Motor Adaptation to Coriolis Force Perturbations of Reaching Movements:
Endpoint but not Trajectory Adaptation Transfers to the Non-exposed Arm.
DiZio, Paul and James R. Lackner.
Ashton Graybiel Spatial Orientation Laboratory, Brandeis University,
Waltham, Massachusetts 02254-9110.
APStracts 2:0192N, 1995.
SUMMARY AND CONCLUSIONS
1. Reaching movements made in a rotating room generate Coriolis forces that
are directly proportional to the cross product of the room's angular velocity
and the arm's linear velocity. Such Coriolis forces are inertial forces not
involving mechanical contact with the arm. 2. We measured the trajectories of
arm movements made in darkness to a visual target that was extinguished at the
onset of each reach. Pre-rotation subjects pointed with both the right and
left arms in alternating sets of 8. During rotation at 10 ? rpm, the subjects
reached only with the right arm. Post-rotation, the subjects pointed with the
left and right arms, starting with the left, in alternating sets of 8
movements. 3. The initial per-rotary reaching movements of the right arm were
highly deviated both in movement path and endpoint relative to the pre-
rotation reaches of the right arm. With additional movements, subjects rapidly
regained straight movement paths and accurate endpoints despite the absence of
visual or tactile feedback about reaching accuracy. The initial post-rotation
reaches of the left arm followed straight paths to the wrong endpoint. The
initial post-rotation reaches of the right arm had paths with mirror image
curvature to the initial per-rotation reaches of the right arm but went to the
correct endpoint. 4. These observations are inconsistent with current
equilibrium point models of movement control. Such theories predict accurate
reaches under our experimental conditions. Our observations further show
independent implementation of movement and posture, as evidenced by transfer
of endpoint adaptation to the non-exposed arm without transfer of path
adaptation. Endpoint control may occur at a relatively central stage that
represents general constraints such as gravitoinertial force background or
ego-centric direction relative to both arms, and control of path may occur at
a more peripheral stage that represents moments of inertia and muscle dynamics
unique to each limb. 5. Endpoint and path adaptation occur despite the absence
both of mechanical contact cues about the perturbing force and visual or
tactile cues about movement accuracy. These findings point to the importance
of muscle spindle signals, monitoring of motor commands, and possibly joint
and tendon receptors in a detailed trajectory-monitoring process. Muscle
spindle primary and secondary afferent signals may differentially influence
adaptation of movement shape and endpoint, respectively. Coriolis forces
generated by movements in a rotating environment can serve as a way of
studying the influence of transient perturbations on limb movement control
without providing mechanical contact with the limb. Coriolis forces are
proportional to limb velocity, F cor = -2m( ? x ? v ), where m is the mass
of the arm (or any other object), , the angular velocity of the rotating
environment in radians, and v , the linear velocity of the arm. This means
that there is not a Coriolis force acting at the very beginning nor at the
very end of a limb movement, but during motion a Coriolis force orthogonal and
proportional to v is generated (see Figure 1A). Such forces are inertial ones
that act without physical contact. Local contact, if present, necessarily
activates a variety of somatosensory and mechano-receptors in the limb that
potentially provide information about the nature of the perturbation. The use
of Coriolis forces to perturb limb movements avoids this and permits the study
of how natural, unencumbered movements respond to transient perturbations of
trajectory.
Received 28 February 1995; accepted in final form 6 July 1995.
APS Manuscript Number J131-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 July 1995.