The most fearless woman alive—someone scientists thought literally could not feel fear, ever, under any circumstances—has a weakness after all, it turns out. What’s her kryptonite? Carbon dioxide.

First, some background. A few decades ago, at around age ten, an Iowa woman named S.M. came down with a rare disorder called Urbach-Wiethe disease, in which the cells in the amygdala calcify and die. Nowadays she has nothing but two “black holes” where her left amygdala and right amygdala should be.

The amygdala plays a crucial role in processing fear, and minus her two amygdalae, S.M. became unflappable. Studies of her are actually a hoot to read, since they basically consist of scientists concocting ever-more-elaborate ways of trying to scare her. They tried clips from The Shining, The Blair Witch Project, and other terrifying films. She merely asked for the names of the movies, so she could rent them later. They drove her to an exotic pet store, since she claimed to hate snakes. As soon as she saw the serpents, however, she grabbed for them, and repeatedly asked to handle the venomous ones. (She also tried to grab the snakes’ tongues, which snakes do not enjoy.) Finally, scientists took S.M. to an elaborate haunted house. Nada. In fact, while other people in their tour group screamed and ducked, she was constantly running ahead to see what was next, and one time even grabbed a monster on duty—an actor in costume—because she wanted to know what his mask felt like. She ended up scaring him.

While often amusing, S.M.’s deficit has serious consequences in her daily life, since she lacks the healthy fear that keeps normal people safe. For instance, she almost got her throat slit because she crossed a city park alone after dark—a man grabbed her, pressed a blade against her neck, and hissed, “I’m going to cut you, bitch.” S.M. felt annoyed by this, but little more. She never felt afraid, and when her attacker set her free (she talked him out of harming her), rather than run away, she walked. The next day she even returned to the park.

If almost getting murdered doesn’t scare the bejesus out of you, it’s a safe bet that nothing will, and S.M. seemed to confirm the notion that the amygdala alone can conjure up fear.

A new paper in Nature Neuroscience, though, has found an important exception. A few of the same scientists that had dragged S.M. to the haunted house and pet store tried to frighten her one last time. They had her lie down on a reclining chair, and they strapped a plastic mask connected to an air tank over her nose and mouth. (Two other women, twins with the same disease and the same blithe lack of fear, were also tested, as were healthy controls.) With the controls, even this prep work of strapping on the mask started to stir up some low-level anxiety, since we don’t like having our airways blocked: their heart rates increased a little, and they began to sweat slightly. Not so with S.M. (or the twins): without her amygdalae, she couldn’t perceive the mask as even a potential threat.

Then she took a breath.

The air tank connected to the mask contained 21 percent oxygen, like normal air. It also contained 35 percent carbon dioxide, almost a thousand times higher than normal air. Despite what you might guess, when monitoring your breathing, your body doesn’t care whether you’re inhaling enough oxygen. It cares only whether you’re expelling enough carbon dioxide—that’s the gas that sets off the panic button when you’re suffocating. So when S.M. and the twins inhaled the CO2-rich air, they immediately felt a rush of horror, anxiety, dread, panic, and every other synonym for fear. Their minds raced, their eyes bulged, their neck muscles contracted; they even began displaying animalistic “escape behavior” and clawing at the mask. None of them could ever remember feeling such pure, raw terror.

So why can invisible, odorless carbon dioxide induce fear, when venomous snakes and maniacs with knives cannot? The scientists traced this difference back to how we sense each threat. When we see or hear something around us that’s potentially dangerous—an external threat—our brains have to run the data through the amygdala, which then (in a healthy person) triggers the brain’s panic circuits and prepares us to fight or flee. The CO2, in contrast, represents an internal threat, which the body processes differently. The sensors that tell our brains that we might be suffocating don’t plug into the amygdala; they plug into our reptilian brainstem, among other places. Since those parts of S.M.’s brain had never suffered damage, she responded normally to this threat, by having the sh!t scared out of her.

The amygdala is one of those brain structures that a lot of people know a little bit about, and there’s a definite tendency to conflate the amygdala and the fear response itself—as if the amygdala, and the amygdala alone, causes fear. As this study shows, that’s not quite right. The amygdala is indeed crucial for monitoring our environment and deciding what’s worth getting worked up over. Once the amygdala determines this, however, it merely trips another circuit to actually produce the panic. And as S.M. has now revealed, there are other ways to access that circuit, and those alternative ways can throw even the most fearless woman in the world into hysterics.

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