On-line control of the limbs during bimanual reaching is independent with directly- and symbolically-cued target perturbations

Using both hands at the same time is an important ability of the human action system. This is referred to as bimanual coordination, and complex cases of coordination are often tested to reveal its limitations. A common limitation is that the limbs cannot make independent movements but are drawn to follow the same spatial trajectories with similar temporal properties. These examples of bimanual interference are called spatial and temporal interference. Another type of interference is seen in the initiation of bimanual reaching movements. When a reaching movement is directly-cued by illuminating the targets, the reaction time is the same for symmetric or asymmetric movements. However, the reaction time is longer for asymmetric compared to symmetric movements if they are symbolically-cued. The leading hypothesis for this reaction time cost is that the increased processing demands on response selection for symbolically-cued asymmetric movements results in bimanual interference (Diedrichsen, Hazeltine, Kennerley, & Ivry, 2001). In two experiments, we investigated the effect of this interference when it occurred as the result of a perturbation during a movement that required an on-line correction. We sought to determine if there was larger spatial interference in one limb when the other limb responded to a symbolically-cued on-line correction compared to a directly-cued correction. Participants made bimanual reaches to targets that were occasionally perturbed at movement onset. These perturbations required on-line corrections with one limb to the new target location. The new target location was indicated by illuminating the new target as a direct cue (experiments 1 and 2) or symbolically cueing the target with a colour change (experiment 1) or displaying the letter L or S (experiment 2). We found larger spatial interference for symbolically-cued on-line corrections compared to directly-cued corrections. Although there was greater interference with symbolic cues, the interference was small and transient with direct and symbolic cues. It was also subtle in comparison to spatial interference during preplanned bimanual reaches. Since a correction in one limb can be accomplished without a large or lasting effect on the other limb, we conclude that on-line control of the limbs during bimanual reaching is largely independent.Education, Faculty ofKinesiology, School ofGraduat