Humans, Like Animals, Behave Fearlessly Without the Amygdala

In the 1930s, researchers discovered that when a certain part of monkeys’ brains was removed, the animals became fearless. They approached snakes, started batting them around like sticks and played with their hissing tongues.

This experiment has been repeated in animals numerous times, and the scientific consensus is that when the amygdala is removed, an animal loses any sense of fear.

Now, scientists have confirmed that a missing amygdala results in similar behavior in humans, according to a study in the journal Current Biology.

“There’s not very many humans with this sort of brain damage,” said Justin Feinstein, the study’s lead author and a clinical neuropsychologist at the University of Iowa. “Luckily for us, we had access to a patient, SM, and we studied her different fear behaviors and we read her personal diaries.”

Patient SM, because of a rare condition called lipoid proteinosis, has holes where her amygdala would normally reside. Researchers found that she, like the monkeys, has no fear of creatures like snakes and spiders, which ordinarily alarm most people.

But while this behavior is relatively benign, the researchers also found that SM put her life at risk. In one instance, she walked through a park alone at night and was attacked by a man with a knife.

“The following day, she again walked through the same park,” Mr. Feinstein said.

Mr. Feinstein and his colleagues are trying to coach the patient to behave in a more cautious manner. They also believe that understanding how the mind of a patient like SM works could help researchers develop therapies for individuals who express excessive amounts of fear, like war veterans.

“We may be able to dampen the effects of the amygdala,” he said. “We can do that through psychotherapy and possibly through medication.”