The hippocampus is a curved structure in the medial temporal lobe that is essential for the formation of new explicit memories. Without it, the past beyond a few minutes largely disappears.

The famous patient H.M., who had his hippocampus surgically removed to control epilepsy, retained old memories and the ability to learn new motor skills, yet could not form new lasting memories of facts or events. That dissociation established the hippocampus as critical for declarative memory while leaving other memory systems intact. The structure is also involved in spatial navigation; place cells within it fire when an animal is in particular locations, contributing to the cognitive map.

The hippocampus is vulnerable to stress, hypoxia, and the early stages of Alzheimer’s disease. Its neurons remain capable of limited adult neurogenesis, a finding that has generated intense research interest. Memory consolidation appears to involve a gradual transfer of dependence from the hippocampus to neocortical networks, so that older memories can survive hippocampal damage.

New episodes and facts pass through the hippocampus on their way to lasting storage. When that gateway is damaged, the ongoing record of life is interrupted.