Partially blind people can 'unconsciously' sense the facial expressions of others. Credit: Punchstock

Two patients, partially blind because of damage to one side of their brain, were able to sense, and respond to, emotions expressed by people in pictures presented to their blind sides.

A study by an international team of researchers found that the patients unconsciously twitched a facial muscle uniquely involved in smiling when a picture showed a happy person, and a muscle involved in frowning when the person depicted looked fearful1.

The patients, both from the United Kingdom, 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. This means that they cannot process information from the visual field on the opposite side of their nose.

The scientists, who were led by Marco Tamietto and Beatrice de Gelder at Tilburg University in the Netherlands, say the results 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," says neuroscientist Christian Keysers, an expert in the neurophysiology of emotion at the University of Groningen, the Netherlands, who was not involved in the study.

Second sight

The two patients were shown random mixtures of images of people looking happy or fearful, each for two seconds, in rapid succession. The pictures were presented on the side of their visual field that they could see, then on the side they could not consciously see. They were asked to press a button after each picture to indicate the emotion they had recognized, or guessed at.

They could sense emotion through an unconscious mechanism, and resonate with it. Marco Tamietto , Tilburg University

The scientists attached special electrodes to the patients' faces, allowing them to measure subtle contractions — of which we are usually unaware — of the tiny muscles involved in expressing emotion.

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. The response was the same whether the pictures were presented on the side they could see or the side they could not see. But the response was faster when the pictures were presented to the blind side — perhaps because there was no conscious emotional evaluation to delay things, says Tamietto.

"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," he says. "They could sense emotion through an unconscious mechanism, and resonate with it."

Mixed picture?

Much of the visual input from the retina goes directly to the visual cortex, which processes the information so that we consciously perceive the image we are looking at. But a small part goes directly to the midbrain, through an evolutionarily primitive subcortical pathway that processes emotion and other information central to survival — and that is intact in the two patients.

So it seems that emotional contagion can be implemented via evolutionarily ancient neural structures, says Tamietto, 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.

But Keysers cautions that it remains to be determined whether the subcortical and higher cortical pathways for recognizing emotions operate in parallel. Emotion recognition could use several types of available information, he says.