How does the mirror neuron system change during development?

It has been suggested that the mirror neuron system (MNS) enables the understanding of others’ intentions by simply observing their actions. As Lepage and Théoret’s paper highlights, while the MNS has been extensively studied in adult humans and in monkeys, very little is known about its development. What properties of the MNS change during development, or even when in development a functioning MNS is present, is unknown. Lepage and Théoret review the literature and, on this basis, suggest that the MNS is present by infancy. Here, we argue that, even if this is the case, this does not preclude further developmental change. An understanding of how the MNS develops would shed light on its mechanisms and functions. The notion that actions are intrinsically linked to perception was proposed by William James, who suggested that ‘every mental representation of a movement awakens to some degree the actual movement which is its object’ (James, 1890, p. 293). The implication is that observing, imagining, preparing, or in any way representing an action excites the motor program used to execute that same action (Jeannerod, 1994; Prinz, 1997). Interest in this idea has grown recently, in part due to the neurophysiological discovery of mirror neurons and, in turn, the mirror neuron system (MNS). Mirror neurons in monkey premotor area F5 and inferior parietal lobule (PF) discharge not only during action execution but also during action observation, which has led many to suggest that these neurons are the neural substrate for action understanding. As Lepage and Théoret’s paper highlights, although the MNS has been extensively studied in both adult humans and monkeys, very little is known about the development of the MNS. What properties of the MNS change during development, or even when in development a functioning MNS is present, is unknown. Lepage and Théoret’s paper addresses this latter issue and they suggest that there are sufficient data to conclude that the MNS is present in infancy. However, this does not preclude further developmental change to the MNS, and an understanding of this would shed light on the mechanisms and functions of the MNS. A recent theoretical account of the MNS predicts that it is the strength of the connections between areas active during action observation that are modulated during development. This theoretical account proposes that the role of the MNS in reading or recognizing the goals of observed actions is best understood within a predictive coding framework (Kilner, Friston & Frith, 2007). The predictive coding framework is a form of recognition model that enables perceptual inference when the mapping between causes and inputs is ill-posed. For example, if while walking along the street someone suddenly waves their arm, are they hailing a taxi or swatting a wasp? In other words, this form of recognition model can infer the goal of an observed action even when the same observed kinematics can be caused by more than one goal or intention. In this type of model, recognition is the inverse of generating sensory data from their causes. Generative models can be framed in terms of a deterministic non-linear generative function: