Structural Constraints on the Performance of Symmetrical Bimanual Movements with Different Amplitudes

Symmetry and synchrony of bimanual movements are not predicated on interlimb control coupling

Previous studies suggest that bimanual coordination recruits neural mechanisms that explicitly couple control of the arms, resulting in symmetric kinematics. However, the higher symmetry for actions that require congruous joint motions compared with noncongruous joint motions calls into question the concept of control coupling as a general policy. An alternative view proposes that codependence might emerge from an optimal feedback controller that minimizes control effort and costs in task performance. Support for this view comes from studies comparing conditions in which both hands move a shared or independent virtual objects. Because these studies have mainly focused on congruous bimanual movements, it remains unclear if kinematic symmetry emerges from such control policies. We now examine movements with congruous or noncongruous joint motions (inertially symmetric or asymmetric, respectively) under shared or independent cursors conditions. We reasoned that if a control policy minimizes kinematic differences between limbs, spatiotemporal symmetry should remain relatively unaffected by inertial asymmetries. As shared tasks reportedly elicit greater interlimb codependence, these conditions should elicit higher bilateral covariance regardless of inertial asymmetries. Our results indicate a robust spatiotemporal symmetry only under inertially symmetric conditions, regardless of cursor condition. We simulated bimanual reaching using an optimal feedback controller with and without explicit costs of kinematic asymmetry, finding that only the latter mirrored our empirical data. Our findings support the hypothesis that bimanual control policies do not include kinematic asymmetry as a cost when it is not demanded by task constraints suggesting that kinematic symmetry depends critically on mechanical movement conditions.NEW & NOTEWORTHY Previously, the control coupling hypothesis and task-dependent control hypothesis have been shown to be robust in the bimanually symmetrical movement, but whether the same policy remains robust in the bimanually asymmetrical movement remains unclear. Here, with evidence from empirical and simulation data, we show that a spatiotemporal symmetry between the arms is not predicated on control coupling, but instead it is predicated on the symmetry of mechanical conditions (e.g. limb inertia) between the arms.

Effects of visual feedback and force level on bilateral ankle-dorsiflexion force control

This study investigated the potential effects of visual feedback and force level on bilateral force control capabilities in the lower limbs. Thirty-nine healthy young adults performed bilateral ankle-dorsiflexion isometric force control tasks for different visual feedback conditions, including continuous visual feedback (CVF) and withdrawal of visual feedback (WVF), indicating the removal of visual feedback on force outputs during the task and force level conditions (i.e., 10 % and 40 % of the maximum voluntary contraction). Bilateral force control capabilities were estimated using force accuracy, variability, regularity, and absolute power in 0-4 Hz and interlimb coordination by cross-correlation with time lag and uncontrolled manifold (UCM) variables. Correlation analyses determined the relationship between changes in bilateral force control capabilities and interlimb coordination from the CVF to WVF conditions. The findings revealed better bilateral force control capabilities in the CVF condition as indicated by less force error, variability, regularity, absolute power in 0-4 Hz, and advanced interlimb force coordination. From CVF to WVF conditions, increased bad variability correlated with greater force control deficits. These findings suggest that visuomotor processing is an important resource for successful fine motor control in the lower limbs.

Neuromuscular Fatigue in Unimanual Handgrip Does Not Completely Affect Simultaneous Bimanual Handgrip

Simultaneous bimanual movements are not merely the sum of two unimanual movements. Here, we considered the unimanual/bimanual motor system as comprising three components: unimanual-specific, bimanual-specific, and overlapping (mobilized during both unimanual and bimanual movements). If the force-generating system controlling the same limb differs between unimanual and bimanual movements, unimanual exercise would be expected to fatigue the unimanual-specific and overlapping parts in the force-generating system but not the bimanual-specific part. Therefore, we predicted that the decrease in bimanual force generation induced by unimanual neuromuscular fatigue would be smaller than the decrease in unimanual force generation. Sixteen healthy right-handed adults performed unimanual and bimanual maximal handgrip measurements before and after a submaximal fatiguing handgrip task. In the fatigue task, participants were instructed to maintain unimanual handgrip force at 50% of their maximal handgrip force until the time to task failure. Each participant performed this task in a left-hand fatigue (LF) condition and a right-hand fatigue (RF) condition, in a random order. Although the degree of neuromuscular fatigue was comparable in both conditions, as expected, the decrease in bimanual right handgrip force was significantly smaller than those during unimanual right performance in the RF condition, but not in the LF condition. These results indicate that for the right-hand, neuromuscular fatigue in unimanual handgrip does not completely affect simultaneous bimanual handgrip. Regarding the underlying mechanisms, we propose that although neuromuscular fatigue caused by unimanual handgrip reduces the motor output of unimanual-specific and overlapping parts in the force-generating system, when simultaneous bimanual handgrip is performed, the overlapping part (which is partially fatigued) and the bimanual-specific part (which is not yet fatigued) generate motor output, thus decreasing the force reduction.

Bilateral Interference in Motor Performance in Homologous vs. Non-homologous Proximal and Distal Effectors

Performance of bimanual motor actions requires coordinated and integrated bilateral communication, but in some bimanual tasks, neural interactions and crosstalk might cause bilateral interference. The level of interference probably depends on the proportions of bilateral interneurons connecting homologous areas of the motor cortex in the two hemispheres. The neuromuscular system for proximal muscles has a higher number of bilateral interneurons connecting homologous areas of the motor cortex compared to distal muscles. Based on the differences in neurophysiological organization for proximal vs. distal effectors in the upper extremities, the purpose of the present experiment was to evaluate how the level of bilateral interference depends on whether the bilateral interference task is performed with homologous or non-homologous effectors as the primary task. Fourteen participants first performed a unilateral primary motor task with the dominant arm with (1) proximal and (2) distal controlled joysticks. Performance in the unilateral condition with the dominant arm was compared to the same effector’s performance when two different bilateral interference tasks were performed simultaneously with the non-dominant arm. The two different bilateral interference tasks were subdivided into (1) homologous and (2) non-homologous effectors. The results showed a significant decrease in performance for both proximal and distal controlled joysticks, and this effect was independent of whether the bilateral interference tasks were introduced with homologous or non-homologous effectors. The overall performance decrease as a result of bilateral interference was larger for proximal compared to distal controlled joysticks. Furthermore, a proximal bilateral interference caused a larger performance decrement independent of whether the primary motor task was controlled by a proximal or distal joystick. A novel finding was that the distal joystick performance equally interfered with either homologous (distal bilateral interference) or non-homologous (proximal bilateral interference) interference tasks performed simultaneously. The results indicate that the proximal–distal distinction is an important organismic constraint on motor control and for understanding bilateral communication and interference in general and, in particular, how bilateral interference caused by homologous vs. non-homologous effectors impacts motor performance for proximal and distal effectors. The results seem to map neuroanatomical and neurophysiological differences for these effectors.

The influence of accuracy constraints on bimanual and unimanual sequence learning

An experiment was designed to determine whether accuracy constraints can influence how unimanual and bimanual motor sequences are produced and learned. The accuracy requirements of the task were manipulated using principles derived from Fitts' Law to create relatively low (ID = 3) and high (ID = 5) accuracy demands. Right-limb dominant participants (N = 28, age = 21.9 yrs; 15 females and 13 males) were required to produce unimanual left, unimanual right or bimanual movement sequences using elbow extension and flexion movements to hit a series of illuminated targets. The targets were illuminated in a repeating sequence of 16 elements. Participants performed 20 practice trials. Thirty minutes following the practice trials participants performed a retention test. Element duration (time interval between target hits) and segment harmonicity (hesitations/adjustments in movement pattern) were calculated. The results indicate longer element duration and lower harmonicity values (more adjustments) when the task required higher accuracy demands (ID = 5) compared to low accuracy demands (ID = 3). Element duration was shorter and harmonicity was higher at ID = 5 for both unimanual groups than the bimanual group. However, element duration was shorter and harmonicity was higher at ID = 3 for the bimanual group than for both unimanual groups. These results indicate that the accuracy demands of the task can influence both performance and learning of motor sequences and suggest differences between unimanual and bimanual motor sequence learning. It appears there is a bimanual advantage for tasks with lower accuracy demands whereas performance is more accurate with unimanual performance, regardless of limb, with higher accuracy demands. These results are consistent with recent research indicating that accuracy requirements change the control processes for bimanual performance differently than for unimanual tasks.

Motor control characteristics of upper limbs in response to assistive forces during bilateral tasks

Most research on power assist suits (PASs) that concerned PAS-human interactions has used human physical reactions as criteria to evaluate the mechanical function, however, with minimal emphasis on human reactions in response to PASs. In this study, we focused on the physiological responses of the upper limbs including muscle activity of the biceps brachii and the triceps brachii, co-activation, force steadiness (CV) and rated perceived exertion (RPE) to various patterns of bilateral assistive force, such as unilateral assistance (L0% & R67% [% = percentage of workload force, L = left arm, R = right arm], L67% & R0%, L0% & R33%, L33% & R0%), symmetrical (L0% & R0%, L33% & R33%, L67% & R67%) and asymmetrical bilateral assistance (L33% & R67%, L67% & R33%), during bilateral isometric force-matching tasks. The results showed a similar muscular response of the two arms to bilateral assistive conditions, and the muscle activity of the arm that was being observed decreased only when the assistive force that applied on itself increased, indicating that both arms may have adopted similar but independent motor control mechanisms to acclimate to the bilateral assistive forces. Comparison between the two unilateral assistances (L0% & R33% and L33% & R0%) and the two asymmetrical bilateral assistances (L33% & R67%, L67% & R33%) showed no significant differences in muscular responses, CV and RPE, indicating that bilateral assistances with bilateral interchanged assistive levels may be equally effective regardless of which arm the higher assistive force is applied to. Comparison between unilateral and symmetrical assistive conditions that have similar overall workloads (L67% & R0%, L33% & R33%, L0% & R67%) showed a lower CV and RPE score at symmetrical assistance compared with unilateral assistance, suggesting that assisting both arms with the same level simultaneously improves task performances compared with applying the assistive force to only one arm.

Perceived effort affects choice of limb and reaction time of movements

The decision regarding which arm to use to perform a task reflects a complex process that can be influenced by many factors, including effort requirements of acquiring the goal. In this study, we considered a virtual reality environment in which people reached to a visual target in three-dimensional space. To vary the cost of reaching, we altered the visual feedback associated with motion of one arm but not the other. This altered the extent of motion that was required to reach, thus changing the effort required to acquire the goal. We then measured how that change in effort affected the decision regarding which arm to use, as well as the preparation time for the movement that ensued. As expected, with increased visual amplification of one arm (reduced effort to reach the goal), subjects increased the probability of choosing that arm. Surprisingly, however, the reaction times to start these movements were also reduced: despite constancy of the visual representation of the target, reaction times were shorter for movements with less effort. Thus, as the perceived effort associated with accomplishing a goal was reduced for a given limb, the decision-making process was biased toward use of that limb. Furthermore, movements that were perceived to be less effortful were performed with shorter reaction times. These results suggest that visual amplification can alter the perceived effort associated with using a limb, thus increasing frequency of use. This may provide a useful method to increase use of a limb during rehabilitation.NEW & NOTEWORTHY We report that visual amplification may serve as an effective means to alter the perceived effort associated with use of a limb. This method may provide an effective tool with which use of the affected limb can be encouraged noninvasively after neurological injury.

Inherent Kinematic Features of Dynamic Bimanual Path Following Tasks

Bimanual coordination is critical in many robotic and haptic systems, such as surgical robots and rehabilitation robots. While these systems often incorporate two robotic manipulators for each limb, there may be a missed opportunity to leverage overarching models of human bimanual coordination to improve the way in which the robotic manipulators are controlled and respond to the dynamic human operator. In this paper, we study the influences of several bimanual motion factors (e.g., symmetry and direction) on kinematic human joint-space features and performance outcome task-space features in a user study with eleven subjects and two haptic devices. Additionally, we evaluated the ability to use joint-space features to classify types of bimanual movement, showing the potential for a robotic system to predict how users coordinate their limbs. Three classifiers: (1) likelihood ratio, (2) k-nearest neighbor, and (3) support vector machine, were evaluated for classification accuracy in regards to the factor of number of targets. Likelihood ratio resulted in an accuracy of 79.6% with the majority of correct predictions occurring immediately at the start of movement. The task-space performance results reveal that despite the relative direction of both hands, reaching two targets results in lower performance than a single target, and symmetry alone does not contribute to performance disparity. Also, dimensionless integrated absolute jerk (DIAJ) is an indicator of superior performance for this particular task. Furthermore, these results align with current bimanual coordination theory by showing manual performance disparities are a consequence of task constraints and conceptualization.

More Pronounced Bimanual Interference in Proximal Compared to Distal Effectors of the Upper Extremities

Bimanual performance depends on effective and modular bilateral communication between the two bodysides. Bilateral neural interactions between the bodysides could cause bimanual interference, and the neuromuscular system for proximal and distal muscles is differently organized, where proximal muscles have more bilateral interneurons at both cortical and spinal level compared to distal muscles. These differences might increase the potential for bimanual interference between proximal arm muscles, because of greater proportions of bilateral interneurons to proximal muscles. The purpose of the present experiment was to evaluate potential differences in bimanual interference between proximal versus distal effectors in the upper extremities. 14 participants first performed a unilateral primary motor task with dominant arm with (1) a proximal and (2) distal controlled joysticks (condition A). Performance in condition A, was compared with the same effector’s performance when a bimanual interference task was performed simultaneously with the non-dominant arm (condition B). The results showed a significant bimanual interference for both the proximal and distal controlled joysticks. Most interestingly, the bimanual interference was larger for the proximal joystick compared to the distal controlled joystick. The increase in spatial accuracy error was higher for the proximal controlled joystick, compared with the distal controlled joystick. These results indicate that the proximal-distal distinction is an important organismic constraint on motor control, and especially for bilateral communication. There seem to be an undesired bilateral interference for both proximal and distal muscles. The interference is higher in the case of proximal effectors compared distal effectors, and the results seem to map the neuroanatomical and neurophysiological differences for these effectors.

A Re-Appraisal of the Effect of Amplitude on the Stability of Interlimb Coordination Based on Tightened Normalization Procedures

The stability of rhythmic interlimb coordination is governed by the coupling between limb movements. While it is amply documented how coordinative performance depends on movement frequency, theoretical considerations and recent empirical findings suggest that interlimb coupling (and hence coordinative stability) is actually mediated more by movement amplitude. Here, we present the results of a reanalysis of the data of Post, Peper, and Beek (2000), which were collected in an experiment aimed at teasing apart the effects of frequency and amplitude on coordinative stability of both steady-state and perturbed in-phase and antiphase interlimb coordination. The dataset in question was selected because we found indications that the according results were prone to artifacts, which may have obscured the potential effects of amplitude on the post-perturbation stability of interlimb coordination. We therefore redid the same analysis based on movement signals that were normalized each half-cycle for variations in oscillation center and movement frequency. With this refined analysis we found that (1) stability of both steady-state and perturbed coordination indeed seemed to depend more on amplitude than on movement frequency per se, and that (2) whereas steady-state antiphase coordination became less stable with increasing frequency for prescribed amplitudes, in-phase coordination became more stable at higher frequencies. Such effects may have been obscured in previous studies due to (1) unnoticed changes in performed amplitudes, and/or (2) artifacts related to inappropriate data normalization. The results of the present reanalysis therefore give cause for reconsidering the relation between the frequency, amplitude, and stability of interlimb coordination.

An inverse optimization approach to understand human acquisition of kinematic coordination in bimanual fine manipulation tasks

Tasks that require the cooperation of both hands and arms are common in human everyday life. Coordination helps to synchronize in space and temporally motion of the upper limbs. In fine bimanual tasks, coordination enables also to achieve higher degrees of precision that could be obtained from a single hand. We studied the acquisition of bimanual fine manipulation skills in watchmaking tasks, which require assembly of pieces at millimeter scale. It demands years of training. We contrasted motion kinematics performed by novice apprentices to those of professionals. Fifteen subjects, ten novices and five experts, participated in the study. We recorded force applied on the watch face and kinematics of fingers and arms. Results indicate that expert subjects wisely place their fingers on the tools to achieve higher dexterity. Compared to novices, experts also tend to align task-demanded force application with the optimal force transmission direction of the dominant arm. To understand the cognitive processes underpinning the different coordination patterns across experts and novice subjects, we followed the optimal control theoretical framework and hypothesize that the difference in task performances is caused by changes in the central nervous system's optimal criteria. We formulated kinematic metrics to evaluate the coordination patterns and exploit inverse optimization approach to infer the optimal criteria. We interpret the human acquisition of novel coordination patterns as an alteration in the composition structure of the central nervous system's optimal criteria accompanied by the learning process.

Accessing interpersonal and intrapersonal coordination dynamics

Both intrapersonal and interpersonal coordination dynamics have traditionally been investigated using relative phase patterns of in-phase (ϕ = 0°) and/or anti-phase (ϕ = 180°). Numerous investigations have demonstrated that coordination tasks that require other relative phase patterns (e.g., 90°) are difficult or near impossible to perform without extended practice. Recent findings, however, have demonstrated that an individual can produce a wide range of intrapersonal bimanual patterns within a few minutes of practice when provided integrated feedback. The present experiment was designed to directly compare intra- and interpersonal coordination performance and variability when provided Lissajous feedback or pacing metronome. Single participants (N = 12) and pairs of participants (N = 24, 12 pairs) were required to produce relative phase patterns between 0° and 180° in 30° increments using either pacing metronomes or Lissajous displays. The Lissajous displays involved a goal template and a cursor providing integrated feedback regarding the position of the two effectors. The results indicated both single and pairs of participants could effectively produce a large range of coordination patterns that typically act as repellers after only 6 min of practice when provided integrated feedback. However, single participants performed the in-phase coordination pattern more accurately and with less variability than paired participants, regardless of the feedback condition. These results suggest an advantage for intrapersonal coordination when performing in-phase coordination, possibly due to the stabilizing effect occurring via the neuro-muscular linkage between effectors.

Co-actors represent the order of each other's actions

Previous research has shown that people represent each other’s tasks and actions when acting together. However, less is known about how co-actors represent each other’s action sequences. Here, we asked whether co-actors represent the order of each other’s actions within an action sequence, or whether they merely represent the intended end state of a joint action together with their own contribution. In the present study, two co-actors concurrently performed action sequences composed of two actions. We predicted that if co-actors represent the order of each other’s actions, they should experience interference when the order of their actions differs. Supporting this prediction, the results of six experiments consistently showed that co-actors moved more slowly when performing the same actions in a different order compared to performing the same actions in the same order. In line with findings from bimanual movement tasks, our results indicate that interference can arise due to differences in movement parameters and due to differences in the perceptual characteristics of movement goals. The present findings extend previous research on co-representation, providing evidence that people represent not only the elements of another’s task, but also their temporal structure.

Handedness results from complementary hemispheric dominance, not global hemispheric dominance: evidence from mechanically coupled bilateral movements

Two contrasting views of handedness can be described as 1) complementary dominance, in which each hemisphere is specialized for different aspects of motor control, and 2) global dominance, in which the hemisphere contralateral to the dominant arm is specialized for all aspects of motor control. The present study sought to determine which motor lateralization hypothesis best predicts motor performance during common bilateral task of stabilizing an object (e.g., bread) with one hand while applying forces to the object (e.g., slicing) using the other hand. We designed an experimental equivalent of this task, performed in a virtual environment with the unseen arms supported by frictionless air-sleds. The hands were connected by a spring, and the task was to maintain the position of one hand while moving the other hand to a target. Thus the reaching hand was required to take account of the spring load to make smooth and accurate trajectories, while the stabilizer hand was required to impede the spring load to keep a constant position. Right-handed subjects performed two task sessions (right-hand reach and left-hand stabilize; left-hand reach and right-hand stabilize) with the order of the sessions counterbalanced between groups. Our results indicate a hand by task-component interaction such that the right hand showed straighter reaching performance whereas the left hand showed more stable holding performance. These findings provide support for the complementary dominance hypothesis and suggest that the specializations of each cerebral hemisphere for impedance and dynamic control mechanisms are expressed during bilateral interactive tasks. NEW & NOTEWORTHY We provide evidence for interlimb differences in bilateral coordination of reaching and stabilizing functions, demonstrating an advantage for the dominant and nondominant arms for distinct features of control. These results provide the first evidence for complementary specializations of each limb-hemisphere system for different aspects of control within the context of a complementary bilateral task.

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.

Intermanual Cross–Talk Effects in Unimanual Choice Reactions

Intermanual interactions originate at different levels of motor control. Interactions during specification of movement characteristics should affect reaction time for choice between left–hand and right–hand movements. In two experiments combinations of short and long target amplitudes for reversal movements of the left and right hand were cued with variable precueing intervals. Upon presentation of the response signal a unimanual left–hand or right–hand movement had to be produced. Reaction time was faster when same target amplitudes were precued than when different target amplitudes were. At short precueing intervals the longer reaction time with different target amplitudes (early effect) was accompanied by an amplitude assimilation: Short amplitudes were too long, and long amplitudes were too short. At longer precueing intervals the longer reaction time with different target amplitudes (late effect) was accompanied by a higher choice accuracy. These findings are taken to indicate a transient parametric coupling of amplitude specifications, which produces the early and the late effects by way of different mechanisms–namely different degrees of advance specification and generalized de–coupling, which affects the process of choice between hands.

Bimanual Lifting: Do Fingertip Forces Work Independently or Interactively?

Bimanual coordination is a commonplace activity, but the consequences of using both hands simultaneously are not well understood. The authors examined fingertip forces across 4 experiments in which participants undertook a range of bimanual tasks. They first measured fingertip forces during simultaneous lifts of 2 identical objects, noting that individuals held the objects with more force bimanually than unimanually. They then varied the mass of the objects held by each hand, noting that when both hands lifted together performance was equivalent to unimanual lifts. The authors next measured one hand's static grip force while the other hand lifted an object. They found a gradual reduction of grip force throughout the trial, but once again no evidence of one hand influencing the other. In the final experiment the authors tested whether tapping with one hand could influence the static grip force of its counterpart. Although the authors found no changes in static grip force as a direct consequence of the other hand's actions, they found clear differences from one task to the other, suggesting an effect of task instruction. Overall, these results suggest that fingertip forces are largely independent between hands in a bimanual lifting context, but are sensitive to different task requirements.

Neural coupling between homologous muscles during bimanual tasks: effects of visual and somatosensory feedback

While the effects of sensory feedback on bimanual tasks have been studied extensively at two ends of the motor control hierarchy, the cortical and behavioral levels, much less is known about how it affects the intermediate levels, including neural control of homologous muscle groups. We investigated the effects of somatosensory input on the neural coupling between homologous arm muscles during bimanual tasks. Twelve subjects performed symmetric elbow flexion/extension tasks under different types of sensory feedback. The first two types involve visual feedback, with one imposing stricter force symmetry than the other. The third incorporated somatosensory feedback via a balancing apparatus that forced the two limbs to produce equal force levels. Although the force error did not differ between feedback conditions, the somatosensory feedback significantly increased temporal coupling of bilateral force production, indicated by a high correlation between left/right force profiles (P < 0.001). More importantly, intermuscular coherence between biceps brachii muscles was significantly higher with somatosensory feedback than others (P = 0.001). Coherence values also significantly differed between tasks (flexion/extension). Notably, whereas feedback type mainly modulated coherence in the α- and γ-bands, task type only affected β-band coherence. Similar feedback effects were observed for triceps brachii muscles, but there was also a strong phase effect on the coherence values (P < 0.001) that could have diluted feedback effects. These results suggest that somatosensory feedback can significantly increase neural coupling between homologous muscles. Additionally, the between-task difference in β-band coherence may reflect different neural control strategies for the elbow flexor and extensor muscles.

NEW & NOTEWORTHY

This study investigated the effects of somatosensory feedback during bimanual tasks on the neural coupling between arm muscles, which remains largely unexplored. Somatosensory feedback using a balancing apparatus, compared with visual feedback, significantly increased neural coupling between homologous muscles (indicated by intermuscular coherence values) and improved temporal correlation of bilateral force production. Notably, feedback type modulated coherence in the α- and γ-bands (more subcortical pathways), whereas task type mainly affected β-band coherence (corticospinal pathway).

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.

Bimanual motor coordination controlled by cooperative interactions in intrinsic and extrinsic coordinates

Although strong motor coordination in intrinsic muscle coordinates has frequently been reported for bimanual movements, coordination in extrinsic visual coordinates is also crucial in various bimanual tasks. To explore the bimanual coordination mechanisms in terms of the frame of reference, here we characterized implicit bilateral interactions in visuomotor tasks. Visual perturbations (finger-cursor gain change) were applied while participants performed a rhythmic tracking task with both index fingers under an in-phase or anti-phase relationship in extrinsic coordinates. When they corrected the right finger's amplitude, the left finger's amplitude unintentionally also changed [motor interference (MI)], despite the instruction to keep its amplitude constant. Notably, we observed two specificities: one was large MI and low relative-phase variability (PV) under the intrinsic in-phase condition, and the other was large MI and high PV under the extrinsic in-phase condition. Additionally, using a multiple-interaction model, we successfully decomposed MI into intrinsic components caused by motor correction and extrinsic components caused by visual-cursor mismatch of the right finger's movements. This analysis revealed that the central nervous system facilitates MI by combining intrinsic and extrinsic components in the condition with in-phases in both intrinsic and extrinsic coordinates, and that under-additivity of the effects is explained by the brain's preference for the intrinsic interaction over extrinsic interaction. In contrast, the PV was significantly correlated with the intrinsic component, suggesting that the intrinsic interaction dominantly contributed to bimanual movement stabilization. The inconsistent features of MI and PV suggest that the central nervous system regulates multiple levels of bilateral interactions for various bimanual tasks.

Bimanual force control: cooperation and interference?

Three experiments were designed to determine the level of cooperation or interference observed from the forces generated in one limb on the forces exhibited by the contralateral limb when one or both limbs were producing a constant force (Experiment 1), one limb was producing a dynamic force while the other limb was producing a constant force (Experiment 2), and both limbs were producing dynamic force patterns (Experiment 3). The results for both Experiments 1 and 2 showed relatively strong positive time series cross correlations between the left and right limb forces indicating increases or decreases in the forces generated by one limb resulted in corresponding changes in the forces produced by the homologous muscles of the contralateral limb. Experiment 3 required participants to coordinate 1:1 and 1:2 rhythmical bimanual force production tasks when provided Lissajous feedback. The results indicated very effective performance of both bimanual coordination patterns. However, identifiable influences of right limb forces on the left limb force time series were observed in the 1:2 coordination pattern but not in the 1:1 pattern. The results of all three experiments support the notion that neural crosstalk is partially responsible for the stabilities and instabilities associated with bimanual coordination.

Problems in planning bimanually incongruent grasp postures relate to simultaneous response specification processes

The purpose of the current experiments was to examine whether the problems associated with grasp posture planning during bimanually incongruent movements are due to crosstalk at the motor programming level. Participants performed a grasping and placing task in which they grasped two objects from a table and placed them onto a board to targets that required identical (congruent) or non-identical degrees of rotation (incongruent). The interval between the presentation of the first stimulus and the second stimulus (stimulus onset asynchrony: SOA) was manipulated. Results demonstrate that the problems associated with bimanually incongruent grasp posture planning are reduced at SOA durations longer than 1000ms, indicating that the costs associated with bimanual incongruent movements arise from crosstalk at the motor programming level. In addition, reach-to-grasp times were shorter, and interlimb limb coupling was higher, for congruent, compared to incongruent, object end-orientation conditions in both Experiment 1 and 2. The bimanual interference observed during reach-to-grasp execution is postulated to arise from limitations in the visual motor system or from conceptual language representations. The present results emphasize that bimanual interference arises from constraints active at multiple levels of the neurobiological-cognitive system.

Complexity of central processing in simple and choice multilimb reaction-time tasks

The default mode of the motor system is a coupling between limbs. However, in some movements, a decoupling is required and thus calls for selection and facilitation/inhibition processes. Here, we investigate the relative contribution of recruitment versus selection processes to the overall processing complexity. To this aim we proposed a new multilimb reaction-time task (MUL-RT). Simple, choice and normalized (choice minus simple) RT were analysed together with error rates in thirty-six young adults for 15 coordination modes including all possible configuration of limb recruitment. Simple and normalized RTs were respectively assumed to be indicative of the recruitment and selection processes. Results supported a model of coupling/decoupling interactions respectively reporting weak, intermediate and strong interaction for selecting diagonal, ipsilateral and homologous limbs. Movement laterality (left vs. right) had no effect on selection complexity, whereas selecting upper limbs was less challenging than selecting lower limbs. Results in the different coordination modes suggested that recruitment complexity decreased as follows: 3 limbs = 4 limbs>2 limbs (homologous, ipsilateral and diagonal)>1 limb, and selection complexity as follows: 2 diagonal limbs>3 limbs>2 ipsilateral limbs>1 limb = 2 homologous limbs>4 limbs. Based on these ordinal scales of recruitment and selection complexity, we extrapolated the overall processing complexity of the simple and choice MUL-RT. This method was efficient in reproducing the absolute results we obtained on a ratio scale (ms) and demonstrated that processing complexity in simple RT was mainly governed by the ‘recruitment principle’ (the more limbs recruited the lower the performance), whereas contributions of recruitment and ‘selection principle’ (nature of the coordination determines performance) to overall processing complexity were similar in choice RT.

Spatiotemporal coupling between speech and manual motor actions

Much evidence has been found for pervasive links between the manual and speech motor systems, including evidence from infant development, deictic pointing, and repetitive tapping and speaking tasks. We expand on the last of these paradigms to look at intra- and cross-modal effects of emphatic stress, as well as the effects of coordination in the absence of explicit rhythm. In this study, subjects repeatedly tapped their finger and synchronously repeated a single spoken syllable. On each trial, subjects placed an emphatic stress on one finger tap or one spoken syllable. Results show that both movement duration and magnitude are affected by emphatic stress regardless of whether that stress is in the same domain (e.g., effects on the oral articulators when a spoken repetition is stressed) or across domains (e.g., effects on the oral articulators when a tap is stressed). Though the size of the effects differs between intra-and cross-domain emphases, the implementation of stress affects both motor domains, indicating a tight connection. This close coupling is seen even in the absence of stress, though it is highlighted under stress. The results of this study support the idea that implementation of prosody is not domain-specific but relies on general aspects of the motor system.

Comparing movement preparation of unimanual, bimanual symmetric, and bimanual asymmetric movements

The goal of this study was to determine the process or processes most likely to be involved in reaction-time costs for spatially cued bimanual reaching. We used reaction time to measure the cost of bimanual symmetric movements compared to unimanual movements (a bimanual symmetric cost) and the cost for bimanual asymmetric movements compared to symmetric movements (a bimanual asymmetric cost). The results showed that reaction times were comparable for all types of movements in simple reaction time; that is, there was neither a bimanual symmetric cost nor an asymmetric cost. Therefore, unimanual, bimanual symmetric, and bimanual asymmetric movements have comparable complexity during response initiation. In choice conditions, there was no bimanual symmetric cost but there was a bimanual asymmetric cost, indicating that the preparation of asymmetric movements is more complex than symmetric movements. This asymmetric cost is likely the result of interference during response programming.

Reacting while moving: influence of right limb movement on left limb reaction

An experiment was designed to determine whether the activation of a muscle group (flexors or extensors) used to produce an ongoing movement of one limb influenced the reaction time and associated initiation of elbow flexion or extension movements of the contralateral limb. Right-handed participants in the bimanual groups were asked to produce a pattern of flexion/extension movements defined by a sine wave (period = 2 s, amplitude = 16°) with the right limb. While performing the right limb movement, participants were instructed that they were to react as quickly as possible by making a flexion or extension movement with their left limb when the cursor they were using to track the sine wave changed color. Participants in the unimanual groups performed the left limb reaction time task but were not asked to make right limb movements. The reaction time stimulus occurred once in each trial and was presented at one of six locations on one of the six cycles comprising the sinusoidal waveform. Participants performed 7 blocks of 6 test trials. Reaction time was calculated as the time interval between the color change of the cursor and the initiation of the response with the left limb. Movement time was calculated as the interval of time between the initiation of the response and the left limb cursor crossing the upper or lower boundary line. Mean reaction of the left limb was significantly influenced by the concurrent type of movement (flexion/extension) of the right limb. Reaction times were shorter on trials in which both limbs were initiating movement with homologous muscles as compared to trials in which the limbs were initiating movement with non-homologous muscles. No differences were detected when the stimuli were presented during the ballistic phase of the right limb movement, and no differences at any position were detected for the unimanual groups. This result is consistent with the notion that neural crosstalk can influence the time required to react to a stimulus but this influence occurs when contralateral muscles are activated.

Interlimb coordination during a cooperative bimanual object manipulation task

This experiment examined asymmetries in the execution of an object manipulation task that requires the coordinated use of both hands. To this end, twenty right-hand-dominant participants performed a bimanual object manipulation task, which required that they reach for and grasp two objects located on a tabletop, fit the two objects through a hole in a horizontally or vertically oriented fitting board, and then rotate the objects 180° to produce a "beep" tone. Overall, the two hands were highly synchronized at the start, but not at end, of each movement phase. The decrease in interlimb coupling at later stages of the movement phase was primarily driven by the shorter movement time values for the dominant right hand. In addition, degree of left object rotation was greater than the right object, irrespective of board orientation. In sum, the results suggest that manual asymmetries and role assignment are not hardwired constraints, but depend on the overall task constraints and the manner in which the task is conceptualized.

The effects of task demands on bimanual skill acquisition

Bimanual coordination is essential for everyday activities. It is thought that different degrees of demands may affect learning of new bimanual patterns. One demand is at the level of performance and involves breaking the tendency to produce mirror-symmetric movements. A second is at a perceptual level and involves controlling each hand to separate (i.e., split) goals. A third demand involves switching between different task contexts (e.g., a different uni- or bimanual task), instead of continuously practicing one task repeatedly. Here, we studied the effect of these task demands on motor planning (reaction time) and execution (error) while subjects learned a novel bimanual isometric pinch force task. In Experiment 1, subjects continuously practiced in one of the two extremes of the following bimanual conditions: (1) symmetric force demands and a perceptually unified target for each hand or (2) asymmetric force demands and perceptually split targets. Subjects performing in the asymmetric condition showed some interference between hands, but all subjects, regardless of group, could learn the isometric pinch force task similarly. In Experiment 2, subjects practiced these and two other conditions, but in a paradigm where practice was briefly interrupted by the performance of either a unimanual or a different bimanual condition. Reaction times were longer and errors were larger well after the interruption when the main movement to be learned required asymmetric forces. There was no effect when the main movement required symmetric forces. These findings demonstrate two main points. First, people can learn bimanual tasks with very different demands on the same timescale if they are not interrupted. Second, interruption during learning can negatively impact both planning and execution and this depends on the demands of the bimanual task to be learned. This information will be important for training patient populations, who may be more susceptible to increased task demands.

Does the central nervous system learn to plan bimanual movements based on its expectation of availability of visual feedback?

The correlation between gaze strategy and kinematics of bimanual movements was assessed using repetitive bimanual object transfers as an experimental paradigm. The hypothesis was that visual demand in such tasks is a critical bottleneck in determining bimanual coordination. Kinematics and eye movements were compared before and after practice of this repetitive task. New eye-hand coordination strategies emerged with practice. Also, with practice a systematic prioritization of the left hand movement to be 'primary' and the right hand movement to be 'secondary' emerged. This choice implied that the left hand movement kinematics was similar to that of the unimanual left hand movements, whereas the performance of the right hand task was contingent on successful completion of the primary task. This was revealed by 'anticipatory adjustments' of the right hand kinematics (right-hand peak velocity ranged from 100-70% of the left-hand, and the scaling was dependent on task conditions and the corresponding eye-hand coordination strategies used). We used this evidence to argue that the CNS, aware of an inherent asymmetry between the two hand systems, learns to anticipate the need and availability of visual feedback for successful task completion, and uses this knowledge to optimize movement coordination, specifically such that the right-hand control was modulated to take visual constraints into account.

Effects of force levels on error compensation in periodic bimanual isometric force control

The authors examined whether force level interacts with the presence or absence of vision in bimanual force control. Participants produced periodic isometric forces such that the sum of the 2 finger forces was the target force under 4 force levels cycling between lower levels (5-40%) of maximum voluntary contraction with an interval of 1000 ms. Without vision, the correlation between the 2 finger forces was strongly positive over all force levels. However, with vision the correlation changed from negative to positive with force level. The result with vision indicated that the strategy of the bimanual force control changed from force error compensation to force coupling and the available redundancy thus decreased with an increase in force.

Effects of stimulus cueing on bimanual grasp posture planning

The purpose of the present study was to investigate whether difficulties in bimanual grasp posture planning arise from conflicts in response selection. Forty-five participants were assigned to one of three groups (symbolic cueing, semi-symbolic cueing, and direct cueing) and instructed to reach for, grasp, and place two objects on a board in various end-orientations, depending on condition. In general, the tendency to adopt initial grasps that resulted in end-state comfort was significantly higher for the semi-symbolic, than that for the other two groups. There were, however, noticeable individual differences in grip behavior in the symbolic and direct cueing groups. Although the majority of participants performed the task in a similar fashion to the semi-symbolic group, there was a subset of participants (40% in each group) who grasped the two objects using an overhand grip in virtually all trials, regardless of condition. It is hypothesized that the observed individual differences in grasp posture strategy arise from differences in motor planning abilities, or the strategies participants employ in order to comply with task demands. A secondary finding is that the degree of interlimb coupling was larger for congruent, than incongruent, conditions irrespective of stimulus cueing. This finding indicates that the interference in the execution of bimanual grasping and placing tasks arises from interference during the specification of movement parameters specific to planning and execution of bimanual movements, or neuronal cross-talk in efferent pathways, rather than response selection conflicts.

Effect of Cognitive Load on Working Memory Forgetting in Aging

Functional approaches to working memory (WM) have been proposed recently to better investigate “maintenance” and “processing” mechanisms. The cognitive load (CL) hypothesis presented in the “Time-Based Resource-Sharing” model (Barrouillet & Camos, 2007) suggests that forgetting from WM (maintenance) can be investigated by varying the presentation rate and processing speed (processing). In this study, young and elderly participants were compared on WM tasks in which the difference in processing speed was controlled by CL manipulations. Two main results were found. First, when time constraints (CL) were matched for the two groups, no aging effect was observed. Second, whereas a large variation in CL affected WM performance, a small CL manipulation had no effect on the elderly. This suggests that WM forgetting cannot be completely accounted for by the CL hypothesis. Rather, it highlights the need to explore restoration times in particular, and the nature of the refreshment mechanisms within maintenance.

The stability of rhythmic movement coordination depends on relative speed: the Bingham model supported

Following many studies showing that the coupling in bimanual coordination can be perceptual, Bingham (Ecol Psychol in 16:45-53, 2001; 2004a, b) proposed a dynamical model of such movements. The model contains three key hypotheses: (1) Being able to produce stable coordinative movements is a function of the ability to perceive relative phase, (2) the information to perceive relative phase is relative direction of motion, and (3) the ability to resolve this information is conditioned by relative speed. The first two hypotheses have been well supported (Wilson and Bingham in Percept Psychophys 70:465-476, 2008; Wilson et al. in J Exp Psychol Hum 36:1508-1514, 2010a), but the third was not supported when tested by de Rugy et al. (Exp Brain Res 184:269-273, 2008) using a visual coordination task that required simultaneous control of both the amplitude and relative phase of movement. The purposes of the current study were to replicate this task with additional measures and to modify the original model to apply it to the new task. To do this, we conducted two experiments. First, we tested the ability to produce 180° visual coordination at different frequencies to determine frequencies suitable for testing in the de Rugy et al. task. Second, we tested the de Rugy et al. task but included additional measures that yielded results different from those reported by de Rugy et al. These results were used to elaborate the original model. First, one of the phase-driven oscillators was replaced with a harmonic oscillator, so the resulting coupling was unidirectional. This change resulted in the model producing less stable 180° coordination behavior beyond 1.5 Hz consistent with the results obtained in Experiment 1. Next, amplitude control and phase correction elements were added to the model. With these changes, the model reproduced behaviors observed in Experiment 2. The central finding was that the stability of rhythmic movement coordination does depend on relative speed and, thus, all three of the hypotheses contained in the original Bingham model are supported.

Visual information interacts with neuromuscular factors in the coordination of bimanual isometric force

The role of perceptual-motor processes in the coordination and control of movement is a long standing issue. Nevertheless, there is no coherence on theoretical perspectives with their being frameworks that emphasize perceptual, motor and perceptual-motor processes in coordination and control. The purpose of this study was to examine the interactive effects of visual information and factors of neuromuscular organization (force level, force direction, and homologous muscle pairs) on coordination patterns in bimanual isometric force production. In Experiment 1, the participants were required to abduct two index fingers isometrically and produce simultaneous forces such that their sum matched the constant force target specified at two force levels (10 and 40% of maximum voluntary contraction (MVC)). Visual information of the force outputs was either present or absent between conditions. The results showed that the coordination patterns interact with visual feedback in that the two finger forces exhibit negative correlation with vision and positive correlation without vision, with stronger correlation in each case found at higher force levels. In Experiment 2, the force direction and muscles involved in the task were different between the hands. In comparison with Experiment 1, the negative correlation was stronger with vision at 40% MVC (but equal at 10% MVC), and positive correlation was weaker without vision at 10% MVC (but equal at 40% MVC). The findings provide further evidence that the coordination patterns in bimanual isometric force production are specified by the interaction of task-relevant visual information and force level and, to a lesser degree by force direction and the muscles involved in the task. The capacity to exploit information mediates coordination and control, and the effective utilization of information is dependent on the specific action.

Inter-limb Coupling in Coordinated Bimanual Movement: Attention and Asymmetries

Inter-limb coupling, a phenomenon whereby each of the upper limbs tends to take on characteristics of the intended movement of the other, represents a limitation on the ability to perform asymmetrical bimanual movements. Two experiments each employing 16 dextral and 16 sinistral normal subjects are reported. In the first experiment evidence of inter-limb coupling was observed during a continuous bimanual rotary task. This coupling appeared to be asymmetrical, with the nonpreferred hand contributing more to coupling than the preferred hand, especially in dextrals. In the second experiment asymmetries in inter-limb coupling were found to be modified by the conscious direction of attention to one or other hand. This suggests that the often reported strong inter-limb asymmetry associated with dextrality, and the weaker assymetry associated with sinistrality, may be partly due to an underlying inter-limb attentional asymmetry in the former, and a relative lack of attentional asymmetry in the latter.

Conceptual unifying constraints override sensorimotor interference during anticipatory control of bimanual actions

Traditional approaches to research on bimanual coordination focus on sensorimotor interference, motor programming, and effects of perception and feedback guidance; surprisingly, little is known about high-level conceptual constraints that might unify separate movements into coordinated actions. We investigated two possible forms of high-level unifying representations on anticipatory control (i.e., reaction time: RT) in two-limb (bimanual) movements. Specifically, we adapted a paradigmatic bimanual task involving reaching to targets by adding two novel manipulations. One involved a visual-perceptual manipulation in which target-objects were presented either separately (i.e., two circles) or as a unified object (i.e., two circles connected by a bar). The other involved variants on language representation to elicit separate action plans (i.e., separate instructional commands joined by 'and') or unified action plans (i.e., a single verb applying to both hands). Typical forms of sensorimotor interference were virtually abolished when these unifying constraints were available. These findings provide strong support for the theoretical account that unifying conceptual representations are primary forms of bimanual constraint. Findings further suggest that the organization and content of the language used to form action representations can strongly influence anticipatory planning of bimanual actions.

Comparing unilateral and bilateral upper limb training: the ULTRA-stroke program design

BACKGROUND

About 80% of all stroke survivors have an upper limb paresis immediately after stroke, only about a third of whom (30 to 40%) regain some dexterity within six months following conventional treatment programs. Of late, however, two recently developed interventions--constraint-induced movement therapy (CIMT) and bilateral arm training with rhythmic auditory cueing (BATRAC)--have shown promising results in the treatment of upper limb paresis in chronic stroke patients. The ULTRA-stroke (acronym for Upper Limb TRaining After stroke) program was conceived to assess the effectiveness of these interventions in subacute stroke patients and to examine how the observed changes in sensori-motor functioning relate to changes in stroke recovery mechanisms associated with peripheral stiffness, interlimb interactions, and cortical inter- and intrahemispheric networks. The present paper describes the design of this single-blinded randomized clinical trial (RCT), which has recently started and will take several years to complete.

METHODS/DESIGN

Sixty patients with a first ever stroke will be recruited. Patients will be stratified in terms of their remaining motor ability at the distal part of the arm (i.e., wrist and finger movements) and randomized over three intervention groups receiving modified CIMT, modified BATRAC, or an equally intensive (i.e., dose-matched) conventional treatment program for 6 weeks. Primary outcome variable is the score on the Action Research Arm test (ARAT), which will be assessed before, directly after, and 6 weeks after the intervention. During those test sessions all patients will also undergo measurements aimed at investigating the associated recovery mechanisms using haptic robots and magneto-encephalography (MEG).

DISCUSSION

ULTRA-stroke is a 3-year translational research program which aims (1) to assess the relative effectiveness of the three interventions, on a group level but also as a function of patient characteristics, and (2) to delineate the functional and neurophysiological changes that are induced by those interventions.The outcome on the ARAT together with information about changes in the associated mechanisms will provide a better understanding of how specific therapies influence neurobiological changes, and which post-stroke conditions lend themselves to specific treatments.

TRIAL REGISTRATION

The ULTRA-stroke program is registered at the Netherlands Trial Register (NTR, http://www.trialregister.nl, number NTR1665).

Conceptual binding: integrated visual cues reduce processing costs in bimanual movements

In discrete reaction time (RT) tasks, it has been shown that nonsymmetric bimanual movements are initiated slower than symmetric movements in response to symbolic cues. By contrast, no such RT differences are found in response to direct cues ("direct cue effect"). Here, we report three experiments showing that the direct cue effect generalizes to rhythmical bimanual movements and that RT cost depends on different cue features: 1) symbolic versus direct or 2) integrated (i.e., action of both hands is indicated as one entity) versus dissociated (i.e., action of each hand is indicated separately). Our main finding was that dissociated symbolic cues were most likely processed serially, resulting in the longest RTs, which were substantially reduced with integrated symbolic cues. However, extra RT costs for switching to nonsymmetrical bimanual movements were overcome only when the integrated cues were direct. We conclude that computational resources might have been exceeded when the response needs to be determined for each hand separately, but not when a common response for both hands is selected. This supports the idea that bimanual control benefits from conceptual binding.