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Amygdala Damage Can Eliminate All Fear, Study Suggests: What's an Amygdala?

african bush viper, snake, isockphoto, 4x3
Venonous snakes like this viper trigger no fear in 44-year-old "SM" (istockphoto) istockphoto

(CBS/AP) Most people are afraid of something. But "SM," a 44-year-old mother of three, knows no fear. Literally. And doctors say it's all because a disease damaged her amygdala (uh-MIG'-duh-luh). That's a part of the brain believed to process fear in the face of scary situations.

Snakes? No problem for SM. Ditto for scary movies, like the "The Blair Witch Project," "The Shining," and "Arachnophobia." When she visited a haunted house, it was a monster who was afraid of her.

A study of SM's fearlessness was published online Thursday in "Current Biology" by Justin Feinstein of the University of Iowa. Feinstein declined to make SM available for an interview with The Associated Press, citing lab policy about confidentiality.

SM scores normally on tests of intelligence, memory and language, and she experiences emotions other than fear. But because of her lack of fear, she has been studied for more than 20 years.

In an experiment published in 1995, she was blasted with a loud horn every time she saw a blue-colored square appear on a screen. Despite the repeated horn, she never developed the fear an ordinary person would feel when seeing the blue square.

SM recalls being afraid as a child, like the time she was cornered by a snarling Doberman pinscher. But maybe that was before the disease wiped out the amygdala on both sides of her brain, researchers say.

She apparently hasn't felt fear as an adult, not even 15 years ago in an incident described by the researchers: a man jumped up from a park bench, pressed a knife to her throat and hissed, "I'm going to cut you."

SM, who heard a church choir practicing in the distance, looked coolly at him and replied, "If you're going to kill me, you're going to have to go through my God's angels first."

The man let her go. She didn't run home. She walked.

But if her lack of fear helped her get out of that scrape, it might also have gotten her into it in the first place. SM had willingly approached the man when he asked her to, even though it was late at night and she was alone, and even though she thought he looked "drugged out."

Given how many other dangerous situations SM has walked into, Feinstein considered SM's predicament and offered a frank appraisal: It's remarkable she's still alive.

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