New views on old memories: re-evaluating the role of the hippocampal complex

Evidence of temporally graded retrograde amnesia (RA) following hippocampal damage has fuelled the long-standing belief that memory undergoes a consol idation process, whereby memories are progressively modified in neocortical regions until they are independent of the hippocampal (HPC) complex. Suppo rt for this position derives from both the animal and human RA literature, although the results are not consistent, Specifically, consolidation theory does not account for loss of episodic (detail) information in humans and c ontext-dependent information in animals, which often extend back for much o f the life span. We discuss an alternative approach, the Multiple Trace The ory, which suggests that the HPC complex contributes to the retrieval of re cent and remote episodic and context-dependent memories. According to this view, such memory traces are represented as spatially distributed interacti ons between the HPC and neocortex that persist for as long as those memorie s exist. On the other hand, semantic, or context-free, memories can become independent of the HPC as consolidation theory predicts. In support of this view, we report recent accounts of relatively flat RA gradients in autobio graphical and spatial detail loss in patients and animal models with extens ive bilateral HPC lesions. By comparison, temporally graded RA was observed in tests of semantic and context-free memory. We also report neuroimaging studies in which hippocampal activity, elicited during recollection of auto biographical memories, did not distinguish recent from remote episodes. Our discussion suggests ways to reconcile discrepancies in the literature and guide predictions of the occurrence of flat versus temporally limited gradi ents of remote episodic and semantic memory loss following lesions to HPC. (C) 2001 Elsevier Science B.V. All rights reserved.