Facilitation and interference during the preparation of bimanual movements: contributions from starting locations, movement amplitudes, and target locations

Individual differences in processing resources modulate bimanual interference in pointing

Coordinating both hands during bimanual reaching is a complex task that can generate interference during action preparation as often indicated by prolonged reaction times for movements that require moving the two hands at different amplitudes. Individual processing constraints are thought to contribute to this interference effect. Most importantly, however, the amount of interference seems to depend considerably on overall task demands suggesting that interference increases as the available processing resources decrease. Here, we further investigated this idea by comparing performance in a simple direct cueing and a more difficult symbolic cueing task between three groups of participants that supposedly vary in their processing resources, i.e., musicians, young adults and older adults. We found that the size of interference effects during symbolic cueing varied in the tested groups: musicians showed the smallest and older adults the largest interference effects. More importantly, a regression model, using processing speed and processing capacity as predictor variables, revealed a clear link between the available processing resources and the size of the interference effect during symbolic cueing. In the easier direct cueing task, no reliable interference was observed on a group level. We propose that the susceptibility to bimanual interference is modulated by the task-specific processing requirements in relation with the available processing resources of an individual.

Bimanual coordination: A missing piece of arm rehabilitation after stroke

Inability to use the arm in daily actions significantly lowers quality of life after stroke. Most contemporary post-stroke arm rehabilitation strategies that aspire to re-engage the weaker arm in functional activities have been greatly limited in their effectiveness. Most actions of daily life engage the two arms in a highly coordinated manner. In contrast, most rehabilitation approaches predominantly focus on restitution of the impairments and unilateral practice of the weaker hand alone. We present a perspective that this misalignment between real world requirements and intervention strategies may limit the transfer of unimanual capability to spontaneous arm use and functional recovery. We propose that if improving spontaneous engagement and use of the weaker arm in real life is the goal, arm rehabilitation research and treatment need to address the coordinated interaction between arms in targeted theory-guided interventions. Current narrow focus on unimanual deficits alone, difficulty in quantifying bimanual coordination in real-world actions and limited theory-guided focus on control and remediation of different coordination modes are some of the biggest obstacles to successful implementation of effective interventions to improve bimanual coordination in the real world. We present a theory-guided taxonomy of bimanual actions that will facilitate quantification of coordination for different real-world tasks and provide treatment targets for addressing coordination deficits. We then present evidence in the literature that points to bimanual coordination deficits in stroke survivors and demonstrate how current rehabilitation approaches are limited in their impact on bimanual coordination. Importantly, we suggest theory-based areas of future investigation that may assist quantification, identification of neural mechanisms and scientifically-based training/remediation approaches for bimanual coordination deficits post-stroke. Advancing the science and practice of arm rehabilitation to incorporate bimanual coordination will lead to a more complete functional recovery of the weaker arm, thus improving the effectiveness of rehabilitation interventions and augmenting quality of life after stroke.

Effects of integrated feedback on discrete bimanual movements in choice reaction time

The ability to coordinate the simultaneous movements of our arms is limited by a coalition of constraints. Some of these constraints can be overcome when the task conceptualisation is improved. The present study investigated how the movement preparation of bimanual reaching movements was affected by integrated visual feedback of the responses. Previous research has shown that the preparation of bimanual asymmetric movements takes longer than bimanual symmetric movements. The goal of the present study was to determine whether integrated, Lissajous feedback could eliminate this bimanual asymmetric cost. Fifteen participants made unimanual and bimanual symmetric and asymmetric reaches with separate feedback, where there was a cursor and a target for each hand. Participants also made bimanual symmetric and asymmetric movements with integrated feedback; a single cursor and a single target represented the locations and goals of both arms in this condition. The results showed a bimanual asymmetric cost with separate feedback, and that this cost persisted with integrated feedback. We suggest that integrated feedback improved continuous and discrete bimanual movements in other experiments by facilitating error detection and correction processes. We hypothesise that the bimanual asymmetric cost persisted in the present experiment because the uncertainty associated with choice reaction time prevented the facilitated error processing from improving the preparation of the next trial.

Goal conceptualization and symmetry of arm movements affect bimanual coordination in individuals after stroke

Coordination during goal-directed movements emerges from an interaction of task and individual constraints. It is not known how individuals with unilateral stroke and age-matched controls coordinate their arms when performing symmetric and asymmetric movements to accomplish common task goals compared to independent task goals. Eleven individuals with chronic stroke and ten age-matched controls executed a bimanual task under virtual conditions that allowed systematic manipulation of symmetry and goal conditions. Spatial and temporal bimanual coordination was characterized using the cross-correlation coefficients and time lag between the tangential velocities between the two hands. While task conditions had little effect on coordination of control participants, individuals with stroke were less coordinated in space and time during common-goal bimanual actions employing asymmetric arm movements. Further, patients demonstrated lesser contribution of their paretic arm compared to their non-paretic arm during common-goal conditions. These findings indicate that conceptualization of task goals (common vs. independent) and symmetry of arm movements influence coordination and contribution of the two hands during bimanual tasks in patients with stroke.

Increased cognitive demands boost the spatial interference effect in bimanual pointing

It is beyond controversy that in bimanual coordination tasks, parameter planning related to the movements of one hand influences the planning and execution of movements simultaneously performed with the other hand. A well-researched example of such bimanual interference is the finding that reaction times tend to be longer when preparing bimanual pointing movements with different amplitudes than for equal amplitude movements. Interestingly, these reaction time costs were found to increase when movement targets were cued symbolically (e.g., using letters) as compared to spatially. Therefore, it was suggested that interference may be primarily related to cue translation and response selection processes rather than resulting from cross-talk at the motor programming level. Here, we argue that spatial interference effects do not necessarily depend on the type of cues used but instead depend on the general task demands (difficulty). In two experiments we show that bimanual interference effects can (1) be abolished in symbolic cueing conditions when highly compatible cues placing minimal demands on response selection processes are used and (2) occur in direct/spatial cueing conditions when a secondary cognitively demanding, but movement-unrelated task is performed. Thus, our findings suggest that whether or not interference effects emerge during movement planning depends on the overall task difficulty and hence the resources available during movement preparation.