The hippocampus is a seahorse-shaped structure within the brain’s temporal lobe that plays a vital role in forming memories, spatial navigation, and emotional processing.
Its name comes from the Greek word hippokampos, meaning “seahorse,” after its distinctive curved shape.
Key Takeaways
- Memory: The hippocampus helps form new memories and recall past experiences.
- Navigation: It builds spatial maps that let you find your way around familiar places.
- Social Maps: This region also helps you process emotions and track how people are connected.
- Damage: Losing hippocampal tissue causes memory loss and disorientation.
- Protection: Exercise, stress management and mental stimulation support its health.
- Not a Store: The hippocampus indexes memories and trains the cortex to hold them. Old memories survive its loss.
- Neurogenesis: Whether adult humans grow new hippocampal neurons is still disputed.
Where is the hippocampus located?
The hippocampus sits within the medial temporal lobes of your brain, just above each ear.
Everyone has two hippocampi, one in each hemisphere. The name comes from the Greek word for “sea horse,” which its curved shape resembles.
Each hippocampus sits immediately behind the amygdala, which caps its front end. Around it lie the parahippocampal and entorhinal cortices.
Together with these neighbours, it forms the hippocampal formation. This is the functional unit memory researchers usually mean when they say “hippocampus.”
Its tissue is allocortex, an older, simpler cortex with only three cell layers. The neocortex that covers most of the brain has six. That simple, ordered design made it a workhorse for studying memory at the level of single cells.
It’s part of the limbic system, a group of structures that help regulate emotions, memory, and learning.
The hippocampus also connects with other brain regions, including the amygdala and the prefrontal cortex, forming a network essential for processing experiences.
It is also unusual in another way. The hippocampus is one of the few brain regions where neurogenesis happens, meaning new neurons are still produced there in adulthood.
Hippocampus Functions
The hippocampus is best known for its role in memory. It does more than that, though. Here are its key functions:
1. Memory formation
The hippocampus helps turn short-term experiences into long-term memories. It specializes in declarative memories. These are facts and events you can consciously recall.
For example, remembering what you had for breakfast or the name of a new colleague involves hippocampal processing.
Its close link to the amygdala also explains why emotional memories are often easier to remember.
When something feels important or intense, your brain tags it, and the hippocampus stores it more effectively.
A landmark case shows this. Scoville and Milner (1957) documented a patient known as HM.
- Aim: To record how memory changes after both medial temporal lobes are surgically removed to treat severe seizures.
- Method: In 1953, surgeon William Scoville removed about two-thirds of each hippocampus, plus the amygdala and entorhinal cortex (the hippocampus’s gateway to the cortex). Brenda Milner then tested him for years.
- Results: HM developed anterograde amnesia: he could no longer form new long-term memories of facts and events. His IQ, digit span and pre-surgery memories were intact, and he could still learn motor skills like mirror-drawing.
- Conclusion: The hippocampus is needed to form new conscious memories. It is not needed to store old ones, or to run short-term memory and skill learning.
One caution matters. HM’s surgery cut beyond the hippocampus into neighbouring structures, so his deficits cannot be pinned on that structure alone.
2. Spatial Memory and Navigation
The rear (posterior) hippocampus builds a cognitive map. This is a mental representation of your surroundings that lets you navigate familiar places.
The idea began with place cells, hippocampal neurons that fire only when an animal is in one particular spot.
- Aim: To test what single hippocampal neurons encode in a freely moving animal, and whether the hippocampus represents space.
- Method: O’Keefe and Dostrovsky (1971) used fine microelectrodes to record single neurons in the dorsal hippocampus of unrestrained rats. They related each cell’s firing to the animal’s position and orientation.
- Results: Some cells fired only when the rat was in a particular location, the cell’s “place field.” The pattern of active cells signalled where the animal was.
- Conclusion: The hippocampus holds a spatial representation of the environment, so its memory and spatial roles are closely intertwined.
O’Keefe and Nadel (1978) argued that these cells form a world-centred map of space.
Grid cells supply its scale. In the neighbouring entorhinal cortex, they fire in a regular triangular lattice that works like neural graph paper (Hafting et al., 2005). This work on the brain’s spatial system won the 2014 Nobel Prize in Physiology or Medicine.
The strongest human evidence comes from London taxi drivers.
- Aim: To test whether years of real-world navigation change the structure of the human hippocampus.
- Method: Maguire et al. (2000) took structural MRI scans of 16 licensed London taxi drivers, who must memorise the whole street layout of the city. They compared them with 50 male controls.
- Results: The drivers had more grey matter in the posterior hippocampi than controls, and less in the anterior. Posterior volume grew with the number of years spent driving.
- Conclusion: The adult hippocampus reshapes itself in response to navigational demand, which is evidence of neuroplasticity.
The original study was correlational. Later work closed that gap. Bus drivers, matched for driving but not for flexible navigation, showed no such difference (Maguire et al., 2006).
Trainees who passed “The Knowledge” grew posterior grey matter, while those who failed did not (Woollett & Maguire, 2011). Navigation itself drove the change.
3. Memory Consolidation
The hippocampus does not store memories permanently. It binds the scattered fragments of an experience into one indexed trace, then prepares that trace for long-term storage in the cortex.
This slow hand-over is called systems consolidation (Squire, 1992).
Much of it happens during sleep. The hippocampus replays the trace again and again until the cortex can hold the memory on its own.
The evidence comes from amnesia. After hippocampal damage, people cannot form new declarative memories, and recent memories are lost. Remote memories laid down long before are relatively spared (Scoville & Milner, 1957).
That pattern fits a two-stage view. The hippocampus is needed to make long-term memories, but not to store the oldest ones.
The need is also selective. Skills learned by doing, called procedural memory, survive hippocampal loss. So do working memory and classically conditioned responses. Human memory is therefore a set of separable systems, and only one of them depends on the hippocampus.
The synaptic mechanism is separate. Long-term potentiation strengthens the link between two neurons that fire together, and it was first demonstrated in the hippocampus (Bliss & Lømo, 1973).
4. Social Mapping
Recent research suggests the hippocampus also helps you track social relationships.
It creates social maps. These are mental representations of how people relate to one another, and they help you remember who is connected to whom (Montagrin et al., 2018).
The same relational machinery appears to organise non-spatial knowledge and imagined futures. Spatial navigation may simply be its original use.
Hippocampus Anatomy
The hippocampus is made up mainly of pyramidal cells, large excitatory neurons with many branches that send signals on to other regions. Smaller granule cells in the dentate gyrus and many inhibitory interneurons join them.
The hippocampus is not a uniform mass. It is divided into fields called CA1, CA2, CA3 and CA4, curled around a separate structure, the dentate gyrus.
Information travels through these fields along a mostly one-way, three-synapse loop known as the trisynaptic circuit. The entorhinal cortex feeds the dentate gyrus, the dentate gyrus feeds CA3, and CA3 feeds CA1.
Each link in the loop has its own fibre pathway:
- Perforant path: carries signals from the entorhinal cortex to the dentate gyrus.
- Mossy fibres: link the dentate gyrus to CA3.
- Schaffer collaterals: link CA3 to CA1.
CA1 then compares the CA3 output with direct input from the entorhinal cortex. It relays the result back out through the subiculum and entorhinal cortex to the neocortex.
Each stage does different work. The dentate gyrus performs pattern separation, keeping similar experiences apart so this morning’s breakfast is not confused with yesterday’s. CA3 performs pattern completion, rebuilding a whole memory from a fragment.
That is why one smell can revive a whole evening.
The entorhinal cortex is the hippocampus’s main gateway. It funnels highly processed input from every sense into the hippocampus and carries the results back out to the neocortex.
Its reach is wider still. It receives input from the prefrontal cortex, the anterior cingulate gyrus and the premammillary region. It can send signals back to each of them.
Four more connections shape what it does with that input:
- Fornix: a C-shaped fibre tract carries output to the mammillary bodies, anterior thalamus and septal nuclei.
- Amygdala: a strong two-way link prioritises emotionally significant events for storage.
- Prefrontal cortex: two-way links let past experience guide decisions and support the hand-over of memories to the cortex.
- Modulators: the septum, dopamine and stress hormones help switch it between forming and retrieving memories.
Long-Term Potentiation
How does a memory physically change the brain? The leading answer is long-term potentiation, or LTP: a lasting strengthening of the connection between two neurons after a brief burst of activity.
LTP was first demonstrated in the hippocampus. Bliss and Lømo (1973) stimulated the pathway into the dentate gyrus of anaesthetised rabbits and found that the synaptic response stayed enlarged for hours afterwards.
Two glutamate receptors explain the effect. AMPA receptors carry ordinary fast signals, while NMDA receptors act as coincidence detectors.
An NMDA receptor is normally plugged by a magnesium ion. It opens only when the sending neuron releases glutamate and the receiving neuron is already active, letting calcium flood in and strengthen the synapse.
So a synapse strengthens only when both cells fire together. This is the Hebbian principle, often summarised as “cells that fire together wire together.”
What Happens When the Hippocampus Is Damaged?
Damage to the hippocampus can seriously disrupt memory and spatial awareness.
Some common effects include:
- Difficulty forming new memories (especially recent events)
- Forgetting familiar places or losing your sense of direction
- Struggling to remember words or learn new information
This kind of dysfunction is common in several conditions:
Alzheimer’s Disease
Alzheimer’s pathology starts in the entorhinal cortex, the hippocampus’s own gateway, and spreads into the hippocampus itself (Anand & Dhikav, 2012).
That anatomy explains the symptom order. Memory loss and confusion come first because the structure that forms new memories is damaged first.
Over time the region shrinks and its neurons deteriorate. Older memories are lost as well, and everyday orientation becomes difficult.
Depression and Chronic Stress
The hippocampus is packed with glucocorticoid receptors, the docking sites for stress hormones, so it is unusually sensitive to them. Sustained high cortisol can suppress and damage its neurons.
Videbech and Ravnkilde (2004) pooled 12 MRI studies of unipolar depression, covering 351 patients and 279 healthy controls. Hippocampal volume was smaller in the patients, by 8% on the left and 10% on the right.
Individual studies differ, though. Some find no loss at all, while in others the loss is linked to memory problems (Sheline et al., 2002).
The traffic runs both ways. The hippocampus normally feeds back to dampen the body’s stress response, so damage here weakens the brake on stress itself.
Hippocampal volume has also increased after electroconvulsive therapy (ECT) for depression (Nordanskog et al., 2010).
Psychiatric Disorders
People with schizophrenia or bipolar disorder often show reduced hippocampal volume and abnormal neural activity (Anand & Dhikav, 2012).
Reduced hippocampal volume is also reported in post-traumatic stress disorder. Whether trauma shrinks the hippocampus, or a smaller hippocampus raises the risk of PTSD, is still debated.
Volume differences of this kind are correlations. They cannot on their own tell you which came first.
Other Causes
Injury, stroke and oxygen deprivation can all harm the hippocampus, usually causing memory and orientation problems.
Epilepsy deserves a separate mention. The hippocampus is one of the most seizure-prone structures in the brain. Scarring of its tissue, called hippocampal sclerosis, is a common cause of temporal lobe epilepsy.
Memory can also fail when the hippocampus is intact. Damage further along its output pathway, to the mammillary bodies and thalamus, produces the amnesia of Korsakoff’s syndrome.
Can you Strengthen your Hippocampus?
While you can’t change your brain’s structure overnight, certain habits can support hippocampal health and slow decline:
- Exercise regularly: In aged mice, running improved learning and increased neurogenesis (new brain cell growth) in the hippocampus (Van Praag et al., 2005). Human evidence is less direct.
- Manage stress: Practices like mindfulness and deep breathing can protect the hippocampus from stress-related shrinkage.
- Stimulate your brain: Reading, writing, puzzles, and memory games help keep hippocampal pathways active.
Critical Evaluation
The hippocampus is often called the brain’s memory centre. That shorthand hides three real problems.
- Not a Store: memories are consolidated into the cortex, so the hippocampus indexes them rather than holding them.
- Memory or Space: whether its core job is memory or spatial mapping has never been settled.
- Fragile Evidence: the human case rests on a few rare patients, and the mechanism on rodents.
Not a Memory Store
Squire (1992) drew this conclusion from the pattern of amnesia itself. Patients who lose the hippocampus keep memories laid down years earlier, which could not happen if those memories lived there.
The structure works as a temporary binding-and-indexing hub. It ties together the scattered cortical fragments of an event, then trains the cortex to hold the memory without help.
Old memories survive its loss because they no longer need it.
The practical consequence for a student is precise. Damage to the hippocampus should predict lost recent memories and no new ones, not a wiped-out past, and that is exactly what amnesic patients show.
Calling it the brain’s memory centre therefore mistakes an index for a library, and it is the most persistent misconception about the structure.
Memory also resists localisation to a single structure. Hippocampus-dependent memory needs the fornix, mammillary bodies and thalamus too (Squire, 1992). The hippocampus is better seen as one hub in a wider memory network.
Memory or Spatial Map?
Two rival accounts have coexisted for decades. Squire (1992) treats the hippocampus as a declarative-memory system. O’Keefe and Nadel (1978) treat it as a spatial map.
Each camp has strong evidence.
Amnesic patients lose facts and events, not just their way around. Place cells, by contrast, fire for location, not for facts. Neither finding fully explains the other.
Cohen and Eichenbaum (1993) argued for a relational view. Hippocampal damage, they said, impairs memory for any flexible relation, not only place. Examples include the order of events and the link between a person and an object.
Place cells fit this view. They fire to locations because space is the dominant relation an animal meets.
The modern synthesis says both are special cases of something more general: the hippocampus stores relational representations, records of how items connect to one another. Space is simply the most obvious kind of relation.
That reconciliation is useful. It is not a decisive win for either camp, and the dispute still shapes how new experiments are designed and interpreted.
Standard Consolidation vs Multiple Trace Theory
A second dispute concerns how long the hippocampus stays involved in a memory it helped to form. Standard consolidation theory (Squire & Alvarez, 1995) says the involvement is time-limited. Multiple trace theory (Nadel & Moscovitch, 1997) says it is permanent for detailed episodic memories.
| Question | Standard consolidation theory | Multiple trace theory |
|---|---|---|
| Hippocampal role over time | Time-limited. Cortical links strengthen with repeated reactivation until the memory no longer needs the hippocampus. | Permanent for detailed episodic memories. Each retrieval lays down a new hippocampal trace. |
| Remote episodic memory after damage | Should eventually be spared. | Should stay vulnerable if it keeps rich detail. |
| Remote semantic memory | Can become hippocampus-independent. | Can become hippocampus-independent. |
Standard consolidation theory explains why remote memories survived in HM while recent ones did not (Scoville & Milner, 1957). Multiple trace theory predicts that old memories keep depending on the hippocampus if they hold rich episodic detail.
Patient and imaging evidence has been cited on both sides. The dispute remains open.
Rare Cases and Rodent Brains
HM’s surgery removed the amygdala and entorhinal cortex as well as most of the hippocampus. His deficits therefore cannot be pinned on one structure.
He was also one man with an unusual operation, which makes generalising to healthy brains a real inferential leap.
The mechanistic backbone has the opposite problem. Place cells (O’Keefe & Dostrovsky, 1971) and long-term potentiation (Bliss & Lømo, 1973) were both found in rodents. Rodent circuits may not map cleanly onto human conscious memory.
LTP carries a further caveat.
It is induced artificially in the laboratory, so showing that the same synaptic change encodes a natural memory remains an open problem.
None of this sinks the account. It is the convergence of lesion cases, animal lesions, single-cell recording and synaptic physiology, rather than any one study, that licenses the general conclusion.
Contemporary Research
Recent work has moved from asking which structure supports memory to asking how its circuits do it, and where the evidence is weak.
The sharpest open question is adult neurogenesis, the birth of new neurons in the mature brain. Eriksson et al. (1998) first reported it in humans, finding dividing neural precursors in cancer patients who had received a DNA-labelling marker.
Two 2018 papers reached opposite conclusions from human autopsy tissue.
Boldrini et al. (2018) counted markers of newly born neurons in whole hippocampi from 28 healthy people aged 14 to 79. Numbers of immature neurons were similar at every age.
Sorrells et al. (2018) ran a similar marker analysis and found young neurons dropping sharply through childhood and becoming undetectable in adults.
The disagreement turns on method: how tissue is preserved, how long after death it is sampled, and whether the markers are specific to new neurons.
Neither should be quoted as settled fact.
Plasticity in the healthy adult hippocampus is on firmer ground. The taxi-driver work progressed from a cross-sectional difference to a longitudinal before-and-after design (Woollett & Maguire, 2011).
The third theme is generalisation. The relational map that codes space also appears to organise social relationships, abstract knowledge and imagined futures (Montagrin et al., 2018).
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