Nothing to fear but suffocation: People with a rare brain disorder don’t get scared — except when they breathe carbon dioxide

Not all fear is the same. A woman who laughs at horror movies, grabs dangerous snakes and calmly deals with knife-wielding men nonetheless surrenders to terror at a single puff of suffocating carbon dioxide.

This woman, known as SM, has a disease that damaged her amygdala, a brain structure implicated in fear. But the new results involving her and two others with the same disease, published online February 3 in Nature Neuroscience, show that a certain kind of danger signal can bypass the amygdala, hitting the panic button in other parts of the brain.

The need to breathe is one of the most fundamental requirements for survival. Clinical neuropsychologist Justin Feinstein of the University of Iowa in Iowa City believes that the instinct to get air might tap into a brain system that’s more primal than the amygdala.

Feinstein and colleagues work with SM and other patients who suffer from a rare genetic disorder called Urbach-Wiethe disease. In late childhood, this disease destroys the amygdala, a pair of almond-shaped structures deep in the brain. SM shows no fear when confronted with haunted houses, ominous spiders and scary movies (SN: 1/15/11, p. 14). Now, the scientists have found something that does scare her.

A breath of gas that is 35 percent carbon dioxide can immediately provoke a strong, panicky fear. (By contrast, normal air is less than one percent carbon dioxide.) When the gas hits the body, specialized proteins sense that something is amiss and send an urgent “must have air, now” message to the brain.

"It’s automatic,” says Feinstein, who has subjected himself to the procedure multiple times. “Your body’s alarms are firing like crazy within the first 10 seconds.”

A recent study using mice showed that the amygdala detects carbon dioxide and helps produce fearlike behavior. So the researchers thought that SM and the two other women in the study might likewise show a blunted response to the gas.

more here: http://www.sciencenews.org/view/generic/id/348032/description/Nothing_to_fear_but_suffocation