The Effect of PTSD on the Brain

The Size of the Hippocampus Differs Between People With and Without PTSD

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Advances in medical technology such as magnetic resonance imaging (MRI), have allowed us to better understand the role the brain may play in different mental disorders, such as PTSD (posttraumatic stress disorder). Researchers have focused specific attention on the hippocampus in cases of PTSD.

What is the Hippocampus?

The hippocampus is a part of the limbic system of the brain. The limbic system describes a group of brain structures that surround the brain stem.

The brain structures that make up the limbic system play a major role in experiencing certain emotions (fear and anger), motivations and memory.

The hippocampus is responsible for the ability to store and retrieve memories. People who have experienced some kind of damage to their hippocampus experience difficulties in or the complete inability to store and recall information. Along with other limbic structures, the hippocampus also plays a role in a person's ability to overcome fear responses.

Why Should We Look at the Hippocampus in PTSD?

Many people with PTSD experience memory-related difficulties. They may have difficulty recalling certain parts of their ​traumatic event, alternatively, memories may be vivid and always present. People with PTSD may also have problems overcoming their fear response to thoughts, memories or situations that are reminiscent of their traumatic event. Due to the hippocampus' role in memory and emotional experience, it is thought that some of the problems people with PTSD experience may lie in the hippocampus.

How Might PTSD Affect the Hippocampus?

There are some studies which suggest that the constant experience of stress may actually damage the hippocampus. When we experience stress, the body releases a hormone called cortisol, which is helpful in mobilizing the body to respond to a stressful event. Some animal studies, though, show that high levels of cortisol may damage or destroy cells in the hippocampus.

Researchers have also looked at the size of the hippocampus in people with and without PTSD. They have found that people who have severe, chronic cases of PTSD have smaller hippocampi. The researchers have taken this to suggest that experiencing constant stress as a result of severe and chronic PTSD may ultimately damage the hippocampus, making it smaller.

Is There Another Possibility?

Not everyone who experiences a traumatic event develops PTSD. Therefore, it has also been proposed that the hippocampus may play a role in determining who is at risk for developing PTSD. Specifically, it is possible that having a smaller hippocampus may be a sign that a person is vulnerable to developing a severe case of PTSD following experiencing a traumatic event. Some people may be born with a smaller hippocampus, which could interfere with their ability to recover from a traumatic experience, putting them at risk for developing PTSD.

To examine this, one study looked at monozygotic twins, often referred to as "identical twins," where one twin had been exposed to a traumatic event (combat) and the other had not.

Since they share the same genes, studying monozygotic twins can tell us about the influence of genetics on developing certain conditions. For example, in this case, if the person who developed PTSD has a smaller hippocampus and has a non-trauma exposed twin who has a smaller hippocampus, it would suggest that a smaller hippocampus may be a sign of a genetic vulnerability for developing PTSD following a traumatic experience.

In fact, this is exactly what they found. People with severe PTSD had a smaller hippocampus, and they also had a non-trauma exposed twin with a smaller hippocampus. Consequently, a smaller hippocampus may be a sign that a person is vulnerable or more likely to develop PTSD after a traumatic experience.

Of course, it is important to remember that twins often share the same environment growing up, so it is difficult to tease apart the role nature versus nurture plays in the size of a person's hippocampus. So the verdict is still out on the true relationship between the hippocampus and PTSD.

How Can This Information Be Used?

There is still a lot more to learn about the role certain parts of the brain play in PTSD. Knowing how PTSD affects the brain (and vice versa), however, is very important to study. Understanding which parts of the brain may impact PTSD can lead to developing better, more effective medications for treating PTSD. In addition, this information may also help us better identify who is at-risk for developing PTSD following experiencing a traumatic event, leading to better ways of preventing PTSD.


Kolassa, I.T., & Elbert, T. (2007). Structural and functional neuroplasticity in relation to traumatic stress. Current Directions in Psychological Science, 16, 321-325.

Wingenfeld K, Wolf OT. Stress, memory, and the Hippocampus. In: The Hippocampus in Clinical Neuroscience. S. Karger AG; 2014:109–120.

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