Learning a pointing task with a kinematically redundant limb: Emerging synergies and patterns of final position variability

The study tested a hypothesis that practice of arm pointing movement can le ad to a reorganization of the joint coordination reflected in the emergence of several synergies based on the same set of joints. In particular, invol vement of the wrist may represent a choice by the central nervous system an d not be driven by the typical "freezing-to-freeing" sequence. The effects of practice on the kinematic patterns and variability of a "fast and accura te" pointing movement using a pointer were studied. An obstacle was placed between the initial position and the target to encourage a curvilinear traj ectory and larger wrist involvement. Practice led to a decrease in variabil ity indices accompanied by an increase in movement speed of the endpoint an d of the elbow and the shoulder, but not of the wrist joint. Five out of si x subjects decreased the peak-to-peak amplitude of wrist motion. Before pra ctice, the variability along the line connecting the endpoint to the should er (extent) was similar to that in the direction orthogonal to this line. A fter practice, variability was reduced along the extent, but not along the orthogonal direction perpendicular to this line. Prior to practice, indices of variability of the endpoint were lower than those of the marker placed over the wrist; after practice, the endpoint showed higher variability indi ces than the wrist. We interpret the data as consequences of the emergence of two synergies: (a) Pointing with a non-redundant set of the elbow and sh oulder joints; and (b) keeping wrist position constant. The former synergy is based on a structural unit involving the elbow and the shoulder, while t he latter is based on a structural unit that includes all the major arm joi nts. (C) 1999 Elsevier Science B.V. All rights reserved. PsycINFO classific ation: 2330: 2520.