The hippocampus is a curved-shaped structure in the temporal lobe associated with learning and memory. The name being derived from the Greek words for ‘sea monster’ but is more commonly recognizable for being shaped like a seahorse.
The hippocampus is considered to be a part of the limbic system, a group of structures involved in the processing and regulating of emotions and memories.
The hippocampus, which is most strongly associated with the formation of memories, is an early storage place for new long-term memories and is involved in the transition of these long-term memories to more permanent memories.
There are two hippocampi in each hemisphere of the brain, located within the temporal lobe, just above each ear.
The hippocampus is one of the most studied areas of the brain and is also one of the few places where neurogenesis occurs, the process by which new neurons are produced.
Historically, the hippocampus was believed to be primarily involved in olfaction (sense of smell), falsely believed to be receiving direct input from the olfactory bulb.
However, there continues to be some interest in olfactory responses due to the hippocampus’ role in the memory of odors (e.g., a certain smell bringing up memories of a particular event from the past).
The hippocampus is comprised primarily of pyramidal cells, which are multipolar neurons that have excitatory and projective functions. This structure is also divided into different regions called fields named CA1, CA2, CA3, and CA4.
The hippocampus receives input and sends output to the rest of the brain via the entorhinal cortex. This is an area strongly and reciprocally connected with many other parts of the cerebral cortex and serves as an interface between the hippocampus and other parts of the brain.
The hippocampus receives input from regions such as the prefrontal cortex, anterior cingulate gyrus, and premammillary region of the brain, meaning it can respond to changes in these areas and send output.
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The hippocampus is important in the organization and storage of new memories, especially those which are declarative memories (e.g., memories relating to facts and events).
This area is also responsible for making memories stronger by connecting sensations and emotions to these memories. For instance, the hippocampus has links with, and is approximate to the amygdala, a structure associated with emotions, especially fear.
The amygdala can work with the hippocampus to associate emotions with new memories to strengthen them. Essentially, if memory has an element of fear attached to it, this makes it more likely to be remembered.
A famous case study from 1953 of a patient referred to as HM had portions of his medial temporal lobes (including the hippocampus) removed. They were removed in an attempt to cure his epileptic seizures.
It was found that HM’s cognitive functions remained intact, but after the procedure, he lost the ability to form new memories (also known as anterograde amnesia).
This case study strengthened the view that the hippocampus is essential in the creation of new memories.
Spatial Memory and Navigation
Another critical function of the hippocampus is its role in spatial navigation.
The rear part of the hippocampus is believed to be involved in processing spatial memories; this is so we can encode the environment around us and remember where everything is.
Within our spatial memories, we have a cognitive map, a mental representation containing information on relative locations in specific environments.
Through using cognitive maps, we can code, store, recall, and decode information about our environment, which can be used when navigating environments we are familiar with.
For instance, taxicab drivers would typically be considered to have extensive and detailed cognitive maps, so they can navigate around streets and landmarks without getting lost.
Maguire et al. (2000) studied 16 London taxi drivers and found an increase in the volume of grey matter in the posterior hippocampus compared to a control group. This area of the brain is involved in short-term memory and spatial navigation.
Transfer of Long-Term Memories
Although the hippocampus is involved in memory, long-term memories are not thought to be stored within this structure.
Instead, the hippocampus is believed to be a transfer center for long-term memories. this means it can take in memory information, register it, then temporarily store it before it is then transferred to long-term memory stores.
Sleep is thought to play a critical role in the process of this.
Information in the hippocampus circulates in this area whilst neurons start to encode this information through a process called long-term potentiation.
Long-term potentiation is a form of neural plasticity and is a vital mechanism in the storage of memory.
When this occurs in the hippocampus, the strongest of the circulating information then returns to the brain area where it originated from in order to turn the short-term memories into long-term memories.
Damage to the Hippocampus
If someone is experiencing dysfunction with their hippocampus, they may be experiencing one or more of the following symptoms:
- Memory loss – this can be anything from either mild to severe impairment
- Inability to form new memories, but memories from a long time ago are not affected
- Inability to remember directions
- Inability to remember locations that should be familiar
- Inability to recall words
- Inability to memorize new information such as memorizing a speech
It has been found that the classic symptoms of Alzheimer’s disease, including memory loss and disorientation, have been associated with a decline in hippocampal activity.
Other symptoms of early-onset of Alzheimer’s are a loss of short-term memory and difficulty following directions. As this condition develops, the volume of the hippocampus tends to reduce, and it becomes harder to function in daily life. Similarly, cell degeneration in the hippocampus has been linked to the onset of early Alzheimer’s (Aand & Dhikav, 2012).
Hippocampal weakening could simply result from aging, leading to this cognitive decline in older adults. There can, however, be other causes of weakening or dysfunction.
The hippocampus contains high levels of glucocorticoid receptors which makes it more susceptible to long-term stress. Long-term stress can harm the hippocampus and there is evidence that individuals who have experienced severe traumatic stress show atrophy of the hippocampus.
Damage to the hippocampus could also be the result of trauma to this area, oxygen starvation, stroke, or medial temporal lobe epilepsy. It is suggested that around 50-75% of people who have epilepsy have some damage to their hippocampus, implying that these individuals may have memory issues as a result.
There is evidence that links hippocampal atrophy to cognitive disorders such as depression, bipolar disorder, and schizophrenia (Weis et al., 2007; Sheline et al., 2002).
It has been found that those who have severe depression have reduced volumes of their hippocampus, causing it to shrink by up to 10% of the typical size (Videbech & Ravnkilde, 2004).
Similarly, the duration that someone has suffered from depression has been correlated with the severity of hippocampal atrophy. It is suggested that a reason for this could be that depression may be causing prolonged stress because of their depression.
Equally, hippocampal volume reduction has consistently been found to be a major finding in the brains of people who have schizophrenia. Functional and biochemical abnormalities have also been found in the hippocampus of these individuals (Anand & Dhikav, 2012).
Whilst conditions such as Alzheimer’s and schizophrenia cannot always be prevented, there are some lifestyle methods which could aid in strengthening the hippocampus.
For instance, it has been found that physical exercise is a way that can strengthen the hippocampus as it stimulates neurogenesis (Van Praag, Shubert, Zhao, & Gage, 2005).
Being able to manage stress can reduce the number of stress-related neurotransmitters being taken in by the hippocampus. Some simple methods could be to practice deep breathing exercises, mindfulness, or meditation.
As the hippocampus is more concerned with memory, completing memory-based activities can help to keep the hippocampus active.
Exercises such as trying to memorize lists of words or reading and writing could all help to keep the hippocampus working properly.
- Nordanskog et al., (2010) conducted research into the treatment of depression, with a focus on attempting to increase the hippocampal volumes of patients. The researchers used electroconvulsive therapy (ECT), a procedure in which small electric currents are passed through the brain, deliberately causing a brief seizure. ECT has found to highly effective for the relief of depression.
- After a series of ECT, there was a significant increase in the hippocampal volume of the patients. As there was an increase in volume, this could imply that the hippocampus may play a key role in the treatment of depression.
- Montagrin, Saiote & Schiller (2017) have proposed that the hippocampus may hold more than just cognitive maps about spatial information.
- They hypothesize that the hippocampus may be involved in forming social maps due to evidence suggesting the hippocampus’ role in supporting social memory, representing different dimensions of social space, tracking dynamic social behavior, and maintaining a flexible map allowing an adaption to new social contexts.
- The researchers suggest that we use social maps to guide us through social spaces in the same way as cognitive maps can help us physically navigate our way around the environment.
- Neuroscientists from the University of South California, one of which named Ted Berger have been developing an artificial hippocampus which may one day help people to stop losing their memories. Berger and his colleagues were able to mimic the structure of the hippocampus by stimulating the brain in a particular way to form memories in rats and monkeys.
- In a small pilot study on humans, people fitted with this implant had improved performance on memory tasks. This implant works by communicating with the brain via electrodes placed on either side of the hippocampus. If this research continues to be successful, it could help in stopping the onset of conditions affecting memory such as Alzheimer’s disease and dementia.
- Maguire et al. (2000) investigated whether London taxi drivers and non-London taxi drivers had differences in their brain. The experimental group was 16 male London taxi drivers and the control group was 50 MRI scans from right-handed males.
- MRI scans were done on the experimental group and compared with the control group. They found an increase in grey matter in the brains of taxi drivers.
- There was no difference in the size of the hippocampus, but the difference in shape was visible. They concluded that the hippocampus changes in line with demand on spatial memory which supports brain plasticity.
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