Updating hippocampal representations: CA2 joins the circuit

The hippocampus integrates the encoding, storage and recall of memories, binding the spatio-temporal and sensory information that constitutes experience and keeping episodes in their correct context. The rapid and accurate processing of such daunting volumes of continuously changing data relies on dynamically assigning different aspects of mnemonic processing to specialized, interconnected networks corresponding to the anatomical sub-fields of dentate gyrus (DG), CA3 and CA1. However, differentially processed information ultimately has to be reintegrated into conjunctive representations, and this is unlikely to be achieved by unidirectional, sequential steps through a DG-CA3-CA1 loop. In this Review, we highlight recently discovered anatomical and physiological features that are likely to necessitate updates to the hippocampal circuit diagram, particularly by incorporating the oft-neglected CA2 region.