INTEGRATED CONTROL OF HAND TRANSPORT AND ORIENTATION DURING PREHENSION MOVEMENTS

At a descriptive level, prehension movements can be partitioned into t hree components ensuring, respectively, the transport of the arm to th e vicinity of the target, the orientation of the hand according to obj ect tilt, and the grasp itself. Several authors have suggested that th is analytic description may be an operational principle for the organi zation of the motor system. This hypothesis, called ''visuomotor chann els hypothesis,'' is in particular supported by experiments showing a parallelism between the reach and grasp components of prehension movem ents. The purpose of the present study was to determine whether or not the generalization of the visuomotor channels hypothesis, from its in itial form, restricted to the grasp and transport components, to its a ctual form, including the reach orientation and grasp components, may be well founded. Six subjects were required to reach and grasp cylindr ical objects presented at a given location, with different orientation s. During the movements, object orientation was either kept constant ( unperturbed trials) or modified at movement onset (perturbed trials). Results showed that both wrist path (sequence of positions that the ha nd follows in space), and wrist trajectory (time sequence of the succe ssive positions of the hand) were strongly affected by object orientat ion and by the occurrence of perturbations. These observations suggest ed strongly that arm transport and hand orientation were neither plann ed nor controlled independently. The significant linear regressions ob served, with respect to the time, between arm displacement (integral o f the magnitude of the velocity vector) and forearm rotation also supp orted this view. Interestingly, hand orientation was not implemented a t only the distal level, demonstrating that all the redundant degrees of freedom available were used by the motor system to achieve the task . The final configuration reached by the arm was very stable for a giv en final orientation of the object to grasp. Tn particular, when objec t tilt was suddenly modified at movement onset, the correction brought the upper limb into the same posture as that obtained when the object was initially presented along the final orientation reached after per turbation. Taken together, the results described in the present study suggest that arm transport and hand orientation do not constitute inde pendent visuomotor channels. They also further suggest that prehension movements are programmed, from an initial configuration, to reach smo othly a final posture that corresponds to a given ''location and orien tation'' as a whole.