Transfer of motor learning across arm configurations

Abstract

It has been suggested that the learning of new dynamics occurs in intrinsic coordinates. However, it has also been suggested that elements that encode hand velocity, and hence act in an extrinsic frame of reference, play a role in the acquisition of dynamics. To reconcile claims regarding the coordinate system involved in the representation of dynamics, we have used a procedure involving the transfer of force-field learning between two workspace locations. Subjects made point-to-point movements while holding a two-link manipulandum. Subjects were first trained to make movements in a single direction at the left of the workspace. They were then tested for transfer of learning at the right of the workspace. Two groups of subjects were defined. For the subjects in group j, movements at the left and right workspace locations were matched in terms of joint displacements. For the subjects in group h, movements in the two locations had the same hand displacements. Workspace locations were chosen such that for group j, the paths (for training and testing) that were identical in joint space were orthogonal in hand space. The subjects in group j showed good transfer between workspace locations, whereas the subjects in group h showed poor transfer. These results are in agreement with the idea that new dynamics are encoded in intrinsic coordinates and that this learning has a limited range of generalization across joint velocities.

Fig. 1.

Motion paths and associated forces and torques for movements at the left and right of the workspace. A andC show movement directions used in training trials.B and D show movement directions in transfer trials. 1 and 2 refer to experiments 1 and 2. In A and C,j and h denote training directions that have the same joint or hand paths as the directions tested in the transfer trials shown in B and D.

Fig. 2.

Transfer of learning between workspace locations is observed for movements involving the same joint displacements. No transfer is observed when movements in different locations are matched in terms of hand displacement. A and Bgive results for single subjects in experiments 1 and 2. In each case, training trials at the left of the workspace and transfer trials at the right are shown.

Fig. 3.

A, Mean values across subjects (experiment 1) of kinematic error for the first and last training trials and for the transfer trials. Error bars denote ±1 SE.Gray bars are for group h; white bars are for group j. B, Mean values across subjects for each of the 20 trials in the first and last training sets and the transfer set. In both panels transfer is observed when movements involve the same joint displacements.

Fig. 4.

Asymmetries in the effect of the force field are observed for movements in different directions. Shown are movement paths for center-out movements in which a clockwise or counterclockwise curl field is unexpectedly introduced. The effects at the left and right of the workspace are similar once they are corrected for the angle at the shoulder.