Constrained and unconstrained movements INVOLVE DIFFERENT control strategies. Desmurget, Michel, Michael Jordan, Claude Prablanc, and Marc Jeannerod. Vision et motricite, INSERM U94, 16 av. du Doyen Lepine, 69500 Bron. France, Dept of Brain and Cognitive Sciences, M.I.T, Cambridge, MA 02139.
APStracts 3:0250N, 1996.
This experiment was carried out to test whether or not the rules governing the execution of compliant and unconstrained movements are different (a compliant motion is defined as a motion constrained by external contact). To answer this question we examined the characteristics of visually directed movements performed with either the index fingertip (unconstrained) or a hand held cursor (compliant). For each of these categories of movements, two experimental conditions were investigated: no instruction about hand path, and instruction to move the fingertip along a straight line path. The results of the experiment showed that: 1) The spatio-temporal characteristics of the compliant and unconstrained movements were fundamentally different when the subjects were not required to follow a specific hand path. 2) the instruction to perform straight movements modified the characteristics of the unconstrained movements, but not those of the compliant movements. 3) the target eccentricity influenced selectively the curvature of the "unconstrained-no path instruction" movements. Taken together, these results suggest that compliant and unconstrained movements involve different control strategies. Our data support the hypothesis that unconstrained motions are, unlike compliant motions, not programmed to follow a straight line path in the task space. This observation provide a theoretical reference frame within which some apparently contradictory results reported in the movement generation literature may be explained.

Received 9 September 1996; accepted in final form 22 November 1996.
APS Manuscript Number J720-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 31 December 1996