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J Neurophysiol (October 15, 2003). doi:10.1152/jn.00652.2003
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Submitted on July 8, 2003
Accepted on October 10, 2003

The role of execution noise in movement variability

Robert J. van Beers1*, Patrick Haggard2, and Daniel M. Wolpert3

1 Institute of Cognitive Neuroscience, University College London, London, United Kingdom; Sobell Department of Motor Neuroscience, Institute of Neurology, London, United Kingdom; Department of Neuroscience, Erasmus MC, Rotterdam, The Netherlands
2 Institute of Cognitive Neuroscience, University College London, London, United Kingdom
3 Sobell Department of Motor Neuroscience, Institute of Neurology, London, United Kingdom

* To whom correspondence should be addressed. E-mail: r.vanbeers{at}erasmusmc.nl.

The origin of variability in goal-directed movements is not well understood. Variability can originate from several neural processes such as target localization, movement planning and movement execution. Here we examine variability resulting from noise in movement execution. In several experiments, subjects moved their unseen hand to visual targets, under conditions which were designed to minimize the variability expected from localization and planning processes. We tested short movements in 32 directions in a center-out reaching task. The variability in the movement endpoints and in the initial movement direction varied systematically with the movement direction, with some directions having up to twice the variability of others. In a second experiment we tested four movements in the same direction but with different extents. Here, the longer movements were systematically curved and the endpoint ellipses were not aligned with the straight line between starting and end position, but they were roughly aligned with the last part of the trajectory. We show that the variability observed in these experiments cannot be explained by planning noise but is well explained by noise in movement execution. A combination of both signal-dependent and signal-independent noise in the amplitude of the motor commands, and temporal noise in their duration can explain the observed variability. Our results suggest that, in general, execution noise accounts for at least a large proportion of movement variability.




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