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.
"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.