A woman who cannot feel afraid because of a missing structure in her brain could help scientists discover treatments for post-traumatic stress disorder (PTSD).
Research published in Current Biology showed the woman felt no fear in a variety of scary situations.
These included exposure to snakes and spiders, horror films and a "haunted house". The woman feels other emotions but said as an adult, she had never felt afraid. She is the first known case of someone who is unable to process fear.
Researchers at the University of Iowa said her inability to feel frightened was because she is missing a structure in her brain called the amygdala.
The structure has long been associated with emotional learning - experiments in animals have shown that removing it makes them fearless. However, it has never been observed in a human before.
The woman experienced fear as a child and knows that some situations should be frightening. As an adult she has been in various frightening situations, including being threatened with a knife and held at gunpoint.
Researchers at the University of Iowa, in Iowa City, observed and recorded the woman's responses in situations that would make most people feel fear.
She watched a series of horror films, went to a reputedly haunted house and to an exotic pet store - where she handled dangerous snakes and asked to handle a tarantula.
She showed no fear in any of the situations and had to be prevented from touching the tarantula because of the high risk of being bitten.
When asked why she wanted to touch something that she knows is dangerous, she replied that she was overcome with curiosity.
Lead researcher Justin Feinstein said: "Because she 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."
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