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The Journal of Neuroscience, February 15, 2001, 21(4):1361-1369
The Inertial Anisotropy of the Arm Is Accurately Predicted
during Movement Planning
J. Randall
Flanagan and
Sarah
Lolley
Department of Psychology and Canadian Institutes of Health Research
Group in Sensory-Motor Systems, Queen's University, Kingston, Ontario,
K7L 3N6, Canada
An important theoretical concept in motor control is the idea that
the CNS uses an internal model of the motor system and environment to
predict the sensory consequences of motor commands. In arm movement
control, a critical factor affecting the transformation from motor
commands to sensory consequences is limb dynamics, including the
inertial anisotropy of the arm, which refers to the fact that the
inertial resistance of the arm depends on hand movement
direction. Here we show that the CNS maintains an accurate internal model of the inertial anisotropy of the arm by demonstrating that the motor system can precisely predict direction-dependent variations in hand acceleration. Subjects slid an object, held beneath
the index finger, across a frictionless horizontal surface to radially
located targets. We recorded the normal (vertical) force exerted by the
fingertip, as well as the tangential (horizontal) force proportional to
hand acceleration. We found that normal force was precisely scaled in
anticipation of tangential force, which, as expected, varied with
direction. The peak rates of change of the normal and tangential
forces, observed early in the movement, were highly correlated. Similar
results were obtained regardless of whether the start position of the
hand was located directly in front of the subject or rotated 45° to
the right. Finally, we observed reduced force correlations under
reaction time conditions. This suggests that the process of prediction,
based on an internal model of the limb, is not fully completed within
the reaction time interval.
Key words:
internal models; arm movement; sensorimotor prediction; object manipulation; reaching; motion planning
Copyright © 2001 Society for Neuroscience 0270-6474/01/2141361-09$05.00/0
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