The case of a literally fearless woman — rendered so after brain damage — has helped scientists confirm that a part of the brain structure regulating emotions is the key to human and animal phobia.

Scientists had to prevent SM, a patient unable to experience fear due to damage to her brain, from touching a tarantula in a pet store. (Victoria Arocho/Associated Press)

The 44-year-old mother of three, referred to in the journal Current Biology as SM, has a rare psychological impairment due to a genetic disease called lipoid proteinosis that left holes where her amygdala — the brain’s danger detector — would normally reside.

The patient felt excitement, but never fear in a series of threatening scenarios monitored by researchers with the University of Iowa, led by Justin Feinstein, a clinical neuropsychologist.

The American woman was taken to a pet store to interact with snakes and spiders, where she was reported to have exclaimed, “This is so cool!” while rubbing a snake’s scales and touching its tongue.

Pet store employees had warned her about the danger of the exotic pets. Previously, the patient also admitted she hated snakes and spiders. During the tests, however, she had to be coaxed away from reaching towards a tarantula.

Asked by researchers to rate her fear from zero to 10, she never exceeded two.

'“It is quite remarkable that she is still alive.'—Justin Feinstein, University of Iowa

Led by scientists into a “haunted house,” SM eagerly approached staff dressed as monsters, even startling one masked performer by poking at the head, reasoning that she was “curious” about how the mask would feel.

While she considered the experience to be “highly exciting and entertaining,” comparing it to the feeling of a roller-coaster ride, her fear rating was zero.

Horror films such as The Blair Witch Project and The Shining also registered minimal fear readings from her.

Spoke calmly to knife-wielding stranger

Scientists have studied SM for more than two decades, with multiple papers published about her fear-related anomaly.

While she recalled experiencing fear as a child, after a scary encounter with a snarling Doberman pinscher that cornered her, she could not remember fearing anything as an adult.

One night in 1995, a strange man leapt from a park bench and pressed a knife to her throat, threatening to cut her.

“If you’re going to kill me, you’re going to have to go through my God’s angels first,” she replied, looking at him coolly. She then walked away and returned to the park the next day.

The scientists are trying to coach SM’s behaviour to react differently in such dangerous situations.

“It is quite remarkable that she is still alive,” said Feinstein, noting that she has submitted herself needlessly into risky scenarios due to her inability to process fear.

Researchers believe that understanding how SM’s mind works may help with the discovery of treatments for post-traumatic stress disorder.

With files from The Associated Press