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updated 12/16/2010 4:45:16 PM ET 2010-12-16T21:45:16

A woman who feels no fear is teaching scientists about the connections between our brains and our behaviors, and offering hope for people whose lives are destroyed by trauma.

In a new study, researchers exposed the unusual woman -- well known to science as SM -- to as many scary situations they could come up with. From an infamously fright-filled haunted house to the spider section of a pet store, SM, who completely lacks a brain structure called the amygdala, failed to show any signs of being afraid.

Instead, she held a snake, pet its scales and touched its tongue. She enjoyed intensely scary horror movies like they were thrilling but fun roller coaster rides. And she revealed through her stories and her diary a lifetime of behaviors that would strike most people as odd and qualify, by all standards, as dangerous.

Once, while SM was walking through an urban park at night, a man yelled at her to come over to the bench where he was sitting. Instead of bolting or freezing with fear, as most people would, SM walked over to him. When he then pulled out a knife and held it to her throat, she remained calm.

"She didn't freak out like anyone else would with a knife held to her throat by a drugged-out man," said Justin Feinstein, a clinical neuropsychologist at The University of Iowa in Iowa City.

"In the background, there was a nearby church choir finishing up practice," he continued. "She looks at the man and ever so gently says, 'If you're going to kill me, you're gonna have to go through my God's angels first.' This freaks the man out and he lets her go. The next day, she goes back on her walk through the park."

That wasn't the only occasion that SM, who was living in the projects in a seedy part of town, got herself into a precarious situation. There was the time, for example, that a man approached her from behind, put a gun to her head, yelled "Boom!" and then ran away. She figured her attacker was a drug dealer whom she had repeatedly turned in to the police, but she didn't consider calling the cops after the incident.

"She described the situation as strange," Feinstein said. "She's not reacting in normal ways that anyone who had fear would."

The amygdala, Feinstein and colleagues suspect, is at the root of SM's odd behavior. Now 44 and a mother of three, SM has a rare, congenital disorder that began to destroy her amygdala in childhood. She remembers being afraid of the dark when she was little. But by the time she was a pre-teen, the brain structure was completely dysfunctional.

Animal studies have clearly linked the amygdala with fear. Monkeys that lack the structure, for example, will approach and touch snakes, even though normal monkeys recoil from snakes in fear. Amygdala-less mice, likewise, do not learn to fear a sound after being repeatedly exposed to a shock upon hearing that sound.

Even human studies, including some 20 years of research on SM, have shown that amygdala damage makes it hard for people to do things like recognize fear in faces, though SM can use words like "terror" and "panic" appropriately in conversation and her other emotions appear unharmed.

Feinstein and colleagues wanted to look beyond simple behaviors and reactions into the deeper and underlying feeling of fear that no animal but humans can express.

For the new study, they put SM through a battery of experiences and questionnaires. They analyzed her reactions, descriptions and diary entries. And in a paper published today in the journal Current Biology, they describe their complete failure to induce in her any sense of fright.

In a notoriously scary haunted house, she laughed at monsters while her companions screamed with fear. She even scared one of the frightening figures when she touched it out of curiosity. In the same way, she was compulsively drawn to the snakes in a pet store, even though she claimed to hate the animals and knew they were dangerous.

The findings offer yet more confirmation that the amygdala is at the root of fear, said Stephan Hamann, a psychologist at Emory University in Atlanta.

"It's the first time there's been a systematic study that has really documented this link with one subject," he said, adding that there is still a possibility that other parts of the brain might be involved in fear, too.

"It's a provocative case study, and it has all of the plusses and minuses of a case study," he said. "What this really shows is that this is a really important issue that needs to be followed up in larger studies with other patients."

SM's immunity to fear also suggests a tantalizing line of research that might some day help people with post-traumatic stress. One study found that soldiers who had brain damage in the amygdala had a lower risk of developing PTSD. Together with the new work, the findings suggest that the amygdala might be a good target for treatment.

"We don't suggest by any means to remove this area of the brain," Feinstein said. "We don't want a million SM's walking around getting in trouble. This one case highlights why we do need an amygdala."

Instead, it's possible that drugs or behavioral methods could quiet the amygdala just enough to free people whose lives have been taken over by fear.

© 2012 Discovery Channel

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