To select one hand while using both : neural mechanisms supporting flexible hand dominance in bimanual object manipulation

Abstract: In daily activities, the brain regularly assigns different roles to the hands dependingon task and context. Yet, little is known about the underlying neural processes. Thiscertainly applies to how the brain, where each hemisphere primarily controls onehand, manages the between-hand coordination required in bimanual objectmanipulation. By using behavioral, neurophysiological and functional magneticresonance imaging techniques, the present thesis examines neural mechanisms thatsupport hand coordination during tasks where the two hands apply spatiotemporallycoupled but opposing forces for goal attainment, e.g., as when removing the cap froma bottle. Although the two hands seem to operate symmetrically in such tasks, Study Ishowed that one hand primarily acts while the other assists. Moreover, this roledifferentiation was found to be flexible with the brain appointing either hand asprime actor depending on the spatial congruency between hand forces and desiredmovement consequences. Accordingly, when we remove a cap from a bottle, the handthat grasps the cap, be it left or right depending on overall task constraints, isappointed as prime actor because the twist forces it generates are aligned with thegoal to remove the cap, while the other hand, holding the bottle, applies stabilizingforces in the opposite direction. Changes in hand assignments are caused by amidline shift of lateralized activity throughout the motor system, from distal handmuscles to corticospinal pathways and primary sensorimotor and cerebellar corticalareas (Study I). Although the bimanual actions examined involved both within- andbetween-hand coordination, Study II failed to reveal additional brain activity duringbimanual as compared to matching unimanual actions, except for the primarysensorimotor areas where subpopulations of neurons were preferentially engagedduring either bimanual or unimanual actions. Thus, dedicated neurons in the motorcortices might support critical bimanual coordinative operations. While imagingresults indicated that a mainly left-lateralized parietal-premotor network managedthe task irrespective of prime actor, premotor areas presumably established handassignment by allocating the lead either to the left or the right primary sensorimotorareas (Study I and II). Regarding the process of prime actor selection and hence thecontrol of these premotor networks, imaging results indicate a transitory involvementof prefrontal cortical areas (Study III). The detected areas belong to a networkconsidered critical for cognitive operations such as judgment and decision-making,and for evaluation of utility of actions, including conflict detection. The implicitselection of prime actor during bimanual tasks thus seems to be supported by corticalareas traditionally associated primarily with complex cognitive challenges.