How a Lateralized Brain Supports Symmetrical Bimanual Tasks

Abstract

A large repertoire of natural object manipulation tasks require precisely coupled symmetrical opposing forces by both hands on a single object. We asked how the lateralized brain handles this basic problem of spatial and temporal coordination. We show that the brain consistently appoints one of the hands as prime actor while the other assists, but the choice of acting hand is flexible. When study participants control a cursor by manipulating a tool held freely between the hands, the left hand becomes prime actor if the cursor moves directionally with the left-hand forces, whereas the right hand primarily acts if it moves with the opposing right-hand forces. In neurophysiological (electromyography, transcranial magnetic brain stimulation) and functional magnetic resonance brain imaging experiments we demonstrate that changes in hand assignment parallels a midline shift of lateralized activity in distal hand muscles, corticospinal pathways, and primary sensorimotor and cerebellar cortical areas. We conclude that the two hands can readily exchange roles as dominant actor in bimanual tasks. Spatial relationships between hand forces and goal motions determine hand assignments rather than habitual handedness. Finally, flexible role assignment of the hands is manifest at multiple levels of the motor system, from cortical regions all the way down to particular muscles.

Selection of Prime Actor in Humans during Bimanual Object Manipulation

“…The appointment of a prime actor in a context-dependent manner apparently reflects a choice made by the brain in executing one of potentially available action representations that differentially transform goal motions represented in a visuospatial reference frame into hand motor commands. Change in hand assignment parallels a midline shift of lateralized activity in distal hand muscles, corticospinal pathways, and primary sensorimotor cortex (SMC), and related cerebellar areas (Johansson et al, 2006). Consistent with a central role of left-hemisphere processes for righthanders in performing skilled manual actions (Geschwind, 1975;Leiguarda and Marsden, 2000;Schluter et al, 2001), a leftlateralized dorsal parietal-premotor network provides critical neural substrates of the implemented action representation regardless of prime actor (Theorin and Johansson, 2007).…”

“…All were right-handed (Oldfield, 1971). A detailed description of the experimental procedure and the apparatus can be found in previous reports that address other aspects of data obtained in the same functional magnetic resonance imaging (fMRI) experiments (Johansson et al, 2006;Theorin and Johansson, 2007). In short, the target-chasing task involved moving a cursor (filled circle, diameter of 0.2°visual angle) on a computer screen to hit, as fast as possible, successively displayed targets (open square, side of 0.8°).…”

“…The brain appoints a prime actor even in tasks that require the hands to apply coupled symmetrical opposing forces on a single object and, notably, the choice of prime actor is flexible and depends on the spatial mapping between hand forces and desired movement outcomes (Johansson et al, 2006). For example, when removing the lid from a jar, the hand that grasps the lid, be it left or right depending on overall task constraints, serves as prime actor because the twist force it generates is aligned with the goal motion, that is, the rotation of the lid.…”

“…For example, when removing the lid from a jar, the hand that grasps the lid, be it left or right depending on overall task constraints, serves as prime actor because the twist force it generates is aligned with the goal motion, that is, the rotation of the lid. Likewise, when moving a cursor by symmetrically applying forces to a tool held freely between the hands to chase targets on a screen, after learning the task (Sailer et al, 2005), participants appoint either the left or the right hand as prime actor depending on whether the cursor moves directionally with left-or right-hand forces (Johansson et al, 2006). The appointment of a prime actor in a context-dependent manner apparently reflects a choice made by the brain in executing one of potentially available action representations that differentially transform goal motions represented in a visuospatial reference frame into hand motor commands.…”

Synergy betwee n the frontal and parietal lobule during motor global coordination tests

“…There were no differences between electrodes for the tasks, including the fine motor coordination. A study tried to identify differences in the cortical activation in bimanual tasks, noticing that the brain chose the hand as a protagonist of the movement, when the strength employed was of same amplitude in both 31,32 . These data reaffirm our findings when noticing symmetry between the cortical hemispheres, non-disclosed data, even if it has not been the objective of the study.…”