By K J Foxhall 
Reference reading: How To Become A London Taxi Driver: How to Pass the London Taxi Driver Knowledge: 1
Tucked deep within your brain’s temporal lobe is a dynamic, shape-shifting routing switch that commands how you experience the world.
Firstly, you have two of them (one in each hemisphere), so while hippocampus is the singular noun, hippocampi is the plural.
For centuries, this structure was a complete enigma, but today, it is recognised as one of the most intensely studied and mind-bending regions in all of neuroscience.
The origin story, from myth to mistake
In 1587, Venetian anatomist Julius Caesar Aranzi was dissecting a human brain when he stumbled upon a strange, curved structure. He initially thought it looked like a silkworm, but later settled on a cooler comparison: a seahorse.
He named it the hippocampus, borrowing from the Greek words hippos (horse) and kampos (sea monster).
The mythical side of the story is that in ancient Greek and Roman lore, hippocamps were the majestic, fish-tailed beasts that pulled the underwater chariot of Poseidon.
The biological side of the story is also the official scientific genus name for actual seahorses.
Despite finding it, early scientists had absolutely no idea what it did. For hundreds of years, anatomists mistakenly believed the hippocampus was responsible for your sense of smell (olfaction), simply because it sat near the brain’s sensory input pathways, but as it was eventually discovered, they couldn’t have been more wrong.
How the hippocampus is wired
The hippocampus is not a uniform blob; it is a highly sophisticated, multi-zoned machine often referred to as the hippocampal formation. Think of it as a factory with distinct assembly lines, the 3 main subregions include:
- Dentate Gyrus (DG) which is the entry gate. It receives raw data from the cerebral cortex, processes new information, and makes rapid-fire decisions to form spatial memories.
- Hippocampus Proper (Cornu Ammonis / CA) which is the core processor. Divided into four layers (CA1 through CA4), this complex relay network organises, stores, and retrieves memories.
- Subiculum which is the exit hub. Once a memory is processed, the subiculum gathers the data and fires it back to the cerebral cortex for permanent storage.
There is much more detail we can go into here, and this is just the tip of the iceberg, but that is for another article.
If you look at the hippocampus along its length, it divides into three functional zones:
- Anterior (the head) which regulates emotion and stress and chats directly with the amygdala (emotion and fear).
- Intermediate (the body) is the bridge that blends emotional processing with spacial memory.
- Posterior (the tail) which handles cognitive and spatial navigation and builds and individual’s mental maps.
The hippocampus is similarly named to the hypothalamus, and while they sound alike, the hypothalamus is an entirely separate structure located at the base of the brain that controls survival basics like body temperature, hunger, and sleep.
What does the hippocampus actually do?
The modern breakthrough in understanding the hippocampus didn’t happen until the mid-20th century, and it shattered several long-held scientific dogmas.
It’s a memory chef, not a filing cabinet
A common misconception is that your memories live inside the hippocampus. In reality, it acts like a printing press or a chef. It takes temporary, short-term sensations from your day, processes them, and then ships them off to the cerebral cortex to be hard-wired into long-term storage, (also known as ‘memory consolidation’).
This mostly happens while you sleep, which is exactly why pulling an all-nighter before an exam can backfire spectacularly and also why at times we feel like we have a sort of memory loss when we wake up.
The case of Patient H.M.
Much of what we know comes from Henry Molaison who is famous in medical literature as ‘Patient H.M.’.
In 1953, Henry underwent an experimental brain surgery to cure severe epilepsy, resulting in the removal of both of his hippocampi.
The surgery stopped his seizures, but it froze him in time. Henry lost the ability to form any new explicit memories (ie, facts, names, events). He could meet his doctor, step out of the room, return a minute later, and have no idea who she was. This is the same for reading a magazine, he could read the same articles over again, but to him each time he read them, was like the first.
Crucially, though, his intellect remained intact, and he could still form procedural memories (eg, motor skills). He could learn to play a new song on the piano through repetition, even though he would swear he had never seen the piano before. This proved definitively that the hippocampus is the gatekeeper for turning short-term experiences into permanent data, but different types of memory live in different parts of the brain.
The brain’s internal GPS
In 1971, landmark studies on rats revealed that the hippocampus contains ‘place cells’ that fire only when you are in a specific, recognisable location. It literally builds a cognitive map of your surroundings.
This was famously proven by the London taxi driver study.
To get licensed in London, drivers must memorise ‘The Knowledge’, an exhaustive labyrinth of 25,000 streets and thousands of landmarks. Even now, this is still a requirement even with the copious amounts of technology available.
Brain scans revealed that these drivers had significantly larger posterior hippocampi than the general public. What makes this topic more fascinating is that the longer a driver had been on the job, the larger that specific region of the brain had grown to accommodate their massive internal GPS.1
The shape-shifter: Stress vs. exercise
For a long time, mainstream science believed you were born with all the brain cells you would ever have. The hippocampus turned that theory completely upside down. It is one of the very few areas of the adult brain capable of neurogenesis, the creation of brand-new neurons.
Because it is constantly regenerating, it is incredibly sensitive to your lifestyle:
The bad news is that the hippocampus is highly vulnerable to cortisol (the stress hormone). Prolonged, chronic stress or trauma can completely halt neurogenesis and cause your hippocampus to physically shrink, leading to memory loss and disorientation.
The good news is that the hippocampus is profoundly resilient. Engaging in regular aerobic exercise stimulates neurogenesis, pumping out new brain cells, increasing the physical volume of your hippocampus, and sharpening your memory.
- This section aligns with the topic of neuroplasticity. More articles will be added in the June 2026 issue ↩︎