Intense fear is commonly believed to require a reaction by the brain structure called the amygdala, but that may not always be the case.

WILLIAM CREAMER/PARAMOUNT/THE KOBAL COLLECTION

People seem to have more than one way to work themselves into a panic. Contrary to a long-standing assumption of neuroscientists, humans can experience fear even when they lack the brain structure widely regarded as the brain's 'fear centre'.

Many studies on animals over the years have shown that the amygdala, a small, almond-shaped structure located deep inside the brain, is crucial for the fear response. This finding has been confirmed in studies of humans.

Justin Feinstein at the University of Iowa in Iowa City and his colleagues have now found that in certain situations the fear response may occur even in people who do not have a working amygdala. Their work is published online today inNature Neuroscience1.

Different fears

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.

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