Hands can play equal roles in bimanual single-object rotations.

When the two hands carry out tasks simultaneously, interactions arise between them, but the way those interactions relate to the specializations of the hands is not fully understood. Much of the existing work on this topic has examined bimanual tasks in which the two hands can act independently. Here, we took a different approach by focusing on actions in which the two hands are physically coupled through the manipulation of a single object. We asked college students to grasp the rim of a disk to rotate the disk repeatedly in 90-degree turns. The disk was light enough to be turned with one hand or two. Participants were instructed to turn the disk unimanually or bimanually either clockwise or counterclockwise. It was found that in the one-hand task, participants used the hand that pulled the disk inward, not the hand that was otherwise preferred. Where participants placed their hands in the bimanual conditions was well predicted by where they placed their hands in the unimanual conditions. Globally, the results indicate that the hands the results indicate a preference for pulling movements, and hand placement is unaffected by whether one or two hands are used, revealing more of a cooperative than a competitive form of control.