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.

We have previously proposed a model of motor lateralization, in which the two arms are differentially specialized for complementary control processes. During aimed movements, the dominant arm shows advantages for coordinating intersegmental dynamics as required for specifying trajectory speed and direction, while the nondominant arm shows advantages in controlling limb impedance, as required for accurate final position control. We now directly test this model of lateralization by comparing performance of the two arms under two different tasks: one in which reaching movement is made from one fixed starting position to three different target positions; and the other in which reaching is made from three different starting positions to one fixed target position. For the dominant arm, performance was most accurate when reaching from one fixed starting position to multiple targets. In contrast, nondominant arm performance was most accurate when reaching toward a single target from multiple start locations. These findings contradict the idea that motor lateralization reflects a global advantage of one "dominant" hemisphere/limb system. Instead, each hemisphere/limb system appears specialized for stabilizing different aspects of task performance.

Mechanisms underlying interlimb transfer of adaptation to visuomotor rotations have recently been explored in depth. However, little data are available regarding interlimb transfer of adaptation to novel inertial dynamics. The present study thus investigated interlimb transfer of dynamics by examining the effect of initial training with one arm on subsequent performance with the other in adaptation to a 1.5-kg mass attached eccentrically to the forearm. Using inverse dynamic analysis, we examined the changes in torque strategies associated with adaptation to the extra mass, and with interlimb transfer of that adaptation. Following initial training with the dominant arm, nondominant arm performance improved substantially in terms of linearity and initial direction control as compared with naïve performance. However, initial training with the nondominant arm had no effect on subsequent performance with the dominant arm. Inverse dynamic analysis revealed that improvements in kinematics were implemented by increasing flexor muscle torques at the elbow to counter load-induced increases in extensor interaction torques as well as increasing flexor muscle torques at the shoulder to counter the extensor actions of elbow muscle torque. Following opposite arm adaptation, the nondominant arm adopted this dynamic strategy early in adaptation. These findings suggest that dominant arm adaptation to novel inertial dynamics leads to information that can be accessed and utilized by the opposite arm controller, but not vice versa. When compared with our previous findings on interlimb transfer of visuomotor rotations, our current findings suggest that adaptations to visuomotor and dynamic transformations are mediated by distinct neural mechanisms.

We previously reported that opposite arm training improved the initial direction of dominant arm movements, whereas it only improved the final position accuracy of non-dominant arm movements. We now ask whether each controller accesses common, or separate, short-term memory resources. To address this question, we investigated interlimb transfer of learning for visuomotor rotations that were directed oppositely [clockwise (CW)/counterclockwise (CCW)] for the two arms. We expected that if information obtained by initial training was stored in the same short-term memory space for both arms, opposite arm training of a CW rotation would interfere with subsequent adaptation to a CCW rotation. All subjects first adapted to a 30° rotation (CW) in the visual display during reaching movements. Following this, they adapted to a 30° rotation in the opposite direction (CCW) with the other arm. In contrast to our previous findings for interlimb transfer of same direction rotations (CCW/CCW), no effects of opposite arm adaptation were indicated in the initial trials performed. This indicates that interlimb transfer is not obligatory, and suggests that short-term memory resources for the two limbs are independent. Through single trial analysis, we found that the direction and final position errors of the first trial of movement, following opposite arm training, were always the same as those of naive performance. This was true whether the opposite arm was trained with the same or the opposing rotation. When trained with the same rotation, transfer of learning did not occur until the second trial. These findings suggest that the selective use of opposite arm information is dependent on the first trial to probe current movement conditions. Interestingly, the final extent of adaptation appeared to be reduced by opposite arm training of opposing rotations. Thus, the extent of adaptation, but not initial information transfer, appears obligatorily affected by prior opposite arm adaptation. According to our findings, it is plausible that the initiation and the final extent of adaptation involve two independent neural processes. Theoretical implications of these findings are discussed.

When exposed to novel visuomotor rotations, subjects readily adapt reaching movements, such that the virtual display of the hand is brought to the target. Whereas this clearly reflects remapping of the relationship between hand movements and the visual display, the nature of this remapping is not well understood. We now examine whether such adaptation results in remapping of the position of the visually displayed target and the final limb position or between the target vector and the movement vector. The latter is defined relative to a starting position, whereas the former should be independent of the starting position. Subjects first adapted to a 30°rotation during reaching movements made from a single starting location to four different target locations. After adaptation, generalization trials were introduced, during which reaching movements were made under the same visual rotation condition but started from one of two locations outside the practiced workspace. These trials were directed to either the previously practiced targets or new targets that reflected the direction and distance of the practiced trials. Generalization was greatest for movements made in similar directions, regardless of changes in spatial location. Most significantly, when reaching to the previously adapted targets, subjects did not reach to the previously learned limb positions but rather to positions that reflected a near 30°rotation of the new target vector. These results indicate that learned visuomotor rotations remap the representations of movement vectors and not final positions of the limb in the workspace.