Hand preference, practice order, and spatial assimilations in rapid bimanual movement

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.

The complementary dominance hypothesis: a model for remediating the 'good' hand in stroke survivors

The complementary dominance hypothesis is a novel model of motor lateralization substantiated by decades of research examining interlimb differences in the control of upper extremity movements in neurotypical adults and hemisphere-specific motor deficits in stroke survivors. In contrast to earlier ideas that attribute handedness to the specialization of one hemisphere, our model proposes complementary motor control specializations in each hemisphere. The dominant hemisphere mediates optimal control of limb dynamics as required for smooth and efficient movements, whereas the non-dominant hemisphere mediates impedance control, important for countering unexpected mechanical conditions and achieving steady-state limb positions. Importantly, this model proposes that each hemisphere contributes its specialization to both arms (though with greater influence from either arm's contralateral hemisphere) and thus predicts that lesions to one hemisphere should produce hemisphere-specific motor deficits in not only the contralesional arm, but also the ipsilesional arm of stroke survivors - a powerful prediction now supported by a growing body of evidence. Such ipsilesional arm motor deficits vary with contralesional arm impairment, and thus individuals with little to no functional use of the contralesional arm experience both the greatest impairments in the ipsilesional arm, as well as the greatest reliance on it to serve as the main or sole manipulator for activities of daily living. Accordingly, we have proposed and tested a novel intervention that reduces hemisphere-specific ipsilesional arm deficits and thereby improves functional independence in stroke survivors with severe contralesional impairment.

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.

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.

The left cerebral hemisphere may be dominant for the control of bimanual symmetric reach-to-grasp movements

Previous research has established that the left cerebral hemisphere is dominant for the control of continuous bimanual movements. The lateralisation of motor control for discrete bimanual movements, in contrast, is underexplored. The purpose of the current study was to investigate which (if either) hemisphere is dominant for discrete bimanual movements. Twenty-one participants made bimanual reach-to-grasp movements towards pieces of candy. Participants grasped the candy to either place it in their mouths (grasp-to-eat) or in a receptacle near their mouths (grasp-to-place). Research has shown smaller maximum grip apertures (MGAs) for unimanual grasp-to-eat movements than unimanual grasp-to-place movements when controlled by the left hemisphere. In Experiment 1, participants made bimanual symmetric movements where both hands made grasp-to-eat or grasp-to-place movements. We hypothesised that a left hemisphere dominance for bimanual movements would cause smaller MGAs in both hands during bimanual grasp-to-eat movements compared to those in bimanual grasp-to-place movements. The results revealed that MGAs were indeed smaller for bimanual grasp-to-eat movements than grasp-to-place movements. This supports that the left hemisphere may be dominant for the control of bimanual symmetric movements, which agrees with studies on continuous bimanual movements. In Experiment 2, participants made bimanual asymmetric movements where one hand made a grasp-to-eat movement while the other hand made a grasp-to-place movement. The results failed to support the potential predictions of left hemisphere dominance, right hemisphere dominance, or contralateral control.

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.

The Interaction of Tactile Information and Movement Amplitude in a Multijoint Bimanual Circle-Tracing Task: Phase Transitions and Loss of Stability

Adaptive behaviour in bimanual coordination was examined with the use of a bimanual circle-tracing task. Circle diameter and tactile information were manipulated to form four tracing conditions: tracing a pair of 3-cm diameter circles with the tips of the index fingers (3F) or hand-held styli (3S) and tracing a pair of 10-cm diameter circles with the tips of the index fingers (10F) or hand-held styli (10S). Movement frequency was increased in all conditions. In the 3F, 3S, and 10S tracing conditions, an abrupt transition from asymmetric to symmetric coordination was the main adaptive response, while in the 10F tracing condition, phase wandering was the main adaptive response. Enhancement of fluctuations in relative phase, a signature of loss of stability, occurred before the transition from asymmetric to symmetric coordination. Movement frequency and movement amplitude interact as control parameters in this task. The results are discussed with reference to tactile surface contact and joint motion as sources of sensory information that can be used to stabilize bimanual coordination patterns. The presence or absence of tactile information is directly linked to the specific form of adaptive behaviour (phase transition or phase wandering) that emerges as a function of required movement amplitude and required pacing frequency.

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

In bimanual movements the amplitude of each hand's movement often depends on the concurrent amplitude of the other hand's movement such that both amplitudes become similar (amplitude coupling). We tested the hypothesis that the strength of amplitude coupling depends on the tempo of performance of a movement sequence, a hypothesis based on a model of bimanual coordination that holds that cross-talk occurs at the execution level as well as at the programming level. Subjects performed bimanual periodic arm movements on two digitizers. In nine conditions constant small, constant large, and alternating small and large amplitudes of each arm were orthogonally combined. Overall tempo was varied by instructing subjects to increase the tempo progressively by 10%. Clear tempo-dependent modulations of the amplitude were observed in movements with instructed constant amplitude when the other hand performed alternating amplitudes. The effect of the size of constant-amplitude movements on the mean amplitude of the other hand indicated cross-talk at the execution level. Cross-talk at the programming level was revealed by the dependence of the current amplitude on the change in the amplitude of the other hand in the preceding cycle. Finally, asymmetric cross-manual effects were observed.

Unintentional force changes in cyclical tasks performed by an abundant system: Empirical observations and a dynamical model

The study explored unintentional force changes elicited by removing visual feedback during cyclical, two-finger isometric force production tasks. Subjects performed two types of tasks at 1Hz, paced by an auditory metronome. One - Force task - required cyclical changes in total force while maintaining the sharing, defined as relative contribution of a finger to total force. The other task - Share task - required cyclical changes in sharing while keeping total force unchanged. Each trial started under full visual feedback on both force and sharing; subsequently, feedback on the variable that was instructed to stay constant was frozen, and finally feedback on the other variable was also removed. In both tasks, turning off visual feedback on total force elicited a drop in the mid-point of the force cycle and an increase in the peak-to-peak force amplitude. Turning off visual feedback on sharing led to a drift of mean share toward 50:50 across both tasks. Without visual feedback there was consistent deviation of the two force time series from the in-phase pattern (typical of the Force task) and from the out-of-phase pattern (typical of the Share task). This finding is in contrast to most earlier studies that demonstrated only two stable patterns, in-phase and out-of-phase. We interpret the results as consequences of drifts of parameters in a dynamical system leading in particular to drifts in the referent finger coordinates toward their actual coordinates. The relative phase desynchronization is caused by the right-left differences in the hypothesized drift processes, consistent with the dynamic dominance hypothesis.

Separate Movement Planning and Spatial Assimilation Effects in Sequential Bimanual Aiming Movements

This study extended earlier work by showing spatial assimilations in sequential bimanual aiming movements when the participant preplanned only the first movement of a two-movement sequence. Right-handed participants ( n = 20, aged 18 to 22 years) made rapid lever reversals of 20° and 60° singly and sequentially with an intermovement interval of 2.5 sec. Following blocked single practice of both movements in each hand (15 trials each), two sets of 30 sequential practice trials were completed. The sequences began with either the long or the short movement and the participant always knew the goal of the first movement. During the intermovement interval, the experimenter gave instructions to complete the sequence with a short movement, a long movement, or no movement in a random order. Compared to the single trials, both movements in the sequence overshot the short-distance and undershot the longdistance goal. Spatial errors increased when a change in the movement goal was required for the second movement in the sequence. The experiment demonstrated that separate planning of sequential aiming movements can reduce spatial assimilation effects, but interference due to practice organization and switching the task's goal must also be overcome in order to produce accurate aiming movements.

Inter-limb coupling during diadochokinesis in Parkinson's and Huntington's disease

Patients with neurodegenerative diseases often exhibit deficits in bimanual coordination. One characteristic of bimanual movements is inter-limb coupling. It is the property of motor performance harmonization between hands during a bimanual task. The objective of this study was to identify whether spatial and temporal inter-limb coupling occurred in Parkinson's disease (PD) and Huntington's disease (HD) patients. Twenty-three PD patients and 15 healthy controls were tested. Data from 12 choreic HD patients were also taken from a databank. Participants were asked to perform a unimanual and bimanual rapid repetitive diadochokinesis task. The difference between hands in mean amplitude and mean duration of cycles was computed in the unimanual and bimanual tasks for each group. Results show that healthy controls exhibited temporal and spatial inter-limb coupling during the bimanual diadochokinesis task. Conversely, PD and HD patients exhibited temporal inter-limb coupling; but failed to exhibit spatial inter-limb coupling during the bimanual diadochokinesis task. Furthermore, HD patients exhibited reduced levels of structural coupling compared to controls and PD patients. These results suggest that alterations in basal ganglia-thalamo-cortical networks due to PD and HD do not affect temporal inter-limb coupling. However, common pathophysiological changes related to PD and HD may cause altered spatial inter-limb coupling during a rapid repetitive bimanual diadochokinesis task.

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.

Interactions between brain structure and behavior: the corpus callosum and bimanual coordination

Bimanual coordination skills are required for countless everyday activities, such as typing, preparing food, and driving. The corpus callosum (CC) is the major collection of white matter bundles connecting both hemispheres that enables the coordination between the two sides of the body. Principal evidence for this brain-behavior relationship in humans was first provided by research on callosotomy patients, showing that sectioning (parts of) the CC affected interactions between both hands directly. Later, new noninvasive in vivo imaging techniques, such as diffusion tensor imaging, have energized the study of the link between microstructural properties of the CC and bimanual performance in normal volunteers. Here we discuss the principal factors (such as age, pathology and training) that mediate the relationship between specific bimanual functions and distinct anatomical CC subdivisions. More specifically, the question is whether different bimanual task characteristics can be mapped onto functionally distinct CC subregions. We review the current status of this mapping endeavor, and propose future perspectives to inspire research on this unique link between brain structure and behavior.

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.

Interlimb differences of directional biases for stroke production

Directional preferences during center-out horizontal shoulder-elbow movements were previously characterized for the dominant arm. These preferences were attributed to a tendency to actively accelerate one joint, while exploiting largely passive motion at the other joint. Since the non-dominant arm is known for inefficient coordination of inter-segmental dynamics, here we hypothesized that directional preferences would differ between the arms. A center-out free-stroke drawing task was used that allowed freedom in the selection of movement directions. The task was performed both with and without a secondary cognitive task that has been shown to increase directional biases of the dominant arm. Mirror-symmetrical directional preferences were observed in both arms, with similar bias strength and secondary task effects. The preferred directions were characterized by maximal exploitation of interaction torques for movement production, but only in the dominant arm. The non-dominant arm failed to benefit from interaction torques. The results point to a hierarchical architecture of control. At the higher level, a movement capable to perform the task while satisfying preferences in joint control is specified through forward dynamic transformations. This process is mediated for both arms from a common neural network adapted to the dominant arm and, specifically, to its ability to exploit interaction torques. Dynamic transformations that determine actual control commands are specified at the lower level of control. An alternative interpretation that strokes might be planned evenly across directions, and biases emerge during movement execution due to anisotropic resistance of intrinsic factors that do not depend on arm dominance is also discussed.

Preparation for voluntary movement in healthy and clinical populations: evidence from startle

In this review we provide a summary of the observations made regarding advance preparation of the motor system when presenting a startling acoustic stimulus (SAS) during various movement tasks. The predominant finding from these studies is that if the participant is prepared to make a particular movement a SAS can act to directly and quickly trigger the prepared action. A similar effect has recently been shown in patients with Parkinson's disease. This "StartReact" effect has been shown to be a robust indicator of advance motor programming as it can involuntarily release whatever movement has been prepared. We review the historical origins of the StartReact effect and the experimental results detailing circumstances where advance preparation occurs, when it occurs, and how these processes change with practice for both healthy and clinical populations. Data from some of these startle experiments has called into question some of the previously held hypotheses and assumptions with respect to the nature of response preparation and initiation, and how the SAS results in early response expression. As such, a secondary focus is to review previous hypotheses and introduce an updated model of how the SAS may interact with response preparation and initiation channels from a neurophysiological perspective.

Cross talk in implicit assignment of error information during bimanual visuomotor learning

When a neural movement controller, called an “internal model,” is adapted to a novel environment, the movement error needs to be appropriately associated with the controller. However, their association is not necessarily guaranteed for bimanual movements in which two controllers—one for each hand—result in two movement errors. Considering the implicit nature of the adaptation process, the movement error of one hand can be erroneously associated with the controller of the other hand. Here, we investigated this credit-assignment problem in bimanual movement by having participants perform bimanual, symmetric back-and-forth movements while displaying the position of the right hand only with a cursor. In the training session, the cursor position was gradually rotated clockwise, such that the participants were unaware of the rotation. The movement of the right hand gradually rotated counterclockwise as a consequence of adaptation. Although the participants knew that the cursor reflected the movement of the right hand, such gradual adaptation was also observed for the invisible left hand, especially when the cursor was presented on the left side of the display. Thus the movement error of the right hand was implicitly assigned to the left-hand controller. Such cross talk in credit assignment might influence motor adaptation performance, even when two cursors are presented; the adaptation was impaired when the rotations imposed on the cursors were opposite compared with when they were in the same direction. These results indicate the inherent presence of cross talk in the process of associating action with consequence in bimanual movement.

Limitations on coupling of bimanual movements caused by arm dominance: when the muscle homology principle fails

Studies of bimanual movements typically report interference between motions of the two arms and preference to perform mirror-symmetrical patterns. However, recent studies have demonstrated that the two arms differ in the ability to control interaction torque (INT). This predicts limitations in the capability to perform mirror-symmetrical movements. Here, two experiments were performed to test this prediction. The first experiment included bimanual symmetrical and asymmetrical circle drawing at two frequency levels. Unimanual circle drawing was also recorded. The increases in cycling frequency caused differences between the two arms in movement trajectories in both bimanual modes, although the differences were more pronounced in the asymmetrical compared with the symmetrical mode. Based on torque analysis, the differences were attributed to the nondominant arm's decreased capability to control INT. The intraarm differences during the symmetrical pattern of bimanual movements were similar (although more pronounced) to those during unimanual movements. This finding was verified in the second experiment for symmetrical bimanual oval drawing. Four oval orientations were used to provide variations in INT. Similar to the first experiment, increases in cycling frequency caused spontaneous deviations from perfect bimanual symmetry associated with inefficient INT control in the nondominant arm. This finding supports the limitations in performing mirror-symmetrical bimanual movements due to differences in joint control between the arms. Based on our results and previous research, we argue that bimanual interference occurs during specification of characteristics of required motion, whereas lower-level generation of muscle forces is independent between the arms. A hierarchical model of bimanual control is proposed.

The potential for utilizing inter-limb coupling in the rehabilitation of upper limb motor disability due to unilateral brain injury

PURPOSE

Symmetry tendencies in human movement have generally been regarded as a constraint to upper limb motor performance. However, several recent studies have suggested that this phenomenon might be utilized in the rehabilitation of individuals with motor disability due to unilateral brain injury. In this paper the efficacy of such a rehabilitative approach is explored by reviewing: (i) examples of symmetry tendencies in healthy individuals, (ii) the potential neurophysiological mechanisms responsible for inter-limb coupling, and (iii) recent studies which have directly assessed the effects of inter-limb coupling on individuals with unilateral brain injury.

METHOD

A thorough review of current published evidence was conducted utilizing various electronic search engines (Medline, PreMedline, Embase and Cinahl). Studies included those that focused on symmetry tendencies and/or inter-limb coupling in the upper limbs with a particular emphasis placed on studies of individuals with unilateral brain injury.

RESULTS

Based on the current literature it seems that motor function of the affected upper limb in individuals with unilateral brain injury can be improved through a rehabilitation approach that incorporates inter-limb coupling.

CONCLUSION

This approach should be considered as an adjunct to more common rehabilitation strategies with future research aimed at determining the most effective means of employing this paradigm.

Coordination of complex bimanual multijoint movements under increasing cycling frequencies: the prevalence of mirror-image and translational symmetry

The present study examined the principles underlying inter and intralimb coordination constraints during performance of bimanual elbow-wrist movements at different cycling frequencies (from 0.75 Hz to 2.50 Hz). Participants performed eight coordination tasks that consisted of a combination of in-phase (IN) and/or anti-phase (AN) coordination modes between both elbows and wrists (interlimb), with isodirectional (Iso) or non-isodirectional (NonI) coordination modes within each limb (intralimb). As expected, the principle of muscle homology (in-phase coordination), giving rise to mirror symmetrical movements with respect to the mid-sagittal plane, had a powerful influence on the quality of global coordinative behavior both between and within limbs. When this principle was violated (i.e., when the anti-phase mode was introduced in one or both joint pairs), the non-isodirectional intralimb mode exhibited a (de)stabilizing role in coordination, which became more pronounced at higher cycling frequencies. However, pattern loss with increasing cycling frequency resulted not only in convergence toward the more stable in-phase patterns with the elbows and wrists but also to the anti-phase patterns (which were associated with directional compatibility of within-limb motions). Moreover, participants generally preserved their initial mode of coordination (either in-phase or anti-phase) in the proximal joints (i.e., elbows) while shifting from anti-phase to in-phase (or vice versa) with their distal joint pair (i.e., wrists). Taken together, these findings reflect the impact of two immanent types of symmetry in bimanual coordination: mirror-image and translational symmetry.

Asymmetric control mechanisms of bimanual coordination: an application of directed connectivity analysis to kinematic and functional MRI data

Mirror-symmetrical bimanual movement is more stable than parallel bimanual movement. This is well established at the kinematic level. We used functional MRI (fMRI) to evaluate the neural substrates of the stability of mirror-symmetrical bimanual movement. Right-handed participants (n=17) rotated disks with their index fingers bimanually, both in mirror-symmetrical and asymmetrical parallel modes. We applied the Akaike causality model to both kinematic and fMRI time-series data. We hypothesized that kinematic stability is represented by the extent of neural "cross-talk": as the fraction of signals that are common to controlling both hands increases, the stability also increases. The standard deviation of the phase difference for the mirror mode was significantly smaller than that for the parallel mode, confirming that the former was more stable. We used the noise-contribution ratio (NCR), which was computed using a multivariate autoregressive model with latent variables, as a direct measure of the cross-talk between both the two hands and the bilateral primary motor cortices (M1s). The mode-by-direction interaction of the NCR was significant in both the kinematic and fMRI data. Furthermore, in both sets of data, the NCR from the right hand (left M1) to the left (right M1) was more prominent than vice versa during the mirror-symmetrical mode, whereas no difference was observed during parallel movement or rest. The asymmetric interhemispheric interaction from the left M1 to the right M1 during symmetric bimanual movement might represent cortical-level cross-talk, which contributes to the stability of symmetric bimanual movements.

Re-examining structural constraints on the initiation of bimanual movements: the role of starting locations, movement amplitudes, and target locations

Structural constraints affect the coordination of bimanual movements in ways that have been taken to suggest that the specification of different movement amplitudes is subject to strong intermanual interference effects. Most experiments taken to support this notion, however, confounded variations of movement amplitudes with symmetry in starting locations and variations in target location. The present experiment was designed to further investigate the relative influence of the parameters starting location, movement amplitude, and target location on bimanual movement coordination. Participants performed simultaneous reaching movements with the left and right hand from same and different starting locations to same and different target locations. On each trial, two movements could match on none, one, or all of the parameters. We assessed the influence of each parameter by comparing conditions in which only a single parameter matched between the two hands with conditions in which all parameters differed. The reaction-time data revealed some challenging results for previous studies: (1) same starting locations significantly delayed movement initiation; (2) specifying movement amplitudes had virtually no effect on movement initiation, whereas (3) selecting same target locations significantly benefited the bimanual responses. These findings cannot be taken to support the notion that amplitude specification affects the initiation of bimanual movements. Rather, they support the notion that the initial starting locations of the two hands and the selection of target locations decide about the ease with which we perform bimanual reaching movements.

Simultaneous specification of amplitudes and directions of bimanual reversal movements

Intermanual interactions are modulated by task requirements in the course of motor preparation. In particular, amplitude coupling is strong when identical amplitudes are specified concurrently for the 2 hands but relaxed when different amplitudes are specified. Similarly, directional coupling is symmetric when symmetric directions are specified concurrently but turns to parallel when parallel directions are specified. Here, the author investigated whether the modulations of amplitude coupling and directional coupling in the course of motor preparation are independent or interact. Application of the timed-response procedure, which enables one to manipulate the time available for motor preparation, in 9 participants revealed a weakly interactive pattern. Directional coupling tended to be reduced when different rather than same amplitudes were specified concurrently, and amplitude coupling tended to be reduced when parallel rather than symmetric directions were specified concurrently. In general, interactive effects were also apparent in the rates at which directions and amplitudes were specified. Those observations are consistent with the notion that intermanual amplitude and direction interference are mediated by different but partially overlapping neural structures.

Multiple frames of reference for bimanual co-ordination

Different movement characteristics can be governed by different frames of reference. The present study serves to identify the frames of reference, which govern intermanual interactions with respect to movement directions. Previous studies had shown that intermanual interactions are adjusted to task requirements during motor preparation: for parallel movements directional coupling becomes parallel, and for symmetric movements it becomes symmetric. The timed-response procedure allows to trace these adjustments as they are reflected in the intermanual correlations between left-hand and right-hand directions. In the present study the adjustments remained unchanged when all target directions were rotated laterally, indicating a critical role of hand-centered frames of reference. The additional role of a body-centered frame of reference was indicated by the finding of overall higher intermanual correlations with the rotated target configurations. Intermanual interference at long preparation intervals was absent even when eccentricities in the body-centered frame of reference were different. These findings converge with results on the frames of reference that govern intermanual interactions with respect to movement amplitudes. They suggest a role of both body-centered and hand-centered frames of reference for the adjustments of intermanual interactions to task requirements, but of a hand-centered frame of reference only for the intermanual interference, which remains in spite of the adjustments.

Intermovement interval and spatial assimilation effects in sequential bimanual and unimanual movements

This study extended earlier work by showing spatial assimilations in sequential bimanual and unimanual movements separated by 1.5-3.5 s. In Experiments 1 and 2, 30 right-handed participants (18-22 years of age) made rapid single and bimanual lever reversals of 20 degrees and 60 degrees assigned to 1.5, 2.5, or 3.5 s intermovement interval groups. Participants self-timed the intermovement interval in the first experiment, but it was provided in the second experiment using separate auditory stimuli. In the third experiment, participants performed both the 20 degree and 60 degree movement with the same hand. In all experiments, the shorter-distance limb overshot and the longer-distance limb undershot the targets in both bimanual and unimanual sequential movements relative to single movements in all three intermovement interval groups, particularly in the non-dominant left limb. The results suggest that assimilation effects in sequential movements are caused by command interactions at the planning level, but the effects are reduced by practice.

Intermanual interactions related to movement amplitudes and endpoint locations

In almost all studies of bimanual movements with same and different amplitudes, the difference between amplitudes has been confounded with a difference between endpoint locations. The present authors varied those parameters orthogonally. In addition, they presented target locations on the surface on which the movements were produced (direct cues) and on a monitor (indirect cues). Participants' (N = 12) reaction times were longer when both amplitudes and endpoint locations differed than when they were the same. Intermanual amplitude correlations were reduced whenever 1 of the movement parameters differed for the 2 hands; only when cues were presented on the monitor was the amplitude correlation further reduced when both movement parameters were different. The results indicate that structural constraints on bimanual movements take effect on both amplitudes and endpoint locations. The relative importance of those 2 parameters is largely independent of the type of cue.

Speed-accuracy tradeoffs in rapid bimanual aiming movements

Two experiments reported the effect of movement time and knowledge of results on overall spatial errors in rapid simultaneous bimanual aiming movements. In Exps. 1 (n=32) and 2 (n=32), participants used light, aluminum levers oriented vertically in the sagittal plane to make reversal movements over the same distance (20 degrees - 20 degrees or 60 degrees - 60 degrees) or different distances (20 degrees - 60 degrees) in each arm in 250, 350, or 450 msec. to the reversal point. The participants in Exp. 1 were given knowledge of results on the spatial and temporal accuracy for both arms, while in Exp. 2 knowledge of results was provided for one arm only. Strong speed-accuracy tradeoffs were shown for all groups in both experiments, but errors were larger in the different distance movements compared to the same distance groups. Spatial errors were also elevated in Exp. 2 when knowledge of results was not available compared to those conditions where knowledge of results was available. Overall, bimanual speed-accuracy tradeoffs are similar to single arm movements when one moves the same distance in each arm and when knowledge of results is available.

The modulation of intermanual interactions during the specification of the directions of bimanual movements

In two experiments bimanual movements with various combinations of target directions were studied by means of the timed-response procedure. The findings revealed an adaptive modulation of intermanual interactions during direction specifications depending on particular target directions. For symmetric movements intermanual correlations of movement directions are positive, indicating a symmetric coupling. For parallel movements the positive intermanual correlations, observed at short preparation intervals, turn into negative correlations as the time available for motor preparation increases. Biases of mean directions, that can be observed for movements to targets with different eccentricities, reflect one or the other kind of coupling, symmetrical for symmetric target directions and parallel for parallel target directions. These biases are static, that is, they are present at long preparation times, and they are phasically enhanced at shorter preparation intervals. The task-adaptive modulation of intermanual interactions is superposed on a basic symmetry bias.

Spatial interference during bimanual coordination: differential brain networks associated with control of movement amplitude and direction

Bimanual interference emerges when spatial features, such as movement direction or amplitude, differ between limbs, as indicated by a mutual bias of limb trajectories. Although first insights into the neural basis of directional interference have been revealed recently, little is known about the neural network associated with amplitude interference. We investigated whether amplitude versus directional interference activates differential networks. Functional magnetic resonance imaging (fMRI) was applied while subjects performed cyclical, bimanual joystick movements with either the same vs. different amplitudes, directions, or both. The kinematic analysis confirmed that subjects experienced amplitude interference when they moved with different as compared to the same amplitude, and directional interference when they moved along different as compared to the same direction. On the brain level, amplitude and directional interference both resulted in activation of a bilateral superior parietal-premotor network, which is known to contribute to sensorimotor transformations during goal-directed movements. Interestingly, amplitude but not directional interference exclusively activated a bilateral network containing the dorsolateral prefrontal cortex, anterior cingulate, and supramarginal gyrus, which was shown previously to contribute to executive functions. Even though the encoding of amplitude and directional information converged and activated the same neural substrate, our data thus show that additional and partly independent mechanisms are involved in bimanual amplitude as compared to that in directional control.

Neural correlates of the spontaneous phase transition during bimanual coordination

Repetitive bimanual finger-tapping movements tend toward mirror symmetry: There is a spontaneous transition from less stable asymmetrical movement patterns to more stable symmetrical ones under frequency stress but not vice versa. During this phase transition, the interaction between the signals controlling each hand (cross talk) is expected to be prominent. To depict the regions of the brain in which cortical cross talk occurs during bimanual coordination, we conducted event-related functional magnetic resonance imaging using a bimanual repetitive-tapping task. Transition-related activity was found in the following areas: the bilateral ventral premotor cortex, inferior frontal gyrus, middle frontal gyrus, inferior parietal lobule, insula, and thalamus; the right rostral portion of the dorsal premotor cortex and midbrain; the left cerebellum; and the presupplementary motor area, rostral cingulate zone, and corpus callosum. These regions were discrete from those activated by bimanual movement execution. The phase-transition-related activation was right lateralized in the prefrontal, premotor, and parietal regions. These findings suggest that the cortical neural cross talk occurs in the distributed networks upstream of the primary motor cortex through asymmetric interhemispheric interaction.

Intermanual interactions in discrete and periodic bimanual movements with same and different amplitudes

In two experiments we compared intermanual interactions in discrete and periodic movements with same and different amplitudes. In the first experiment there was only a weak amplitude assimilation in first cycles of movements with 1, 3, and 10 cycles, but a strong assimilation in later cycles. Whereas movement times of concurrent short-amplitude and long-amplitude movements were different in first cycles, in the later cycles they were essentially identical. In the second experiment the timed-response procedure was used to study the specification of same and different amplitudes of discrete reversal movements and periodic movements with three cycles. Differences in the time courses of amplitude specifications were only small. In periodic movements a dependence of amplitudes on the preparation interval was seen not only in the first cycles, but also in the later ones. However, in the later cycles the characteristic dependence of assimilation effects and intermanual correlations on the preparation interval was absent. Taken together, these findings strongly suggest that intermanual interactions arise transiently in the specification of both discrete and periodic movements, and that additional kinds of interactions become effective during execution of periodic movements.

The influence of movement cues on intermanual interactions

In two experiments, we studied intermanual interactions in bimanual reversal movements and bimanual aiming movements. Targets were presented on a monitor or directly on the table on which the movements were produced. Amplitudes for each hand were cued symbolically or spatially either in advance of an imperative signal or simultaneous with it. In contrast to findings of Diedrichsen et al. (Psychological Science, 12, 493-498, 2001), reaction times for different-amplitude movements were longer than for same-amplitude movements both for symbolic and spatial cues presented on the monitor and directly on the table. However, with symbolic cues the effect of the relation between target amplitudes was considerably stronger than with spatial cues, no matter where the cues were presented. Intermanual correlations of amplitudes, movement times, and reaction times were smaller with different than with same target amplitudes, and this modulation was more pronounced when targets and cues were presented on the monitor than when they were presented on the table. The findings are taken to suggest that the basic reaction-time disadvantage of different-amplitude movements results from interference between concurrent processes of amplitude specification. Additional factors like interference between concurrent processes of mapping cues on movement characteristics may add strongly to it.

Hand skill and hand preference in blind and sighted children

Congenitally blind and sighted blindfold children between the ages of 6 and 15 years were compared with each other for hand preferences and hand ability. All the children performed a 20-item hand preference test and every child performed three hand ability tasks: a sorting task, a finger dexterity task, and the Minnesota rate of manipulation task, each separately with the left and the right hand. Results indicated no differences between the hand preferences of the two groups. The sighted children were faster than the blind children on some of the hand ability tasks. There were no differences between the left and right hands for any of the tasks for either group. Results indicate an equipotentiality between the hands and suggest the possibility of training both hands during development on tasks that require tactile ability.

Independent on-line control of the two hands during bimanual reaching

Many studies on bimanual coordination have shown that people exhibit a preference for mirror-symmetric movements. We demonstrate that this constraint is absent when bimanual reaching movements are made to visual targets. We investigated the ability of humans to make on-line adjustments during such movements when one or both targets were displaced during the initial phase of the movements. Adjustments were as efficient during bimanual as unimanual movements, even when two adjustments had to be made simultaneously. When one target was displaced in the bimanual condition, the hand reaching to that target adjusted efficiently to the displacement. However, a small transient perturbation in the trajectory of the other hand was also observed. This perturbation was in the same direction as the displacement, rather than in mirror-symmetric direction. A control experiment demonstrated that these perturbations could be elicited by visual information alone, but that they were also influenced by whether an adjustment was required in the trajectory of the other hand. Our results demonstrate near independent control of the two arms during visually guided reaching. The subtle interference observed between the arms reflects interactions between target-related representations in visual coordinates rather than between movement-related representations in joint- or muscle-coordinates.

Bimanual Directional Interference: The Effect of Normal versus Augmented Visual Information Feedback on Learning and Transfer

When performing movements with different spatial trajectories in both upper limbs simultaneously, patterns of interference emerge that can be overcome with practice. Even though studies on the role of augmented feedback in motor learning have been abundant, it still remains to be discovered how overcoming such specific patterns of spatial interference can be optimized by instructional intervention. In the present study, one group acquired a bimanual movement with normal vision, whereas a second group received augmented feedback of the obtained trajectories on a computer screen in real time. Findings revealed that, relative to normal vision, the augmented feedback hampered skill learning and transfer to different environmental conditions. These observations are discussed in view of the benefits and pitfalls of augmented feedback in relation to task context and instructional condition.

Directional interference during bimanual coordination: is interlimb coupling mediated by afferent or efferent processes

The role of afferent information in bimanual directional interference was studied by means of a modulation of the response-produced information in one of both limbs. In Experiment 1, visual information was either present, withdrawn, or shown with a directional transformation on a LCD screen. In Experiment 2, the technique of muscle tendon vibration was used to bias the kinesthetic afferent information associated with movement. The findings revealed strong evidence for directional interference between both limbs. Nevertheless, no evidence could be advanced that the observed interference from the right onto the left limb movement was modulated by manipulation of the afferent sources of information. It is concluded that directional interference primarily emerges at the efferent level of movement planning and organization.