Edvard Munch's "The Scream." (AP Photo/Sotheby’s Auction House)

Fear is one of our most basic evolutionary instincts, a sudden physical jolt to help us react to danger more quickly. In the modern world, fear often seems excessive -- in the absence of wild animals to flee, we're left screaming over roller coasters and scary movies. But for at least one woman, fear is unobtainable. And while she lives a normal life, her fearlessness is actually a handicap.

The researchers who study her keep her closely guarded, using the code-name "SM" when publishing papers about her brave brainpower. And until this year, she'd never been interviewed.

On this week's episode of "Invisibilia," a new podcast from NPR, that changed. But SM didn't pop into the studio for her radio debut. This unprecedented access still came with the buffer of one of her doctors (University of Iowa neuroscientist Daniel Tranel), who conducted and recorded the interview before passing it along to "Invisibilia."

"Tell me what fear is," Tranel began.

"Well, that's what I'm trying to -- to be honest, I truly have no clue," SM said, her voice raspy. That's actually a symptom of the condition that stole fear from her. Urbach-Wieth disease, which is characterized by a hoarse voice, small bumps around the eyes, and calcium deposits in the brain is rare in its own right -- only 400 people on the planet are known to have it -- but in SM's case, some of those brain-deposits happened to take over her amygdalae.

These almond-shaped structures deep inside the brain are crucial to human fear response. And in SM's case, they've been totally calcified since she was a young woman. Now in her 40s, her fear-center is as good as gone.

"It's a little bit as if you would go to this region and literally scoop it out," Antonio Damasio, another neuroscientist who studies SM, told "Invisibilia" hosts Lulu Miller and Alix Spiegel.


The amygdalae, shown in red. (Life Sciences Database/Wikimedia Commons)

So SM isn't exaggerating. When she tries to imagine what fear feels like, she draws a blank. Sometimes literally. Even though she's a talented artist, she always has trouble drawing (or reading) a fearful facial expression.

"I wonder what it's like, you know, to actually be afraid of something," she told Tranel.

Once upon a time, when her amygdalae were still healthy, SM felt fear of animals that she thought might hurt her. But these days, when researchers expose her to creepy crawlies in the hope of inducing some kind of response, her innate curiosity means she has to be held back from touching poisonous creatures.

Obviously someone with SM's condition (or those with the unrelated inability to feel pain, which helps prevent serious injury) would have had a rough time surviving a few thousand years ago. But when avoiding poisonous snakes isn't an immediate concern, why is lack of fear such a bad thing? In her NPR interview, SM recalls one particularly harrowing event:

"Okay. I was walking to the store, and I saw this man on a park bench. He said, 'come here please.' So I went over to him. I said, 'what do you need?' He grabbed me by the shirt, and he held a knife to my throat and told me he was going to cut me. I told him -- I said, 'go ahead and cut me.' And I said, 'I'll be coming back, and I'll hunt your ass.' Oops. Am I supposed to say that? I'm sorry... I wasn't afraid. And for some reason, he let me go. And I went home."

That's actually just one of two times that SM has been held at knife point. She's also been held at gunpoint twice. And after the above incident, she didn't feel like she should call the police. The threat had passed. She didn't have any lasting trauma, because the event had failed to faze her.

SM isn't stupid. She understands what can and can't kill her. But she lacks the quick, subconscious, visceral response that the rest of us feel when we're exposed to danger. In some ways, she leads a charmed life; Everyone she meets wishes her well, and the world is a sunny place. But because she has to consciously process danger, she can put herself into unfortunate situations -- no poisonous snakes required.

After years of trying, one thing did make SM panic. Excess carbon dioxide concentration in the blood (which is a sign of suffocation) is known to induce fear and panic in healthy individuals. Sure enough, elevating SM's carbon dioxide levels did manage to give her a fright. Even then, she described her feelings as a sense of loss of control and unsteadiness -- not exactly a scream fest.

To find out more about fear, check out the whole episode.

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