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Hierarchies of synergies: an example of two-hand, multi-finger tasks

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Abstract

We explored the ability of the central nervous system (CNS) to assemble synergies stabilizing the output of sets of effectors at two levels of a control hierarchy. Specifically, we asked a question: can the CNS organize both two-hand and within-a-hand force stabilizing synergies in a simple two-hand force production task that involves two fingers per hand? Intuitively, one could expect a positive answer; that is, forces produced by each hand are expected to co-vary negatively across trials to bring down the total force variability, while forces produced by each finger within-a-hand are expected to co-vary negatively to reduce the variability of that hand’s contribution to the total force. The subjects were instructed to follow a trapezoidal time profile with the signal corresponding to the force produced by a set of instructed fingers in one-hand tasks with two-finger force production and in two-hand tasks with involvement of both symmetrical and asymmetrical finger pairs in the two hands. Finger force co-variation across trials was quantified and used as an index of stabilization of the force produced by all the instructed fingers, and of the force produced by finger pairs within-a-hand. No major differences were seen between the dominant and the non-dominant hand and between the two-hand tasks with symmetrical and asymmetrical finger involvement. Stronger synergies were seen in the index–middle finger pair as compared to the ring-little finger pair. The main result of the study is the significantly weaker or even lacking two-finger force stabilizing synergies within-a-hand during two-hand tasks while such synergies were present in one-hand tasks. This observation points at a potential limitation in the ability of the CNS to organize synergies at two levels of a control hierarchy simultaneously. It also allows suggesting a hypothesis on two types of synergies in the human motor repertoire, well-practiced synergies that form a library serving as the foundation for all novel actions, and freshly assembled synergies.

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Authors and Affiliations

  1. Department of Kinesiology, The Pennsylvania State University, Rec.Hall-268N, University Park, 16802, PA, USA

    Stacey L. Gorniak, Vladimir M. Zatsiorsky & Mark L. Latash

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  1. Stacey L. Gorniak
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  2. Vladimir M. Zatsiorsky
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  3. Mark L. Latash
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Correspondence to Mark L. Latash.

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Gorniak, S.L., Zatsiorsky, V.M. & Latash, M.L. Hierarchies of synergies: an example of two-hand, multi-finger tasks. Exp Brain Res 179, 167–180 (2007). https://doi.org/10.1007/s00221-006-0777-z

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