A second class of grasping-related neurons, mirror neurons, has been described in area F5 of the macaque monkey (Gallese et al. 1996; Rizzolatti et al. 1996). These neurons, although having the same motor properties as canonical neurons, differ sharply in the nature of their visual properties. Mirror neurons are not activated during the observation of objects but only during the observation of an agent (a human being or a monkey) acting in a purposeful way upon objects with his hand or his mouth. Neither the sight of the object alone nor of the agent alone is effective in driving these neurons. Mimicking the action in the absence of the target object or using a tool to execute the object-related action is similarly ineffective in driving mirror neurons' activity.
In a relevant percentage of mirror neurons a strict congruence between the observed action effective in triggering the neural visual response and the executed action effective in driving the motor response has been observed. In other words, the observed action performed by another individual evokes in the observer the same neural pattern that occurs during the active execution of that action. Grasping, holding, manipulating, and tearing objects are the actions that, both when observed and when executed, most frequently activate these neurons.
On the basis of their functional properties, mirror neurons can be considered as constituting an action observation/execution matching system. What is the link between acting and observing someone else acting? This link is constituted by the presence in both instances of a goal. This goal is recognized and "understood" by the observer by mapping it on a shared motor representation. Again, as in the case of canonical neurons, the motor system exhibits a double function. On one side it supervises action execu tion, and on the other it "validates" what is perceived in motor terms.
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