How to induce fear in "fearless" people

The amygdala is popularly referred to as the brain's "fear center," due to the central role it plays in regulating the body's fear response. So pivotal is this small, almond-shaped structure's involvement, that S.M. (a woman whose amygdala has been ravaged by a rare condition known as Urbach–Wiethe disease), has been said not to suffer feelings of panic whatsoever — even in extreme, potentially life-threatening situations. But newly published research suggests that the brain has more than one way to process and experience terror — even in people dispossessed of their supposed fear centers.

Over at Nature News, Mo Costandi examines the latest findings of neuropsychologist Justin Feinstein (much of whose work has involved S.M.), which show "the fear response may occur even in people who do not have a working amygdala":

Feinstein and his team had been studying a 44-year-old woman with an extremely rare genetic condition called Urbach-Wiethe disease, in which the amygdala hardens and shrivels up2. The woman, known as S.M., showed only minimal levels of fear when shown clips from horror films and when exposed to large spiders, snakes and other things that many people find terrifying.

One situation in which the amygdala triggers fear and panic attacks is when it detects unusually high concentrations of carbon dioxide - a sign of possible suffocation - by sensing increased acidity in the blood. This may occur even if CO2 is inhaled in concentrations that are not lethal. Feinstein and his colleagues therefore predicted that patients with damaged amygdalas would not feel fear after inhaling the gas.

To test this, they asked S.M., two other patients with Urbach-Wiethe disease, and 12 healthy controls to inhale 35% carbon dioxide through a mask. To their surprise, the researchers found that the brain-damaged patients did experience fear immediately after inhalation - and, in fact, became even more fearful and panicky than did the healthy volunteers.

Read more about this surprising find over at Nature News.