The amygdala, an almond-shaped structure in the brain, was believed to be central to our experience and perception of fear. But the role of the structure turned out to be a bit more complicated, as research scientist Dr. Lisa Feldman Barrett explains in a new paper.

Dr. Barrett, a University Distinguished Professor of Psychology at Northeastern University, was invited to write the paper for the 40th-anniversary issue of the journal Trends in Neurosciences.

The amygdala has been studied for its association with fear since the 1930s. At the time, researchers Heinrich Klüver and Paul C. Bucy published a study showing changes in the behavior of rhesus monkeys after the removal of their amygdalae — they were able to display a willingness to approach snakes and other dangerous animals.

Originally, scientists believed that the amygdala contained the circuitry necessary for fear and other related behaviors. Subsequent findings from both human studies and animal studies added to the hypothesis, suggesting that the circuitry was not just linked to fear but also negative emotions or even emotions in general.

"Through the natural process of systematic scientific investigation, it's become clearer that the amygdala plays a role in signaling the rest of the brain to information that is important to learn because it is relevant to allostasis," said Barrett, who also works in the Department of Psychiatry at Massachusetts General Hospital (MGH). 

Allostasis is a process by which our brain anticipates the energy needs of the body and tries to meet those needs before they arise. "Whether threatening, rewarding or novel, this to-be-learned information will help the brain better predict future occasions," she added.

But during the 1990s, a new perspective regarding the function of the amygdala was put forward by the University of Iowa College of Medicine. The team from the university published a series of studies on a woman, referred to as S.M, with rare bilateral lesions of the amygdala.

First, in a 1994 study, they showed that the woman was not able to recognize the facial expressions of fear. Further studies discovered that S.M. actually failed to recognize any facial expression that involved the widening of eyes, putting the "fearful eye whites" on prominent display.

So rather than directly mediating fear, the brain structure simply helps by instructing us to attend to the whites of another person's widened eyes. Barrett noted that this is more important in social functioning.

"The amygdala is not necessary to experience or perceive fear," she explained. "Amygdala neurons very likely contribute to fear in some instances, but the neurons can't be said to actually compute fear. More likely, amygdala neurons act as a context-sensitive sentinel for learning threat and reward."