Our brain makes snap judgements about a face's trustworthiness even before we are aware of it, a new study has found.
"Our findings suggest that the brain automatically responds to a face's trustworthiness before it is even consciously perceived," said Jonathan Freeman, an assistant professor in New York University's Department of Psychology and the study's senior author.
"The results are consistent with an extensive body of research suggesting that we form spontaneous judgements of other people that can be largely outside awareness," said Freeman, who conducted the study as a faculty member at Dartmouth College.
The researchers focused on the workings of the brain's amygdala, a structure that is important for humans' social and emotional behaviour.
Previous studies have shown this structure to be active in judging the trustworthiness of faces. However, it had not been known if the amygdala is capable of responding to a complex social signal like a face's trustworthiness without that signal reaching perceptual awareness.
The study's authors conducted a pair of experiments in which they monitored the activity of subjects' amygdala while the subjects were exposed to a series of facial images.
These images included both standardised photographs of actual strangers' faces as well as artificially generated faces whose trustworthiness cues could be manipulated while all other facial cues were controlled.
Prior to the start of these experiments, a separate group of subjects examined all the real and computer-generated faces and rated how trustworthy or untrustworthy they appeared. As previous studies have shown, subjects strongly agreed on the level of trustworthiness conveyed by each given face.
In the experiments, a new set of subjects viewed these same faces inside a brain scanner, but were exposed to the faces very briefly - for only a matter of milliseconds.
This rapid exposure, together with another feature known as "backward masking," prevented subjects from consciously seeing the faces.
In the first experiment, the researchers examined amygdala activity in response to three levels of a face's trustworthiness: low, medium, and high.
In the second experiment, they assessed amygdala activity in response to a fully continuous spectrum of trustworthiness.
Across the two experiments, the researchers found that specific regions inside the amygdala exhibited activity tracking how untrustworthy a face appeared, and other regions inside the amygdala exhibited activity tracking the overall strength of the trustworthiness signal (whether untrustworthy or trustworthy) - even though subjects could not consciously see any of the faces.
The findings appear in the Journal of Neuroscience.
