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Mirror neurons were discovered at the Università degli Studi di Parma in the 1990s, and first described as nerve cells in the frontal and premotor cortex of macaques that fire both when the monkey performs a movement and when it observes the same movement by another monkey or a human. The presence of a ‘mirror’ system (describing a close match between execution and observation of movements) was then also confirmed in humans, but not in other species, indicating that this might be a peculiar feature of primates. Scientists have theorised that mirror neurons allow understanding of the meaning of another’s actions, and constitute the neural basis of imitation learning and empathy.

The original description of mirror neurons was that they are not activated when watching a non-biological stimulus, such as a tool or object performing an action. A team led by Luca Bonini, also at the Università degli Studi di Parma, had adult macaques perform different grasping experiments, while recording the activity of several brain regions involved in the execution and observation of actions. As expected, mirror neurons fired both when the monkeys were grasping objects of different shapes, and when they were watching a human grasp them. But the same neurons also fired when the monkey was watching a metal cylinder move towards the object with an unnatural movement. “We looked at the whole neural population in the region and realised that, though the activation level is different, mirror neurons are activated regardless of how the observed task is performed”, explains Davide Albertini, first author of the study in the Journal of Neurophysiology1.

In fact, there was more overlap between the neural activation patterns for the observation of biological and non-biological stimuli, than between the patterns for execution and observation of biological movements. “Our work shows that the activation of those neurons in the premotor and motor area is a passive response to the observation of an action,” says Albertini.

"For the first time, a non-matching between observing and executing domains is reported," says Vassilis Raos, associate professor at the University of Crete and co-author of some of the seminal studies on mirror neurons, who was not involved in the study. “This finding opens a new era in studying the fundamental properties of mirror neurons”.

According to Gregory Hickok, a professor of cognitive science at the University of California Irvine and a critic of the original mirror neuron theory, “this is additional confirmation that the mirror neuron system is not the basis of action understanding”. The new study, he notes, shows not only that mirror neurons respond to non-biological stimuli, but that these do not even have to include the core actions (here, a kind of grasp) to activate the network.