A much-studied woman was thought to be fearless—literally unable to experience the emotion after having part of her brain, the amygdala, damaged. Nothing from snakes to assaults could scare the woman, dubbed SM, until, in a recent study, she was faced with the feeling of suffocation. That prompted a panic attack, which fascinated scientists. It seemed to support a theory that there are different pathways in our brains for external fears—of robberies, for instance—or internal ones, like an inability to breathe or a heart attack.
Two other women with damaged amygdalas also had panic symptoms under the experiment, which involved breathing air with extra carbon dioxide. Though the study wasn't dangerous, the body responds to a perceived increase in the gas, as when one holds one's breath, the New York Times notes. When there's too much carbon dioxide, "your body’s alarms are firing like crazy within the first 10 seconds," a researcher notes. SM herself said she was surprised by her response, Science News reports. It may be that the brain stem and other parts of the brain are quick to respond to the feeling of suffocation, the Times notes. (Read more fear stories.)