Flexible, task-dependent bimanual coordination of movement direction and extent

Recent work suggests that coordination of bilateral arm movements is mediated through flexible, task-dependent control policies rather than coupled motor commands that activate homologous muscle groups in the two arms. Here, we examined whether such flexibility in bimanual control extends independently to movement direction and extent. We employed a bimanual reach task in which we altered the contribution of each arm to the perpendicular (direction axis) and/or parallel (extent axis) position of a single feedback cursor controlled by both arms together. We first replicated findings of Kitchen et al. (J Neurophysiol 130: 497-515, 2023) showing that when one arm contributed more to perpendicular cursor motion, its lateral variability was reduced, whereas variability of the other arm increased. We then extended this result to perturbations affecting movement extent, observing similar asymmetric adjustments in variability in the parallel direction. Subsequent manipulations, in which contributions along both axes were manipulated simultaneously, revealed that irrespective of the gain combinations, the arm with the higher perpendicular contribution showed reduced lateral variability, and the arm with the higher parallel contribution demonstrated restricted parallel variability, while allowing the corresponding lower contribution arms to compensate for perturbation-induced errors. These results suggest that: 1) the sensorimotor system prioritizes corrections for deviations directly impacting task goals while tolerating higher variability in less relevant dimensions, and 2) that it is capable of adjusting coordination of movement direction and extent largely independently based on peripheral task demands. These findings add to the growing body of evidence supporting task-dependent modulation of bimanual motor coordination.NEW & NOTEWORTHY Human bimanual actions are guided by flexible, task-specific control policies. We show that this flexibility extends independently to movement direction and extent. During bimanual reaching, we manipulated each arm's contribution to the direction and extent dimensions of motion of a shared feedback cursor in a variety of ways. We consistently observed restricted variability in each dimension in the more task-relevant arm, demonstrating that these two dimensions can be independently and flexibly regulated during bimanual actions.