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.”