The Journal of Neuroscience, August 1, 1998, 18(15):5948-5957
The Role of Inertial Sensitivity in Motor Planning
Philip N.
Sabes1,
Michael I.
Jordan1, and
Daniel M.
Wolpert2
1 Department of Brain and Cognitive Sciences,
Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and 2 Sobell Department of Neurophysiology, Institute of
Neurology, London WC1N 3BG, United Kingdom
To achieve a given motor task a single trajectory must be chosen
from the infinite set of possibilities consistent with the task. To
investigate such motor planning in a natural environment, we examined
the kinematics of reaching movements made around a visual obstacle in
three-dimensional space. Within each session, the start and end points
of the movement were uniformly varied around the obstacle. However, the
distribution of the near points, where the paths came closest to the
obstacle, showed a strong anisotropy, clustering at the poles of a
preferred axis through the center of the obstacle. The preferred axes
for movements made with the left and right arms were mirror symmetric
about the midsagittal plane, suggesting that the anisotropy stems from
intrinsic properties of the arm rather than extrinsic visual factors.
One account of these results is a sensitivity model of motor planning,
in which the movement path is skewed so that when the hand passes
closest to the obstacle, the arm is in a configuration that is least
sensitive to perturbations that might cause collision. To test this
idea, we measured the mobility ellipse of the arm. The mobility minor axis represents the direction in which the hand is most inertially stable to a force perturbation. In agreement with the sensitivity model, the mobility minor axis was not significantly different from the
preferred near point axis. The results suggest that the sensitivity of
the arm to perturbations, as determined by its inertial stability, is
taken into account in the planning process.
Key words:
human psychophysics; visuomotor control; motor planning; reaching; obstacle avoidance; optimal control; theoretical model
Copyright © 1998 Society for Neuroscience 0270-6474/98/18155948-10$05.00/0
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