A hallmark of human behavior is the ability to coordinate the two hands for effective manipulation of the environment. Bimanual coordination has been the subject of intensive investigation the last decades. Most of this research has concerned situations in which the two hands have difficulties acting independently [
], rather than on how they are coordinated for common object-oriented goals as in natural manipulations [
]. Studies that address object manipulations have principally dealt with tasks characterized by different and asymmetric engagement of the two hands [
]. A large repertoire of bimanual manipulations, however, involves symmetric forces of the two hands. In particular, whenever we grasp bimanually a single object, whether opening a jar, bending a rod or molding a snowball, the hands must be able to orchestrate equal and opposing forces. In this paper, we ask how the functionally lateralized brain handles this basic problem of spatial and temporal complexities. A fundamental question is whether the brain allocates different functional roles to the hands as previously suggested for many bimanual skills, with one hand primarily acting and the other stabilizing the object as to provide frames of reference for the acting hand [
]. If so, one possibility is that habitual handedness determines acting hand, making the right hand prime actor for right-handed persons. Indeed, handedness is the most recognized human behavioral asymmetry and links to cerebral hemispheric differences [
An alternative possibility is that the functional roles of the hands are flexible and can change across tasks and task phases. Such flexibility could relate to how bimanual actions are conceptualized with reference to their spatial goals, or more specifically, to the mapping between hand forces and desired movement outcomes. For example, when removing a stopper from a bottle held by one hand, the other hand grasping the stopper, be it left or right depending on overall task constraints, may be appointed as prime actor because the forces it generates are aligned with the goal motion, that is, to pull the stopper out of the bottle. However, if the task instead would entail moving the bottle rather than the stopper, as might occur in rare instances, whichever hand that grasps the bottle may be prime actor. Likewise, when we open or close a jar by generating bimanual twist forces, the hand holding the lid would be the prime actor as long as the intention is to rotate the lid rather than the jar. By this view, the brain assigns as prime actor the hand (right or left) whose forces are spatially congruent with the movement goal while the accompanying hand, bound to generate forces directed opposite to the goal motion, is assigned an assisting, stabilizing function.
We addressed critically these alternatives in a task where study participants applied light isometric forces to a rigid unsupported tool (weighing 150 g) held between the two hands in order to control a cursor on a screen (
). The task was to hit successively displayed visual targets as quickly as possible. Compressing and stretching forces applied between the handles of the tool along its longitudinal axis moved the cursor horizontally and twist forces (torques) applied around this axis moved the cursor vertically (
A). Participants had to generate various combinations of longitudinal and twist forces to hit the target, since it reappeared at an unpredictable location on the screen after each hit. Note that, any force generated by one hand must be counterbalanced by opposing forces generated by the other hand because the tool was freely held in the air.
The Bimanual Target-Chasing Task and Flexible Role Assignment of the Hands
To assess whether spatial congruency between hand actions and goal motions might influence possible functional role differentiation between the hands, each participant experienced two different mapping rules relating bimanual forces and cursor movements, one congruent with the left hand and one with the right hand. That is, with the “left-hand map,” the cursor moved in the direction of the forces applied by the left hand and consequently in a direction opposite to the forces applied by the right hand (left panel in
A). Hence, longitudinal compression forces moved the cursor to the right side of the screen and stretch forces to the left side while counterclockwise twist forces moved the cursor to the upper part of the screen and clockwise forces to its lower part. The “right-hand map” was reversed compared to the left-hand map and the cursor thus moved with the forces of the right hand and opposite to those of the left hand (right panel in
A). We report that the brain flexibly appoints one hand as prime actor even though the task required that the hands generated symmetrical forces. Furthermore, the choice of acting hand depended on the mapping rule and correlated with lateralized activity in distal hand muscles, corticospinal pathways, and primary sensorimotor and cerebellar cortical areas. We also report on lateralized engagement of cortical premotor areas in the task.