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How our brain decodes facial expressions has been a matter of debate for neuroscientists. Credit: master1305/ iStock / Getty Images Plus.

A study by researchers in Parma and Milan has shown that we interpret facial expressions using both motor and emotional areas of the brain. The finding could solve a 20-year debate between scientists working on ‘mirror’ neurons.

Mirror neurons fire both when an organism performs an action and when it watches someone else perform the same action. Since their discovery in the 1990s they have been described as the neural basis of our ability to understand another person's actions, by replicating them in our own brain. But two different hypotheses have been proposed by cognitive neuroscientists to explain where the ‘mirroring’ and interpretation of other people’s emotional facial expressions actually happens. One, called motor resonance, suggests it occurs within motor and premotor areas of the brain. The other, called emotional resonance, says that this recognition happens mostly in emotional and interoceptive regions.

The answer, according to clinical experiments carried out on patients being treated for epilepsy at the Niguarda Hospital in Milan, is that both the motor and emotional channels are involved in the empathic processes of face recognition, but they work independently. “This means that we have not one but two autonomous 'mirror systems' hidden in the frontal lobe," explains Fausto Caruana, a researcher at the CNR in Parma, and lead author of the article in Brain1.

"We asked more than 40 epilepsy patients to observe the facial expressions of other people with a positive, negative or neutral valence, while colleagues from Niguarda monitored the different areas of the brain to understand which ones were activated and how," Caruana continues. The study was innovative because it combined three invasive techniques that are only possible in epilepsy surgery patients: intracranial recordings, electrical stimulation and effective connectivity.

The researchers observed that emotional expressions activated both the motor and the emotional pathways independently, along with regions that are neither motor nor emotional, but related to attentional processes.

"The possibility that mirror neurons activate in non-motor areas of the brain has been known for some time, but until now it was not clear which other parts were activated and how, and whether there are connections between these systems,” says Caruana. “The question now is what different functions they might have. Our guess is that the motor channel may contribute to the recognition of ambiguous expressions, while the emotional channel may concur to emotional contagion and social synchronization".