Differential modulation of a common memory retrieval network revealed by positron emission tomography

Functional neuroimaging is uniquely placed to examine the dynamic nature of normal human memory, the distributed brain networks that support it, and h ow they are modulated. Memory has traditionally been classified into contex t-specific memories personally experienced ("episodic memory") and imperson al non-context-specific memories ("Semantic memory"). However, we suggest t hat another useful distinction is whether events are personally relevant or not. Typically the factors of personal relevance and temporal context are confounded, and it is as yet not clear the precise influence of either on h ow memories are stored or retrieved. Here we focus on the retrieval of real -world memories unconfounding personal relevance and temporal context durin g positron emission tomography (PET) scanning. Memories differed along two dimensions: They were personally relevant (or not) and had temporal specifi city (or not). Recollection of each of the resultant four memory subtypes-a utobiographical events, public events, autobiographical facts, and general knowledge-was associated with activation of a common network of brain regio ns. Within this system, however, enhanced activity was observed for retriev al of personally relevant, time-specific memories in left hippocampus, medi al prefrontal cortex, and left temporal pole. Bilateral temporoparietal jun ctions were activated preferentially for personal memories, regardless of t ime specificity. Finally, left parahippocampal gyrus, left anterolateral te mporal cortex, and posterior cingulate cortex were involved in memory retri eval irrespective of person or time. Our findings suggest that specializati ons in memory retrieval result from associations between subsets of regions within a common network. We believe that these findings throw new light on an old debate surrounding episodic and declarative theories of memory and the precise involvement of the hippocampus. Hippocampus 1999; 9:54-61. (C) 1999 Wiley-Liss, Inc.