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Dissociating Variability and Effort as Determinants of Coordination

O'Sullivan, I; Burdet, E; Diedrichsen, J; (2009) Dissociating Variability and Effort as Determinants of Coordination. PLOS COMPUT BIOL , 5 (4) , Article e1000345. 10.1371/journal.pcbi.1000345. Green open access

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Abstract

When coordinating movements, the nervous system often has to decide how to distribute work across a number of redundant effectors. Here, we show that humans solve this problem by trying to minimize both the variability of motor output and the effort involved. In previous studies that investigated the temporal shape of movements, these two selective pressures, despite having very different theoretical implications, could not be distinguished; because noise in the motor system increases with the motor commands, minimization of effort or variability leads to very similar predictions. When multiple effectors with different noise and effort characteristics have to be combined, however, these two cost terms can be dissociated. Here, we measure the importance of variability and effort in coordination by studying how humans share force production between two fingers. To capture variability, we identified the coefficient of variation of the index and little fingers. For effort, we used the sum of squared forces and the sum of squared forces normalized by the maximum strength of each effector. These terms were then used to predict the optimal force distribution for a task in which participants had to produce a target total force of 4-16 N, by pressing onto two isometric transducers using different combinations of fingers. By comparing the predicted distribution across fingers to the actual distribution chosen by participants, we were able to estimate the relative importance of variability and effort of 1:7, with the unnormalized effort being most important. Our results indicate that the nervous system uses multi-effector redundancy to minimize both the variability of the produced output and effort, although effort costs clearly outweighed variability costs.

Type: Article
Title: Dissociating Variability and Effort as Determinants of Coordination
Open access status: An open access version is available from UCL Discovery
DOI: 10.1371/journal.pcbi.1000345
Publisher version: http://dx.doi.org/10.1371/journal.pcbi.1000345
Language: English
Additional information: © 2009 O'Sullivan et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was funded by the Biotechnology and Biological Sciences Research Council (BB/E009174/1) and the National Science Foundation (BSC 0726685).The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Keywords: FORCE PRODUCTION, FEEDBACK-CONTROL, NERVOUS-SYSTEM, ARM MOVEMENTS, NOISE, WRIST, TASKS, MODEL
UCL classification: UCL
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences > Faculty of Brain Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/120847
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