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

ABSTRACT 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 across 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. (2023) showing that when one arm contributed more to perpendicular cursor motion, its lateral variability was reduced, while 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 where 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 always 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 along 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. SIGNIFICANCE STATEMENT Bimanual actions in humans are thought to be controlled via a control policy that can be flexibly tuned to task demands. This work aimed to demonstrate that such flexibility in control extends independently to movement direction and extent. We used a shared-cursor bimanual reaching task to selectively manipulate the contribution of each arm to cursor motion along the direction and extent axes first in isolation, then simultaneously. Our results show that the sensorimotor system restricts variability in the more task-relevant arm along each axis, enabling axis-specific coordination. These findings thus provide evidence that bimanual coordination along movement direction and extent can be governed flexibly and largely independently in response to changing task demands.