The hippocampus is a seahorse shaped organ that sits on the underside of each temporal lobe — the part of the brain near our ears. The hippocampus is a small but important part of the brain that's responsible for storing memories, learning and navigation.

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What does the hippocampus do?

The hippocampus is a heavily investigated part of the brain, but it wasn't until the 1950s that scientists were truly able to grasp what role it plays. In 1953, Henry Molaison consented to an experimental procedure allowing doctors to surgically remove his hippocampus and neighboring regions to address his epilepsy.

The surgery stopped his seizures, but caused Molaison to develop a form of amnesia. He was able to form fresh memories but they lasted minutes, and he could no longer permanently store new information, according to neuroscientist Larry R. Squire's 2009 review of Molaison's case, published in the journal Neuron.

Molaison described his state as "like waking from a dream … every day is alone in itself," Squire wrote. All Molaison could recall were events that occurred years before his surgery. Still, he did eventually improve his performance on certain motor tasks, such as the ability to draw a shape reflected in a mirror even though he had no recollection of ever having done it before.

Molaison's case provided the first scientific evidence that there are multiple types of memory, and that the hippocampus acts in concert with other regions of the brain to encode and store memories. (Molaison lived for another 55 years after his surgery.)

Data on Molaison, who went on to become the most intensively studied subject in neuroscience, alongside other patients with varying degrees of hippocampal damage (triggered by accident or disease), convinced scientists that the hippocampus plays a key role in memory. However, according to a 2009 review published in the journal Nature, scientists are still unsure what, exactly, that role is.

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Research suggests that the hippocampus might store spatial information, and act as an internal GPS — key to remembering where you have been and how to get to where you want to be. Studies on rats have found that an intact hippocampus is required for initial spatial awareness and long-term retention of certain spatial memory tasks, in particular those that require finding the way to a hidden goal.

Human studies indicate that the hippocampus plays a role in finding shortcuts and new routes between familiar places. Researchers from University College London compared MRI scans of London's iconic black cab drivers (notorious for their extensive navigation experience) to control subjects who weren't taxi drivers. The study, published in 2000 in the journal Proceedings of the National Academy of Sciences, found that part of the hippocampus was larger in the taxi drivers versus the control group, and that more experienced drivers had bigger hippocampus organs.

"This increased volume was attributed to them having more neurons in this area of the brain," said Amy Reichelt, a neuroscientist at the University of Adelaide, Australia, who was not associated with the study.

How the hippocampus directs behavior and emotion

The hippocampus is not only implicated in memory, navigation and learning, but can also affect mood and behavior.

Stress itself can impact the hippocampus, and in turn, our demeanor, Reichelt said. "If we're getting any sort of oxidative stress build up — that can start to damage the function of the neurons in the hippocampus, and then lead to forgetting things," she said. And that can lead to frustration or other changes in mood, she added.

Because part of the hippocampus is connected to the amygdala — an almond-shaped region of the brain central to processing fear and other emotions — it, too, is believed to be involved in emotion processing.

Animal data suggest that the hippocampus is one of the few areas of the brain where new nerve cells are generated, even in adulthood, according to a 2011 review published in the journal Neuron. Animal research has also shown that promoting the proliferation of neurons within the hippocampus can improve mood, and these effects might also be reflected in humans, Reichelt said.

Although research suggests that antidepressants act on the serotonin system, they might also be increasing the ability of the brain to modify its connections or rewire itself in areas, such as in the hippocampus. Antidepressants take roughly four weeks to kick in, which coincides with the approximate period of how long it takes for new neurons to integrate into the hippocampus properly, Reichelt said.

What happens when the hippocampus is damaged?

In patients with Alzheimer's disease, one of the first things to falter is the ability to make new memories because of the gradual decrease in size of the hippocampus, according to a 2012 review published in the journal Annals of Indian Academy of Neurology. The gradual decline in size and function of this part of the brain is also associated with a string of other severe mental illnesses, such as depression, schizophrenia and epilepsy.

According to Epilepsy Research UK, hippocampus damage has been observed in 50-75% of patients with epilepsy who had autopsies, but it is not yet clear that the damage is a cause or consequence of recurrent seizures.

In general, the hippocampus is a particularly vulnerable part of the brain and can be adversely affected by many different conditions, including long-term exposure to high levels of stress, or head injury, the 2012 review concluded.

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How to promote a healthy hippocampus

The best way to stimulate the hippocampus — and improve our memory — is exercise.

Physical aerobic exercise increases blood flow to the brain — but it also stimulates the birth of new neurons, as does stimulating the brain by engaging crossword puzzles or games such as chess or sudoku.

Reichelt's research suggests that high fat and high sugar foods also have a rapid, detrimental inflammatory impact on the hippocampus. Eating a healthy diet is key, she said; a diet that is high in anti-inflammatory and antioxidant rich foods, including blueberries, leafy green vegetables, fatty fish, and spices, such as turmeric.

"I think that it's critical that we eat well, and … stay active," she said, adding that while memory declines with age, a healthy lifestyle can help mitigate that decline, although it cannot completely neutralize it.

Continuing research

In order to robustly address problems like memory and cognitive decline, scientists must understand the brain as a whole — a significant challenge, given the organ and its machinations remain an enigma even to those who have long been involved in unpacking its complexity.

One key question scientists are still trying to understand is how memories are formed. Scientists are using new techniques to look at the specific cell types involved in forming memories, Reichelt said, highlighting an approach called optogenetics that uses specific wavelengths of light to switch off key sets of neurons in the hippocampus and related brain structures with high precision.

In a lab setting, this technique is used to switch off neurons in a rat during an event that would normally be remembered. By switching off some neurons, researchers can identify which sets of neurons are imperative for memory encoding.

"By delving deeper into the activity of types of neurons, and the brain circuitry they work with, scientists have a greater understanding of how the hippocampus works," she said.

"But the brain … itself is just such a complex organ — it's a mystery to so many of us and we're still just really scratching the surface."

Additional resources

Watch a short video about the hippocampus from Neuroscientifically Challenged.Learn more about what happens in the hippocampus in this explainer published by The Conversation. Read more about Henry Molaison in "Patient H.M.: A Story of Memory, Madness, and Family Secrets" (Random House, 2017) by Luke Dittrich.