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For the majority of her 40-some years, a woman identified only as "S.M." had never felt fear. Diagnosed with Urbach-Wiethe disease, a genetic condition that causes the amygdala to harden and shrivel up over time, she exhibited no fear from the myriad of experiments researchers had conducted on her - not to snakes, spiders or haunted houses or very real domestic abuse, death threats or robberies at knife point. Though she lived in a crime-ridden neighborhood, she did not learn fear from her experiences. However, for the first time, scientists were able to provoke fear in S.M., perhaps leading to a greater understanding of fear and the amygdala.

So it is with surprise that scientists reported that they were able to successfully provoke fear in S.M. Researchers handed S.M. and a pair of twins - A.M. and B.G. - that had the same condition a mask. Then, according to the New York Times, scientists pumped in carbon dioxide in an amount that was not harmful, but causes people to feel like they are being suffocated.

All three individuals showed symptoms of intense panic and fear. The Guardian reports that S.M. began waving her hand up to the mask and screaming for help. "It felt like my throat was closing up... I couldn't breathe," she said in an interview with the researchers after the experiment. She explained that she felt "[panic,] mostly, because I didn't know what the hell was going on."

Even more interestingly, New Scientist reports that the individuals whose amydalas were impaired demonstrated an even stronger panic than the 12 healthy controls did. Before the experiment began, the controls' heart rates and perspiration increased before the experiment began. For those without a working amgydala, they had no way of anticipating the fear. The only individuals who feel a similar response to the experiment as the amygdala-impaired group are people with panic disorder, indicating that the amygdala does not properly work for individuals with panic disorder.

Researchers believe now that the amygdala controls fear to external responses, like predators. That means that a separate fear pathway controls the body's reaction to internal problems, like respiratory issues. This study marked the first time S.M. was tested for responses to internal trouble.

The study was published in the journal Nature Neuroscience.