One of the best ways to figure out how parts of the brain work is to study people who have damage in those specific areas. Take a patient known as SM, a middle-aged woman with a rare condition called Urbach-Wiethe disease that has damaged the amygdala on both sides of her brain.
A few years ago, SM told Justin Feinstein, then a graduate student at the University of Iowa in Iowa City, that she’d never felt fear, even when she’d been confronted by a knife-wielding assailant.
Feinstein put her claim to the test. His group had SM handle a snake, took her on a tour of a haunted house and showed her scenes from scary movies like The Shining. SM was unfazed.
But another University of Iowa researcher, neuroscientist John Wemmie, had a different test for SM: breathing carbon dioxide. In people without brain damage, exposure to high levels of carbon dioxide evokes fear and a hunger for air. In some, it can even set off a panic attack.