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A face's trustworthiness is gauged in a few milliseconds by the amygdala part of the brain.Reuters

Thank your amygdala for snap judgments that have helped you avoid nasty situations or people in the past.

It transpires that this region of the brain is capable of responding to a complex social signal like a face's trustworthiness even before that signal has evoked perceptual awareness in the person.

Scientists know that the amygdala structure plays an important role in human social and emotional behaviour but not that it responds even before the onset of awareness.

The findings appear in the Journal of Neuroscience.

Subjects in the study were shown a series of facial images which included real faces of strangers as well as manipulated ones.

Computer synthesis of faces was based on previous research that higher inner eyebrows and pronounced cheekbones are seen as trustworthy and lower inner eyebrows and shallower cheekbones are seen as untrustworthy.

A separate group of subjects examined all the real and computer-generated faces and rated how trustworthy or untrustworthy they appeared. They strongly agreed on the level of trustworthiness conveyed by each given face.

The main subjects viewed these same faces inside a brain scanner, but were exposed to the faces for a few milliseconds.

Care was taken to avoid prolonged exposure which could allow the brain to process the face and become aware.

Researchers examined amygdala activity in response to three levels of a face's trustworthiness: low, medium and high. They then assessed amygdala activity in response to a fully continuous spectrum of trustworthiness.

They found that specific regions inside the amygdala exhibited activity tracking how untrustworthy a face appeared, and other regions inside the amygdala showed activity tracking the overall strength of the trustworthiness signal even when not seeing faces consciously.