The hippocampus: what it does for memory, and how to protect it

A seahorse-shaped structure deep in each side of your brain decides much of what you’ll remember. Here’s how it works, what harms it, and what helps it.

By the Daily Recall team · Updated October 3, 2026 · 10 sources

If you remember what you had for breakfast, where you parked, or a conversation from last week, thank your hippocampus. It’s one of the most studied parts of the brain, and for good reason: it sits at the centre of how we form new memories and find our way around.

Where it is, and why it’s called that

You have two hippocampi, one deep in each temporal lobe, roughly level with your ears. Each is a curved, S-shaped structure about 5 cm long. In 1587 the anatomist Julius Caesar Arantius named it after its shape: hippocampus comes from the Greek for seahorse.

What the hippocampus does

• Forms new long-term memories of facts and events (declarative memory), turning short-term experiences into lasting ones. See short-term vs long-term memory.
• Spatial memory and navigation: building mental maps of places.
• Telling similar memories apart, such as today’s parking spot from yesterday’s.

It’s much less involved in skills and habits, like riding a bike, which depend on other brain systems.

Patient H.M.: the man who couldn’t make new memories

In 1953, surgeons removed most of both hippocampi from a young man named Henry Molaison to stop his severe seizures. The seizures improved, but he could no longer form new long-term memories. He could hold a conversation, but forgot it minutes later. He still remembered much of his life before the surgery.

Psychologist Brenda Milner began testing him in 1955. In one famous experiment, he practised tracing a star while looking only at its reflection in a mirror. He got better day after day, yet each time insisted he’d never done the task before. That showed skill memory and fact-and-event memory use different systems. Studied for decades as “H.M.”, he died in 2008, and his case reshaped our understanding of memory.

The brain’s GPS

In 1971, John O’Keefe discovered place cells in the hippocampus: neurons that fire when an animal is in a particular spot. In 2005, May-Britt and Edvard Moser found grid cells in a neighbouring region that act like a coordinate system. Together they form an inner positioning system, and the three shared the 2014 Nobel Prize in Physiology or Medicine.

Heavy use of that system leaves a mark. London taxi drivers, who memorize about 25,000 streets, have a larger rear hippocampus than non-drivers. A study that followed trainees over about four years found only those who passed the exam gained grey matter there, which suggests the training caused the change. See neuroplasticity.

Sleep: when the hippocampus replays your day

During deep sleep and quiet rest, the hippocampus produces brief bursts of activity called sharp-wave ripples, in which neurons replay the day’s experiences at high speed. In a 2009 experiment, blocking these ripples during sleep made rats worse at remembering a maze. It’s a big reason sleep matters for memory.

What harms the hippocampus

Chronic stress. The hippocampus is packed with receptors for stress hormones. In older adults, higher cortisol levels over time have been linked to hippocampal shrinkage and memory problems. The good news: in Cushing’s syndrome, where the body makes too much cortisol, hippocampal volume has been shown to increase after treatment. See stress and memory.

Alzheimer’s disease. According to the Nobel committee’s summary, the hippocampus and the neighbouring entorhinal cortex are often affected early in Alzheimer’s, which helps explain why getting lost and forgetting recent events are common early signs.

Heavy drinking. Alcohol blocks the hippocampus from recording memories, causing blackouts, and long-term drinking is linked to hippocampal shrinkage. See alcohol and the brain.

Other causes listed by Cleveland Clinic include epilepsy, low oxygen, head injury, infections such as encephalitis, PTSD and aging.

Can adults grow new hippocampal neurons?

Part of the hippocampus called the dentate gyrus is the main candidate for new neurons in adults. One estimate suggested about 700 new neurons a day in each hippocampus, but a 2018 study found almost none in adults. Scientists still disagree, partly because of how brain tissue is preserved. It remains an open question.

How to look after your hippocampus

• Exercise: in a 2011 trial, a year of walking 3 times a week increased hippocampal volume by about 2% in older adults. Later reviews suggest exercise mainly slows age-related shrinkage. See exercise and memory.
• Sleep well so replay and consolidation can happen.
• Manage chronic stress with exercise, social contact and, for some people, meditation.
• Limit alcohol.
• Keep learning and exploring: new places and new skills give the hippocampus work to do.

Test your spatial memory

Remembering where things were is classic hippocampus territory. Try this quick tile test:

The daily Recall game is the full version, with a new pattern every day. It’s a game, not a brain scan, but it’s a fun way to give your spatial memory a workout.

Sources

1. Cleveland Clinic: Hippocampus
2. Journal of Neurosurgery (2015): history of the hippocampus and its naming by Arantius
3. MIT News: Suzanne Corkin on patient H.M.
4. Scoville & Milner (1957): loss of recent memory after bilateral hippocampal lesions
5. NobelPrize.org: 2014 Nobel Prize in Physiology or Medicine, the brain’s positioning system
6. Scientific American: London taxi drivers and the hippocampus
7. Journal of Neuroscience (2021): the adult neurogenesis debate
8. Girardeau & Zugaro (2011): hippocampal ripples and memory consolidation
9. NCBI Bookshelf, Brain Aging: stress, glucocorticoids and the hippocampus
10. Erickson et al. (2011), PNAS: exercise training increases size of hippocampus (ScienceDaily)

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