CONSTRAINED AND UNCONSTRAINED MOVEMENTS INVOLVE DIFFERENT CONTROL STRATEGIES

This experiment was carried out to test whether or not the rules gover ning the execution of compliant and unconstrained movements are differ ent (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 c ategories 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 were as fol lows. 1) The spatiotemporal characteristics of the compliant and uncon strained movements were fundamentally different when the subjects were not required to follow a specific hand path. 2) The instruction to pe rform straight movements modified the characteristics of the unconstra ined movements, but not those of the compliant movements. 3) The targe t eccentricity influenced selectively the curvature of the ''unconstra ined-no path instruction'' movements. Taken together, these results su ggest that compliant and unconstrained movements involve different con trol strategies. Our data support the hypothesis that unconstrained mo tions are, unlike compliant motions. not programmed to follow a straig ht-line path in the task space. These observations provide a theoretic al reference frame within which some apparently contradictory results reported in the movement generation literature may be explained.