"SM" is a bit of an emotional anomaly. The 44-year-old mother, given those initials to preserve her anonymity, isn't scared of snakes. She doesn't shriek when she sees a scary movie. Even haunted houses don't give her chills. SM is pretty much fearless—and now scientists think they've figured out why.
The study's lead authors met SM, who has a rare genetic condition called Urbach-Wiethe disease, more than 2 decades ago. As a result of her illness, she has "two perfectly symmetrical black holes" where her amygdala should be, says Justin Feinstein, a graduate student in clinical psychology at the University of Iowa in Iowa City. The amygdala is a pair of almond-shaped clusters of neurons in the brain that play a role in fear and anxiety. And indeed, when the researchers examined SM, they found that she could not recognize fear on others' faces.
In the new study, the researchers—who now included Feinstein—tested whether SM could experience fear. They took her to a pet store filled with snakes and spiders, showed her clips from horror films (including The Silence of the Lambs and The Shining), and brought her to the annual haunted house at the Waverly Hills Sanatorium in Louisville, Kentucky, a notoriously scary place.
In each situation, SM failed to act fearful. Instead, she seemed excited and curious. In the pet store, for example, she held a snake and rubbed its scales despite telling the researchers that she "hates" snakes. In the haunted house, SM led the way, smiling and laughing. SM didn't report feeling scared. Throughout each experience, the researchers asked her to rate her fear on a scale of 1 to 10. In each case, she selected low values, 2 or lower. But SM isn't an unfeeling robot. She reports experiencing other emotions—surprise, happiness, disgust—and understands that scary movies might induce fear in others.
The researchers also gave SM an electronic diary. Three times each day, the diary displayed a list of 50 questions asking her to rate her current emotional state. The emotion that received the highest average rating during the 3 months SM had the diary was "fearless." She never reported being scared, fearful, or afraid. Similarly, SM's score was much lower than normal on a series of questionnaires aimed at assessing recent fear and how much fear she would feel in a series of hypothetical situations, such as talking to other people or getting lost.
When the researchers delved into SM's past, they found the same fearlessness. The woman says she is scared of snakes, but her son once saw her pick up a large snake and move it off the road. When SM was held at knifepoint in a dark park, she recalls remaining calm and not being scared. As an adult, SM has been the victim of numerous crimes, but the only fearful experiences she could recall happened when she was a child. The researchers posit that the bulk of the damage to SM's amygdala occurred at about the age of 10.
The results suggest that "the amygdala is a critical brain region for triggering a state of fear when an individual encounters threatening stimuli," Feinstein and his co-authors write today in Current Biology. It's the first human study to show that amygdala damage can wipe out fearful feeling, they say. It also contradicts a 2002 paper that showed that patients with damage to one or both halves of the amygdala had no deficit in their ability to feel fear.
The authors point out that the amygdala communicates with other regions of the brain to orchestrate the fear response. "Because SM is missing her amygdala, she doesn't have this cascade of responses that comprise a state of fear," Feinstein says. "And because of that, she's unable to feel fear."
"It's an important observation," says David Anderson, a neurobiologist at the California Institute of Technology in Pasadena who studies the neural circuits involved in fear. But he notes that there's no way to unequivocally prove that SM's responses are the result of damage to her amygdala. "One would like to have more subjects than just one," he says.
Elizabeth Phelps would also like to see evidence in more patients. "I don't believe you can make a general statement about what the amygdala does by a single case study," says the cognitive neuroscientist at New York University and author of the 2002 study that returned opposite results. The authors, she says, were too bold in their conclusions. "The data are mixed."
If confirmed, Feinstein says the findings might lead to new therapies for post-traumatic stress disorder, such as new forms of psychotherapy that hinder the amygdala's activity. Still, fear is an important emotion, notes Anderson. So "you would not want to advocate permanent destruction of the amygdala in soldiers as a way to protect against possible post-traumatic stress disorder," Anderson says.
As for Feinstein's own amygdala, it appears to be intact. "A lot of things scare me, including snakes and spiders," he says. "You couldn't pay me enough money in the world to touch these animals."
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