"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.
But another psychologist who has done extensive research on the amygdala and emotional responses said while the study is interesting, it's difficult to draw too many conclusions from one case study.
"You have to interpret case studies with caution since there's been contradictory work done," said Elizabeth Phelps, professor of psychology and neural science at New York University.
Among the most notable studies, she points out, is the one that found an unusual emotional response in monkeys whose brains were experimentally damaged, but only during specific stages of development.
She also made reference to the well-documented case of H.M., a patient who had part of his brain, including the amygdala, removed to treat severe epilepsy. His main problem, according to experts, was his memory loss.
"It's probably not the amygdala involved in all aspects of fear," said Phelps.
Despite the disagreement over the true impact of the amygdala on the fear response, experts say the findings could be valuable in terms of developing treatments for conditions like post-traumatic stress disorder (PTSD).
"[This] puts us in a position to design treatments to target that structure to treat conditions marked by pathological fear reactions," said Tranel. "Probably the most well-known such condition is PTSD."
"Understanding the amygdala is important to understanding PTSD," said Phelps.
Because S.M. seems to be immune to the experience of fear, the study authors say her unusual case strengthens the hypothesis that combat veterans with amygdala lesions do not experience the effects of PTSD.
S.M.'s case is also important to understand because the ability to experience fear and respond to it is essential to survival. The researchers believe that because she could not detect threats to her safety and avoid them, she wound up in numerous life-threatening situations.
"Indeed, it appears that without the amygdala, the evolutionary value of fear is lost," the authors wrote.