Neural bases of hand synergies

TLDR

The human hand has many degrees of freedom, making control challenging, and synergy control offers dimensionality reduction while preserving flexibility, a concept widely applied across tasks. This review aims to chart the evolution of hand synergy concepts, elucidate the neuronal circuitry underlying observed synergy phenomena, and propose a theoretical framework reconciling fixed versus flexible synergies. The authors synthesize evidence from kinematic and force synergy studies, cortical and spinal neuronal recordings, and electromyographic analyses, and integrate these findings into a mechanistic model of synergy organization.

Abstract

The human hand has so many degrees of freedom that it may seem impossible to control. A potential solution to this problem is 'synergy control' which combines dimensionality reduction with great flexibility. With applicability to a wide range of tasks, this has become a very popular concept. In this review, we describe the evolution of the modern concept using studies of kinematic and force synergies in human hand control, cortical and spinal neurons, and electromyographic activity of hand muscles. We go beyond the often purely descriptive usage of the concept of synergy by reviewing the organization of the underlying neuronal circuitry in order to provide mechanistic explanations for various observed synergy phenomena. Finally, we propose a theoretical framework to reconcile important and still debated concepts such as the definitions of 'fixed' versus 'flexible' synergies and mechanisms underlying the combination of synergies for hand control.