Gateway to memory
The name comes from the 16th-century Italian anatomist Arantius, who thought the structure resembled a seahorse (hippocampus). Connected to the cortical areas that gather sensory information, the hippocampus first captures declarative memories of facts and events, such as the name of someone you met today or what you ate last night. At first those memories depend heavily on the hippocampus, but during sleep it is thought to replay the day's experiences, gradually transferring them to the cortex in a process called consolidation. Skills learned through practice, such as riding a bicycle, and the short-term juggling of working memory rely more on other brain regions.
Patient H.M. and place cells
In 1953 Henry Molaison, a 27-year-old American with severe epilepsy known in the literature as H.M., had the medial temporal lobes, including both hippocampi, surgically removed. His seizures eased, but for the rest of his life he could not form new memories of facts or events, while older memories and motor skills such as mirror drawing remained. Neuropsychologist Brenda Milner's studies of H.M. showed that memory depends on specific brain regions and comes in different kinds. In 1971 John O'Keefe discovered hippocampal "place cells" that fire only when a rat is in a particular location, and shared the 2014 Nobel Prize in Physiology or Medicine with May-Britt and Edvard Moser, who found grid cells in the neighbouring entorhinal cortex. A study of London taxi drivers found that those with longer careers memorising the city's streets had a larger posterior hippocampus.
What harms it and what protects it
The hippocampus is especially vulnerable to lack of oxygen, low blood sugar and epileptic seizures, and it is one of the first regions to shrink in Alzheimer's disease, which is why forgetting recent events is often an early sign of dementia. Animal and human studies suggest that chronically high cortisol from long-term stress can damage hippocampal neurons and reduce its volume. On the other hand, it is one of the few places where new neurons may form in the adult brain, although how much this happens in humans is still debated. In a 2011 trial of 120 older adults, walking three times a week for a year increased hippocampal volume by about 2 percent. Evidence is building that regular aerobic exercise, adequate sleep and learning new things support hippocampal health (neuroplasticity).
FAQ
- Does hippocampal damage erase old memories?
- Usually the ability to form new memories is hit hardest, while very old memories are often relatively spared, although memories from the few years before the damage may be lost too.
- Are there habits that help the hippocampus?
- Regular aerobic exercise has been shown to increase hippocampal volume, and good sleep, stress management and learning new skills are also thought to help. A sudden decline in memory should be checked by a doctor.
Sources reviewed: 2026-10-26
References
- Scoville, W. B., & Milner, B. (1957). Loss of recent memory after bilateral hippocampal lesions. Journal of Neurology, Neurosurgery & Psychiatry, 20(1), 11–21.
- O'Keefe, J., & Dostrovsky, J. (1971). The hippocampus as a spatial map. Brain Research, 34(1), 171–175.
- Maguire, E. A., et al. (2000). Navigation-related structural change in the hippocampi of taxi drivers. PNAS, 97(8), 4398–4403.
- Erickson, K. I., et al. (2011). Exercise training increases size of hippocampus and improves memory. PNAS, 108(7), 3017–3022.