Deficits in compensatory trajectory adjustments after unilateral sensorimotor stroke

In a previous study, we demonstrated that the time-course for amplitude spe cification of goal-directed aiming movements is similar for individuals wit h and without a unilateral sensorimotor (SM) area lesion. However, subjects with a SM lesion performing with the arm ipsilateral to the side of the br ain lesion were significantly less accurate than control subjects in an unp redictable condition. The unpredictable condition requires that subjects bo th formulate an initial plan for movement as well as adjust the response la ter as additional information about the target (i.e., the goal) is gained. It has been demonstrated that premovement planning and compensatory adjustm ents are the processes contributing largely to accuracy in targeted, isomet ric force responses. A statistical model has been described, which partitio ns response trajectories into the planned and compensatory adjustment compo nents. The purpose of this study was to apply the statistical model to our previous data to determine if the difference in accuracy in those with unil ateral stroke was due to a deficit in premovement planning, compensatory ad justments, or a combination of these two factors. We compared the performan ce of six subjects with unilateral stroke to that of matched control subjec ts participating in a timed-response movement paradigm. Subjects rapidly fl exed or extended the forearm in order to capture a short (20 degrees) or lo ng (45 degrees) target presented in either a fixed (predictable condition) or a random sequence (unpredictable condition). For individuals with stroke , the limb used was that ipsilateral to the side of the SM lesion. Time to prepare the response was manipulated by varying the time of target presenta tion relative to an auditory cue for movement initiation. Velocity was deri ved from the displacement data, and multiple regression was used to determi ne the effect of premovement planning and compensatory adjustments on end-p oint accuracy. In the predictable condition, premovement planning contribut ed to final position more for the subjects with stroke [mean (SEM) =0.50 (0 .02)] than for the control subjects [0.36 (0.03)]. In the unpredictable con dition, there were no differences between groups in percent variance due to planning [0.54 (2.1) for the stroke group and 0.45 (2.8) for the control g roup]. This suggests that the ipsilateral (i.e., intact, undamaged) SM hemi sphere significantly participates in the premovement planning of an aiming action. In contrast, for both predictable and unpredictable conditions, com pensatory adjustments accounted for a smaller percentage of the variability in final position for the subjects with stroke than for the control subjec ts [0.09 (2.2) for the stroke group and 0.25 (4.8) for the control group]. Therefore, the less accurate responses for the stroke group can be explaine d by deficits in the compensatory adjustment component. This suggests a sub stantial role for SM areas in the preparation and implementation of correct ive actions while the effects of the pre-planned action are unfolding. In p articular, we discuss the role of the ipsilateral SM areas in relation to p arallel feedforward processing in unimanual aiming.