Memory Digest

The Hippocampus

The hippocampus is a seahorse-shaped brain structure vital for forming new memories of facts and events—and one of the first regions hurt in Alzheimer's.

Last reviewed: October 2026

The hippocampus (plural: hippocampi) is a curved structure deep in each temporal lobe of the brain, named for its resemblance to a seahorse (hippocampus in Greek). It is central to forming new memories of facts and personal events—the kind of memory you use when you learn a name, recall what you ate for breakfast, or remember a doctor's appointment.

When families hear that Alzheimer's "starts in the hippocampus," this is the structure they mean. Understanding what it does—and what it does not do—makes early symptoms easier to interpret and less mysterious.

What the hippocampus does

The hippocampi sit in the medial temporal lobes and work with nearby cortex as a hub for:

• Encoding new episodic memories – experiences tied to a time and place ("I met Sarah at the café yesterday")
• Spatial memory and navigation – building mental maps of rooms, routes, and neighborhoods
• Linking new information to existing knowledge so it can be stored more permanently in the cortex

They are less critical for:

• Skills and habits (riding a bike, typing)—more dependent on other systems
• Very short working memory of a few seconds (though attention networks interact with the hippocampus)
• Old, well-consolidated memories from decades ago—those live largely in the cortex

A famous case in neuroscience, patient H.M., had both hippocampi surgically removed to treat epilepsy. He kept his personality, skills, and childhood memories—but could not form lasting new memories of events after the surgery. That case cemented the hippocampus's role in memory formation.

How memories get stored

New experiences first leave a fragile trace. The hippocampus helps bind sights, sounds, emotions, and context into a coherent episode. Over time—especially during deep sleep—important patterns are replayed and gradually transferred into longer-term cortical networks. This process is called consolidation.

That is why a good night's sleep after learning often improves recall the next day, and why sleep disorders and chronic insomnia are linked with poorer memory performance.

The hippocampus and Alzheimer's disease

In Alzheimer's Disease, abnormal proteins (amyloid plaques and tau tangles) and loss of connections between neurons typically affect the medial temporal lobe early. The hippocampi shrink—a change visible on MRI and used clinically as one clue among many.

Early consequences often include:

• Forgetting recent conversations while older memories remain
• Repeating questions
• Misplacing items and not recalling the last place they were used
• Getting lost in unfamiliar—or sometimes familiar—places
• Trouble learning new names or routes

These overlap with mild cognitive impairment (MCI) when daily independence is still largely intact. Other conditions also shrink or injure the hippocampus, including some forms of vascular injury, prolonged seizures, certain infections, oxygen deprivation, and chronic severe stress.

Stress, cortisol, and the hippocampus

The hippocampus is rich in receptors for stress hormones. Short bursts of stress can sharpen attention; chronic, intense stress is associated with weaker memory performance and, in research studies, with measurable hippocampal changes. Depression is also linked with smaller hippocampal volume in some studies—volume that may partly recover with successful treatment. See Depression and Memory ("Pseudodementia") and Stress, Anxiety and Memory.

This does not mean everyday worry "destroys" the hippocampus. It does mean that treating chronic stress, sleep problems, and mood disorders is brain care, not just "mental health" in isolation.

Other conditions that affect the hippocampus

• Transient global amnesia – a sudden, temporary inability to form new memories lasting hours; usually benign but warrants urgent evaluation to rule out stroke
• Limbic encephalitis and autoimmune conditions – inflammation targeting memory circuits
• Alcohol-related brain injury and thiamine deficiency (Wernicke–Korsakoff spectrum)—severe memory encoding problems
• Temporal lobe epilepsy – seizures arising near the hippocampus can impair memory
• Normal aging – modest volume loss occurs with age; large or rapid change is not "just aging." See Normal Aging vs. Memory Problems

What imaging and tests show

MRI can measure hippocampal size and rule out tumors, strokes, or hydrocephalus. It cannot by itself diagnose Alzheimer's. Doctors combine imaging with history, cognitive testing, blood work for Reversible Causes of Memory Problems, and sometimes PET or fluid biomarkers for amyloid and tau. See Getting a Diagnosis.

Protecting hippocampal health

No lifestyle change replaces medical care when disease is present, but research and public-health guidance support:

• Regular aerobic exercise
• Quality sleep and treatment of sleep apnea
• Blood pressure, blood sugar, and hearing care
• Social and mental engagement
• A brain-healthy eating pattern such as the The MIND Diet
• Avoiding heavy alcohol use and smoking
• Managing depression and chronic stress

See Reducing Dementia Risk: The 14 Risk Factors and How to Improve Your Memory for practical steps. For people with MCI or early Alzheimer's due to confirmed amyloid, disease-modifying options such as Lecanemab (Leqembi) and Donanemab (Kisunla) may be discussed with specialists.

Bottom line

The hippocampus is the brain's gateway for turning today's experiences into lasting memories of facts and events, and for navigating space. It is vulnerable in Alzheimer's and sensitive to sleep, stress, and vascular health—but it is also part of a resilient network. Understanding its role helps families recognize why recent memory fails first in many dementias, and why sleep, mood, and heart-brain health are not side issues—they are core memory care.

Sources

1. National Institute on Aging: What Happens to the Brain in Alzheimer's Disease?
2. Mayo Clinic: Alzheimer's disease
3. Squire LR, Zola-Morgan S. The medial temporal lobe memory system. Science, 1991

Not medical advice. This article is general information, not a diagnosis or treatment plan for any individual. Talk with a doctor about your situation.

Related articles

• How Memory Works: An Overview
• Types of Memory
• Alzheimer's Disease
• Sleep and Memory
• Mild Cognitive Impairment (MCI)