Medial temporal lobe activation during episodic encoding and retrieval: A PET study

Recent neuroimaging studies have obtained evidence of activation in the med ial temporal lobe (MTL) during episodic encoding and retrieval. On the basi s of a meta-analysis of MTL activations in studies that used positron emiss ion tomography (PET), Lepage et al. (Hippocampus 1998;8:313-322) suggested that episodic encoding tends to involve the anterior MTL, whereas episodic retrieval tends to involve the posterior MTL. In a meta-analysis of studies that used PET and functional magnetic resonance imaging, Schacter,and Wagn er (Hippocampus 1999;9:7-24) reported weaker evidence for such a rostrocaud al distribution of encoding and retrieval activations. However, these meta- analyses were based largely on studies that examined encoding or retrieval separately. Here, we report a direct, within-subjects comparison of MTL act ivation during episodic encoding and retrieval by using PET. Results indica ted that both encoding and retrieval were associated with blood flow increa ses in similar MTL regions with little indication that encoding and retriev al are preferentially associated with activity in the anterior versus the p osterior MTL. Direct comparisons revealed greater blood flow increases in p osterior MTL during encoding than retrieval. (C) 1999 Wiley-Liss, Inc.