Experts who used technology developed in Glasgow say they have discovered that the eyes can be a giveaway when it comes to identifying fear.
Experts say the eyes are all-important for identifying fear
Scientists, including Glasgow University's Professor Philippe Schyns, looked at the case of a woman who had rare damage to part of her brain.
They found she could only identify fear if asked to look at a person's eyes.
The research, which used 30 control subjects as well as the case study, was published in science journal Nature.
Part of the experiment saw participants being shown images of faces with their eyes erased.
The 38-year-old female case study had suffered bilateral damage to her amygdala - an almond-shape part of the brain in the temporal lobe.
Although she is completely unable to recognise fear from facial expressions, the scientists found the problem could be overcome by instructing her to concentrate on a person's eyes.
The researchers found, however, that the woman, named SM in the study, only succeeded in directing her gaze to the eye regions of facial images when given explicit reminders.
SM's problem was that although she could physically see facial features, she could not recognise the emotion of fear.
Her impairment stemmed from an inability to make normal use of information from the eye region within faces when judging emotions, although she could read emotions from the mouth.
The woman's selective impairment in recognising fear is explained by the fact that the eyes are the most important feature for identifying fear, say the experts.
However, the subject's recognition of fearful faces became entirely normal when she was instructed to look at the eyes.
The scientists say that the study shows there are mechanisms in the brain that allow us to actively seek out information in the environment in the first place.
They believe that the positive effect of explicit reminders may help those who suffer from disorders such as autism, which affects about half a million people in the UK.