks on the left and right PFC respectively. We investigated the patterns of PFC fMRI activations associated with verbal and non-verbal episodic memory in Temporal Lobe Epilepsy (TLE) patients.Methods: We studied 27 healthy controls and 42 patients with TLE due to unilateral hippocampal sclerosis (HS), (21 right) on a 3T GE Signa Excite MRI scanner using an fMRI memory encoding paradigm of faces and words with a subsequent out of scanner assessment of recognition. fMRI analysis was performed with SPM8. We examined the main effects and interactions of both word and face encoding compared to baseline. Neural correlates of performance were explored in a regression analysis using individual recognition accuracy scores. In addition, the effect of age at onset of epilepsy on encoding activity was investigated in a separate regression analysis. All results of the main effects are corrected for multiple corrections at family wise error, p< 0.05. Group comparisons and correlations are shown at p< 0.005 unless otherwise stated.Results: Word encoding: controls and right HS patients activated the left PFC and left hippocampus (HC). Patients with left HS showed bilateral HC activations and significant additional right PFC activations greater than in controls and the right HS group. Younger age at onset of epilepsy in the left HS group was associated with right PFC activations whereas later onset correlated with increased left PFC activations. Successful word recognition correlated with activation in the left PFC. Face encoding: controls showed right lateralised PFC activation and bilateral HC activations. Patients with left HS activated the right HC and right PFC. The right HS group activated the left HC and bilateral PFC. Significantly greater left PFC activation was seen in the right HS group compared to the left HS group. There was no correlation between age at onset of epilepsy and face encoding for either of the patient groups. Successful face recognition correlated with left PFC activations in right HS patients. Conclusions: We showed material specific PFC lateralisation for verbal and non-verbal memory encoding in healthy controls. Patients with HS ipsilateral to the side of material encoding showed additional contralateral PFC activations. In the left HS group, we showed that age at onset of epilepsy influences the degree of contralateral PFC reorganization of the memory network during word encoding however this reorganization was not related to successful encoding for words. In contrast, contralateral PFC reorganization proved functionally relevant in right HS patients during face encoding.