POSTURAL CONTROL OF THREE DIMENSIONAL PREHENSION MOVEMENTS.
Desmurget, Michel and Claude Prablanc.
MIT, dept of brain and cognitive sciences, Cambridge, MA 02139.
APStracts 3:0234N, 1996.
This experiment was carried out to test the hypothesis that 3D upper limb
movements could be initiated and controlled in the joint-space, via a
mechanism comparing an estimate of the current postural state of the upper arm
with a target value, determined by one specific inverse static transform
converting the coordinates of the object into a set of arm, forearm and hand
angles. This hypothesis involves two main predictions: a) despite joint
redundancy, the posture reached by the upper limb should be invariant for a
given context; b) a movement programmed in joint space should exhibit
invariant characteristics of the joint covariation pattern as well as a
corresponding variable hand path curvature in the task space. To test these
predictions, we examined prehension movements towards a cylindrical object
presented at a fixed spatial location and at various orientations, without
vision of the moving limb. Once presented, the object orientation was either
kept constant (unperturbed trials) or suddenly modified at movement onset
(perturbed trials). Three-D movement trajectories were analyzed in both joint
and task spaces. For the unperturbed trials, the task space analysis showed a
variable hand path curvature depending upon object orientation. The joint
space analysis showed that the seven final angles characterizing the upper
limb posture at hand-to-object contact varied monotonically with object
orientation. At a dynamic level movement onset and end were nearly identical
for all joints. Moreover for all joints having a monotonic variation, maximum
velocity occured almost simultaneously. For the elbow, (the only joint
presenting a reversal), the reversal was synchronized with the time to peak
velocity of the other joint angles. For the perturbed trials, a smooth and
complete compensation of the movement trajectory was observed in the task
space. At a static level the upper limb final posture was identical to that
obtained when the object was initially presented at the orientation following
the perturbation. This result was particularly remarkable considering the
large set of comfortable postures allowed by joint redundancy. At a dynamic
level, the joints covariation pattern was updated to reach the new target
posture. The initial synergies were not disrupted by the perturbation, but
smoothly modified, the different joints movements ending nearly at the same
time. Taken together these results support the hypothesis that prehension
movements are initiated and controlled in the joint space on the basis of a
joint angular error vector, rather than a spatial error vector.
Received 15 March 1996; accepted in final form 25 September 1996.
APS Manuscript Number J246-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 5 November 1996