Humans are judgmental creatures. In a sometimes frightening world, it is perhaps understandable we feel the need to form an immediate opinion about each new person we encounter.

However, a new study has found this process of deciding who might be friend or foe takes place so instantaneously it can only be described as unconscious, even entirely beyond our control. The researchers found faces flashed before participants’ eyes so briefly as to not be identifiable prompted spontaneous and specific responses in the amygdala, a deeply buried set of neurons implicated in social and emotional behavior. “These findings provide evidence that the amygdala’s processing of social cues in the absence of awareness may be more extensive than previously understood,” said Dr. Jonathan Freeman, who conducted the study while a faculty member at Dartmouth College.

Before beginning their experiments, researchers asked a group of participants to rate a series of real and computer-generated faces for trustworthiness. The images included actual photographs of strangers’ faces as well as computer-generated faces the researchers had synthesized based on familiar cues, including higher inner eyebrows and pronounced cheekbones signal trustworthiness, while lower inner eyebrows and shallower cheekbones point to untrustworthiness. As previous studies have shown, the level of trustworthiness conveyed by any given face is highly consistent across people; most of us rate the exact same faces as threatening, others as trustworthy. Once the researchers had gathered this preliminary information, the real fun began.

Faces, ranging from low to high trustworthiness, Courtesy of Journal of Neuroscience Faces, ranging from low to high trustworthiness, Courtesy of Journal of Neuroscience

In twin experiments, two new sets of participants viewed target faces while undergoing brain scans. However, the faces they saw appeared only briefly — milliseconds! Added to this, the researchers followed each rapid exposure with a "mask" — a neutral face, consistently rated as neither trustworthy nor untrustworthy — as a way to terminate each participant’s ability to further process the face in their minds. For the first experiment, participants saw a stream of target faces displaying varying levels of trustworthiness, while in the second, they saw an unbroken stream of trustworthy faces. Meanwhile, the researchers observed and recorded amygdala responses to all.

In both experiments, even though participants never consciously saw any of the target faces, their amygdalas consistently responded in ways indicating trust or lack of trust. In other words, this tiny region of our brains spontaneously decides friend or foe for us. The essential amygdala, responsible for aggression and fear, appears to influence our lives profoundly.

Source: Freeman JB, Stolier RM, Ingbretsen ZA, Hehman EA.  Amygdala Responsivity to High-Level Social Information from Unseen Faces. Journal of Neuroscience. 2014.