Mirror Neurons

Mirror Neurons in Monkeys

In the early 1990s, a new class of premotor neurons, “mirror neurons,” was discovered in the anterior sector of the macaque monkey's ventral premotor cortex, known as area F5. Mirror neurons discharge not only when the monkey executes goal-related hand motor acts such as grasping objects, but also when it observes other individuals (monkeys or humans) executing similar motor acts. Neurons with similar properties were subsequently discovered in regions of the posterior parietal cortex reciprocally connected with area F5.

Action observation causes in the observer the automatic activation of the same neural mechanism triggered by action execution. For the first time, a neural mechanism allowing for a direct matching between the visual perception of an action and its execution has been identified. By means of the mirror matching mechanism, the results of the visual analysis of the observed action—which, in principle, has no meaning for the observer—can be translated into an account that the individual is able to understand. It was proposed that this mechanism could underlie a direct form of action understanding. If mirror neurons do in fact mediate action understanding, their activity should reflect the meaning of the observed action, not its visual features.

Two sets of experiments were carried out to verify this hypothesis. The first experiments tested whether the mental representation of an action triggers F5 mirror neurons, the second whether mirror neurons can respond to the sound produced by actions. The results of these experiments answered both questions in the affirmative and showed that what drives mirror neurons’ discharge is not the mere visual description of a motor act, but rather its goal.

In the most lateral part of area F5, a class of mirror neurons responding to the execution and observation of mouth actions has been found. The majority of these neurons discharge when the monkey executes and observes mouth-related, object-related motor acts, such as grasping, biting, or licking. However, a small percentage of mouth-related mirror neurons discharge during the observation of communicative facial actions performed by the experimenter in front of the monkey (“communicative mirror neurons”). Macaque monkeys show an initial capacity to control and “voluntarily” emit social signals; this is mediated by the frontal lobe. It is interesting that this capacity develops in a cortical area—area F5—that in humans became Brodmann's area 44, a key area for verbal communication.

More recently, premotor and parietal mirror neurons have been found to have a role in intention [Page 521]understanding. The discharge of mirror neurons during the observation of an act (e.g., grasping an object) is conditioned by the type of subsequent act (e.g., bringing the object to the mouth) that specifies the overall action intention. In addition to recognizing the goal of the observed motor act, mirror neurons allow the observing monkey to predict the agent's next action, and hence its overall basic motor intention. This neural mechanism could provide scaffolding for more sophisticated social cognitive abilities, such as those that characterize the human species.

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