Editorial: Modularity in motor control: from muscle synergies to cognitive action representation.

Abstract

Mastering a rich repertoire of motor behaviors, as humans and other animals do, is a surprising and still a poorly understood outcome of evolution, development, and learning. Many degrees-of-freedom, non-linear dynamics, and sensory delays provide formidable challenges for controlling even simple actions. Modularity as a functional element, both structural and computational, of a control architecture might be the key organizational principle that the central nervous system employs for achieving versatility and adaptability in motor control. Recent investigations of muscle synergies, motor primitives, compositionality, basic action concepts, and related work in machine learning have contributed, at different levels, to advance our understanding of the modular architecture underlying rich motor behaviors. However, the existence and nature of the modules comprising the control architecture is far from settled. For instance, regularity and low-dimensionality of the motor output are often taken as an indication of modularity but they could simply be a byproduct of optimization and task constraints. Moreover, what are the relationships between modules at different levels, such as muscle synergies, kinematic invariants, and basic action concepts? One important reason for the new interest in understanding modularity in motor control from different perspectives is the impressive development in cognitive robotics. In comparison to animals and humans, the motor skills of today's best robots are limited and inflexible. However, robot technology is maturing to the point at which it can start approximating a reasonable spectrum of different perceptual, cognitive, and motor capabilities. These advances allow researchers to explore how these motor, sensory, and cognitive functions might be integrated into meaningful architectures and to test their functional limits. Such systems provide a new test bed to explore different concepts of modularity and to experimentally investigate possible interactions between motor and cognitive processes. Thus, the goal of this Research Topic is to review, compare, and debate theoretical and experimental studies of the modular organization of the motor control system at different levels. By bringing together researchers seeking to understand the building blocks of coordinating many muscles, planning endpoint and joint trajectories, and representing motor and behavioral actions in memory we aim at promoting new interactions between often disconnected research areas and approaches and providing a broad perspective on the notion of modularity in motor control.