Limbic System · Medial Temporal Lobe
Named for its seahorse-like shape, the hippocampus is crucial for forming new memories and converting short-term memories into long-term storage. It also plays an important role in spatial navigation and the cognitive map of our environment.
The hippocampus is a curved, elongated structure buried within the medial (inner) surface of each temporal lobe - there are two, one in each hemisphere. Its name comes from the Greek for "seahorse," which its cross-sectional shape was thought to resemble. It's part of the limbic system, a group of interconnected structures involved in memory and emotion, and sits adjacent to the amygdala.
The hippocampus is the brain's gateway for forming new explicit (conscious, factual and episodic) memories. When you learn something new - a name, an event, a route through a city - the hippocampus is where that information is initially bound together from signals arriving across the cortex, before being gradually transferred to distributed storage across the cerebral cortex for long-term keeping, a process that can take months to years. Critically, the hippocampus itself isn't thought to be the permanent storage site for old memories - it's more like an index or assembly point.
The hippocampus is equally essential for spatial navigation. It contains "place cells," neurons that fire when an animal or person is in a specific location, effectively forming an internal cognitive map of physical space. This dual role - memory and navigation - reflects the deep, ancient link between remembering the past and knowing where you are.
Much of what we know about the hippocampus comes from the case of Henry Molaison ("H.M."), who in 1953 had both hippocampi surgically removed to treat severe epilepsy. The surgery controlled his seizures but left him unable to form any new long-term explicit memories for the rest of his life, while his memories from before the surgery, and his ability to learn new physical skills, remained largely intact. H.M.'s case, studied for decades by researchers including Brenda Milner, became the foundation of modern memory science, demonstrating that different types of memory are handled by different, dissociable brain systems.
Decades later, a landmark 2000 study by Eleanor Maguire and colleagues found that London taxi drivers - who must memorise the labyrinthine layout of London's streets to pass "The Knowledge," a notoriously difficult licensing exam - had measurably larger posterior hippocampi than control subjects, with the size correlating to years of driving experience. It remains one of the clearest demonstrations of neuroplasticity, the brain's ability to physically change in response to sustained mental demands.
The hippocampus is highly vulnerable to oxygen deprivation, certain infections, chronic stress, and neurodegeneration, and its dysfunction is central to several conditions. It's one of the earliest structures affected in Alzheimer's disease, which is why short-term memory loss is typically an early symptom. Damage to it - whether from injury, surgery, or disease - is the most common cause of amnesia. Chronic stress and depression have also been linked to reduced hippocampal volume, an active area of ongoing research.
The hippocampus is discussed in connection with the following conditions on this site:
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