A woman with a rare genetic disorder, Urbach-Wiethe disease, isn't frightened by anything – haunted houses, spiders, snakes, movie monsters, death threats, being attacked or robbed, the Live Science website reports.
Researchers at the University of Iowa have done their best to scare the 44-year-old woman, identified only as "SM" for confidentiality reasons.
Writing in the journal Current Biology, study co-author Justin Feinstein indicates that SM's Urbach-Wiethe condition has damaged her amygdala, an almond-shaped portion of the brain that may be connected with "abnormal fear reactions and a reduced experience of fear."
"We conducted a new study in a rare human patient, SM, who has focal bilateral amygdala lesions," Feinstein and his colleagues wrote.
"To provoke fear in SM, we exposed her to live snakes and spiders, took her on a tour of a haunted house, and showed her emotionally evocative films."
Past research has suggested that the amygdala is strongly linked to how animals respond to fear. And the study with SM shows signs that the amygdala may control the fear factor in humans.
SM is considered very special in the ongoing study.
"On no occasion did SM exhibit fear, and she never endorsed feeling more than minimal levels of fear," the experts wrote.
They indicated that, over a three-month period of investigation "and a life history replete with traumatic events, SM repeatedly demonstrated an absence of overt fear manifestations."
But this lack of an ability to experience fear can have its drawbacks.
Over the years, SM has faced dangerous situations that would incite fear in most people, including being held up at both knifepoint and gunpoint, and almost killed by domestic violence.
"She is aware that she does not have appropriate or normal responses to situations that would normally induce fear," University of Iowa researcher Daniel Tranel said.
"For example, let's say you walk up to your apartment door and there's a stranger sitting on the park bench nearby that's dressed in a threatening manner. It's dark and you've never seen the person before. That's a situation that would normally inspire fear in people. This patient has been in such situations numerous times and has not experienced fear in those situations."
"Their lives are marred by fear, and they are oftentimes unable to even leave their home due to the ever-present feeling of danger," he said.
And because SM appears to be unafraid of any kind of danger or traumatic-induced stress, Feinstein and his colleagues hope to better understand how the amygdala is connected to human fear, leading to more effective treatments for those who suffer from PTSD.
Read more at Live Science and Current Biology.
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