Evidence for sparse synergies in grasping actions

• Published in: Scientific reports Vol. 8; no. 1; pp. 616 - 16
• Main Authors: Prevete, Roberto, Donnarumma, Francesco, d’Avella, Andrea, Pezzulo, Giovanni
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 12.01.2018; Nature Publishing Group
• Subjects: 631/378/116; 631/378/2649/1773; 639/705/117; Biomechanical Phenomena; Grasping; Hand - physiology; Hand Strength - physiology; Humanities and Social Sciences; Humans; Hypotheses; Mapping; Movement - physiology; multidisciplinary; Muscles; Psychomotor Performance; Science; Science (multidisciplinary); Sparsity
• ISSN: 2045-2322; 2045-2322
• DOI: 10.1038/s41598-017-18776-y

Converging evidence shows that hand-actions are controlled at the level of synergies and not single muscles. One intriguing aspect of synergy-based action-representation is that it may be intrinsically sparse and the same synergies can be shared across several distinct types of hand-actions. Here, adopting a normative angle, we consider three hypotheses for hand-action optimal-control: sparse-combination hypothesis (SC) – sparsity in the mapping between synergies and actions - i.e., actions implemented using a sparse combination of synergies; sparse-elements hypothesis (SE) – sparsity in synergy representation – i.e., the mapping between degrees-of-freedom (DoF) and synergies is sparse; double-sparsity hypothesis (DS) – a novel view combining both SC and SE – i.e., both the mapping between DoF and synergies and between synergies and actions are sparse, each action implementing a sparse combination of synergies (as in SC), each using a limited set of DoFs (as in SE). We evaluate these hypotheses using hand kinematic data from six human subjects performing nine different types of reach-to-grasp actions. Our results support DS, suggesting that the best action representation is based on a relatively large set of synergies, each involving a reduced number of degrees-of-freedom, and that distinct sets of synergies may be involved in distinct tasks.