Dec. 16, 2010 -- Snakes, spiders, haunted houses and scary movies may evoke shrieks of fright in many people, but there's one woman who doesn't seem to be afraid of them -- or anything else.

She goes only by the initials S.M. to protect her identity and she is the subject of a study published in the latest edition of the journal Current Biology that looked at how a certain part of the brain experiences fear.

Neurology researchers at the University of Iowa in Iowa City have been studying the woman for a number of years because she is one of very few people known to have damage to the amygdala on both sides of her brain. The amygdala is an almond-shaped structure that studies have shown plays a role in processing fear and other emotions, though experts say its exact role is unclear.

The damage to S.M.'s amygdala was caused by another rare condition, Urbach-Wiethe disease, which caused calcium deposits to form and cause lesions on the amygdala.

"We wanted to know: Can such a patient experience fear normally in response to fear-inducing stimuli?" said Daniel Tranel, one of the study authors and professor of neurology and psychology at the University of Iowa.

To determine the impact of this rare condition on the fear response, Tranel and his colleagues exposed the woman to things that normally frighten people -- spiders, snakes, a place ranked as one of the "most haunted," and frightening film clips. They used questionnaires to assess whether she experienced the symptoms of fear over a three-year period and also asked her to rate the level of different emotions at random times for a three-month period.

S.M. reported little-to-no symptoms of fear on the questionnaires and also rated herself as fearless most often during the emotional experience sampling. However, she did experience other emotions, such as joy, happiness and sadness normally, the researchers said.

S.M. also had exposure to fearful situations in her past. She was held up at gunpoint and at knifepoint and was almost killed during a domestic incident. S.M. told researchers she did not feel fear during these life-threatening situations. She was also aware that her inability to react to fearful stimuli was not normal.

"It's very striking that she has only a rational response, not a physiological one," said Dr. Jon Shaw, professor of psychiatry at the University of Miami School of Medicine. "The body is not prepared for a physiological response because the amygdala has been taken out of the loop."

Tranel said this study, along with S.M.'s recollection of past incidents, show that the amygdala plays a vital role in how people respond to fear.

"This gives us, for the first time, really solid empirical evidence that the amygdala is critical for the induction and the experience of fear as triggered by visual and auditory stimuli," said Tranel.

Experts Disagree on Value of Findings

"If you look at past neuroimaging studies, there is clear evidence that the amygdala is extremely sensitive to fearful stimuli," said Ruben Gur, professor in the Departments of Psychiatry, Radiology & Neurology at the University of Pennsylvania School of Medicine in Philadelphia.

"There has been lots of work in animals and humans suggesting that the amygdala is critical to the fear response," said Dr. Dennis Charney, dean of the Mount Sinai School of Medicine in New York.

But another psychologist who has done extensive research on the amygdala and emotional responses said while the study is interesting, it's difficult to draw too many conclusions from one case study.

"You have to interpret case studies with caution since there's been contradictory work done," said Elizabeth Phelps, professor of psychology and neural science at New York University.

Among the most notable studies, she points out, is the one that found an unusual emotional response in monkeys whose brains were experimentally damaged, but only during specific stages of development.

She also made reference to the well-documented case of H.M., a patient who had part of his brain, including the amygdala, removed to treat severe epilepsy. His main problem, according to experts, was his memory loss.

"It's probably not the amygdala involved in all aspects of fear," said Phelps.

Implications for PTSD Treatment?

Despite the disagreement over the true impact of the amygdala on the fear response, experts say the findings could be valuable in terms of developing treatments for conditions like post-traumatic stress disorder (PTSD).

"[This] puts us in a position to design treatments to target that structure to treat conditions marked by pathological fear reactions," said Tranel. "Probably the most well-known such condition is PTSD."

"Understanding the amygdala is important to understanding PTSD," said Phelps.

Because S.M. seems to be immune to the experience of fear, the study authors say her unusual case strengthens the hypothesis that combat veterans with amygdala lesions do not experience the effects of PTSD.

S.M.'s case is also important to understand because the ability to experience fear and respond to it is essential to survival. The researchers believe that because she could not detect threats to her safety and avoid them, she wound up in numerous life-threatening situations.

"Indeed, it appears that without the amygdala, the evolutionary value of fear is lost," the authors wrote.