A woman who knows no fear because of damage to a key emotion centre in her brain is lucky to be alive, scientists said today.
The US woman, known only as SM, lives in a poor and dangerous neighbourhood and has been the victim of numerous crimes.
Over the years she has shrugged off being held at knife and gunpoint, death threats, and assaults, and was once nearly killed by an act of domestic violence.
Yet even when her life was in peril her behaviour "lacked any sense of desperation or urgency", said researchers. She was simply unafraid.
In tests for which she gave her consent, SM showed no hint of fear when exposed to snakes and spiders, during a trip to one of the world's scariest haunted houses, or while watching clips of spine-chilling horror films.
But other emotions such as happiness and sadness remained entirely intact.
SM suffers from a rare condition that has destroyed her amygdala, an almond-shaped region of the brain strongly linked to fear reactions.
Reporting on her case today in the journal Current Biology, US scientists said this had left her vulnerable to walking into dangerous situations.
Neuropsychologist Justin Feinstein, one of the researchers at the University of Iowa, said: "The nature of fear is survival and the amygdala helps us stay alive by avoiding situations, people, or objects that put our life in danger.
"Because SM is missing her amygdala, she is also missing the ability to detect and avoid danger in the world. It is quite remarkable that she is still alive."
The biggest surprise for the scientists was SM's reaction to spiders and snakes, both of which she claimed to "hate".
Yet at an exotic pet store she had to be prevented from touching a venomous tarantula, and displayed a "compulsive desire" to handle the larger and more dangerous snakes.
The researchers also took her to Waverly Hills Sanatorium in Louisville, Kentucky, reputed to be one of the most haunted sites in the world.
Once a year staff at the sanatorium add to the spooky atmosphere by dressing as monsters and ghosts that attempt to frighten visitors.
Amid screams from other members of her tour group, SM could not be scared - and even gave one of the "monsters" a shock by poking it in the head.
SM "showed no signs of nervousness or apprehension while walking through dark passageways, and was never visibly frightened by any of the numerous attempts to scare her", the scientists wrote.
On a more serious note, the researchers believe the work could lead to better treatments for post-traumatic stress disorder (PTSD) and other anxiety conditions.
"This past year, I've been treating veterans returning home from Iraq and Afghanistan who suffer from PTSD," said Mr Feinstein. "Their lives are marred by fear, and they are oftentimes unable to even leave their home due to the ever-present feeling of danger.
"In striking contrast, the patient in this study is immune to these states of fear and shows no symptoms of post-traumatic stress. The horrors of life are unable to penetrate her emotional core. In essence, traumatic events leave no emotional imprint on her brain."Reuse content