Partially blind people can 'unconsciously' sense the facial expression of others shows a study by an international team of researchers.
Led by Marco Tamietto and Beatrice de Gelder at Tilburg University in the Netherlands, the study involved two patients from the United Kingdom who have the very rare condition known as partial cortical blindness.
Their eyes are intact but they have damage to the visual cortex on one side of their brain, which means that they cannot process information from the visual field on the opposite side of their nose.
However, during the study, the researchers observed that the patients were able to sense, and respond to, emotions expressed by people in pictures presented to their blind sides.
The scientists say that their findings show that our spontaneous tendency to synchronize our facial expressions with those of other people in face-to-face situations - known as emotional contagion - occurs even if we cannot consciously see them.
"This is interesting evidence that we can recognize the emotions of others without needing to be visually aware of them," Nature magazine quoted neuroscientist Christian Keysers, an expert in the neurophysiology of emotion at the University of Groningen, the Netherlands, who was not involved in the study, as saying.
During the study, the patients were shown random mixtures of images of people looking happy or fearful, each for two seconds, in rapid succession.
The researchers presented the pictures on the side of their visual field that they could see, then on the side they could not consciously see.
The patients were asked to press a button after each picture to indicate the emotion they had recognized, or guessed at.
Special electrodes were attached to the patients' faces, which allowed the researchers to measure subtle contractions - of which we are usually unaware - of the tiny muscles involved in expressing emotion.
The researchers observed that the patients twitched their smiling-specific zygomaticus major muscle when presented with happy pictures, and the frowning-specific corrugator supercilii muscle when presented with fearful pictures.
According to them, the response was the same whether the pictures were presented on the side they could see or the side they could not see.
However, says Tamietto, the response was faster when the pictures were presented to the blind side - perhaps because there was no conscious emotional evaluation to delay things.
"The subjects were not simply imitating the expression of others, because their faces responded whether the emotion was conveyed to them via facial expression or body language. They could sense emotion through an unconscious mechanism, and resonate with it," he says.
Analysing the results, Tamietto say that it seems that emotional contagion can be implemented via evolutionarily ancient neural structures, and does not necessarily require the involvement of higher brain regions, visual awareness or the mirror neurons that are active when we recognize the physical actions of others.
Keysers, however, insists that there is need for further studies to determine whether the subcortical and higher cortical pathways for recognizing emotions operate in parallel.
Emotion recognition could use several types of available information, he says.