Hand synergies: Integration of robotics and neuroscience for understanding the control of biological and artificial hands
Physics of Life Reviews
The term 'synergy' -from the Greek synergia -means 'working together'. The concept of multiple elements working together towards a common goal has been extensively used in neuroscience to develop theoretical frameworks, experimental approaches, and analytical techniques to understand neural control of movement, and for applications for neuro-rehabilitation. In the past decade, roboticists have successfully applied the framework of synergies to create novel design and control concepts for
... ial hands, i.e., robotic hands and prostheses. At the same time, robotic research on the sensorimotor integration underlying the control and sensing of artificial hands has inspired new research approaches in neuroscience, and has provided useful instruments for novel experiments. The ambitious goal of integrating expertise and research approaches in robotics and neuroscience to study the properties and applications of the concept of synergies is generating a number of multidisciplinary cooperative projects, among which the recently finished 4-year European project "The Hand Embodied" (THE). This paper reviews the main insights provided by this framework. Specifically, we provide an overview of neuroscientific bases of hand synergies and introduce how robotics has leveraged the insights from neuroscience for innovative design in hardware and controllers for biomedical engineering applications, including myoelectric hand prostheses, devices for haptics research, and wearable sensing of human hand kinematics. The review also emphasizes how this multidisciplinary collaboration has generated new ways to conceptualize a synergy-based approach for robotics, and provides guidelines and principles for analyzing human behavior and synthesizing artificial robotic systems based on a theory of synergies. M. Santello et al. / Physics of Life Reviews to the nervous system would interfere with the ability of the sensorimotor system to flexibly combine synergies, thus leading to abnormal synergies ([15,16]; for a review see  ). As the number of studies of hand synergies in animal and human models grew and refined analytical and experimental approaches, over the past decade roboticists have started to investigate the potential applications of hand synergies for artificial hands [17, 18] . This work has led to the successful application of the concept of synergies  and the creation of novel design and control concepts for robotic hands and prostheses, while providing experimental tools and new research approaches for neuroscience. The aims of this review are to provide an overview of the neuroscientific bases of hand synergies -meant as common patterns of actuation of the human hand -and to report how robotics has leveraged the insights from neuroscience for innovative design in hardware and controllers for biomedical engineering applications, including myoelectric hand prostheses, devices for haptics research and human-machine interactions, and wearable sensing of human hand kinematics. We also describe insights that robotics research on artificial hand design and control has provided to neuroscience research. This review mainly focuses on the most significant achievements of the international cooperation project called "THE Hand Embodied" 1 and is organized in four sections. The first section describes the synergistic organization underlying motor control of the human hand. We will review how such an organization can be observed at different levels -postural, muscular, and neural -and quantified through different techniques -motion tracking, electromyography (EMG), and brain functional imaging. The second section describes how the concept of synergy has been exploited in robotics to develop tools for the analysis, modeling, and synthesis of artificial robotic hands, with potential applications for human-robot interaction and prosthetics. The third section focuses on the relation between sensing and human hand synergies, and in particular on how the concept of dimensionality reduction might apply to the sensory domain of hand control, in the framework we define as sensory synergies. Furthermore, we consider how sensory and motor elements can be combined together into the paradigm of sensory-motor synergies. Analogies and applications in robotics are also discussed. The fourth and last section introduces open questions and directions for future research built on leveraging the complementary questions and approaches used in robotics and neuroscience to advance our understanding of neural control of the hand, as well as biomedical and robotic applications.