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The Journal of Neuroscience, October 7, 2009, 29(40):12606-12616; doi:10.1523/JNEUROSCI.2826-09.2009

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Behavioral/Systems/Cognitive
Impedance Control Reduces Instability That Arises from Motor Noise

Luc P. J. Selen, David W. Franklin, and Daniel M. Wolpert

Computational and Biological Learning Laboratory, Department of Engineering, University of Cambridge, Trumpington Street, CB2 1PZ, Cambridge, United Kingdom

Correspondence should be addressed to Luc P. J. Selen at his present address: Donders Institute for Brain, Cognition, and Behaviour, Radboud University, Montessorilaan 3, 6525 HR Nijmegen, The Netherlands. Email: L.Selen{at}donders.ru.nl

There is ample evidence that humans are able to control the endpoint impedance of their arms in response to active destabilizing force fields. However, such fields are uncommon in daily life. Here, we examine whether the CNS selectively controls the endpoint impedance of the arm in the absence of active force fields but in the presence of instability arising from task geometry and signal-dependent noise (SDN) in the neuromuscular system. Subjects were required to generate forces, in two orthogonal directions, onto four differently curved rigid objects simulated by a robotic manipulandum. The endpoint stiffness of the limb was estimated for each object curvature. With increasing curvature, the endpoint stiffness increased mainly parallel to the object surface and to a lesser extent in the orthogonal direction. Therefore, the orientation of the stiffness ellipses did not orient to the direction of instability. Simulations showed that the observed stiffness geometries and their pattern of change with instability are the result of a tradeoff between maximizing the mechanical stability and minimizing the destabilizing effects of SDN. Therefore, it would have been suboptimal to align the stiffness ellipse in the direction of instability. The time course of the changes in stiffness geometry suggests that modulation takes place both within and across trials. Our results show that an increase in stiffness relative to the increase in noise can be sufficient to reduce kinematic variability, thereby allowing stiffness control to improve stability in natural tasks.

Received June 16, 2009; revised Aug. 24, 2009; accepted Aug. 26, 2009.

Correspondence should be addressed to Luc P. J. Selen at his present address: Donders Institute for Brain, Cognition, and Behaviour, Radboud University, Montessorilaan 3, 6525 HR Nijmegen, The Netherlands. Email: L.Selen{at}donders.ru.nl






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Copyright 2009 by Society for Neuroscience ONLINE ISSN: 1529-2401
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