Neuromuscular-skeletal constraints upon the dynamics of unimanual and bimanual coordination

In the first of three experiments, 11 participants generated pronation and supination movements of the forearm. in time with an auditory metronome. The metronome frequency was increased in eight steps (0.25 Hz) from a base frequency of 1.75 Hz. On alternating trials, participants were required to coordinate either maximum pronation or maximum supination with each beat of the metronome. In each block of trials, the axis of rotation was either coincident with the long axis of the forearm, above this axis, or below this axis. The stability of the pronate-on-the-beat pattern, as indexed by the number of pattern changes, and the time of onset of pattern change, was greatest when the axis of rotation of the movement was below the long axis of the forearm. In contrast, the stability of the supinate-on-the-beat pattern was greatest when the axis of rotation of the movement was above the long axis of the forearm. In a second experiment, we examined how changes in the position of the axis of rotation alter the activation patterns of muscles that contribute to pronation and supination of the forearm. Variations in the relative dominance of the pronation and supination phases of the movement cycle across conditions were accounted for primarily by changes in the activation profile of flexor carpi radialis (FCR) and extensor carpi radialis longus (ECR). In the final experiment we examined how these constraints impact upon the stability of bimanual coordination. Thirty-two participants were assigned at random to one of four conditions, each of which combined an axis of rotation configuration (bottom or top) for each limb. The participants generated both inphase (both limbs pronating simultaneously, and supinating simultaneously) and antiphase (left limb pronating and right limb supinating simultaneously, and vice versa) patterns of coordination. When the position of the axis of rotation was equivalent for the left and the right limb, transitions from antiphase to inphase patterns of coordination were frequently observed. In marked contrast, when the position of the axis of rotation for the left and right limb was contradistinct, transitions from inphase to antiphase patterns of coordination occurred. The results demonstrated that when movements are performed in an appropriate mechanical context, inphase patterns of coordination are less stable than antiphase patterns.     

The effect of postural stability and spatial orientation of the upper limbs on interlimb coordination

It has recently been reported that the spatial orientation of two moving limbs has a determining influence on the relative accuracy and stability of coordination patterns. The purpose of the present experiments was to test perceptual and neuromuscular explanations of these spatial orientation effects. Experiment 1 was an initial test of the hypotheses and an extension of a previous study [Lee et al. (2002) Exp Brain Res 146:205-212] that required participants to coordinate inphase and antiphase movement patterns in four spatial orientations: two symmetric orientations (90 degrees and 180 degrees separation between the limbs) and two asymmetric orientations (90 degrees and 135 degrees separation between the limbs). Results of Experiment 1 suggest that the symmetry of movement may be a key factor influencing spatial orientation effects observed during interlimb coordination. In Experiment 2, participants again performed inphase and antiphase movement patterns in symmetric and asymmetric spatial orientations. However, one-half of the participants in Experiment 2 were provided with mechanical constraints during the performance of the desired coordination patterns. The mechanical constraints provided postural support but did not influence the visual experience. Results showed that the addition of the postural support improved performance. These findings suggest that neuromuscular, and perhaps biomechanical, constraints contribute more to the influence of spatial orientation than visual-perceptual constraints.     

Local and global stabilization of coordination by sensory information

In studies of rhythmic coordination, where sensory information is often generated by an auditory stimulus, spatial and temporal variability are known to decrease at points in the movement cycle coincident with the stimulus, a phenomenon known as anchoring (Byblow et al. 1994). Here we hypothesize that the role of anchoring may be to globally stabilize coordination under conditions in which it would otherwise undergo a global coordinative change such as a phase transition. To test this hypothesis, anchoring was studied in a bimanual coordination paradigm in which either inphase or antiphase coordination was produced as auditory pacing stimuli (and hence movement frequency) were scaled over a wide range of frequencies. Two different anchoring conditions were used: a single-metronome condition, in which peak amplitude of right finger flexion coincided with the auditory stimulus; and a double-metronome condition, in which each finger reversal (flexion and extension) occurred simultaneously with the auditory stimuli. Anchored reversal points displayed lower spatial variation than unanchored reversal points, resulting in more symmetric phase plane trajectories in the double- than the single-metronome condition. The global coordination dynamics of the double-metronome condition was also more stable, with transitions from antiphase to inphase occurring less often and at higher movement frequencies than in the single-metronome condition. An extension of the Haken-Kelso-Bunz model of bimanual coordination is presented briefly which includes specific coupling of sensory information to movement through a process we call parametric stabilization. The parametric stabilization model provides a theoretical account of both local effects on the individual movement trajectories (anchoring) and global stabilization of observed coordination patterns, including the delay of phase transitions.     

Bimanual coordination between isometric contractions and rhythmic movements: an asymmetric coupling

 Interactions between rhythmically moving limbs typically result in attraction to a limited number of coordination modes, which are distinguished in terms of their stability. In addition, the stability of coordination typically decreases with elevations in movement frequency. To gain more insight into the neurophysiological mechanisms underlying these stability characteristics, the effects of phasic voluntary muscle activation onto the movement pattern of the contralateral limb as well as onto the stability of interlimb coordination were examined. This was done in circumstances in which a minimal degree of movement-elicited afferent information was available to mediate the coupling influences. The task involved rhythmic application of isometric torque by one hand, while the other hand was moving rhythmically with unconstrained amplitude. The effects of two levels of applied torque, two coordination patterns (inphase and antiphase), and two movement frequencies were determined, both at the behavioural level (movement kinematics and kinetics) and the neuromuscular level (EMG). The isometric applications of torque clearly influenced the muscle-activation profile and movement pattern of the other limb, affecting both temporal variability and amplitude. Surprisingly, there were no differences between the two coordination patterns or between the tempo conditions. As such, the results did not conform to the Haken-Kelso-Bunz model for rhythmic movement coordination. These data suggest that the archetypal differences in stability of rhythmic bimanual coordination are contingent upon a correspondence between the limbs in terms of their respective tasks. This interpretation is elaborated in terms of the role of sensory feedback and the functional specificity of motor unit recruitment in rhythmic interlimb coordination. Received: 6 November 1998 / Accepted: 7 July 1999     

Comparing the attractor strength of intra- and interpersonal interlimb coordination using cross-recurrence analysis

Previous research has demonstrated that intra- and interpersonal rhythmic interlimb coordination are both constrained by the self-organizing entrainment process of coupled oscillators. Despite intra- and interpersonal coordination exhibiting the same stable macroscopic movement patterns the variability of the coordination is typically found to be much greater for inter- compared to intrapersonal coordination. Researchers have assumed that this is due to the interpersonal visual-motor coupling producing a weaker attractor dynamic than the intrapersonal neuromuscular coupling. To determine whether this assumption is true, two experiments were conducted in which pairs of participants coordinated hand-held pendulums swung about the wrist, either intra- and interpersonally. Using the cross-recurrence statistics of percent recurrence and maxline to independently index the level of noise and the attractor strength of the coordination, respectively, the results confirmed that the attractor strength was significantly weaker for inter- compared to intrapersonal coordination and that a similar magnitude of noise underlies both.     

Phase transitions and postural deviations during bimanual kinesthetic tracking

Upper limb coordination was studied by examining pattern stability of between-hand rhythmical coordination. In the first of two experiments, relative phase of rhythmical wrist flexion-extension was examined within a kinesthetic tracking paradigm. Eight right-handed subjects actively tracked a driven hand being flexed and extended by a computer-controlled AC servo-motor. Hand movements were constrained in flexion or extension. The simultaneous contraction of wrist flexors and extensors was defined as inphase (IP) and the alternating contraction of wrist flexors and extensors as antiphase (AP). Phase transitions (from AP to IP) were observed in 16% of trials prepared in AP. Fewer phase transitions occurred when the right wrist was constrained in flexion, and also when the left wrist was constrained in extension. IP patterns were performed with greater stability than AP patterns. These effects were explored further in a second experiment with the addition of a secondary probe reaction time task to assess demands on central capacity, and the analysis of wrist flexor and extensor electromyographic activity. Subjects returned longer reaction times for AP than IP movement, suggesting the AP movement pattern placed a greater demand on central capacity than the IP movement pattern. During this kinesthetic tracking task, similar dynamic principles emerged as those observed during bilaterally active bimanual rhythmical coordination. The greater stability of the hand-posture combination where the driven left hand was constrained in extension and the active right hand was constrained in flexion may be a demonstration of unique central control of coupled activity. Electronic Publication     

Interpersonal Fitts�� law: when two perform as one

Intra- and interpersonal coordination was investigated using a bimanual Fitts' law task. Participants tapped rhythmically between pairs of targets. Tapping was performed with one hand (unimanual), two hands (intrapersonal coordination), and one hand together with another participant (interpersonal coordination). The sizes and distances of targets in a pair were manipulated independently for each hand. When target difficulty was unequal across hands, movement times were similar in the coordination conditions, in violation of Fitts' law. Processing speed (measured by index of performance) increased for more difficult tasks, suggesting increased attention, even for dyads. These findings suggest that similar processes, not captured by centralized control, guide coordination for both individuals and dyads. Measures of coordination, though, still showed stronger coordination tendencies for intrapersonal coordination, indicating a possible role for centralized mechanisms.     

Unraveling interlimb interactions underlying bimanual coordination

Three sources of interlimb interactions have been postulated to underlie the stability characteristics of bimanual coordination but have never been evaluated in conjunction: integrated timing of feedforward control signals, phase entrainment by contralateral afference, and timing corrections based on the perceived error of relative phase. In this study, the relative contributions of these interactions were discerned through systematic comparisons of five tasks involving rhythmic flexion-extension movements about the wrist, performed bimanually (in-phase and antiphase coordination) or unimanually with or without comparable passive movements of the contralateral hand. The main findings were the following. 1) Contralateral passive movements during unimanual active movements induced phase entrainment to interlimb phasing of either 0 degrees (in-phase) or 180 degrees (antiphase). 2) Entrainment strength increased with the passive movements' amplitude, but was similar for in-phase and antiphase movements. 3) Coordination of unimanual active movements with passive movements of the contralateral hand (kinesthetic tracking) was characterized by similar bilateral EMG activity as observed in active bimanual coordination. 4) During kinesthetic tracking the timing of the movements of the active hand was modulated by afference-based error corrections, which were more pronounced during in-phase coordination. 5) Indications of in-phase coordination being more stable than antiphase coordination were most prominent during active bimanual coordination and marginal during kinesthetic tracking. Together the results indicated that phase entrainment by contralateral afference contributed equally to the stability of in-phase and antiphase coordination, and that differential stability of these patterns depended predominantly on integrated timing of feedforward signals, with only a minor role for afference-based error corrections.     

Joint action in a cooperative precision task: nested processes of intrapersonal and interpersonal coordination

The authors determined the effects of changes in task demands on interpersonal and intrapersonal coordination. Participants performed a joint task in which one participant held a stick to which a circle was attached at the top (holding role), while the other held a pointer through the circle without touching its borders (pointing role). Experiment 1 investigated whether interpersonal and intrapersonal coordination varied depending on task difficulty. Results showed that interpersonal and intrapersonal coordination increased in degree and stability with increments in task difficulty. Experiment 2 explored the effects of individual constraints by increasing the balance demands of the task (one or both members of the pair stood in a less stable tandem stance). Results showed that interpersonal coordination increased in degree and stability as joint task demands increased and that coupling strength varied depending on joint and individual task constraints. In all, results suggest that interpersonal and intrapersonal coordination are affected by the nature of the task performed and the constraints it places on joint and individual performance.     

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.     

Bimanual coordination affects motor task switching

Task-switching paradigms have generally been used to investigate cognitive processes involved in decision making or allocating attention. This work extended the task-switching paradigm into the motor domain in order to investigate the consequences of an unexpected environmental perturbation on reaction time and movement time. Typically, task-switching paradigms have investigated consequences of rearranging task sets from one trial to the next; this work explored rearranging planned movements within the context of a single trial. Of particular interest was how the motor system reorganizes coordination patterns when reaching amplitude congruency is manipulated between the two hands. Results for Experiment 1 and the far distance in Experiment 2 indicated that reaction time switch costs were the smallest during congruent task-switch trials, where reaching amplitudes between the two hands were the same. This implies that a planned movement parameter for one hand is accessible for the other hand in the circumstance of an unexpected task switch. However, the reversed congruency effects found for the near distance in Experiment 2 suggest that the ability to capitalize on stored parameter information to decrease reaction time is dependent on environmental factors and task instructions. Movement time results showed that even if a movement with one hand is aborted in mid-execution, it can still influence the performance of the other hand during a task switch. This suggests that bimanual coordination can affect prehensile performance even though only one hand has a goal to achieve.     

Location but not amount of stimulus occlusion influences the stability of visuomotor coordination

The current study examined whether the amount and location of available movement information influenced the stability of visuomotor coordination. Participants coordinated a handheld pendulum with an oscillating visual stimulus in an inphase and antiphase manner. The effects of occluding different amounts of phase at different phase locations were examined. Occluding the 0°/180° phase locations (end-points) significantly increased the variability of the visuomotor coordination. The amount of occlusion had little or no affect on the stability of the coordination. We concluded that the end-points of a visual rhythm are privileged and provide access to movement information that ensures stable coordination. The results are discussed with respect to the proposal of Bingham (Ecol Psychol 16:45–43, 2004) and Wilson et al. (Exp Brain Res 165:351–361, 2005) that the relevant information for rhythmic visual coordination is relative direction information.     

Instabilities during antiphase bimanual movements: are ipsilateral pathways involved?

The spatial and temporal coupling between the hands is known to be very robust during movements which use homologous muscles (in-phase or symmetric movements). In contrast, movements using nonhomologous muscles (antiphase or asymmetric movements) are less stable and exhibit a tendency to undergo a phase transition to in-phase movements as movement frequency increases. The instability during antiphase movements has been modeled in terms of signal interference mediated by the ipsilateral corticospinal pathways. In this study we report that participants in whom distal ipsilateral motor-evoked potentials could be elicited with transcranial magnetic stimulation (TMS), exhibited higher variability during a bimanual circling task than participants whose ipsilateral pathways could not be transcranially activated. These results suggest that ipsilateral control of the limb affects the level of bimanual coupling, and may contribute to uncoupling phenomena observed during asymmetric coordination.     

Sharing a bimanual task between two: evidence of temporal alignment in interpersonal coordination

In recent years, researchers have made many new discoveries in the field of social interaction and have attempted to understand the mechanisms of interpersonal coordination. This research is marked by two streams: On the one hand, there are attempts to explain spontaneous, incidental interpersonal coordination in terms of the behavioral dynamics perspective, and on the other, to explain instructed, intentional interpersonal coordination in terms of joint action. Other paradigms fall somewhere between incidental and intentional coordination, e.g. task sharing paradigms. The present study has two major objectives. First, we wanted to explore to what extent a dyadic scenario for bimanual coordination mimics typical signatures of bimanual coordination performance as obtained in the classical individual scenario. Second, if such mimicking is obtained, we wanted to investigate the kind of information on which the coordination between the two individuals may be grounded. To do so, we used a bimanual aiming task, which enabled us to assess measurements of two levels of coordination: global (operating over longer periods of time) and local (operating on each particular trial). In Experiment 1, this task was performed in an individual and in a dyadic setting. In the dyadic scenario, we observed strong global coordination and weak local coordination. In Experiment 2, we replicated this pattern and showed that different kinds of feedback had no impact on interpersonal coordination. Based on these findings, we propose that interpersonal coordination in a non-rhythmic choice response task is based on weak interpersonal coordination.     

Time dependence of coupling in frequency-scaled bimanual coordination

Prior research has shown that fluctuations in the relative phase of bimanual coordination do not reflect a white Gaussian noise process. The present study furthered the examination of time-dependent properties in bimanual coordination by comparing the magnitude of relative phase variability and the degree of effector independence within the time domain. The original Kelso (1984) [10] bimanual frequency-scaling protocol was reproduced in which phase transitions from antiphase to in-phase were induced with increasing movement frequency. The results showed that as movement frequency was scaled-up the amount of relative phase variability increased and the effector movements became more dependent prior to the transition. This is consistent with previous modeling showing that stronger effector coupling can prevent the occurrence of phase transitions when long range correlations in relative phase are present. It appears that, as movement frequency is scaled up, increases in effector coupling strength minimize loss of pattern stability and delay the onset of phase transitions.     

Location but not amount of stimulus occlusion influences the stability of visuo-motor coordination

The current study examined whether the amount and location of available movement information influenced the stability of visuo-motor coordination. Participants coordinated a hand-held pendulum with an oscillating visual stimulus in an inphase and antiphase manner. The effects of occluding different amounts of phase at different phase locations were examined. Occluding the 0°/180° phase locations (end-points) significantly increased the variability of the visuo-motor coordination. The amount of occlusion had little or no affect on the stability of the coordination. We concluded that the end-points of a visual rhythm are privileged and provide access to movement information that ensures stable coordination. The results are discussed with respect to the proposal of Bingham and colleagues (e.g., Bingham GP. Ecol Psychol 16:45–53, 2004a; Wilson AD, Collins DR, Bingham GP. Exp Brain Res 165:351–361, 2005a) that the relevant information for rhythmic visual coordination is relative direction information.     

Relative phase destabilization during interlimb coordination: the disruptive role of kinesthetic afferences induced by passive movement

The disruption of three patterns of two-limb coordination, involving cyclical flexion-extension movements performed in the same or in different directions, was investigated through application of passive movement to a third limb by the experimenter. The three patterns referred to the homologous, homolateral, and heterolateral (diagonal) limb combinations which were performed in the sagittal plane. The passive movement involved a spatiotemporal trajectory that differed from the movements controlled actively. Even though subjects were instructed to completely ignore the passive limb movement, the findings of experiment 1 demonstrated a moderate to severe destabilization of the two-limb patterns, as revealed by analyses of power spectra, relative phase, cycle duration, and amplitude. This disruption was more pronounced in the homolateral and heterolateral than in the homologous effector combinations, suggesting stronger coupling between homologous than nonhomologous limb pairs. Moreover, passive mobilization affected antiphase (nonisodirectional) movements more than inphase (isodirectional) movements, pointing to the differential stability of these patterns. Experiment 2 focused on homolateral coordination and demonstrated that withdrawal of visual information did not alter the effects induced by passive movement. It was therefore hypothesized that the generation of extra kinesthetic afferences through passive limb motion was primarily responsible for the detriment in interlimb coordination, possibly conflicting with the sensory information accompanying active movement production. In addition, it was demonstrated that the active limbs were more affected by their homologous passive counterpart than by their non-homologous counterpart, favoring the notion of specific interference. The findings are discussed in view of the potential role of kinesthetic afferences in human interlimb coordination, more specifically the preservance of relative phasing through a kinesthetic feedback loop.     

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

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

Increasingly complex bimanual multi-frequency coordination patterns are equally easy to perform with on-line relative velocity feedback

An experiment was conducted to determine whether multi-frequency continuous bimanual circling movements of varying difficulty (1:2. 2:3, 3:4, and 4:5) could be effectively performed following relatively little practice when on-line continuous relative velocity feedback is provided. The between-subjects results indicate extremely effective bimanual multi-frequency performance for all coordination patterns with relatively stable and continuous movements of both limbs. The findings suggest that the previous performance effects using Lissajous feedback with reciprocal movement can be extended to circling movements using on-line relative velocity feedback. Contrary to the long-held position that these coordination patterns result in increasing difficulty, we failed to find systematic relative velocity error, variability, or bias differences between the participants performing the various multi-frequency coordination patterns. Indeed, coordination error, variability, and biases were remarkably low for each of the tasks. The results clearly indicate the ease with which participants are able to produce bimanual coordination patterns typically considered difficult if not impossible when salient visual information is provided that allows the participants to detect and correct their coordination errors.     

Postural context alters the stability of bimanual coordination by modulating the crossed excitability of corticospinal pathways

The tendency for movements of the upper limbs to be drawn systematically toward one another and to follow similar spatiotemporal trajectories is well known. Although suppression of this tendency is integral to tasks of daily living, its exploitation may prove to be critical in the rehabilitation of acquired hemiplegias. In general, however, the task-related factors that determine the degree of coupling between the upper limbs and the mechanisms that mediate bilateral interactions between neural pathways projecting to the muscles of the arm and hand are not yet well understood. We present evidence that the postural context in which human participants perform upper limb movements determines the relative stability of patterns of bimanual coordination. Manipulation of the axes of rotation of forearm movements reversed the relative stability of simultaneous and alternating patterns of bimanual coordination. Transcranial magnetic stimulation of motor cortex revealed that these manipulations of postural context altered the crossed modulation of excitability in corticospinal pathways that arises from movement of the opposite limb. Furthermore, modulation of responses to electrical stimulation of the cervicomedullary junction indicated that crossed modulation was also expressed at the level of the spinal motoneurons. Our data support the view that crossed modulation of excitability in corticospinal pathways mediates the stability of bimanual coordination. Furthermore, task-related factors that are sufficient to give rise to changes in the stability of bimanual coordination are accompanied by crossed modulation of excitability at multiple levels of the neuraxis, indicative of a failure of inhibitory control.