Mirror neurons in monkeys
Neurons in a particular area of the monkey brain (F5) discharge when a particular type of action is performed, e.g.,
Some neurons are specific to actions performed with a particular body part, others are more general.
A subset of these neurons fire not only when the monkey performs one of these actions, but also when it observes the experimenter perform it.
The function of these mirror neurons appears to be the (supramodal) representation of action, which can be used for imitation of actions and for 'understanding' them. They form a link between sender and receiver.
Area F5 in the monkey brain is the homolog of Broca's area in humans-frontal cortex that is central to speech and language.
Evidence for a mirror system in humans
During the observation of various actions, a selective increase has been observed in motor evoked potentials in the muscles that subjects use for performing these actions.
Cortical neurons in areas parallel to F5 in monkeys show activation during action observation in PET studies.
Primates appear to have a fundamental mechanism for action recognition and imitation.
Actions can be recognized (and imitated) because the neural pattern elicited in the pre-motor areas during action observation are the similar to those generated internally to produce the action.
The "because" makes sense only if firing patterns of such neurons is innately specified.
But if so, this may be a better account that the M&M account which focuses on organ identification, as an explicit account of the association of self and other's organs is not provided.
Mirror view would also provide an account for why actions are imitated--obsevations of actions activate the same brain areas that control the initiation of self-generated motions. Normally, in adults, spinal cord mechanisms block the motoneurons involved.
Evolution of language
Rizolatti and Arbib argue that the system for action recognition be central to the evolution of human language.