Imagine living without fear - and I mean any fear. That's what happened to a woman called S.M. whose amygdala was damaged by a disease.

She's the fascinating subject of a medical investigation on the brain's response to fearful stimuli:

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."

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