What Scared the Fearless Woman?

Snakes don’t scare patient SM. Haunted houses don’t spook her, nor do horror movies. Not even a walk through a deserted park at night sends a shiver down her spine. In fact, SM doesn’t respond to any fear-provoking stimuli like these. She suffers from Urbach–Wiethe disease, a rare genetic disorder that damages the amygdala, a region of the brain that plays an influential role in our fear response.

Recently, however, SM got the first scare of her adult life.

The culprit wasn’t anything that those of us who shriek at the sight of spiders might have suspected. Rather, SM, as well as two other patients with the disease, exhibited signs of fear and panic after breathing oxygen spiked with carbon dioxide, according to a study published recently in Nature Neuroscience.

Inhaling CO2 causes body fluids to become more acidic and creates a feeling of suffocation, says Justin Feinstein, lead author of the study and now a clinical neuropsychologist at the California Institute of Technology. SM described her reaction to breathing CO2 as a strange new experience, reporting feelings of unsteadiness and a sense of losing control, among other sensations. “I didn’t know what the hell was going on,” she told the researchers. “I [was surprised] cause nothing usually happens to me.”

While the amygdala is crucial to interpreting certain environmental factors as threatening, the findings suggest that it may not be critical in provoking a fearful reaction to internal bodily sensations such as suffocation or chest pain—that is, cues that signal that something is physically wrong with us, says Feinstein. Rather, there may be a different kind of mechanism involved, according to the study.

“One of the big things this paper will lead to is questioning the standard wisdom of how and what the amygdala does,” says Anantha Shekar, a professor of psychiatry at the Indiana University School of Medicine, who was not involved in the study. Researchers will now likely explore what else in the brain besides the amygdala might generate fear, he adds.

The findings could also lead to new treatments for panic disorders, says Shekar. “Until now, the most common focus has been looking for drugs that would shut down the amygdala, but now that there may be some other part of the brain besides the amygdala that plays a role, it will have us look for other kinds of chemicals,” he says.