Scientists at Bangor University have discovered a patient who appears to possess a "sixth sense" that allows him to recognise sad faces.
The researchers scanned people's brains to monitor activity
The 52-year-old "patient X" suffered two strokes which damaged the brain areas which process visual signals.
Although he cannot see, researchers found that that the patient was able to identify angry or happy human faces.
Scans showed that when the man looked at faces with emotion, another part of his brain, the amygdala, was activated.
The small almond-shaped structure is known to respond to non-verbal signs of anger, defensiveness, avoidance and fear.
The results of the study are published in the online edition of Nature Neuroscience on Sunday.
Dr Alan Pegna of the school of psychology at the University of Wales, Bangor, led the research team with colleagues in north Wales and Geneva University Hospital.
They found that when "patient X" was shown images of shapes like circles and squares, he was only able to make wild guesses about what they were.
Nor was he able to identify the sex of "deadpan" male and female faces with any degree of success, or tell the difference between "normal" and jumbled faces.
But when the patient was asked to identify angry or happy human faces, he did so with an accuracy of 59% - significantly higher than would be expected by chance.
There was a similar response when he was asked to distinguish between sad and happy, or fearful and happy, faces.
But pictures of animals which appeared threatening or non-threatening did not give the same kind of results.
Scientists were able to establish that emotion displayed on a human face is registered in an area other than the visual cortex.
The area involved was identified as the right amygdala, an almond-shape structure situated deep within the brain's temporal lobe.
"This discovery is... interesting for behavioural scientists as the right amygdala has been associated with subliminal processing of emotional stimuli in clinically healthy individuals," said Dr Pegna.
"What 'patient X' has assisted us with establishing is that this area undoubtedly processes visual facial signals connected with all types of emotional facial expressions."