Hippocampus

The hippocampus is a paired, curled ridge of gray matter tucked into the inner (medial) side of each temporal lobe, where it bulges into the floor of the lateral ventricle. Seen in cross-section, its coiled shape reminded early anatomists of a seahorse, which gave it its name. With the neighboring dentate gyrus and subiculum it makes up the hippocampal formation, and it is a core part of the limbic system, the group of brain regions tied to memory and emotion.

Its main job is to turn new experiences and facts into lasting memories, a process called consolidation; the memories themselves end up spread across the cerebral cortex, with the hippocampus acting as an index that links their parts. It also builds mental maps of places, which we use to navigate. In many mammals it is one of the few sites where new neurons keep forming during adult life.

Its role became clear in 1953, when a patient known as H.M. had large parts of both hippocampi removed to control his epilepsy and afterward could not form new memories of facts or events, though he could still learn motor skills. The hippocampus is also among the first areas damaged in Alzheimer's disease, which is why trouble remembering recent events is often an early sign.