When Antonio Damasio, a neuroscientist at the University of Southern California, first met a woman now known as SM, he noticed that she would get unusually close to other people. In most people, this might seem like an odd personality quirk, but for SM it was a symptom of her very rare condition. "The woman couldn’t feel fear — literally could not experience that emotion," explains NPR correspondent Alix Spiegel on the radio show "Invisibilia."

In last week’s show, called "Fearless," Spiegel and her co-host, Lulu Miller, explored what living without fear is like. SM has participated in neuroscience research for years, but the show is the first time she has granted an interview, though it was conducted via an intermediary, one of her doctors, Daniel Tranel of the University of Iowa. Her fearlessness actually makes SM vulnerable, "To make the point very clearly, if she would be threatened - and she has been in her life - she would not register the fear that that would immediately cause in you or me," Damasio says.

SM’s condition is due to the rare genetic disorder called Urbach-Wieth disease. Only 400 people in the world have the disorder, which causes a raspy voice, easily damaged skin and calcium deposits in the brain, writes Rachel Feltman for the Washington Post. SM’s deposits have hardened structures deep in the brain that help people feel fear — the amygdalae. "[I]n SM's case, they've been totally calcified since she was a young woman," Feltman writes. "Now in her 40s, her fear-center is as good as gone."

On the show, Miller explains:

That bit of brain couldn't signal to the rest of her body that it was time for her heart to start racing and her palms to sweat. It's also why SM was so profoundly valuable to the scientists who studied her, like Damasio, and the fear researcher Ralph Adolphs that you heard earlier because fear seems critical to survival. But here was SM, alive and also completely normal in other ways. She had normal intelligence and no problem with any other emotion.

Her experience has helped researchers figure out how the amgydalae are involved in fear, writes Ed Yong for Discover. Justin Feinstein, of the University of Iowa, suspects that the brain structure serves as a go-between for the parts of the brain that interpret sensory inputs and the sections of the brainstem that "initiate fearful actions."

Before her amgydalae were calcified, SM remembers experiencing what she thinks was fear when her dad caught a big catfish on a fishing trip. "I didn’t want to touch the doggone fish," she says. But when her ability to experience fear was lost, she had to be held back to keep from touching dangerous snakes researchers showed her in tests.

Fear became alien to her. In one of Damasio’s studies, she couldn’t even figure out how to draw a frightened face, even though she is a talented artist. When a man she encountered in a park held a knife to her throat and threatened to kill her her response was atypical: "I said, go ahead and cut me. And I said, I’ll be coming back and I’ll hunt your ass." He let her go.

"Without fear, trauma is not traumatizing," Speigel says. And perhaps as a result, SM reports that her outlook on life is quite sunny. "You know, there's some days that I could be on top of the world, and there's some days that, you know, I can be — [I’ve] got the blues," she says. "But 9 out of 10, I'd say happy."​