The Dynamics of Bimanual Circle Drawing

A bimanual circle drawing task was employed to elucidate the dynamics of intralimb and interlimb coordination. Right-handed subjects were required to produce circles with both hands in either a symmetrical (mirror) mode (i.e. one hand moving clockwise, the other counter-clockwise) or in an asymmetrical mode (i.e. both hands moving clockwise or counter-clockwise). The frequency of movement was scaled by an auditory metronome from 1.50 Hz to 3.25 Hz in8 (8-sec) steps.In the asymmetrical mode,distortions ofthe movement trajectories, transient departures from the target pattern of coordination, and phase wandering were evident as movement frequency was increased. These features suggested loss of stability. Deviations from circular trajectories were most prominent for movements of the left hand. Transient departures from the required mode of coordination were also largely precipitated by the left hand. The results are discussed with reference to manual asymmetries and mechanisms of interlimb and intersegmental coordination.     

Roles of Visual Information for Control of Reaching Movements in Children

Poststroke hemiparetic individuals (n = 9) and a control group (n = 9) completed a frequency-scaled circle-drawing task in unimanual and bimanual conditions. Measures of intralimb spatial and temporal task accuracy and interlimb coordination parameters were analyzed. Significant reductions in task performance were seen in both limbs of the patients and controls with the introduction of bimanual movement. Spatial performance parameters suggested that the 2 groups focused on different hands during bimanual conditions. In the controls, interlimb coordination variables indicated predictable hand dominance effects, whereas in the patient group, dominance was influenced by the side of impairment and prior handedness of the individual. Therefore, in this particular bimanual task, performance improvements in the hemiplegic side could not be elicited. Intrinsic coupling asymmetries between the hands can be altered by unilateral motor deficits.     

Bimanual coordination dynamics in poststroke hemiparetics

Poststroke hemiparetic individuals (n = 9) and a control group (n = 9) completed a frequency-scaled circle-drawing task in unimanual and bimanual conditions. Measures of intralimb spatial and temporal task accuracy and interlimb coordination parameters were analyzed. Significant reductions in task performance were seen in both limbs of the patients and controls with the introduction of bimanual movement. Spatial performance parameters suggested that the 2 groups focused on different hands during bimanual conditions. In the controls, interlimb coordination variables indicated predictable hand dominance effects, whereas in the patient group, dominance was influenced by the side of impairment and prior handedness of the individual. Therefore, in this particular bimanual task, performance improvements in the hemiplegic side could not be elicited. Intrinsic coupling asymmetries between the hands can be altered by unilateral motor deficits.     

Development of bimanual skill: the search for stable patterns of coordination

In 2 experiments, dynamic systems theory predictions concerning intrinsic dynamics and variability of bimanual coordination were examined at different developmental stages. In Experiment 1, ten 4-, 6-, 7-, 8-, and 10-year-old children and adults performed unimanual dominant, unimanual nondominant, and bimanual continuous circle drawing. All tasks were performed at the participants' preferred rate, size, and mode of coordination. The 4-, 6-, and 7-year-old children produced larger circles with longer durations than those of the 8- and 10-year-olds and the adults. That finding demonstrates that younger children display different intrinsic dynamics than older children and adults. The 4-, 6-, and 7-year-old children also displayed more variability in bimanual coordination (more time in less stable patterns of coordination, higher standard deviation in relative phase) and produced more transitions between coordination patterns than the 8- and 10-year-olds and the adults. In Experiment 2, the same participants performed bimanual circles at increasing rates. Consistent with predictions of the HKB model (H. Haken, J. A. S. Kelso, & H. Bunz, 1985), the number of transitions decreased as speed increased. Some support was found for the notion that age-related variables of attention and rate contribute to the increased variability in young children's bimanual coordination.     

Bimanual Training Reduces Spatial Interference

The authors investigated whether training can reduce bimanual directional interference by using a star-line drawing paradigm, Participants (N = 30) were required to perform rhythmical arm movements with identical temporal but differing directional demands. Moreover, the effectiveness of part-task training in which each movement was practiced in isolation was compared with that of whole-task training in which only combined movements were performed. Findings revealed that bimanual training substantially reduced spatial interference, but unimanual training did not. The authors therefore concluded that the spatial coupling of the limbs is not implemented in a rigid way; instead, the underlying neural correlate can undergo plastic changes induced by training. Moreover, the practical implication that emerged from the present study is that athletic, musical, or ergonomic skills that require a high degree of interlimb coordination are best served by whole-task practice.     

Bimanual training reduces spatial interference

The authors investigated whether training can reduce bimanual directional interference by using a star-line drawing paradigm. Participants (N = 30) were required to perform rhythmical arm movements with identical temporal but differing directional demands. Moreover, the effectiveness of part-task training in which each movement was practiced in isolation was compared with that of whole-task training in which only combined movements were performed. Findings revealed that bimanual training substantially reduced spatial interference, but unimanual training did not. The authors therefore concluded that the spatial coupling of the limbs is not implemented in a rigid way; instead, the underlying neural correlate can undergo plastic changes induced by training. Moreover, the practical implication that emerged from the present study is that athletic, musical, or ergonomic skills that require a high degree of interlimb coordination are best served by whole-task practice.     

Development of Bimanual Skill: The Search for Stable Patterns of Coordination

In 2 experiments, dynamic systems theory predictions concerning intrinsic dynamics and variability of bimanual coordination were examined at different developmental stages. In Experiment 1, ten 4-, 6-, 7-, 8-, and 10-year-old children and adults performed unimanual dominant, unimanual nondominant, and bimanual continuous circle drawing. All tasks were performed at the participants' preferred rate, size, and mode of coordination. The 4-, 6-, and 7-year-old children produced larger circles with longer durations than those of the 8- and 10-year-olds and the adults. That finding demonstrates that younger children display different intrinsic dynamics than older children and adults. The 4-, 6-, and 7-year-old children also displayed more variability in bimanual coordination (more time in less stable patterns of coordination, higher standard deviation in relative phase) and produced more transitions between coordination patterns than the 8- and 10-year-olds and the adults. In Experiment 2, the same participants performed bimanual circles at increasing rates. Consistent with predictions of the HKB model (H. Haken, J. A. S. Kelso, & H. Bunz, 1985), the number of transitions decreased as speed increased. Some support was found for the notion that age-related variables of attention and rate contribute to the increased variability in young children's bimanual coordination.     

Unilateral vs. bilateral coordination of circle-drawing tasks

The number of joint motions available in the upper extremity provides for multiple solutions to the coordination of a motor task. Making use of these abundant joint motions provides for task flexibility. Controlling bimanual movements poses another level of complexity because of possible tradeoffs between coordination within a limb and coordination between the limbs. We examined how flexible patterns of joint coordination were used to stabilize the hand's path when drawing a circle independently compared to a bimanual pattern. Across-trial variance of joint motions was partitioned into two components: goal-equivalent variance (GEV), representing variance of joint motions consistent with a stable hand path and non-goal-equivalent variance (NGEV) representing variance of joint motions that led to deviations of the hand's path. GEV was higher than NGEV in both unimanual and bimanual drawing, with one exception. Both GEV and NGEV, related to control of the individual hands' motion, decreased when engaged in the bimanual compared to unimanual drawing. Moreover, NGEV, leading to variability in the vectorial distance between the hands, was higher when the two hands drew circles in a bimanually asymmetric vs. symmetric pattern, consistent with reported differences in the relative phasing of the two hands. Our results suggest that the nervous system controls the individual hands' motions by separate intra-limb synergies during both unimanual and bimanual drawing, and superimposes an additional synergy to achieve stable relative motion of the two hands during bimanual drawing.     

Coordination dynamics and attentional costs of continuous and discontinuous bimanual circle drawing movements

It has been suggested that the temporal control of rhythmic unimanual movements is different between tasks requiring continuous (e.g., circle drawing) and discontinuous movements (e.g., finger tapping). Specifically, for continuous movements temporal regularities are an emergent property, whereas for tasks that involve discontinuities timing is an explicit part of the action goal. The present experiment further investigated the control of continuous and discontinuous movements by comparing the coordination dynamics and attentional demands of bimanual continuous circle drawing with bimanual intermittent circle drawing. The intermittent task required participants to insert a 400ms pause between each cycle while circling. Using dual-task methodology, 15 right-handed participants performed the two circle drawing tasks, while vocally responding to randomly presented auditory probes. The circle drawing tasks were performed in symmetrical and asymmetrical coordination modes and at movement frequencies of 1Hz and 1.7Hz. Intermittent circle drawing exhibited superior spatial and temporal accuracy and stability than continuous circle drawing supporting the hypothesis that the two tasks have different underlying control processes. In terms of attentional cost, probe RT was significantly slower during the intermittent circle drawing task than the continuous circle drawing task across both coordination modes and movement frequencies. Of interest was the finding that in the intermittent circling task reaction time (RT) to probes presented during the pause between cycles did not differ from the RT to probes occurring during the circling movement. The differences in attentional demands between the intermittent and continuous circle drawing tasks may reflect the operation of explicit event timing and implicit emergent timing processes, respectively.     

Age Changes in Interlimb Coupling and the Development of Bimanual Coordination

Changes in interlimb coupling, and their role in the development of bimanual coordination, were studied longitudinally in 6- to 12-month-old infants (N = 6). Infants were observed while they were reaching for simple objects of 2 different sizes. Their use of a uni-versus bimanual strategy for reaching as well as the coupling of their bimanual movements were compared; progress in bimanual coordination of complementary movements was evaluated on 8 different bimanual tasks. The bimanual tasks involved an asymmetrical cooperation between the 2 hands. Although spatiotemporal coupling of bimanual reaching movements did not decrease during the age period studied, infants around 7 months of age used their 2 hands infrequently for reaching. Occurrences of bimanual reaching were particularly low at the session preceding the first bimanual success at a bimanual task. This suggests that the temporal coincidence between greater independence of the 2 hands and progress in bimanual coordination of complementary movements acts in 2 directions: Infants may be more at ease when using their 2 hands in differentiated patterns as the hands move less in synchrony, but, in turn, they may be less likely to move their hands in synchrony as the anticipate mirror manipulations of the object less. The frequency of bimanual reaches increased toward the end of the 1st year. This might have been caused by an increase in the repertoire of bimanual asymmetrical object manipulations and by the fact that the development of bimanual coordination allows infants to manipulate objects with complementary movements even after a bimanual approach toward the object.     

Spatial topological constraints in a bimanual task.

Previous research has shown that the concurrent performance of two manual tasks results in a tight temporal coupling of the limbs. The intent of the present experiment was to investigate whether a similar coupling exists in the spatial domain. Subjects produced continuous drawing of circles and lines, one task at a time or bimanually, for a 20 s trial. In bimanual conditions in which subjects produced the circle task with one hand and the line task with the other, there was a clear tendency for the movement path of the circle task to become more line-like and the movement path of the line task to become more circle-like, i.e., a spatial magnet effect. A bimanual circle task and a bimanual line task did not exhibit changes in the movement path when compared to single-hand controls. In all bimanual conditions, the hands were tightly temporally locked. The evidence of temporal coupling and concomitant accommodation in the movement path for the conditions in which the hands were producing different shapes suggests that spatial constraints play a role in the governance of bimanual coordinated actions.     

Amplitude scaling in a bimanual circle-drawing task: pattern switching and end-effector variability

The authors manipulated movement amplitude in a bimanual circle-tracing task performed by 11 participants. With pacing frequency fixed, the systematic increase and decrease of circle diameter within a trial induced phase transitions from the asymmetric (33% of trials) to the symmetric bimanual circle-tracing pattern; the transitions resulted from a loss of stability in the asymmetric pattern. Tracing frequency varied inversely with circle diameter so that end-effector variability was minimized in a set of self-paced trials in which the circle diameter in a trial was fixed. In the amplitude-scaling trials, end-effector variability varied directly with circle diameter, a consistent speed-accuracy tradeoff. The results support the conclusion that movement amplitude is a nonspecific control parameter. The findings are discussed with reference to several factors, e.g., tactile feedback, the recruitment and suppression of biomechanical degrees of freedom, and the role those factors may play in stabilizing bimanual coordination patterns     

Effects of movement frequency and joint kinetics on the joint coordination underlying bimanual circle drawing

Ten healthy participants performed bimanual symmetric and asymmetric circle drawing at 4 frequencies. The authors partitioned the variance of the joint configuration across repetitions into 1 component representing equivalent joint configurations with respect to achieving stability of the mean hand path (i.e., goal-equivalent variance, GEV) and 1 component leading to a variable hand path (non-goal-equivalent variance, NGEV) across cycles. Higher frequencies led to increased NGEV related to control of the nondominant hand and to the relative position and orientation between the hands during asymmetric drawing. The results were related to differences in muscle and interaction moments between the arms, and they suggest a possible relationship between the ability to use intersegmental forces and the stability of interlimb synergy.     

Interlimb coordination following stroke

Studies investigating whether simultaneous bilateral movements can facilitate performance of the impaired limb(s) of stroke patients have returned mixed results. In the present study we compared unilateral limb performance (amplitude, cycle duration) with performance during an interlimb coordination task involving both homologous (both arms, both legs) and non-homologous (one arm, one leg) limbs in stroke participants (n=7) and healthy age-matched controls (n=7). In addition, the effect of on-line augmented visual feedback on interlimb coordination was investigated. Participants performed cyclical flexion-extension movements of the arms and legs in the sagittal plane paced by an auditory metronome (1 Hz). Movement amplitudes were larger and cycle durations shorter during homologous limb coordination than non-homologous coordination. Compared with unilateral movements both groups had reduced movement amplitudes and the stroke group increased cycle duration when interlimb coordination tasks were performed. These effects were most evident during non-homologous (arm and leg) coordination. No evidence of facilitation of the impaired limb(s) was found in any of the interlimb coordination conditions. Augmented visual feedback had minimal effect on the movements of control participants but lead to an increase of cycle duration for stroke participants.     

Generation of Bimanual Trajectories of Disparate Eccentricity: Levels of Interference and Spontaneous Changes Over Practice

The authors examined intermanual interactions of 2 hands that were required to concurrently follow trajectories that differed in eccentricity. Ten healthy participants attempted to learn to trace 2 figures, a circle and an ellipse, with bilaterally isochronous (1:1) timing demands. Initial unimanual trials were followed by bilateral practice comprising 750 movement cycles. Two objectives were addressed: The authors' primary aim was to determine if kinematic interlimb interference is evident independent of spatial and temporal interference and to observe the potential practice-related changes in the nature of that interference. That test was afforded by participants' natural tendency to draw a circle with a relatively constant tangential velocity and an ellipse with a systematically varying velocity. A second aim was to observe the nature of spontaneous changes in the performance of each individual effector, and in the relationship between effectors, across practice. Those objectives were specifically addressed in a context in which augmented feedback was not available to direct the learners' attention to a particular feature of performance. The results suggested that interlimb assimilation of spatial features is the primary source of interference for that task and that apparent effects at the kinematic level are the secondary, indirect product of spatial coupling. Those results were found across blocks of practice. With respect to nondirected performance changes, substantially less improvement was evident in the performance of each individual effector than in the reduction of interlimb interference. Specifically, no practice-related changes in temporal variability or velocity bias, and minimal changes in trajectory smoothness, were evident in individual limbs. Conversely, significant reductions were observed in the variability of relative phase between limbs and in the magnitude of interlimb phase lag.     

Attention as a mediating variable in the dynamics of bimanual coordination

The influence of focal attention on the coordination dynamics in a bimanual circle drawing task was investigated. Six right-handed and seven left-handed subjects performed bimanual circling movements, in two modes of coordination, symmetrical or asymmetrical. The frequency of movement was scaled by an auditory metronome from 1.50 Hz to 3.00 Hz in 7 steps. On each trial, subjects were required to attend either to the dominant hand, to a neutral position, or to the nondominant hand.The uniformity of the relative tangential angle was lower in asymmetrical than in symmetrical conditions, but was not influenced by the direction of attention. In the asymmetrical mode, shifts in RTA relations, suggestive of loss of stability, were evident as the movement frequency was increased. Typically, these shifts were mediated by distortions of the trajectory of the nondominant limb. When the nondominant hand was the focus of attention, movements of this hand were more circular, and temporal variability was reduced, at the cost of a greater deviation from the target frequency. Movements of the dominant hand were not affected by the direction of attention. The findings show that although directed attention acts to modify the coordination dynamics, it does so primarily at the level of the individual hands, rather then in terms of the relation between them.     

Generation of bimanual trajectories of disparate eccentricity: levels of interference and spontaneous changes over practice

The authors examined intermanual interactions of 2 hands that were required to concurrently follow trajectories that differed in eccentricity. Ten healthy participants attempted to learn to trace 2 figures, a circle and an ellipse, with bilaterally isochronous (1:1) timing demands. Initial unimanual trials were followed by bilateral practice comprising 750 movement cycles. Two objectives were addressed: The authors' primary aim was to determine if kinematic interlimb interference is evident independent of spatial and temporal interference and to observe the potential practice-related changes in the nature of that interference. That test was afforded by participants' natural tendency to draw a circle with a relatively constant tangential velocity and an ellipse with a systematically varying velocity. A second aim was to observe the nature of spontaneous changes in the performance of each individual effector, and in the relationship between effectors, across practice. Those objectives were specifically addressed in a context in which augmented feedback was not available to direct the learners' attention to a particular feature of performance. The results suggested that interlimb assimilation of spatial features is the primary source of interference for that task and that apparent effects at the kinematic level are the secondary, indirect product of spatial coupling. Those results were found across blocks of practice. With respect to nondirected performance changes, substantially less improvement was evident in the performance of each individual effector than in the reduction of interlimb interference. Specifically, no practice-related changes in temporal variability or velocity bias, and minimal changes in trajectory smoothness, were evident in individual limbs. Conversely, significant reductions were observed in the variability of relative phase between limbs and in the magnitude of interlimb phase lag.     

Kinetic attraction during bimanual coordination

Two experiments examined the effects of independent variations in kinetic and kinematic requirements on interlimb coupling during a bimanual task. The goal of the investigation was to provide preliminary evidence regarding one general class of physical variables that constrains discrete bimanual movements. Subjects attempted to execute a smooth unidirectional movement with the left arm, along with a three-segment reversal movement with the right arm. The first experiment manipulated the torque required to produce the reversal action, while movement duration and average angular velocity were held constant for both limbs. Several indications of increased interlimb coupling, due to the kinetic variation, were evident. The converse manipulation was used in the second experiment, with movement time and kinematics (velocity, acceleration) changed independently of joint torque requirements for the reversal limb. No clear effect of kinematics on coupling strength was noted. The results suggest that one variable influencing interlimb attraction toward common spatiotemporal trajectories may be kinetic in nature.     

Kinetic Attraction During Bimanual Coordination

Two experiments examined the effects of independent variations in kinetic and kinematic requirements on interlimb coupling during a bimanual task. The goal of the investigation was to provide preliminary evidence regarding one general class of physical variables that constrains discrete bimanual movements. Subjects attempted to execute a smooth unidirectional movement with the left arm, along with a three-segment reversal movement with the right arm. The first experiment manipulated the torque required to produce the reversal action, while movement duration and average angular velocity were held constant for both limbs. Several indications of increased interlimb coupling, due to the kinetic variation, were evident. The converse manipulation was used in the second experiment, with movement time and kinematics (velocity, acceleration) changed independently of joint torque requirements for the reversal limb. No clear effect of kinematics on coupling strength was noted. The results suggest that one variable influencing interlimb attraction toward common spatiotemporal trajectories may be kinetic in nature.