Impaired direction and extent specification of aimed arm movements in humans with stroke-related brain damage
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The role of sensorimotor (S-M) areas in the specification of kinematic parameters for aiming movements was studied by comparing the performance of six subjects with unilateral stroke to that of matched control subjects. Rapid arm movements were made to one of four targets by rotating the forearm in a short (20°) or long (45°) arc of motion. Thus, the four targets represented two directions (flexion or extension) and two extents (short or long). Subjects with stroke used the arm ipsilateral to the side of the lesion. A timed-response paradigm was used to dissociate response initiation and specification. Subjects initiated movements in concert with the last of four regularly timed tones. A visual cue of the designated target was presented during the preparation interval (400–0 ms) before the last tone. Targets were presented in a fixed sequence (predictable condition) or a random sequence (unpredictable condition). No significant differences in performance were found between stroke and control groups in the predictable condition. In the unpredictable condition, subjects with stroke produced more direction errors and were less accurate in extent than the control subjects. As specification time increased to 400 ms, the frequency of direction errors attenuated less for stroke than for control groups, but the reduction in magnitude of extent errors was similar for the two groups. When specification was minimal (i.e., <100 ms), default responses were distributed equally between directions and clustered around the short extent. Further, wrong direction responses did not converge on the designated extent as specification time increased. This pattern of findings is consistent with a view of parameterization of planning and executing movements, in which direction and extent can be specified in parallel. Our results suggest that ipsilateral S-M areas contribute to the specification of an optimal motor program, particularly when imperative programming of unimanual goal-directed aiming movements is required.
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