Snakes, spiders, haunted houses and horror movies may frighten most of us, but there is one woman who doesn't seem to be afraid of them or anything else.
Identified only as 'SM', she is the subject of a new study that looked at how a certain part of the brain experiences fear.
Researchers at the University of Iowa have found an almond-shaped structure in the brain, called the amygdala, which develops the emotion of fear.
Earlier studies have shown that the amygdala plays a key role in generating fear in animals.
The new study confirms for the first time that the amygdala triggers fear in human beings too.
The finding offers a powerful take on the connection between the brain and behaviour, specifically in the context of situations that would normally evoke fear, said the researchers.
"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," said Justin Feinstein.
"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," he added.
Feinstein said that the average person may have different definitions of fear, but his goal ultimately is to define emotions including fear based on the biological machinery that triggers them.
"Normally, the amygdala is constantly sorting through all the information coming into our brain through the different senses in order to rapidly detect anything that might impact our survival," he said.
"Once it detects danger, the amygdala orchestrates a rapid full-body response that compels us to stay away from the threat, thereby improving our chances for survival," he added.
Feinstein and his team observed and recorded SM's responses in a variety of situations that would make most people feel fear.
They exposed her to snakes and spiders, took her to one of the world's scariest haunted houses, and had her watch a series of horror films.
They also had her fill out questionnaires probing different aspects of fear, from the fear of death to the fear of public speaking.
SM recorded her emotions at various times throughout the day while carrying around an electronic diary over a 3-month period. Across all questionnaires, measures, and scenarios, SM failed to experience fear.
That apparent lack of fear mirrored her personal experience, Feinstein said.
"In everyday life, SM has encountered numerous traumatic events which have threatened her very existence, and by her report, have caused no fear," he said.
"Yet, she is able to feel other emotions such as happiness and sadness. Taken together, these findings suggest that the human amygdala is a pivotal area of the brain for triggering a state of fear," he added.
Feinstein said he was most surprised at what happened when SM was presented with snakes and spiders in an exotic pet store.
"For many years, SM has been telling us that she 'hates' snakes and spiders and 'tries to avoid them'," he said.
"Going into the experiment, everyone, including myself, thought SM would stay away from these animals. Yet, to our surprise, she immediately started touching them! When asked why she was touching something that she claims to hate, she appeared perplexed by her own behaviour and stated that she was overcome with curiosity," he added.
Feinstein said the findings suggested that methods designed to safely and non-invasively turn off the amygdala might hold promise for those suffering from post-traumatic stress disorder (PTSD).
"Traumatic events leave no emotional imprint on SM's brain. By understanding how the brain processes fear through cases like SM, we may one day be able to create treatments that selectively target the brain areas that allow fear to take over our lives," he said.
The study is published online in Current Biology.