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J Neurophysiol (March 31, 2004). doi:10.1152/jn.00519.2003
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Submitted on May 30, 2003
Accepted on March 16, 2004

Optimal Impedance Control for Task Achievement in the Presence of Signal-dependent Noise

Rieko Osu1, Naoki Kamimura2, Hiroshi Iwasaki2, Eri Nakano3, Chris M. Harris4, Yasuhiro Wada2, and Mitsuo Kawato1*

1 ATR Computational Neuroscience Laboratories, Kyoto, Japan
2 Nagaoka University of Technology, Niigata, Japan
3 ATR Human Information Processing Research Laboratories, Kyoto, Japan
4 Institute of Neuroscience, University of Plymouth, Plymouth, United Kingdom

* To whom correspondence should be addressed. E-mail: kawato{at}atr.jp.

There is an infinity of impedance parameter values, and thus different co-contraction levels, that can produce similar movement kinematics, from which the central nervous system must select one. Although signal-dependent noise (SDN) predicts larger motor-command variability during higher co-contraction, the relationship between impedance and task performance is not theoretically obvious and thus examined here. Subjects made goal-directed, single-joint elbow movements to either move naturally to different target sizes or voluntarily co-contract at different levels. Stiffness was estimated as the weighted summation of rectified EMG signals through the index of muscle co-contraction around the joint (IMCJ) proposed by Osu et al. (2002). When subjects made movements to targets of different sizes, IMCJ increased with the accuracy requirements, leading to reduced end-point deviations. Therefore, without the need for great accuracy, subjects accepted worse performance with lower co-contraction. When subjects were asked to increase co-contraction, the variability of electromyography (EMG) and torque both increased, suggesting that noise in the neuromotor command increased with muscle activation. In contrast, the final positional error was smallest for the highest IMCJ level. Although co-contraction increases the motor-command noise, the effect of this noise on the task performance is reduced. Subjects were able to regulate their impedance and control end-point variance as the task requirements changed, and they did not voluntarily select the high impedance that generated the minimum end-point error. These data contradict predictions of the SDN-based theory, which postulates minimization of only end-point variance, and thus require its revision.




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