Meet The Woman Who Can't Feel Fear

Think about the last time you felt afraid.

Was it a fear of heights?

Did you oversleep on a weekday and fear you'd get in trouble at the office?

Did you nearly walk into oncoming traffic while looking down at your phone?

In any case, you know what it feels like to feel fear.

One woman doesn't.

A new podcast called "Invisibilia," brought to you by NPR, documents this woman's story in an episode.

The woman, code-named "SM," gave her first ever interview after years of being studied by a team of researchers - her real name is concealed because researchers want to protect her from anyone who would take advantage of her inability to pick up on cues that she should be afraid.

The Washington Post reports that SM was not interviewed by the folks at NPR, but rather, one of her doctors (University of Iowa neuroscientist Daniel Tranel) who then passed along the interview to the radio station.

SM can't tell you what fear is because she's never experienced it.

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

The formal name for what took away her ability to fear is Urbach-Wieth disease, "which is characterized by a hoarse voice, small bumps around the eyes, and calcium deposits in the brain," The Washington Post reports.

Only 400 people on the planet are known to have Urbach-Weith disease. In the case of SM, the disease has transformed the part of her brain, the amygdala, that controls the human response to fear.

This part of SM's brain has been calcified for years, rendering it sort of useless.

In the interview, SM talks about several separate events in her life where she was held at knife point and gunpoint:

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?' and 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 a--.' 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.

Even though SM knows inherently that this is a dangerous situation that could end her life, her brain can't make her feel afraid of it.

Doctors who have been studying her condition for years have been trying different things that could strike fear in SM. Finally they figured something out - elevating SM's carbon dioxide levels.

The Washington Post reports that "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 listen to the entire episode of "Invisibilia," click here.