Reach adaptation and final position control amid environmental uncertainty after stroke

We characterized how hemiparetic stroke survivors and neurologically intact individuals adapt reaching movements to compensate for unpredictable environmental perturbations. We tested the hypotheses that like unimpaired subjects, hemiparetic stroke survivors adapt using sensory information obtained during only the most recent movements and that the reliability of target acquisition decreases as the degree of sensorimotor impairment increases. Subjects held the handle of a two-joint robotic arm that applied forces to the hand while reaching between targets in a horizontal plane. The robot simulated a dynamic environment that varied randomly in strength from one trial to the next. The trial sequence of perturbations had a nonzero mean value corresponding to information about the environment that subjects might learn. Stroke subjects were less effective than control subjects at adapting reaches to the perturbations. From a family of potential adaptation models, we found that the compensatory strategy patients used was the same as that used by neurologically intact subjects. However, analysis of model coefficients found that the relative weighting of prior perturbations and prior movement errors on subsequent reach attempts was significantly depressed poststroke. Regulation of final hand position was also impaired in the paretic limbs. Measures of trajectory adaptation and final position regulation deficits were significantly dependent on the integrity of limb proprioception and the amount of time poststroke. However, whereas model coefficients varied systematically with impairment level poststroke, variability of final positioning in the contralesional limb did not. This difference suggests that these two aspects of limb control may be differentially impaired poststroke.