Inference and representations of hand actions through grasping synergies: comment on "Grasping synergies: a motor-control approach to the mirror neuron mechanism" by D'Ausilio, Bartoli, and Maffongelli.

The concept of synergy, denoting the coordination of multiple elements working together toward a common goal, has been extensively studied to understand how the central nervous system (CNS) controls movement (for review see [5,9]). Although this definition is appealing in its simplicity, ‘multiple elements’, ‘working together’, and ‘common goal’ each take different meanings depending on the scale at which a given sensorimotor system is studied, whether the ‘working together’ is defined in spatial and/or temporal domains, and the hypothesized synergy’s ‘common goal’. For example, the elements involved in a synergy can be defined as single motor units, muscles, or joints. Similarly, the goal of a synergy may be defined as a means available to the CNS to ‘simplify’ the control of multiple elements, or to minimize a given cost function or movement feature — all of which may differ across tasks and tasks conditions. These considerations underscore the fact that a universally accepted definition of synergies and their functional role remains to be established (for review see [6]). Thus, the nature and functional role(s) of synergies is still debated in the literature. Nevertheless, it is generally agreed that the reduction in the number of independent degrees of freedom that is manifested through synergies emerges from the interaction of biomechanical and neural factors constraining the spatial and temporal coordination of multiple muscles. The theoretical framework of synergies has motivated extensive investigations of fundamental questions in motor neuroscience about neural representations of complex movements (for review see [1,3,4]). In their review [2], D’Ausilio and colleagues propose an additional and novel perspective on the role that synergies might play in the context of movement observation and the mirror neuron system. Their focus is on grasp synergies and, more specifically, on their temporal evolution during the act of reaching towards an object to be grasped and/or manipulated. Grasp synergies in the kinematic domain emerge from the interaction between biomechanical and neural constraints, e.g. tendons spanning multiple joints of the digits and common neural inputs to spinal motor nuclei [6]. The ‘observable’ outcome of this interaction is the spatial and temporal coordination of multiple joints of the digits that is found not