REVIEW ARTICLE The Ensemble That Plays Together, Stays Together

It has long been known that the medial temporal lobe is crucial for the formation and retrieval of episodic memories. This region includes the hippocampal formation (the hippocampus proper and subiculum) and the underlying entorhinal, perirhinal, and parahippocampal cortices, all of which receive a unique complement of cortical inputs. How best to characterize the precise roles of these different brain regions and their interactions is of central concern in understanding the medial temporal lobe contributions to episodic memory. Using functional magnetic resonance imaging (fMRI) to assess brain activity while subjects performed an episodic retrieval task, Giovanello et al. (2004) have contributed two important pieces to this puzzle. First, their findings show that the hippocampal formation is differentially recruited when subjects are asked to make memory decisions about the conjunctive relationship (in this case, the co-occurrence) of two stimuli compared to decisions about each st...