On the coordination of two-handed movements

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.

Can children with developmental coordination disorder adapt to task constraints when catching two-handed?

PURPOSE

To compare the nature and extent of inter and intralimb coupling during two-handed catching and the effect of manipulating task constraints in children with Developmental Coordination Disorder (DCD) and their typically developing peers (AMC).

METHOD

Twenty children aged 7 - 10 years, ten with DCD and 10 AMC attempted to catch a ball ten times in condition 1 (C1), ball to the midline; condition 2 (C2), ball to the left shoulder and condition 3 (C3), ball to the right shoulder. Both 3D kinematic data and video data were collected.

RESULTS

Children with DCD caught fewer balls than the AMC children, regardless of age or condition (p <or= 0.001). Children with DCD demonstrated a higher degree of linkage between limbs in C1 and a lower degree of between limb coupling in C2 and C3 when compared to the AMC (p <or= 0.05). Differences between the AMC7 - 8 and AMC9 - 10 group were found with respect to interlimb coupling.

CONCLUSIONS

The influence of manipulating task constraints and the individual nature of children with DCD must be considered by those involved in rehabilitation. By doing so, children with DCD may search for appropriate motor solutions to many functional movement tasks required for everyday life.

The influence of perturbing the working surface during reaching and grasping in children with hemiplegic cerebral palsy

PURPOSE

To examine unimanual and bimanual reaching and grasping in children with hemiplegic cerebral palsy with particular emphasis on the nature and extent of interlimb coupling when the working surface is perturbed.

METHOD

Nine children with hemiplegic cerebral palsy and 7 control children with no movement difficulties took part in the study. Children were asked to pick up a cube unimamually and bimanually when the surface it was placed on was either sloping away from the child (Experiment 1) or towards the child (Experiment 2). Both 3D kinematic data and video data were gathered and qualitative descriptions of video data were made.

RESULTS

The working surface did indeed influence the nature and extent of interlimb coupling and this varied from participant to participant. Analysis of the displacement data revealed that during the bimanual condition lower trajectories were produced by both the hemiplegic and non hemiplegic sides, especially in Experiment 2. The control group showed little difference between the unimanual and bimanual condition.

CONCLUSIONS

Evidence of interlimb coupling is found, these studies support the findings of our previous work that indicates that there are some benefits to performing bimanual movements in children with hemiplegic cerebral palsy.

The influence of object size on discrete bimanual co-ordination in children with hemiplegic cerebral palsy

PURPOSE

To examine the influence of perturbation of object size on the nature and extent of interlimb coupling in children with hemiplegic cerebral palsy.

METHOD

Participants completed a number of trials reaching and grasping unimanually and bimanually to a small cube (1.5 cm) and a large cube (6 cm). Both 3D kinematic data and video data were gathered and qualitative descriptions of the video data were made.

RESULTS

It was found that object size did influence the nature and extent of bimanual coupling. As in previous studies this varied from participant to participant and either or a combination of temporal, spatial, or postural coupling was observed. In some cases the hemiplegic hand was influenced by the non hemiplegic hand, while in others it was the reverse.

CONCLUSIONS

The influence of context and the individual nature of children with cerebral palsy observed in this paper must be considered by those in rehabilitation. Additionally, the therapist might be able to use the natural tendency to couple to assist the functional control of the hemiplegic side.

Temporal Coordination during Bimanual Reach-to-Grasp Movements: The Role of Vision

The performance of bimanual movements involving separate objects presents an obvious challenge to the visuo-motor system: Visual feedback can only be obtained from one target at a time. To overcome this challenge overt shifts in visual attention may occur so that visual feedback from both movements may be used directly (Bingham, Hughes, & Mon-Williams, 2008; Riek, Tresilian, Mon-Williams, Coppard, & Carson, 2003). Alternatively, visual feedback from both movements may be obtained in the absence of eye movements, presumably by covert shifts in attention (Diedrichsen, Nambisan, Kennerley, & Ivry, 2004). Given that the quality of information falls with increasing distance from the fixated point, can we obtain the level of information required to accurately guide each hand for precision grasping of separate objects without moving our eyes to fixate each target separately? The purpose of the current study was to examine how the temporal coordination between the upper limbs is affected by the quality of visual information available during the performance of a bimanual task. A total of 11 participants performed congruent and incongruent movements towards near and/or far objects. Movements were performed in natural, fixate-centre, fixate-left, and fixate-right vision conditions. Analyses revealed that the transport phase of incongruent movements was similar across vision conditions for the temporal aspects of both the transport and grasp, whereas the spatial aspects of grasp formation were influenced by the quality of visual feedback. We suggest that bimanual coordination of the temporal aspects of reach-to-grasp movements are not influenced solely by overt shifts in visual attention but instead are influenced by a combination of factors in a task-constrained way.

Interactions between new and pre-existing dynamics in bimanual movement control

Motor skills are commonly acquired through practice. This process not only involves acquisition of the particular task demands but also requires overcoming pre-existing modes. In the present study, interactions between new and intrinsic dynamics were evaluated. Accordingly, bimanual finger tapping with a 2:1 ratio was performed according to two training schedules: continuous (consecutive trials) and interrupted (non-consecutive trials with intermediate 1:1 in-phase performances). In addition, in-phase and anti-phase were probed before and after training. Behavioral output was assessed by means of temporal accuracy and variability, whereas neural activation patterns were determined by EEG coherence. Results showed that continuous practice resulted in improved performance with reduced coherence across the motor network. For interrupted practice, behavioral execution ameliorated, although it was inferior to performance with continuous practice. In terms of neural changes, the degree of intrahemispheric and midline connectivity did not reduce with interrupted practice, whereas interhemispheric connectivity increased. This signifies that short-term motor consolidation of the 2:1 task was disrupted due to intermediate performance of the in-phase mode. Furthermore, the probed in-phase and anti-phase pattern showed no behavioral changes, although neural alterations occurred that depended on training schedule and coordination mode. Overall, the observations illustrate bidirectional interactions between new and inherent dynamics during motor acquisition, raising issues about effective methods for learning skills and scheduling of practices in neurorehabilitation.

Multi-compartment model can explain partial transfer of learning within the same limb between unimanual and bimanual reaching

Multi-limb motor skills, such as swimming and rowing, often involve isolated practice of each limb (unimanual) followed by practice with both limbs together (bimanual). We recently demonstrated that learning a novel load during unimanual reaching is partially, but not completely transferred to the same limb during bimanual reaching (and vice versa), learning can remain hidden and only revealed by the original context, and the ability to learn two conflicting force fields if each was separately associated with unimanual and bimanual reaching (Nozaki et al. 2006). The purpose of the present article is to develop a formal state-space model to conceptualize and interpret these complex experimental results. The model contains three separate compartments for learning, a unimanual-specific, a bimanual-specific, and an overlapping compartment, and the internal state of each compartment is updated context-dependently according to motor errors. The model was able to capture all major aspects of motor learning across these two behaviours and predict further complexities during washout trials when bimanual and unimanual trials are interleaved. We propose that partial, but not complete transfer of motor learning is due to a corresponding partial overlap in neural control processes across these behaviours, and is a general feature of different classes of voluntary motor behaviour, such as postural control, point-to-point reaching, manual tracking and oscillatory movements.

The organization of intralimb and interlimb synergies in response to different joint dynamics

We sought to understand differences in joint coordination between the dominant and nondominant arms when performing repetitive tasks. The uncontrolled manifold approach was used to decompose the variability of joint motions into components that reflect the use of motor redundancy or movement error. First, we hypothesized that coordination of the dominant arm would demonstrate greater use of motor redundancy to compensate for interaction forces than would coordination of the nondominant arm. Secondly, we hypothesized that when interjoint dynamics were more complex, control of the interlimb relationship would remain stable despite differences in control of individual hand paths. Healthy adults performed bimanual tracing of two orientations of ellipses that resulted in different magnitudes of elbow interaction forces. For the dominant arm, joint variance leading to hand path error was the same for both ellipsis orientations, whereas joint variance reflecting the use of motor redundancy increased when interaction moment was highest. For the nondominant arm, more joint error variance was found when interaction moment was highest, whereas motor redundancy did not differ across orientations. There was no apparent difference in interjoint dynamics between the two arms. Thus, greater skill exhibited by the dominant arm may be related to its ability to utilize motor redundancy to compensate for the effect of interaction forces. However, despite the greater error associated with control of the nondominant hand, control of the interlimb relationship remained stable when the interaction moment increased. This suggests separate levels of control for inter- versus intra-limb coordination in this bimanual task.

The relationship between visual orienting and interlimb synchrony in a patient with a superior parietal infarction: a case study

Much work indicates that parietal cortex mediates the transformation of visual information into the motor commands necessary for successful performance of many unimanual tasks. Accumulating evidence suggests that parietal cortex also mediates the coordination of bimanual movements, during which the natural tendency is to couple the limbs temporally. However, the extent to which parietal oculomotor and/or visual processes contribute to temporal coupling of the limbs during bimanual task performance is unknown. In the current study, we monitored the eye movements of a patient with a left parietal infarction as she performed a series of bimanual visuomotor tasks. We demonstrate the impact of an ipsilesional (leftward) orientation bias on her ability to synchronize the onset of bimanual limb movements; the movements were performed in serial fashion, i.e., left limb before right, when the patient was permitted to freely shift saccades and the visual target cuing the left (ipsilesional) limb movement was presented at greater (leftward) eccentricities. Disruption of interlimb synchrony as such was not, however, evident when the patient was required to fixate or when visual targets were presented at lesser ipsilesional eccentricities. Additionally, despite the disruptive influence of oculomotor and visual factors on interlimb synchrony, the patient appeared capable of using visual feedback to straighten the right (contralesional) limb trajectory, thus improving the spatial component of task performance. Results suggest that parietal cortex plays an important role in the coordination of limb movements during performance of bimanual visuomotor tasks. This role appears to involve orienting gaze or attention to the goals of each limb so that the nervous system can synchronize the activity of both limbs and thereby ensure successful task completion.

Bimanual response grouping in dual-task paradigms

In three experiments we measured response time (RT) and peak force (PF) to investigate the grouping of left- and right-hand key press responses in a dual-task paradigm involving two independent go/no-go tasks. Within each task, a go stimulus within one of two modalities (i.e., visual versus auditory) required a response by one hand. In Experiment 1 with simultaneous go stimuli in the two tasks, responses appeared to be grouped in approximately 75-80% of trials, compared with nearly 100% grouping in a single-task condition requiring bimanual responses to the onset of any stimulus in either modality. In Experiment 2 with stimulus onset asynchronies (SOAs) of 0-400 ms between the two go stimuli, response grouping clearly declined as SOA increased, although some grouping was still evident even at the longest SOA. The same pattern was observed in Experiment 3 with the same range of SOAs but unpredictable stimulus order, suggesting that grouping is not strongly dependent on prior knowledge of the likely response order. These results emphasize the pervasiveness of response grouping in bimanual dual-task RT paradigms and provide useful clues as to its nature.

Combined effects of planning and execution constraints on bimanual task performance

In this study we investigated the relative impact of planning and execution constraints on discrete bimanual task performance. In particular, in a bimanual CD-placement task, we compared people's preference to end movements comfortably with their preference to move symmetrically. In "Experiment 1" we examined the degree of interlimb coupling as participants repositioned two CDs in a CD rack by simultaneously moving their arms mirror-symmetrically or asymmetrically into comfortable or uncomfortable end postures. Interlimb coupling was stronger when the arms moved symmetrically towards uncomfortable end postures. In "Experiment 2" participants were asked to realize specific end orientations of the CDs but they were free to choose an initial grip type and subsequent direction of forearm rotation. Surprisingly, the participants did not move their arms symmetrically but preferred to end in a comfortable posture with their right hand but not with their left hand. We conclude that in discrete bimanual task performance the tendency to end movements in a comfortable posture dominates over the tendency to synchronously activate homologous muscle pairs. The lateralized end-state comfort effect suggests a hemispheric specialization for motor planning.

Goal-related planning constraints in bimanual grasping and placing of objects

Our primary objective was to examine the possible interplay of the end-state comfort effect and bimanual temporal and spatial coupling constraints in a grasp-to-place task. Unimanual and bimanual grasping and placing tasks were employed with manipulations on initial comfort (by use of potentially interfering obstacles) and target goals (using various demands on end goal object orientations). Confirming previous temporal findings, incongruent bimanual tasks were considerably slower in initiation time and movement time than congruent ones, reflecting costs in conceptualizing, planning, and completion of the task. With respect to spatial constraints, when the same goal was present for both hands there was strong evidence of the influence of both end-state comfort and bimanual constraints. This was often not the case when the task demands differed for the two hands, although the primary task goals were still attained. We suggest that the implementation of constraints is not based on a strict hierarchy; rather, certain constraints become dominant depending on the task and situation.

Coordination and control of bimanual prehension: effects of perturbing object location

The purpose of the present study was to investigate the coordination of the two effectors when one or both targets were displaced in a bimanual prehension task. Sixteen right-handed volunteers were asked to reach 20 cm to grasp and lift two cubic objects with the right and left hands. Upon initiation of the reach: (1) both objects could remain at the initial position (NN); (2) the right object could be displaced toward the subject (NJ); (3) the left object could be displaced (JN); or (4) both objects could be displaced (JJ). Generally, the results indicated that the hand moving to the perturbed object was reorganized to reach the target efficiently, but hovered to somewhat couple object lift for the two hands. In contrast, adjustments were seen in the velocity profiles of the hand moving to the non-perturbed target, including a premature deceleration phase and corrective movements to reach the target location. Together, these results indicate that when the perturbation of one object occurs during the performance of a bimanual prehension task, visual information is used to independently update the control process for the limb moving to the perturbed object. Additionally, interference causes the limb moving to the non-perturbed target to be inappropriately adjusted in response to the perturbation. Our results also indicated that perceptual and motor factors such as time allotted for the use of feedback and the direction of movement may play a role in the independence/dependence relationship between the hands during bimanual tasks. Furthermore, subjects' expectations about the performance and goal of the task could have a further influence on the level of interference seen during bimanual movements. Finally, despite interference effects which caused multiple accelerations and decelerations, the hand moving to the non-perturbed target still achieved the target location in the same movement time as during control conditions. This final result indicates the efficiency with which subjects can reorganize both limbs in the face of altered task requirements.

Coordination constraints during bimanual versus unimanual performance conditions

Coordinated behaviour is prominent during daily life activities in various combinations and degrees of complexity. Here the influence of coordination constraints upon motor behaviour is evaluated by contrasting two-finger tapping (in-phase and anti-phase) during bimanual and unimanual conditions. Cortical dynamics was assessed by means of EEG coherence in the beta frequency band (13-30 Hz) and included intrahemispheric, interhemispheric and midline connectivity patterns. Results showed that intrahemispheric connectivity varied strongly in the different coordination tasks, with left hemisphere dominance for bimanual and right hand coordination versus right hemisphere dominance for left hand coordination. Interhemispheric connectivity was fairly similar across coordination tasks, except for the bimanual in-phase configuration that comprised the lowest coherence scores. Midline connectivity was equivalent across coordination tasks, with exception of the bimanual anti-phase assignment that was characterized with increased coherence scores. Across connectivity regions, the lowest coherence scores were obtained for bimanual and right hand coordination performed in the in-phase mode, underlining their basic mode of functioning. Furthermore by evaluating the coordination effort, estimated by the discrepancy between the coordination task and the sum of the individual components, an increased processing for intrahemispheric and midline connections was observed, but not for interhemispheric connections, which supports the general significance of interhemispheric communication for voluntary movement. Overall the current findings indicate a dynamic modulation of functional connectivity patterns according to the coordinative context. It suggests that brain regions within a motor network flexibly couple and decouple to implement the processing requirements associated with coordinated behaviour.

Hemispheric coordination is necessary for song production in adult birds: implications for a dual role for forebrain nuclei in vocal motor control

Precise coordination across hemispheres is a critical feature of many complex motor circuits. In the avian song system the robust nucleus of the arcopallium (RA) plays a key role in such coordination. It is simultaneously the major output structure for the descending vocal motor pathway, and it also sends inputs to structures in the brain stem and thalamus that project bilaterally back to the forebrain. Because all birds lack a corpus callosum and the anterior commissure does not interconnect any of the song control nuclei directly, these bottom-up connections form the only pathway that can coordinate activity across hemispheres. In this study, we show that unilateral lesions of RA in adult male zebra finches (Taeniopigia guttata) completely and permanently disrupt the bird's stereotyped song. In contrast, lesions of RA in juvenile birds do not prevent the acquisition of normal song as adults. These results highlight the importance of hemispheric interdependence once the circuit is established but show that one hemisphere is sufficient for complex vocal behavior if this interdependence is prevented during a critical period of development. The ability of birds to sing with a single RA provides the opportunity to test the effect of targeted microlesions in RA without confound of functional compensation from the contralateral RA. We show that microlesions cause significant changes in song temporal structure and implicate RA as playing a major part in the generation of song temporal patterns. These findings implicate a dual role for RA, first as part of the program generator for song and second as part of the circuit that mediates interhemispheric coordination.

Bilateral tremor relations in Parkinson's disease: effects of mechanical coupling and medication

The aim of the present study was to examine the dynamics of bilateral multiple segment resting and postural tremor in 12 young, older subjects and eight Parkinson's (PD) patients in their different medication states. A second aim was to investigate whether bilateral independence of upper limb tremor was preserved for PD patients with amplified tremor under conditions where no upper limb segment was supported. Under these conditions, the likelihood for mechanical transmission between segments was increased. Tremor was recorded, bilaterally, from the hand and finger segments of all subjects. In addition to the PD subjects exhibiting greater tremor under both on/off medication states than the young/old control subjects, the tremor increase within limb from the hand to the finger was 2-3 times greater for the PD group in comparison to the healthy subjects. Despite this increased tremor, no differences were observed in the level of coupling between limbs across groups. Furthermore, the degree of coupling between limbs for the PD group was unaffected by their medication state. Overall, these results demonstrate that bilateral independence of tremor in PD participants is preserved despite conditions which maximised the chance of increased coupling.

The coordination patterns observed when two hands reach-to-grasp separate objects

What determines coordination patterns when both hands reach to grasp separate objects at the same time? It is known that synchronous timing is preferred as the most stable mode of bimanual coordination. Nonetheless, normal unimanual prehension behaviour predicts asynchrony when the two hands reach towards unequal targets, with synchrony restricted to targets equal in size and distance. Additionally, sufficiently separated targets require sequential looking. Does synchrony occur in all cases because it is preferred in bimanual coordination or does asynchrony occur because of unimanual task constraints and the need for sequential looking? We investigated coordinative timing when participants (n = 8) moved their right (preferred) hand to the same object at a fixed distance but the left hand to objects of different width (3, 5, and 7 cm) and grip surface size (1, 2, and 3 cm) placed at different distances (20, 30, and 40 cm) over 270 randomised trials. The hand movements consisted of two components: (1) an initial component (IC) during which the hand reached towards the target while forming an appropriate grip aperture, stopping at (but not touching) the object; (2) a completion component (CC) during which the finger and thumb closed on the target. The two limbs started the IC together but did not interact until the deceleration phase when evidence of synchronisation began to appear. Nonetheless, asynchronous timing was present at the end of the IC and preserved through the CC even with equidistant targets. Thus, there was synchrony but requirements for visual information ultimately yielded asynchronous coordinative timing.

Interhemispheric interactions and redundancy gain: tests of an interhemispheric inhibition hypothesis

In simple reaction time (RT) tasks, responses are faster when stimuli are presented to both the left and right visual hemifields than when a stimulus is presented to a single hemifield. Paradoxically, this redundancy gain with bilateral stimuli is enhanced in split-brain individuals relative to normals. This article reports three experiments testing an account of that enhancement in which normals' responses to bilateral stimuli are slowed by interhemispheric inhibition. In simple RT tasks, normal participants responded bimanually to left, right, or bilateral visual stimuli. In choice RT tasks, they responded to each stimulus with one hand, responding bimanually only when both stimuli were presented. Measurements of response forcefulness (Experiment 1) and electroencephalographic activity (Experiments 2 and 3) showed no evidence of the correlation patterns predicted by the hypothesis of interhemispheric inhibition. The results suggest that such inhibition is unlikely to be the explanation for enhanced redundancy gain in split-brain individuals.

Kinematic analysis of upper limbs and trunk movement during bilateral movement after stroke

OBJECTIVE

To compare the kinematics of the upper limbs and trunk during unilateral and parallel bilateral tasks in subjects with hemiparesis and control subjects.

DESIGN

Comparative study.

SETTING

Geriatric center offering rehabilitation services.

PARTICIPANTS

Convenience sample of 15 persons (age, 69.4 +/- 12.0 y; > or = 3 mo poststroke) recruited in a geriatric center with rehabilitation services, and 13 control persons (67.8 +/- 7.5 y) participated in the study.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Unilateral and bilateral movements toward 1 or 2 targets located beyond arm's length and positioned in 3 directions. Angular changes of both upper limbs and trunk were characterized in the sagittal, frontal, and horizontal planes.

RESULTS

During the bilateral task, the deficits of the kinematic joints of the paretic upper limb persisted in subjects with hemiparesis as compared with the corresponding upper limb in the control subjects (abduction shoulder: subjects with hemiparesis, 5.7 degrees +/- 5.3 degrees; control subjects, 0.7 degrees +/- 4.8 degrees; extension elbow: subjects with hemiparesis, 38.2 degrees +/- 14.2 degrees; control subjects, 52.8 degrees +/- 12.5 degrees) with a marked flexion of the trunk (subjects with hemiparesis, 33.7 degrees +/- 8.7 degrees; control subjects, 26.8 degrees +/- 5.8 degrees). The elbow extension of the nonparetic upper limb was reduced (subjects with hemiparesis, 41.0 degrees +/- 13.6 degrees; control subjects, 52.8 degrees +/- 12.5 degrees).

CONCLUSIONS

The use of parallel bilateral reaching tasks and placing movements of the upper extremities in the subjects with hemiparesis contributed an increase in the trunk flexion rather than improve the motor performance of the paretic upper limb, especially with regard to increasing elbow extension.

Intermanual interactions during initiation and production of rhythmic and discrete movements in individuals lacking a corpus callosum

Three individuals lacking a corpus callosum, two due to callosotomy and one agenesis, and three age-matched healthy controls were tested on a bimanual task in which a discrete or rhythmic arm movement was initiated following a visual signal while the other arm produced continuous, rhythmic movements. The control participants initiated the secondary, rhythmic movement in phase with the ongoing rhythmic base movement and the two limbs were coupled in an inphase mode across the duration of the trial. In contrast, the acallosal individuals failed to show phase entrainment at the initiation of the secondary, rhythmic movements. Moreover, the callosotomy patients exhibited weak coupling between the rhythmically moving limbs while the individual with callosal agenesis consistently synchronized in an antiphase mode. The control participants exhibited increased perturbation of the ongoing base movement when initiating a discrete movement; for the acallosal participants, the base movement was similarly perturbed in both secondary movement conditions. These results are consistent with the hypothesis that intermanual interactions observed during bimanual movements arise from various levels of control, and that these are distinct for discrete and rhythmic movements. Temporal coupling during rhythmic movements arises in large part from transcallosal interactions between the two hemispheres. The imposition of a secondary movement may transiently disrupt an ongoing rhythmic movement even in the absence of the corpus callosum. This may reflect subcortical interactions associated with response initiation, or, due to dual task demands, a transient shift in attentional resources.

Bilateral conjugacy of movement initiation is retained at the eye but not at the mouth following long-term unilateral facial nerve palsy

Voluntary eyelid closure and smiling were studied in 11 normal subjects and 11 patients with long-term unilateral facial nerve palsy (FNP). The conjugacy of eyelid movements shown previously for blinks was maintained for voluntary eye closures in normal subjects, with movement onset being synchronous in both eyes. Bilateral onset synchrony of the sides of the mouth was also observed in smiling movements in normal subjects. In FNP patients, initiation of movement of the paretic and non-paretic eyelids was also synchronous, but markedly delayed relative to normal (by 136 ms = 32%). The initiation of bilateral movements at the mouth was similarly delayed, but in contrast to the eyes, it was not synchronous. Central neural processing in the FNP subjects was normal, however, since unilateral movements at the mouth were not delayed. The delays therefore point to considerable additional information processing needed for initiating bilateral facial movements after FNP. The maintenance of bilateral onset synchrony in eyelid closure and its loss in smiling following FNP is an important difference in the neural control of these facial regions. Bilateral conjugacy of eyelid movements is probably crucial for coordinating visual input and was achieved apparently without conscious effort on the part of the patients. Bilateral conjugacy of movements at the sides of the mouth may be less critical for normal function, although patients would very much like to achieve it in order to improve the appearance of their smile. Since the everyday frequency of eyelid movements is considerably greater than that of smiling, it is possible that the preserved eyelid conjugacy in these patients with long-term FNP is merely a product of greater experience. However, if synchrony of movement onset is found to be preserved in patients with acute FNP, then it would suggest that eyelid conjugacy has a privileged status in the neural organisation of the face.

Target-related coupling in bimanual reaching movements

While bimanual interference effects can be observed when symbolic cues indicate the parameter values of simultaneous reaching movements, these effects disappear under conditions in which the target locations of two movements are cued directly. The present study investigates the generalizability of these target-location cuing benefits to conditions in which symbolic cues are used to indicate target locations (i.e., the end points of bimanual movements). Participants were asked to move to two of four possible target locations, being located either at the same and different distances (Experiment 1), or in the same and different directions (Experiment 2). Circles and crosses served as symbolic target-location cues and were arranged in a symmetric or non-symmetric fashion over the four target locations. Each trial was preceded by a variable precuing interval. Results revealed faster initiation times for equivalent as compared to non-equivalent target locations (same vs. different cues). Moreover, the time course of prepartion suggests that this effect is in fact due to target-equivalence and not to cue-similarity. Bimanual interference relative to movement parameter values was not observed. These findings suggest that cuing target locations can dominate potential intermanual interference effects during the concurrent programming of different movement parameter values.

Exploiting interlimb coupling to improve paretic arm reaching performance in people with chronic stroke

OBJECTIVE

To determine whether paretic arm reaching performance is improved in bilateral compared with unilateral conditions.

DESIGN

Cohort study.

SETTING

University human performance laboratory.

PARTICIPANTS

Thirty-two subjects with chronic stroke (57+/-14y; on Fugl-Meyer Assessment arm score, 37+/-14).

INTERVENTION

Unilateral and bilateral reaching. Bilateral tasks included varying levels of weight on the nonparetic hand.

MAIN OUTCOME MEASURES

An electromagnetic tracking system recorded hand peak acceleration, velocity, and movement time. A 2-way repeated-measures analysis of variance and Tukey-adjusted pairwise comparisons were used to analyze the results (alpha=.05).

RESULTS

Paretic differed significantly from nonparetic peak acceleration and velocity in unilateral reaching but not bilateral reaching. Within limbs, the paretic arm attained a higher peak acceleration (P<.001) and velocity (P=.03) in the bilateral compared with the unilateral task, but movement time was unchanged between tasks. Nonparetic peak acceleration was higher (P=.015), velocity was unchanged, and movement time increased (P=.005) in the bilateral compared with the unilateral task. The addition of a weight to the nonparetic arm during bilateral reaching did not result in further improvement in paretic arm performance.

CONCLUSIONS

Interlimb coupling effects during bilateral reaching are retained even after chronic stroke and can be used to produce an immediate improvement in paretic arm reaching performance.

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.

Temporal coordination of the arms during bilateral simultaneous and sequential movements in patients with chronic hemiparesis

Upper extremity (UE) hemiparesis results in decreased movement speed and impaired coordination leading to functional limitations and disability. The effects of UE hemiparesis on bilateral functional reaching have not been studied even though most activities of daily living are bilateral tasks. We examined the characteristics of bilateral simultaneous (SIM-B) and bilateral sequential paretic-lead (SEQ-P) and nonparetic-lead (SEQ-NP) functional reaching tasks at preferred and fast speeds. Sixteen patients with chronic hemiparesis completed three bilateral reaching tasks as fast as possible. A subset of eight participants attempted to complete the tasks at both preferred and fastest possible speeds. Paretic (P) and nonparetic (NP) arms were not different from each other in movement time (MT) or peak velocity in the SIM-B condition. MT and peak velocity differed between the two arms during both SEQ tasks. P MT was shorter and NP MT longer in the SIM-B task compared to SEQ-P and SEQ-NP. The P arm MT was the shortest when moving with the NP arm in a simultaneous task compared to both P and NP lead sequential movements. Despite hemiparesis, the two arms demonstrate a temporal coupling when moving simultaneously. When attempting to move at fastest speed, P arm MT time is better when reaching before or with the NP arm than when reaching after the NP arm showing coupling to the NP limb and increased speed of movement. These coupling effects support the rationale for bilateral arm training for individuals with UE hemiparesis.

Learning and transfer of bimanual multifrequency patterns: effector-independent and effector-specific levels of movement representation

Current behavioural theories consider that during motor learning, an effector-independent memory representation of the acquired skill is built up. Using a transfer paradigm, we addressed the nature of the memory representation for a 2:1 multifrequency co-ordination task, requiring, for example, the left arm to cycle twice as fast as the right. After learning this 2:1 pattern, transfer to its converse pattern (i.e., the right arm cycles twice as fast as the left) revealed powerful evidence for negative transfer. The converse task arrangement revealed similar effects. These observations suggest a reconsideration of current viewpoints on movement representations, which emphasize effector independence. Based on the present findings, we propose a new model of motor memory, consisting of an abstract, effector-independent and an effector-specific layer. The abstract code is hypothesized to represent general spatiotemporal movement features, whereas the specific representation refers to effector-related movement commands. This concept is consistent with recent neuroscientific evidence in animal and human species, and invites a reconsideration of current behavioural theories of motor learning and memory.

Bimanual adaptation: internal representations of bimanual rhythmic movements

From tying your shoes and clipping your tie to the claps at the end of a fine seminar, bimanual coordination plays a major role in our daily activities. An important phenomenon in bimanual coordination is the predisposition toward mirror symmetry in the performance of bimanual rhythmic movements. Although learning and adaptation in bimanual coordination are phenomena that have been observed, they have not been studied in the context of adaptive control and internal representations-approaches that were successfully employed in the arena of reaching movements and adaptation to force perturbations. In this paper we examine the dynamics of the learning mechanisms involved when subjects are trained to perform a bimanual non-harmonic polyrhythm in a bimanual index finger tapping task. Subjects are trained in this task implicitly, using altered visual feedback, while their performance is continuously monitored throughout the experiment. Our experimental results indicate the existence of significant (p<<0.01) learning curves (i.e., error plots with significantly negative slopes) during training and aftereffects with a washout period after the visual feedback ceases to be altered. These results confirm the formation of internal representations in bimanual motor control. We present a simple, physiologically plausible, neural model that combines feedback and adaptation in the control process and which is able to reproduce key phenomena of bimanual coordination and adaptation.

An object-centred reference frame for control of grasping: effects of grasping a distractor object on visuomotor control

Previous evidence based on perceptual integration and arbitrary responses suggests extensive cross-modal links in attention across the various modalities. Attention typically shifts to a common location across the modalities, despite the vast differences in their initial coding of space. An issue that remains unclear is whether or not these effects of multisensory coding occur during more natural tasks, such as grasping and manipulating three-dimensional objects. Using kinematic measures, we found strong effects of the diameter of a grasped distractor object on the aperture used to grasp a target object at both coincident and non-coincident locations. These results suggest that interference effects can occur between proprioceptive and visuomotor signals in grasping. Unlike other interference effects in cross-modal attention, these effects do not depend on the spatial relation between target and distractor, but occur within an object-based frame of reference.

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.

Video analysis of endoscopic cutting task performed by one versus two operators

BACKGROUND

In the performance of complex laparoscopic tasks, one question is whether the task should be distributed between two operators or accomplished bimanually by one operator. The authors hypothesized that superior task performance results when two operators work collaboratively in a dyad team as opposed to one operator performing the task bimanually. Furthermore, in a visually misaligned condition, the performance of a team will be more robust than that of a single operator working alone.

METHODS

The suture-cutting task was performed by 24 right-handed subjects in a mock surgical setup using a laparoscopic grasper and a pair of laparoscopic scissors. The cutting task was performed by 8 subjects bimanually (using both limbs) and 16 paired subjects unimanually (using their preferred limbs). The image of the work plane was displayed either vertically or superimposed over the work plane. In half of the conditions, the camera was rotated 45 degrees, causing misalignment between the actual and displayed work planes. Movements were videotaped. Important movement events were identified and used to subdivide the task into subtasks. Durations of the subtasks and attempts for grasping and cutting were analyzed using a mixed-design multivariate analysis of variance (MANOVA).

RESULTS

For a number of subtasks, the dyad group showed shorter durations than the bimanual group. The 45 degrees rotation of the camera degraded both bimanual and dyad performance, resulting in prolonged movement times for all subtasks. The learning process was facilitated by the superimposed display in that grasper and scissor reaching times improved over trials, as compared with the vertical display.

CONCLUSION

The results indicate the superior role of team collaboration, as compared with the single operator, in a complex remote manipulation such as a laparoscopic cutting task. This enhanced task performance is achieved because of the larger capacity for information processing. These results may have some relevance for optimizing performance of endoscopic surgery.

Interlimb and within limb force coordination in static bimanual manipulation task

The aim of the study was to compare the coordination of hand grip (G) and load force (a force that tends to cause slippage of a grasped object; L) in static bimanual manipulation tasks with the same data obtained from the similar dynamic tasks. Based on the previous findings obtained from dynamic tasks, it was hypothesized that an increase in the rate of L change would be predominantly associated with a decrease in the coordination of the within limb forces (coordination of G and L of each hand as assessed through the correlation coefficients), while a decrease in coordination of interlimb forces (between two G and two L) will be less pronounced. Regarding the pattern of modulation of G, the same increase in L frequency was also expected to be associated with a decrease in G gain and an increase in G offset (as assessed by slope and intercept of the regression lines obtained from G to L diagrams, respectively), as well as with an increase in average G/L ratio. Subjects exerted oscillatory isometric L profiles by simultaneous pulling out two handles of an externally fixed device under an exceptionally wide range of L frequencies (0.67-3.33 Hz). The results demonstrated relatively high correlation coefficients between both the interlimb and within limb forces that were only moderately affected under sub-maximal L frequencies. Furthermore, the hypothesized changes in G gain and offset appeared only under the highest L frequency, while the G/L ratio remained unaffected. We conclude that, when compared with the dynamic tasks based on the unconstrained movements of hand-held objects that produce similar pattern of L change, the static manipulation tasks demonstrate a consistent and highly coordinated pattern of bilateral G and L under a wide range of frequencies. However, the neural mechanisms that play a role in the revealed differences need further elucidation.

The effects of attention and handedness on coordination dynamics in a bimanual Fitts’ law task

Two experiments were conducted to investigate the effects of attention and handedness on bimanual coordination in the context of a dynamical model of coordinated movements. Participants performed a bimanual, rhythmic Fitts' law task in which the relative amount of attention directed to each task was manipulated by the relative difficulty associated with the pair of targets that each hand tapped. In both experiments, participants tended to lead with their preferred hand. The effects of attention, though, were mixed, which suggested that there was a combined effect of an attentional asymmetry and an asymmetry in the hands' uncoupled frequency, both of which are captured in the dynamical model.

Internal models for bi-manual tasks

Co-ordinated bi-manual actions form the basis for many everyday motor skills. In this review, the internal model approach to the problem of bi-manual co-ordination is presented. Bi-manual coordinative tasks are often regarded as a hallmark of complex action. They are often associated with object manipulation, whether the holding of a single object between the two hands or holding an object in each hand. However, the task of movement and control is deceptively difficult even when we execute an action with a single hand without holding an object. The simplest voluntary action requires the problems of co-ordination, timing and interaction between neural, muscular and skeletal structures to be overcome. When we are making a movement whilst holding an object, a further requirement is that an internal model is able to predict the dynamics of the object that is being held as well as the dynamics of the motor system. There has been extensive work examining the formation of internal models when acting in novel environments. The majority of studies examine uni-lateral learning of a task generally to the participant's dominant hand. However, many everyday motor tasks are bi-manual, and the existing findings regarding the learning of internal models in uni-manual tasks and their subsequent generalization highlights the complexities that must underlie the formation of bi-manual tasks. Our ability to perform bi-manual tasks raises interesting questions about how internal models are specified for co-ordinative actions, and also for how the motor system learns to represent the properties of objects.

Bimanual interference in rapid discrete movements is task specific and occurs at multiple levels of processing

It has been suggested that interference in symbolically cued bimanual reaction time tasks is caused primarily by the perceptual processing of stimuli and not by motor preparation of the required movements. Here subjects made movements of the right and left index fingers that varied in their spatial and motor congruence. Spatial congruence was manipulated by presenting symbolic cues (i.e., pairs of letters) on a computer screen cueing the required movement directions. Motor congruence was manipulated by altering hand orientation. Results showed that interference occurs at both the stage of stimulus processing and the stage of motor preparation. These effects were reflected in the latencies of the different bimanual movements with both motor incongruence and spatial incongruence causing significant increases in reaction time. However, spatially incongruent movements that were made in response to incongruent visual cues demonstrated changes in reaction time that were more than double those of movements that required simultaneous activation of nonhomologous muscles. Therefore in symbolically cued bimanual reaction-time tasks, although both motor and spatial constraints operate, there is a clear dominance of spatial incongruence on performance. While motor congruence effects are likely due to cross-facilitation in corticospinal pathways, spatial incongruence effects are probably due to interference between the mechanisms that identify incongruent stimuli and translate these cues into the appropriate movements.

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

When subjects make rapid bimanual aiming movements over different distances, spatial assimilations are shown; the shorter distance limb overshoots when paired with a longer distance limb. Recent research has also shown spatial assimilations to be greater in the nonpreferred left limb of right-handed subjects, but it is not known whether the increased spatial assimilations represent a handedness effect or one of hemispheric lateralization of motor control. To determine the nature of the asymmetric effect, left- (n = 32) and right- (n = 60) handed subjects part practiced, then whole practiced, short (20 degrees ) and long 60 degrees ) reversal movements. During whole practice, both groups showed spatial assimilations in the shorter distance limb, particularly when the left limb performed the short movement. This asymmetry was greatest for right-handed subjects, but left-handed subjects showed smaller, but systematic effects, providing moderate support for the hypothesis that the asymmetric effect is due to hemispheric lateralization of motor control. All interlimb differences in spatial accuracy for the short and long movements were eliminated with practice, however, suggesting the asymmetric effect was temporary as well. In addition, subjects who part practiced the long movement just prior to whole practice showed greater overshooting in the short distance limb compared with subjects who followed the other practice order throughout whole practice and the no-KR retention trials. Such findings suggest that the part-practice order of bimanual tasks can directionally bias whole-task performance.

Manual asymmetries in bimanual reaching: the influence of spatial compatibility and visuospatial attention

The goal of the present investigation was to explore the possible expression of hemispheric-specific processing during the planning and execution of a bimanual reaching task. Participants (N = 9) completed 80 bimanual reaching movements (requiring simultaneous, bilateral production of arm movements) to peripherally presented targets while selectively attending to either their left or right hand. Further, targets were presented in spatially compatible (ipsilateral to the aiming limb) and incompatible (contralateral to the aiming limb) response contexts. It was found that the left hand exhibited temporal superiority over the right hand in the response planning phase of bimanual reaching, indicating a left hand/right hemisphere advantage in the preparation of a bimanual response. During response execution, and consistent with the view that interhemispheric processing time (Barthelemy & Boulinguez, 2002) or biomechanical constraints (Carey, Hargreaves, & Goodale, 1996) generate temporal delays, longer movement times were observed in response to spatially incompatible target positions. However, no hemisphere-specific benefit was demonstrated for response execution. Based on these findings, we propose lateralized processing is present at the time of response planning (i.e., left hand/right hemisphere processing advantage); however, lateralized specialization appears to be annulled during dynamic execution of a bimanual reaching task.

Coordination of hand grip and load forces in uni- and bidirectional static force production tasks

The purpose of the study was to explore the differences in coordination of grip (G) and load forces (L) in a unidirectional and bidirectional bimanual static force production task. Subjects (N=10) exerted oscillatory isometric L profiles against an externally fixed hand-held device, modulated either in pure tension (unidirectional) or in alternating tension and compression (bidirectional) at a rate of either 1.33 or 2.67 Hz. The unidirectional task revealed a high level of coordination of both the ipsilateral (i.e., G and L of each hand) and contralateral pairs of forces (two Gs and two Ls) as assessed by correlation and stability of force ratios. The bidirectional task demonstrated a low level of inconsistently modulated Gs with respect to the change of L, which resulted in a deteriorated coordination, particularly between the ipsilateral forces. The overall effect of task on the force coordination was higher than the effect of frequency suggesting that the higher frequency of G modulation required in the bidirectional task is not likely to be the main cause of the observed phenomenon. We interpret these differences by a relative simplicity of the control mechanisms of the unidirectional task based on a single synergy of G and L muscles that allows simultaneous coordination of both the ipsilateral and contralateral forces. Due to the switching between two distinctive synergies involving G muscles, the bidirectional task could possess a higher control complexity causing a decoupled coordination of the ipsilateral forces, while retaining the coordination of contralateral forces at a relatively high level.

Intermanual interactions during simultaneous execution and programming of finger movements

In bimanual movements, some differences between the movements performed by the two hands cause interference, while others do not. Similarly, in choice between responses with the left and right hand some differences between the two movements increase RT, while others do not. It is suggested that both kinds of effects are, at least in part, due to the incompatibility between processes that determine characteristics of movements jointly for both hands and that are present during preparation as well as during execution. This hypothesis implies that during execution of one movement, programming of a different movement to be performed with the other hand should be impaired, as compared to a condition in which the successive movements of both hands are the same. this expectation was confirmed for finger movements of different forms where an effect on choice RT had been shown previously. On the other hand, interference between execution and programming is not to be expected when successive movements differ in characteristics that are likely to be specified separately for each hand, as indicated by a lacking effect in choice experiments. This expectation was confirmed for successive movements performed with different fingers of either hand as compared to movements performed with the same fingers.

The timing of natural prehension movements

Prehension movements were studied by film in 7 adult subjects. Transportation of the hand to the target-object location had features very similar to any aiming arm movement, that is, it involved a fast-velocity initial phase and a low-velocity final phase. The peak velocity of the movement was highly correlated with its amplitude, although total movement duration tended to remain invariant when target distance was changed. The low-velocity phase consistently began after about 75% of movement time had elapsed. This ration was maintained for different movement amplitudes. Formation of the finger grip occurred during hand transportation. Fingers were first stretched and then began to close in anticipation to contact with the object. The onset of the closure phase was highly correlated to the beginning of the low velocity phase of transportation. This pattern for both transportation and finger grip formation was maintained in conditions whether visual feedback from the moving limb was present or not. Implications of these findings for the central programming of multisegmental movements are discussed.

Force and timing components of the motor program

Three experiments were undertaken to assess the effects of variations of force and time on both simple and choice reaction time. The first two experiments demonstrated that although latency did not vary as a function of force, timing variations, such as requiring that a response be maintained, led to consistent changes in reaction time. These results led to the development of a model of motor programming in which force and timing are dissociated as separate components. However, the data also indicated that the force component may be further analyzed into two subcomponents-force activation and force deactivation. The model predicts that the latter subcomponent may be programmed on-line provided that sufficient time elapses between the implementation of the two subcomponents. A different pair of movements was used in Experiment 3 to further demonstrate that force activation and deactivation may be preprogrammed into a single component. These results support the aspect of the proposed model that makes a distinction between operations required for program construction from those necessary for program implementation.

Connectivity exploration with structural equation modeling: an fMRI study of bimanual motor coordination

The present fMRI study explores the connectivity among motor areas in a bimanual coordination task using the analysis framework of structural equation modeling (SEM). During bimanual finger tapping at different frequency ratios, temporal correlations of activations between left/right primary motor cortices (MI), left/right PMdc (caudal dorsal premotor area) and supplementary motor cortex (SMA) were detected and used as inputs to the SEM analysis. SEM was extended from its traditional role as a confirmatory analysis to be used as an exploratory technique to determine the most statistically significant connectivity model given a set of cortical areas based on anatomic constraints. The resultant network exhibits coupling from left MI to right MI, links from both PMs to the two MIs, a negative interaction from left PM to right PM, and functional influence from SMA to right MI and right PM, revealing contributions of these areas to bimanual coordination.

Don’t think twice, it's all right—contralesional dependency for bimanual prehension movements

In bimanual coordination when moving the hands to two separate objects, subjects tend to initiate and terminate the movements together, even when the targets are at different distances or are of a different size. Additionally, each hand tends to scale its grasp independently to the object to be grasped. Here, we report the performance of a patient, who had previously shown signs of motor neglect, on two experiments investigating coupling and independence in bimanual coordination. The patient showed relatively normal bimanual behaviour for the transport phase of prehension when objects were placed at different distances (Experiment 1), but abnormal behaviour for the grasp component when objects were of different sizes (Experiment 2). Moreover, the contralesional limb demonstrated a dependency of grasp that was related to the object grasped by the ipsilesional limb. We discuss the possible underlying mechanisms of this dependency in relation to competitive motor programming and attentional bias. The results also reinforce the view that the transport and grasp components of prehension are distinct processes.