Psychology › Brain & biology › Brain regions and localization of function
Hippocampus: forming new memories
· Concept
Well replicatedStrong, repeated evidence.
A seahorse-shaped structure in each temporal lobe, essential for forming new memories of events, facts and places.
See it on the Psychology map →What it says
The hippocampus binds the pieces of an experience (who, what, where) into a memory, then gradually helps move it to the cortex for long-term storage, much of this during sleep. Without it, as patient H.M. showed, people cannot form new memories of events but keep old skills and intelligence. It also builds mental maps: 'place cells' fire at specific locations (Nobel Prize, 2014). London taxi drivers, who memorize thousands of streets, have enlarged rear hippocampi.
Why it matters
It explains why memory for recent events fails first in Alzheimer's disease, and why sleep matters for learning. Chronic stress is linked to a smaller hippocampus.
The evidence
Status: well replicated. Amnesia patients, animal recordings and brain imaging all confirm its role; the taxi-driver study (2000) used MRI scans comparing drivers with non-drivers.
Open questions
Whether adult humans keep growing new hippocampal neurons: two major 2018 studies reached opposite conclusions, and the question is still open.
Inside this (1)
Connections (4)
▸Hippocampus: forming new memories is a textbook case of Brain regions and localization of functionWell replicated
The connection
The hippocampus is a textbook case of a region with a specific job. When it is damaged, people lose the ability to form new memories of events while intelligence, old skills and short-term memory survive: proof that memory is not one thing but several systems housed in different places.
Who showed it and when
Named in 1587 by Bologna anatomist Giulio Cesare Aranzi for its seahorse shape; its memory role was established in 1957 by William Scoville and Brenda Milner.
The evidence
Amnesia cases such as H.M., recordings of place cells in rats (1971) and brain imaging all converge on its role.
Open questions
Whether old memories eventually stop depending on the hippocampus, or always need it to be recalled vividly, is still debated.
▸Patient H.M. (Henry Molaison) revealed the role of Hippocampus: forming new memoriesWell replicated
The connection
Before Henry Molaison, many scientists thought memory was spread across the whole cortex. His surgery removed the hippocampus and nearby tissue on both sides, and he lost the ability to form new long-term memories while keeping intelligence, language and older memories. That pointed to the hippocampal region as the gateway for storing new memories.
Who showed it and when
Hartford Hospital, Connecticut, 1953, surgery by William Scoville; studied by Brenda Milner of the Montreal Neurological Institute, then by Suzanne Corkin at MIT until 2008.
The evidence
Scoville and Milner's 1957 paper compared ten patients: memory loss tracked how much hippocampal tissue was removed. Later patients with damage limited to the hippocampus confirmed it.
Open questions
His lesion also included the amygdala and entorhinal cortex, so the hippocampus's own share was clarified only by later cases and animal studies.
▸Hippocampus: forming new memories is damaged early in Dementia and Alzheimer's diseaseWell replicated
The connection
In Alzheimer's disease, tau tangles appear early in and around the hippocampus, the region that turns new experiences into lasting memories. That is why the first sign is often forgetting recent conversations or events while old memories stay intact. The damage then spreads through wider cortex, affecting language, reasoning and eventually daily tasks.
Who showed it and when
The spread pattern was mapped by Heiko and Eva Braak in Frankfurt in 1991 from autopsied brains; MRI studies since the 1990s show hippocampal shrinkage in early disease.
The evidence
Autopsy staging, MRI volume measurements that track memory decline, and the parallel with patient H.M., who could not form new memories after hippocampal surgery.
Open questions
Why the hippocampal region is so vulnerable, and why some people with heavy plaque and tangle loads stay sharp, perhaps thanks to cognitive reserve.
▸Hippocampus: forming new memories replays the day's memories during Sleep and dreamsWell replicated
The connection
During deep slow-wave sleep the hippocampus replays the day's experiences in fast-forward, firing the same cell sequences seen while awake. These replays, timed with brain rhythms called sleep spindles, appear to transfer memories to the cortex for long-term storage. Learning followed by sleep is retained better than learning followed by the same time awake.
Who showed it and when
Matthew Wilson and Bruce McNaughton recorded replay in rats at the University of Arizona in 1994; sleep-and-memory studies in people go back to Jenkins and Dallenbach in 1924.
The evidence
Recordings in rats and in epilepsy patients; a 2009 French study showed disrupting replay during rat sleep impaired memory, and cueing studies in people boost specific memories.
Open questions
How much REM sleep, versus deep sleep, contributes; and whether memories ever fully move out of the hippocampus or it stays involved indefinitely.