The role of kinematic redundancy in adaptation of reaching

Although important differences exist between learning a new motor skill and adapting a well-learned skill to new environmental constraints, studies of force field adaptation have been used frequently in recent years to identify processes underlying learning. Most of these studies have been of reaching tasks that were each hand position was specified by a unique combination of joint angles. At the same time, evidence has been provided from a variety of tasks that the central nervous system takes advantage of the redundancy available to it when planning and executing functional movements. The current study attempted to determine whether a change in the use of joint motion redundancy is associated with the adaptation process. Both experimental and control subjects performed 160 trials of reaching in each of four adaptation phases, while holding the handle of a robot manipulandum. During the first and last adaptation phases, the robot motors were turned off. During phases 2 and 3 the motors produced a velocity-dependent force field to which experimental subjects had to adapt to regain relatively straight line hand movements during reaching to a target, while the motors remained off for the control group. The uncontrolled manifold (UCM) method was used to partition the variance of planar clavicle-scapular, shoulder, elbow and wrist joint movements into two orthogonal components, one (V(UCM)) that reflected combinations of joint angles that were equivalent with respect to achieving the average hand path and another (V(ORT)) that took the hand away from its average path. There was no change in either variance component for the control group performing 640 non-perturbed reaches across four 'pseudo-adaptation' phases. The experimental group showed adaptation to reaching in the force field that was accompanied initially by an increase in both components of variance, followed by a smaller decrease of V(UCM) than V(ORT) during 320 practice reaches in the force field. After initial re-adaptation to reaching to the null field, V(UCM) was higher in experimental than in control subjects after performing a comparable number of reaches. V(UCM) was also larger in the experimental group after re-adaptation when compared to the 160 null field reaching trials performed prior to initial force field introduction. The results suggest that the central nervous system makes use of kinematic redundancy, or flexibility of motor patterns, to adapt reaching performance to unusual force fields, a fact that has implications for the hypothesis that motor adaptation requires learning of formal models of limb and environmental dynamics.