Encoding and retrieval in human medial temporal lobes: An empirical investigation using functional magnetic resonance imaging (fMRI)

The precise functional role of the hippocampus in human episodic memory is an unresolved question though it has recently been suggested that distinct medial temporal lobe (MTL) regions are involved in encoding and retrieval o perations respectively. For example, a recent meta-analysis of positron emi ssion tomography (PET) literature has suggested a rostral-caudal functional division in the medial temporal lobes (MTL), with rostral MTL mediating en coding and caudal MTL retrieval operations. However, a review of the combin ed PET and fMRI literature, reported in the present issue, while noting sys tematic discrepancies between PET and fMRI, reaches a conclusion that poste rior MTL is involved in encoding. Here we present fMRI data, from a modifie d artificial grammar learning paradigm, that examines two questions concern ing the functional role of the hippocampus, and related MTL structures in e pisodic memory. Firstly, we test a hypothesis that anterior hippocampus is activated during encoding and that this response is greater for novel items . Secondly, we test whether increasing familiarity with stimulus material i s associated with a posterior MTL neural response. Our empirical findings s upport both hypotheses in that we demonstrate a left anterior hippocampal r esponse sensitive to encoding demands and a posterior parahippocampal respo nse sensitive to retrieval demands. Furthermore, we show that both anterior and posterior hippocampal responses are modulated to the degree to which s timuli can be assimilated into a meaningful rule-based framework. Hippocamp us 1999;9:25-34. (C) 1999 Wiley-Liss, Inc.