The Hippocampus: Definition, Function, & Anatomy

The word hippocampus means “seahorse” in Greek, which is what it’s shaped like. Like most things in the brain, the hippocampus is complex. It’s a structure consisting of several subparts and multiple layers, and it connects to several other areas of the brain (Dhikav & Anand, 2012). These connections are what allow your short-term memories to be stored in other parts of the brain to create long-term memories that you can retrieve later.

Hippocampus Location

The brain is divided into four sections, called lobes. The temporal lobe is the second largest lobe, and it sits behind the ears. The hippocampus is located deep in the temporal lobe, toward the middle of the brain. There are actually two hippocampi, one on each side of the temporal lobe. But they’re usually referred to in the singular form of hippocampus. 

Hippocampus Function
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The hippocampus plays important roles in learning, spatial navigation, and emotions. But it’s best known for the formation, organization, and storage of long-term memories, especially episodic memories, which are the events and experiences of your life. 

Another significant function of the hippocampus is spatial recognition. This gives you the ability to navigate your environment. It allows you to determine where your body is in relation to objects around you and to move around effectively. The hippocampus does this by translating what you see into a kind of mental map (Lisman et al., 2017). This process demonstrates its connection to visual areas of the brain (Voss et al., 2017).  

Along with other parts of the brain, the hippocampus plays a significant role in emotional processing (Zhu et al., 2019). This includes impulse control (Noble et al., 2019), which we’ll get into further when we talk about the hippocampus’s connection to other brain areas.

Considering the functions described above, it seems pretty clear why the hippocampus is important. Memory is involved in almost every aspect of our lives. This includes not only remembering factual information like your phone number and information about the world, but also the people in your life and your experiences.

Perhaps the most important role of the hippocampus is the generation of episodic memories (Lisman et al., 2017). Episodic memory is a type of long-term memory that allows you to recall personal past events and experiences. It works by tying together the spatial, emotional, and mental elements of experiences to form these rich memories. Studies suggest that if a memory involves vivid imagery with lots of sensory input (sights, sounds, smells), it’s dependent on the hippocampus (Bird & Burgess, 2008). These types of memories are a huge part of what makes you who you are. 

In addition, the hippocampus’s contribution to emotion regulation is important for emotional well-being (Zhu et al., 2019). Trouble regulating emotions contributes to anxiety and depression. The hippocampus also plays a role in impulse control. And lack of impulse control typically leads to nothing good—overeating, excessive gambling, and drug and alcohol misuse (Noble et al., 2019).

Anatomy of the Hippocampus 

The hippocampus is composed of densely packed neurons. Visually, the hippocampus can be pictured as being divided into head, body, and tail, like a seahorse. More technically, it has three distinct zones—the hippocampus proper, the dentate gyrus, and the subiculum. Each of these is composed of several substructures or layers. (As mentioned earlier, it’s a complex structure.) So without getting too into technical names, the basic structure is that the hippocampus proper and the dentate gyrus together form two C-shaped rings and the subiculum links them together. You may see the whole thing referred to as the hippocampus formation.

There are two types of long-term memory: declarative (or explicit), and nondeclarative (or implicit). Declarative memory is the “what” of memory. It includes facts, dates, your phone number, what you ate yesterday, and experiences (episodic memories). Implicit memory is the “how” of memory. This is when you just know how to do something without having to think about it. This includes things like riding a bike, reading, walking, and getting dressed.  

The hippocampus is essential for creating declarative memories—the “what” part. It turns short-term memories into long-term memories. It “shifts” memories to other parts of the brain for long-term storage. Then when you want to retrieve a memory, it’s the hippocampus that activates those long-term storage areas. 

Research also suggests that the hippocampus reduces memory “interference” (Britannica, 2023). For example, it’s what allows you to remember where you parked your car today rather than yesterday. (Although I’ve made that mistake, so maybe mine needs a tuneup.)

The word amygdala is Greek for “almond,” and like the hippocampus, it’s named for its shape. It sits right next to the hippocampus. It’s responsible for emotions, especially strong emotions like fear, anxiety, and pleasure. The amygdala is what triggers the “fight or flight” response. 

Our episodic memories are often associated with emotions. When our brain creates new memories, the amygdala works with the hippocampus to connect the emotional aspects of an experience to the specifics of what happened. So these two brain structures work together to recall not only the facts of an event but also the emotional flavor. This is why emotional memories are so strong and easy to recall. Everyone can relate to recalling an experience and feeling the same surge of emotions that we felt at the time it happened. It can feel as if you’re experiencing it all over again.
 
Hippocampus vs. Hypothalamus

While the hippocampus is primarily involved in memory and emotions, the hypothalamus regulates involuntary functions such as temperature, breathing, and heart rate. Its job is to keep the body in balance. It also controls impulses such as eating, sleeping, and sex. It does this by releasing hormones that send messages to these other bodily systems. The hypothalamus also plays a role in emotions and survival behaviors. It communicates with the amygdala when something is perceived as scary or threatening. And since an upsetting memory can also trigger changes in automatic functions like increasing breathing or heart rate, we can see how the hippocampus and the hypothalamus can work together. ​

People with damage to the hippocampus can’t store new long-term memories. Studies of people with hippocampal damage show that while they can remember things that were already in long-term memory before the damage, they can’t create new long-term memories.
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If you’ve seen the movie “Momento,” then you have an idea of what it’s like to live with a damaged hippocampus. The main character, Leonard, can remember experiences that happened years ago, but he’s unable to retain recent memories. Parts of the film are shown in reverse sequence to give the audience a taste of what Leonard experiences. Desperate to remember his mission (to take revenge on his wife’s murderer), he writes himself notes and even gets reminder tattoos. But every time he sees them, it’s a mystery that he has to figure out all over again.

The role of the hippocampus in memory is dramatically shown in the real-life case study of a patient named Henry Molaison (also referred to as HM). After an accident, he suffered from terrible and frequent seizures. When nothing else worked, a neurosurgeon removed his hippocampus. Though his seizures stopped, he was unable to form new episodic memories. Interestingly, he could still learn how to do new things, which are nondeclarative memories. He just couldn’t remember the experience of the learning process. Check out this video for more:

Let’s define neurogenesis. Neurons are the cells that make up the nervous system, and neurogenesis is the biological process that creates new neurons (Baptista & Andrade, 2018). 

Until the 1990s, researchers didn’t believe neurogenesis occurred in adult brains (Baptista & Andrade, 2018). However, studies using new techniques now show that neurogenesis happens throughout our lives. And the hippocampus is one of only a few areas capable of generating new brain cells. 

There is a lot of excitement around the discovery that the hippocampus is responsible for long-term memory, because it can lead to new and better treatments for Alzheimer’s disease and other forms of dementia. For anyone that has witnessed the debilitating effects of dementia, which I did with both my parents, this is especially compelling. 

Unfortunately, our rate of neurogenesis naturally declines with age. But there are things you can do to improve it. Studies suggest exercise, a healthy diet rich in specific foods, reducing stress, regular sleep, avoiding alcohol, and learning new things can reduce age-related decline in neurogenesis (Kuhn et al., 2018).

Everyone’s brain shrinks a bit as we age. But depression and Alzheimer’s disease are linked to a higher-than-normal shrinkage rate, especially in the hippocampus (Erickson et al., 2011). This is why memory loss is one of the first symptoms seen in Alzheimer’s patients. 

Amyloid plaques, a type of protein in the brain associated with Alzheimer’s disease, are believed to be the main cause of the loss of volume of the hippocampus in these patients. But there’s debate about whether Alzheimer’s causes amyloid plaques or vice versa.

Research also suggests that the hippocampus can shrink as a result of years of persistent depression, anxiety, or PTSD (Sapolsky, 2001). These conditions trigger the release of stress hormones such as cortisol, which can have harmful effects on the brain as well as other systems in the body. 

Stress has also been linked to decreases in neurogenesis, which also leads to the shrinking of the hippocampus. Some researchers even argue that depression and stress are risk factors for Alzheimer’s disease due to their effects on the hippocampus (Sheline, 2011). So stress reduction techniques such as meditation, exercise, good sleeping habits, and social connection are important for brain health.

As stated above, depression is linked to reduced volume of the hippocampus. Studies report the hippocampus showed as much as a 20% loss in volume among those suffering from depression, and longer depression was associated with more shrinkage (Sapolsky, 2001).

Most researchers believe that depression leads to the decreased size of the hippocampus rather than the other way around. But depression often has underlying biological causes such as a chemical imbalance in the brain. In addition, stress is often associated with depression, so again, reducing stress can help. 

Can the shrinkage be reversed? The good news is, some promising studies indicate that aerobic exercise and cognitive stimulation can slow down the shrinking and possibly even reverse it. Also, antidepressant drugs have been shown to prevent hippocampal shrinking and may increase volume (Sapolsky, 2001). And because depression has been linked to decreased neurogenesis, following lifestyle advice that helps to increase neurogenesis (reducing stress, exercise, a healthy diet, getting enough sleep, learning, and exposure to rich environments) also helps to alleviate depression.

​Want to learn more? Check out these articles:

• Neuroscience: Definition, History, & Examples
• The Cerebellum: Function, Location, & Anatomy​
• ​The Amygdala: Definition, Function, & Location
• ​Areas of The Brain: Definition, Function, & Development
• ​Left Brain vs Right Brain: Definition, Theory, & Differences
• ​Phineas Gage: History, Facts, & Importance in Psychology​​​​​​​​​​​​​

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