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updated 12/16/2010 12:51:54 PM ET 2010-12-16T17:51:54

A 44-year-old woman who doesn't experience fear has led to the discovery of where that fright factor lives in the human brain.

Researchers put out their best foot to try to scare the patient, who they refer to as "SM" in their write-up in the most recent issue of the journal Current Biology. Haunted houses, where monsters tried to evoke an avoidance reaction, instead evoked curiosity; spiders and snakes didn't do the trick; and a battery of scary film clips entertained SM.

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The patient has a rare condition called Urbach–Wiethe disease that has destroyed her amygdala, the almond-shaped structure located deep in the brain. Over the past 50 years studies have shown the amygdala plays a central role in generating fear responses in various animals from rats to monkeys.

The new study involving SM is the first to confirm that brain region is also responsible for experiencing fear in humans. "This is the first study to systematically investigate the experience or feeling of fear in humans with amygdala damage," lead author Justin Feinstein told LiveScience. 

The finding, the researchers say, could lead to treatments for post-traumatic stress disorder (PTSD) in soldiers and others. "My hope is to expand on this work and search for psychotherapy treatments that selectively target and dampen down hyperactivity in the amygdala of patients with PTSD," said Feinstein, who is a doctoral student studying clinical neuropsychology at the University of Iowa.

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Over the past year, Feinstein has been treating PTSD in veterans coming back from Iraq and Afghanistan, seeing first-hand the effects.

"Their lives are marred by fear, and they are oftentimes unable to even leave their home due to the ever-present feeling of danger," Feinstein said. In contrast, SM is immune to this stress. "Traumatic events leave no emotional imprint on her brain," he said.

Are you scared?
Previous studies with this patient revealed she can't recognize fear in facial expressions, but it was unknown if she had the ability to experience fear herself.

To find out, Feinstein and his colleagues measured the patient's experience of fear with several standardized questionnaires that probed different aspects of fear, ranging from the fear of death to the fear of public speaking. [ Fear of Spiders & 9 Other Phobias ]

In addition, for three months SM carried a computerized emotion diary that randomly asked her to rate her current fear level throughout the day. The diary also had her indicate emotions she was feeling from a list of 50 items. Her average score of fear was 0 percent, while for other emotions she showed normal functioning.

Across all of the scenarios, she showed no fear. Looking into her past, the researchers found lots of reasons for her to react with fear. In fact, she told them she didn't like snakes, but when brought into contact with the two characters, she was fearless.

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The good and bad of being fearless
Her eldest son (she has three children) in his early 20s recalls this instance: "Me and my brothers were playing in the yard and mom was outside sitting on the porch. All of a sudden we see this snake on the road. It was a one lane road, and seriously, it touched from one end of the yard all the way to the other side of the road. I was like, 'Holy cow, that's a big snake! ' Well mom just ran over there and picked it up and brought it out of the street, put it in the grass and let it go on its way…"

That's not all. She has been held up at knife point and at gun point, physically accosted by a woman twice her size, nearly killed in an act of domestic violence, and on more than one occasion explicitly threatened with death, the researchers wrote in the journal article. Police reports corroborated these experiences and revealed the poverty-stricken area where she lived. SM has never been convicted of a crime.

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"What stands out most is that, in many of these situations, SM's life was in danger, yet her behavior lacked any sense of desperation or urgency," the researchers wrote.

And when she was asked to recollect how she felt during those situations, SM said she didn't feel fear but did feel upset and angry about what happened. "Without fear, it can be said that SM's distress lacks the deep heartfelt intensity endured by most survivors of trauma," the researchers wrote.

Essentially, due to the amygdala damage the woman is "immune to the devastating effects of posttraumatic stress disorder," they wrote.

As always, there are tradeoffs as such an inability to detect and avoid threatening situations likely contributed to the frequency with which she's had life-threatening run-ins, the researchers suggest.

To firm up the phenomenon, Feinstein says studying other patients with damaged amygdalas would be great. "Unfortunately, such patients are so rare that it is nearly impossible to find them," he said, adding that there is much to be learned from a single patient.  

The National Institutes of Health and a National Science Foundation Graduate Fellowship provided funding for the study.

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You can followLiveScienceManaging Editor Jeanna Bryner on Twitter @jeannabryner.

© 2012 LiveScience.com. All rights reserved.

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