From monkey mirror neurons to primate behaviours: possible 'direct' and 'indirect' pathways

The discovery of mirror neurons (MNs), deemed to be at the basis of action understanding, could constitute the potential solution to the 'correspondence problem' between one's own and others' action that is crucial for of imitative behaviours. However, it is still to be clarified whether, and how, several imitative phenomena, differing in terms of complexity and cognitive effort, could be explained within a unified framework based on MNs. Here we propose that MNs could differently contribute to distinct imitative behaviours by means of two anatomo-functional pathways, subjected to changes during development. A 'direct mirror pathway', directly influencing the descending motor output, would be responsible for neonatal and automatic imitation. This proposal is corroborated by some new behavioural evidences provided here. During development, the increased control of voluntary movements and the capacity to efficiently suppress automatic motor activation during action observation assign to the core MNs regions essentially perceptuo-cognitive functions. These functions would be exploited by an 'indirect mirror pathway' from the core regions of the MN system to prefrontal cortex. This latter would play a key role in parsing, storing and organizing motor representations, allowing the emergence of more efficient and complex imitative behaviours such as response facilitation and true imitation.