Interlimb transfer of visuomotor rotations: independence of direction and final position information

Abstract

Previous findings from our laboratory support the idea that the dominant arm is more proficient than the non-dominant arm in coordinating intersegmental dynamics for specifying trajectory direction and shape during multijoint reaching movements. We also showed that adaptation of right and left arms to novel visuomotor rotations was equivalent, suggesting that this process occurs upstream to processes that distinguish dominant and non-dominant arm performance. Because of this, we speculate that such visuomotor adaptations might transfer to subsequent performance during adaptation with the other arm. We now examine whether opposite arm training to novel visuomotor rotations transfers to affect adaptation using the right and left arms. Two subject groups, RL and LR, each comprising seven right-handed subjects, adapted to a 30 degrees counterclockwise rotation in the visual display during a center-out reaching task performed in eight directions. Each group first adapted using either the right (RL) or left (LR) arm, followed by opposite arm adaptation. In order to assess transfer, we compared the same side arm movements (either right or left) following opposite arm adaptation to those performed prior to opposite arm adaptation. Our findings indicate unambiguous transfer of learning across the arms. Different features of movement transferred in different directions: Opposite arm training improved the initial direction of right arm movements under the rotated visual condition, whereas opposite arm training improved the final position accuracy, but not the direction of left arm movements. These findings confirm that transfer of training was not due to a general cognitive strategy, since such an effect should influence either hand equally. These findings support the hypothesis that each arm controller has access to information learned during opposite arm training. We suggest that each controller uses this information differently, depending on its proficiency for specifying particular features of movement. We discuss evidence that these two aspects of control are differentially mediated by the right and left cerebral hemispheres.

Fig. 1

A Side view: subjects were seated in a dentist-type chair with the arm supported by an airjet system that removed the effects of friction on arm movement. Targets and the cursor representing finger position were backprojected on a screen placed above the arm. A mirror placed below this screen reflected the image, such that the projection was perceived in the plane of the arm. B Top view: the positions of the Flock of Birds sensors are shown

Fig. 2

Representative hand-paths of a subject from the RL group are compared with those of a subject from the LR group to illustrate between group differences. Right-hand paths are shown along the top row, whereas left-hand paths are shown below. The first column shows the last cycle of movements performed during the preexposure session with the right arm (top) and the left arm (bottom). The second column shows the first cycle of movement performed under initial rotation exposure conditions with the right arm (top) and the left arm (bottom). These movements are shown as dashed lines, whereas the baseline movements from column 1 are shown as solid gray lines. The third column shows the first cycle of movements performed following opposite arm training for the right arm (top) and the left arm (bottom). These paths are shown as solid black lines, whereas the paths from column 2 are shown as dashed lines for comparison. The fourth column shows the last cycle of movements performed following opposite arm training for the right arm (top) and the left arm (bottom)

Fig. 3

A–C Mean performance measures, direction error at V_max (top), direction error at A_max (middle) and final position error (bottom) are shown for left and right arms separately. Baseline performance, measured for each subject separately, has been subtracted from each value prior to computing the average across subjects. Thus, the performance measures shown represent a change from baseline performance. Every data point shown on the X-axis represents the average of two consecutive cycles across all subjects (mean ± SE). Performance measures for the LR group (filled circles) and RL group (open circles) are shown separately. Thus, differences in performance between groups represent the effects of opposite arm training. For example, the second row (right column) shows that following opposite arm training the direction error made with the right arm (LR group) is significantly smaller than that made with the right