How Does The Brain Process Fear?
February 5, 2013

Different Areas Of The Brain Responsible For External Versus Internal Threats

Connie K. Ho for — Your Universe Online

A new study from the University of Iowa (UI) recently found the amygdala, an almond-shaped area of the brain, is not the only part of the brain managing fear; other regions, such as the brainstem, the insular cortex and the diencephalon, also play a role.

“This research says panic, or intense fear, is induced somewhere outside of the amygdala,” explained the senior author of the study John Wemmie, who serves as an associate professor of psychiatry at the UI, in a prepared statement. “This could be a fundamental part of explaining why people have panic attacks.”

The researchers believe the findings of the new pathways could help develop treatments for panic attacks, post-traumatic stress syndrome, and other conditions associated with anxiety that may be initiated by internal emotional triggers.

“Our findings can shed light on how a normal response can lead to a disorder, and also on potential treatment mechanisms,” noted Daniel Tranel, who serves as a professor of neurology and psychology at UI, and a corresponding author on the paper, in the statement.

The paper included an anecdote of a female patient in her 40s (known as SM) who suffered from Urbach-Wiethe disease that had significant damage to her amygdala. Interestingly enough, she felt no fear when encountering horror movies, haunted houses, snakes, spiders, and other external threats. However, the researchers never had the opportunity to study her response to internal threats. As a result, the scientists decided to test SM and two other patients whose amygdala was damaged. The participants were requested to inhale a gas mixture that included 35 percent carbon dioxide, which would cause a momentary panic in the patients for approximately 30 seconds to a minute. When the patients breathed in the gas, they demonstrated a panic-stricken response, where they were gasping for air and showing distress. Feeling a boost in their heart rate, they attempted to remove the inhalation masks.

“They were scared for their lives,” commented the study´s first author Justin Feinstein, a clinical neuropsychologist who received his doctorate at UI in 2012, in the statement.

Researcher Wemmie had expected to find a pattern in the brain where the amygdala could directly sense the carbon dioxide, as he had noticed the same behavior in a previous study on mice conducted in 2009.

"We were completely surprised when the patients had a panic attack,” continued Wemmie in the statement.

On the other hand, three of the 12 healthy participants panicked, which is akin to the number of adults who do not have a past of panic attacks. Unlike the healthy patients who began sweating and feeling quickening heart rates prior to the inhalation of carbon dioxide, the patients whose amygdala was damaged had no fear prior to the test. The researchers believe this showcases the amygdala´s ability to detect danger in an external environment and physiologically prepare the organism to face the threat head on.

“Information from the outside world gets filtered through the amygdala in order to generate fear,” noted Feinstein in the statement. “On the other hand, signs of danger arising from inside the body can provoke a very primal form of fear, even in the absence of a functioning amygdala.”

Outside researchers believe the study provides useful information.

"This is a novel and important paper," Joseph LeDoux, a professor of neuroscience and psychology at New York University, told The Guardian. "It adds to a growing body of work showing that there are different systems for responses to different kinds of threats. The mechanisms that produce reactions to threats are different from those that generate fearful feelings. There is lots of evidence that the amygdala contributes to fear responses, but very little evidence that it generates the conscious experience of fear."

The findings of the study were recently featured in the journal Nature Neuroscience.