Is the amygdala really the brain’s fear center?
What would it be like to live a life without fear? What if it were possible to stand face to face with poisonous reptiles without even the slightest flutter in your chest? How would it feel to giggle in the face of monsters, both real and imaginary?
These questions are not hypothetical for the famous patient S.M., a middle-aged woman who is neurologically unable to feel fear.
Living without fear
S.M. was born with a rare genetic disorder known as Urbach-Wiethe disease. Although the most noticeable symptoms of Urbach-Wiethe disease are dermatological – for example, skin lesions, wrinkles and papules around the eyes – the majority of patients also experience neurological symptoms. These neurological symptoms arise when calcium deposits build up in the blood vessels that nourish the medial temporal lobe of the brain. Eventually, these blood vessels are blocked completely, resulting in profound brain damage that can provoke symptoms that resemble epilepsy or even schizophrenia.
This brain damage can also cause fearlessness.
Due to extensive bilateral damage to her amygdala, S.M. has not experienced fear since she was a child. Researchers exposed S.M. to frightening stimuli including horror movies and a haunted house. They also introduced her to a number of poisonous snakes, only to find that S.M. was fascinated by them rather than not afraid of them even though she knew intellectually that the animals were dangerous.
S.M. was not simply a horror movie buff with an enthusiasm for snakes– she also demonstrated fearlessness in genuine life-threatening situations, seemingly immune to all forms of panic.
The amygdala as the “fear center”
Located within the medial temporal lobe, the amygdala is a small, almond-shaped brain region that is informally known as the brain’s “fear center.” When the brain perceives a threat, the amygdala stimulates autonomic functions, such as increased heart rate, sweaty palms and rapid breathing. Increased heart rate and respiration can be excellent in the face of danger – the person is better equipped to either fight or run away – but the same response could prove excessive in a casual office setting or any other safe environment. People with higher anxiety levels have a hyperactive amygdala when compared with people with lower anxiety levels.
Because S.M. received damage to both her left and right amygdala, her ability to respond to stressful situations with fear was almost completely squashed. The key word, however, is “almost.” In fact, people with no amygdala can still feel fear under certain circumstances.
In a 2013 study published in Nature Neuroscience, researchers examined how S.M. and two other patients with Urbach-Wiethe disease responded to inhalation of carbon dioxide. When too much carbon dioxide enters the bloodstream, the brain receives a warning signal – excessive carbon dioxide could indicate suffocation. This can provoke fear or even panic in healthy patients, presumably because their amygdala is able to sense the increase in carbon dioxide. The researchers theorized that because the patients had received damage to their amygdala, they would be unable to experience this fear.
When the three patients inhaled carbon dioxide, they immediately experienced extreme panic. In fact, they panicked significantly more than healthy people who had been exposed to the same task.
In this particular case, it seemed as though having a damaged amygdala actually caused more fear.
What does this mean?
Researchers assumed that people with anxiety had greater activity in their amygdala because it was over-responsive to fear, but what if anxious people have a hyperactive amygdala because the amygdala is trying to keep their fear from getting even worse?
Although it might be tempting to call the amygdala the “fear center,” the brain is a highly complex organ that likely has many neural pathways that contribute to the fear response. As demonstrated in the study with carbon dioxide, a healthy and intact amygdala might even protect against panic, since people without an amygdala panicked after inhaling carbon dioxide even more than their neurologically typical peers. The story of the amygdala is a complicated one and it will probably only get more complicated as research continues to explore this enigmatic region. Only the future knows whether or not defeating the amygdala is the key to defeating fear.
Sovereign Health Group recognizes that mental illness and substance addiction come from one place: the brain. Sovereign follows cutting edge neuroscience research to learn how to better serve its patients. For further questions about our treatment programs for mental health disorders, addiction and dual diagnosis, please contact us by phone 888-530-4614 or through our online chat.
Written by Courtney Lopresti, M.S. neuroscience, Sovereign Health Group writer