Lateralized sensitivity of motor memories to the kinematics of the opposite arm reveals functional specialization during bimanual actions.

It is generally believed that the dominant arm exhibits greater functional advantages over the nondominant arm in every respect, including muscular strength and movement accuracy. Recent studies have proposed that this laterality is due to different underlying control strategies for each limb rather than different limb capabilities constraining performance. However, the functional role and mechanisms of these different control strategies have yet to be elucidated. Here, we report a specialized function of the nondominant arm that plays a significant role only during bimanual movements. Right-handed human participants performed bimanual reaching movements while only one arm was subjected to a force field. Consistent with our previous study, adaptation to the force field decreased gradually as the movement direction of the opposite arm deviated from the trained direction. We also observed that the decrement of the adaptation was significantly greater for the nondominant left arm. According to our previously proposed theory, this poorer generalization of the left arm originated from a difference in parameters characterizing motor memory; the nondominant arm's motor memory was more strongly influenced by the opposite arm's kinematics. Remarkably, a model incorporating this lateralized memory predicted that the nondominant arm would demonstrate greater adaptability to force fields associated with the opposite arm's movement. We confirmed this prediction experimentally and found that this advantage of the left arm disappeared in left-handed human participants. We concluded that the secondary supporting role often played by the nondominant arm in bimanual actions reflects its specialization rather than its inferiority.