Two main questions were addressed in the present study. First, does the existence of kinematic regularities in the extrinsic space represent a general rule? Second, can the existence of extrinsic regularities be related to specific experimental situations implying, for instance, the generation of compliant motion (i.e. a motion constrained by external contact)? To address these two questions we studied the spatio-temporal characteristics of unconstrained and compliant movements. Five major differences were observed between these two types of movement: (1) the movement latency and movement duration were significantly longer in the compliant than in the unconstrained condition; (2) whereas the hand path was curved and variable according to movement direction for the unconstrained movements, it was straight and invariant for the compliant movements; (3) whereas the movement end-point distribution was roughly circular for the unconstrained movements, it was consistently elongated and typically oriented in the movement direction for the compliant movements; (4) whereas constant errors varied as a function of target eccentricity for the unconstrained movements, they were independent of this factor for the compliant movements; (5) the instruction to move the final effector along a straight line path influenced the characteristics of the unconstrained movements but not the characteristics of the compliant movements. When considered together, the previous observations suggest that compliant and unconstrained movements involve different planning strategies. Our data support the hypothesis that unconstrained motions, unlike compliant motions, are not programmed to follow a straight line path in the extrinsic space. This observation provides a theoretical frame of reference within which some apparently contradictory results reported in the movement generation literature may be explained.
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