Directional tuning effects during cyclical two-joint arm movements in the horizontal plane

The present study explored the effect of different movement orientations on the arm end-effector kinematic features, levels of muscle activity and intermuscular coordination between shoulder and elbow muscles during cyclical movement. Subjects were instructed to trace cyclical lines with their dominant arm along vertical, horizontal, and right (low inertia) or left diagonal (high inertia) orientations. EMG activity from the biceps, triceps and anterior and posterior deltoids were monitored along with the displacements of the end-effector of the arm. The results suggested a differential role for the shoulder versus elbow muscles in the manipulation of the hand end-effector trajectory. The activity in the shoulder flexors was predominantly in anti-phase with that of the shoulder extensors and was therefore presumed to manipulate the global features of the trajectory. Biceps and triceps tended to show less orchestrated activity and were therefore assumed to be responsible for making the fine adjustments and to compensate for intersegmental interactions. The most pronounced differences in kinematics and EMG features among the four principal movement orientations were observed between the two diagonal orientations, which differed profoundly in arm inertial resistance. The findings converged upon the principle of 'inertial anisotropy,' as previously identified for discrete movement, suggesting that the central nervous system did not fully preplan the actual kinematic requirements of cyclical task performance. Moreover, inertial anisotropy was evident in spite of the fact that movement was performed under temporal constraints (metronome pacing) and with availability of a visual template of the task, suggesting that enhancement of the feedback loop did not fully eliminate these effects.

Coordination of upper and lower limb segments: deficits on the ipsilesional side after unilateral stroke

Coordination of the ipsilateral limbs was studied in unilateral stroke patients and a control group of healthy age-matched controls. Cyclical single-limb movements of the forearm and lower leg as well as their coordination, with the segments moving either in the same (isodirectional) or in different directions (nonisodirectional), were investigated under normal vision and blindfolded conditions. Findings revealed that stroke patients experienced difficulties with coordination of the limb segments on the ipsilesional side and this effect was more pronounced during nonisodirectional than during isodirectional coordination. In addition, cycle durations were larger and movement amplitudes shorter in stroke patients as compared to controls. Overall, the present findings clearly demonstrated motor control deficits in stroke patients on the so-called "unaffected side." The availability of normal vision did not alleviate these deficits. Therefore, the more general implication of the present findings appears to be that interlimb coordination is a complex function, requiring the integrity of both hemispheres. Comparison of the left- and right-hemispheric stroke groups revealed that patients with a left-hemisphere lesion tended to be more variable in performing the more difficult nonisodirectional pattern than patients with a right-hemisphere lesion. This possibly hints at a more pronounced involvement of the left hemisphere in the organization of ipsilateral coordination. The spatiotemporal features of movement (cycle duration, amplitude), however, did not differ between both stroke groups.

Brain areas involved in interlimb coordination: a distributed network

Whereas behavioral studies have made significant contributions toward the identification of the principles governing the coordination of limb movements, little is known about the role of higher brain areas that are involved in interlimb coordination. Functional magnetic resonance imaging (fMRI) was used to reveal the brain areas activated during the cyclical coordination of ipsilateral wrist and foot movements. Six normal subjects performed five different tasks that were presented in a random order, i.e., isolated flexion-extension movements of the right wrist (WRIST) and right foot (FOOT), cyclical coordination of wrist and foot according to the isodirectional (ISODIR) and nonisodirectional (NON-ISODIR) mode, and rest (REST). All movements were auditory paced at 66 beats/min. During the coordination of both limb segments, a distributed network was identified showing activation levels in the supplementary motor area (SMA), cingulate motor cortex (CMC), premotor cortex (PMC), primary sensorimotor cortex (M1/S1), and cerebellum that exceeded the sum of the activations observed during the isolated limb movements. In addition, coordination of the limb movements in different directions was associated with extra activation of the SMA as compared to movements in the same direction. It is therefore concluded that the SMA is substantially involved in the coordination of the nonhomologous limbs as part of a distributed motor network. Accordingly, the long-standing exclusive association that has been made between this medial frontal area and bimanual (homologous) coordination needs to be abandoned and extended towards other forms of interlimb coordination (nonhomologous).

Proprioceptive regulation of interlimb behavior: interference between passive movement and active coordination dynamics

The coordination of homolateral effectors (right arm/right leg) according to the in-phase or anti-phase mode was perturbed through passive movement of a third segment (left arm or left leg) imposed by the experimenter. The manipulated parameters of the passive segment were frequency and amplitude along with their degree of scaling. Results showed that passive movement degraded anti-phase patterns more than in-phase patterns. Furthermore, the anti-phase mode deteriorated profoundly during frequency manipulation, but scaling did not induce additional effects, whereas a linear association was observed between anti-phase deterioration and amplitude manipulation. Together, these data indicate that passive movement disturbed the coordination dynamics of an actively performed task. The fact that interference depended on the manipulated parameter suggests a distinction in the degree of intrusiveness of the irrelevant afferent information induced by the passive limb. It is concluded that sensory discrimination between irrelevant and relevant input is critical in performing a coordinated task adequately under perturbed conditions.

Proprioceptive control of cyclical bimanual forearm movements across different movement frequencies as revealed by means of tendon vibration

The effect of unilateral tendon vibration on the performance of cyclical bimanual forearm movements was investigated across different cycling frequencies (from 0.67 to 2.53 Hz). The spatiotemporal features of the individual limb motions as well as their coordination were studied. Tendon vibration was found to result in a substantial reduction in the amplitude of the vibrated arm, leaving the nonvibrated arm unaffected. The vibration-induced amplitude reduction decreased from 26% to 11% as cycling frequency increased even though significant reductions were still observed at the highest cycling frequencies. Tendon vibration was also found to result in an increase of the phase lead of the dominant arm with respect to the nondominant arm, but this effect was not modulated by cycling frequency. The data argue in favor of a closed-loop mode of movement control during cyclical high-speed movements. It is suggested that kinesthetic afferent information is processed and used to guide action up to near-maximal movement speeds, reinforcing recent claims with respect to visual information processing.

Constraints during bimanual coordination: the role of direction in relation to amplitude and force requirements

The present study addressed the status of spatial encoding during a bimanual task paradigm. This was based on the premise that patterns of contralateral interference during bimanual coordination provide a window into those movement parameters that are primarily encoded within the central nervous system. Results showed that both direction and amplitude were subject to (bilateral) interference when different specifications were to be generated simultaneously for each limb. Directional interference was found to be partially independent of the amount and pattern of underlying muscle activation, suggesting that direction is encoded at a rather abstract level in the central nervous system. The findings are consistent with single-cell recording studies that have pointed to the role of directional tuning in various brain areas. Moreover, the findings suggest that spatial parameters of movement constrain the coordination of limb movements in addition to temporal parameters.

Dynamic position sense during a cyclical drawing movement: effects of application and withdrawal of tendon vibration

The present study addressed the stability of dynamic position sense across time as well as the resetting of position sense following its disturbance by means of tendon vibration. Blindfolded subjects drew circles within a specific location of the workspace for a duration of 28 s and at a repetition rate of 1 s(-1). To study the stability of dynamic position sense, the changes in circle drawing performance across time were studied (control condition). Resetting of dynamic position sense was studied by application and subsequent withdrawal of biceps tendon vibration during movement (vibration condition). The results showed that the spatial characteristics of circle drawing in the control condition did not change significantly over the course of the 28 s trial, suggesting that dynamic position sense does not drift systematically across time. Whereas vibration leads to a decrease in diameter, a deterioration of the circularity of the pattern, and a drift of the hand movement toward the body, a restoration of these features was obtained during withdrawal of vibration. This suggests that subjects are able to reset dynamic position sense to reasonable values without the help of vision during active cyclical movement.

Spatial interactions during bimanual coordination patterns: the effect of directional compatibility

Whereas previous bimanual coordination research has predominantly focused on the constraining role of timing, the present study addressed the role of spatial (i.e., directional) constraints during the simultaneous production of equilateral triangles with both upper limbs. In addition to coordination modes in which mirror-image and isodirectional movements were performed (compatible patterns), new modes were tested in which the left limb lagged with respect to the right by one triangle side (non-compatible patterns). This resulted in the experimental manipulation of directional compatibility between the limbs. In addition, triangles with either horizontal or vertical orientations were to be drawn in order to assess the role of static images on movement production. Results supported the important role of directional constraints in bimanual coordination. Furthermore, triangles in vertical orientations (with a vertical symmetry axis, i.e., one apex pointing up) were drawn more successfully than those in horizontal orientations (with a horizontal symmetry axis, i.e., one apex pointing left or right), suggesting that the static aspects of a geometric form may affect movement dynamics. Finally, evidence suggested that cognitive processes related to integration of the submovements into a unified plan mediate the performance of new coordination patterns. The implications of the present finding for clinical populations are discussed

Bimanual coordination and limb-specific parameterization in patients with Parkinson's disease

Bimanual coordination and the capability to parameterize the individual limb movements were examined in patients with Parkinson's disease (PD) as compared to healthy control subjects. In-phase and anti-phase patterns were performed while the individual limb movements were subjected to amplitude and loading manipulations. Findings showed that PD patients produced the bimanual configurations with lower degrees of phasing accuracy and consistency than control subjects, indicating an impairment at the global (coordinative) level of simultaneously produced movements. At the local (limb-specific) level, the imposed distances with and without loading were unaffected in PD patients as compared to control subjects, whereas cycle times were prolonged and depended on the task requirements. This illustrates a disturbance at the limb-specific level in complying with the execution of the submovements. The finding that movement slowness only became evident in the more complex conditions, suggests that it did not mainly represent a deficit in the execution of coordinated movements, but rather an inability to accommodate the motor output during stringent spatiotemporal task constraints.

Age-related deterioration of coordinated interlimb behavior

Younger and older participants performed two-limb coordination patterns of homologous (similar) and nonhomologous (dissimilar) effectors during 1:1 synchronization, according to the in-phase or anti-phase mode. The aim of the study was to examine age-related changes during the production of these basic movement patterns and their relative stability difference. The findings revealed that the aging process modulated the coordination dynamics as a function of effector system characteristics. Whereas the homologous system was resistant to age-related deficits, movements of the nonhomologous system showed coordinative degradation that was most apparent during execution of the anti-phase mode. The latter performance regression is argued to be an expression of age-dependent declines in cognitive regulation and afferent information processing. This implies that deterioration in coordinated behavior across the life span may be strongly task dependent because of a combined effect of cognitive and sensory components.

The synchronization of human arm movements to external events

Previous research revealed the existence of coupling mechanisms (e.g. iso-directionality) at the level of perception and action. The present experiment investigated how the strength of the perception-action coupling affected synchronization performance. Arm movements were to be synchronized with a moving light that traveled back and forth from the left to the right side of a runway. Four experimental conditions were administered representing the orthogonal combination of two viewing conditions (intermittent vs. continuous) and two synchronization modes (in-phase, i.e. arm moving in the same direction as the light vs. anti-phase, i.e. arm moving in the opposite direction). Performance outcome measures, movement kinematics, and relative phase were used to examine the data. The results revealed a better synchronization performance when the arm and light traveled in the same direction (iso-directionality) during the continuous viewing condition. Apparently, the strength of the perception-action coupling has a severe impact on the quality of the synchronization of an arm movement to an external event.

Motor learning and Parkinson's disease: refinement of within-limb and between-limb coordination as a result of practice

Even though the basal ganglia have been assigned a role in motor learning, few studies have addressed motor learning capabilities in Parkinson patients. In the present experiment, improvement of bimanual figure drawing across practice was compared between Parkinson patients and normal age-matched controls. At regular intervals during acquisition, performance was assessed under normal vision and blindfolded conditions. At initiation of practice, the typical signs associated with Parkinson's disease became evident, such as bradykinesia and hypometria. Moreover, reduced synchronization between the force-time specifications of both limbs was observed. When vision was withdrawn, Parkinson patients showed a larger drift of drawing performance across the workspace, indicative of a decline in proprioception. In spite of the aforementioned deficits, Parkinson patients made marked improvements in the speed of execution, the consistency of the spatial trajectories, and the synchronization between the limbs across practice, even though they never reached the performance levels obtained in elderly controls. The findings demonstrate that Parkinson patients do benefit from practice to refine their upper limb control and to alleviate their most basic motor deficits.

Intentional switching between behavioral patterns of homologous and nonhomologous effector combinations

Intentional switching between preferred coordination modes (Experiment 1) and between isofrequency and multifrequency conditions (Experiments 2 and 3) was compared across different effector combinations. Experiment 1 showed that homologous limbs switched faster toward the in-phase and anti-phase mode than nonhomologous limbs, supporting their distinct degree of coordinative stability during 1:1 synchronization. Experiments 2 and 3 revealed that switching time between isofrequency and multifrequency conditions depended on the attractiveness of both coordination dynamics associated with the combination of segments involved. These results are consistent with the unique prediction derived from dynamic pattern approach in which the differential stability of the coordination modes determines the switching time.

The identification of coordination constraints across planes of motion

Two dominant coordination constraints have been identified during isofrequency conditions in previous work: the egocentric constraint, i.e., simultaneous activation of homologous muscle groups, and the allocentric constraint, i.e., moving the segments in the same direction in extrinsic space. To verify their generalization, bimanual drawing movements were performed in different planes of motion (transverse, frontal, sagittal, frontal-transverse) according to the in-phase and anti-phase mode along the X- and Y-axes. Convergent findings were obtained across the transverse, frontal, and frontal-transverse planes. The in-phase mode along both axes was performed most accurately/consistently, whereas the anti-phase mode resulted in a deterioration of the coordination pattern and this effect was most pronounced when the latter mode was introduced with respect to both dimensions. For sagittal plane motions, the in-phase mode was again superior but the second most optimal configuration was the anti-phase mode along both axes. This finding was hypothesized to result from the familiarity with the pattern since it resembles cycling behavior. It illustrates how cognitive mapping is superimposed onto the dynamics of interlimb coordination. Overall, these results support the presence of both the egocentric and allocentric constraint during bimanual movement production.

Proprioceptive control of multijoint movement: unimanual circle drawing

The present experiments addressed whether proprioception is used by the central nervous system (CNS) to control the spatial and temporal characteristics of unimanual circle drawing. Circle drawing is a multijoint movement, in which the muscles crossing the elbow and the shoulder are sequentially activated. The spatial and temporal characteristics of circle drawing depend on the precise coordination of these sequential activation patterns, and proprioception is ideally suited to support this coordination. Blindfolded human subjects produced a counterclockwise circular drawing motion (diameter = 16 cm) with the dominant arm at a repetition rate of 1/s. In some trials, 60-70 Hz vibration was applied to the tendons of the biceps brachii and/or the anterior deltoid. Spatial parameters measured from hand-movement data included the x- and y-axis diameters, circularity, and drift of the hand in the workspace. Vibration of either the biceps brachii or the anterior deltoid caused subjects to draw circles with decreased diameter, with changes in circularity, and with a systematic drift of the hand. These distortions to circle drawing by tendon vibration demonstrate that the CNS uses proprioceptive information to accomplish the spatial characteristics of this motor task. Simultaneous vibration of both muscles produced a drift that exceeded the individual vibration effects, which suggests that the CNS combined proprioceptive information related to elbow and shoulder rotation to control the movement of the hand. The temporal characteristics of circle drawing were quantified from joint angle data. While vibration did not significantly influence the relative phase between elbow and shoulder rotation, the variability of the phase relationship increased significantly, which suggests that proprioception contributes to phase stabilization. During circle drawing, elbow flexion-extension movements were produced with limited activation of the biceps. Nevertheless, biceps vibration distorted the circle metrics, suggesting that a muscle's significance as a sensory transducer is independent of its activity level.

Proprioceptive control of multijoint movement: bimanual circle drawing

Proprioception is used by the central nervous system (CNS) in the control of the spatial and temporal characteristics of single joint and multiple joint movement. The present study addressed the role of proprioception in the control of bilateral cyclical movements of the limbs. Normal blindfolded human subjects drew circles simultaneously and symmetrically with the two arms (16 cm diameter, 1 /s) upon two digitizing tablets. In selected trials, vibration (60-70 Hz) was applied to the tendon of the biceps and/or anterior deltoid muscles of the dominant arm to distort the proprioceptive information from muscle spindle afferents. One goal of this study was to identify whether tendon vibration influenced the spatial characteristics of circles drawn by the vibrated, dominant arm and the non-vibrated, non-dominant arm. A second goal was to determine the effect of vibration on the temporal coupling between the two arms during circle drawing. The results revealed that tendon vibration affected the spatial characteristics of circles drawn by the vibrated arm in a manner similar to that previously found for unilateral circle drawing. During bimanual circle drawing, vibration had only a minimal effect on the spatial characteristics of the non-vibrated, non-dominant arm. Temporal interlimb coupling was quantified by the relative phasing between the arms. Without tendon vibration, the dominant arm led the non-dominant arm. Vibration of the dominant arm increased the average phase lead. In a first control experiment, vibration of the non-dominant arm decreased the phase lead of the dominant arm, or even reversed it to a non-dominant arm phase lead. In a second control experiment, the subjects performed the bimanual circle-drawing task with vision of only the vibrated arm, in which case there was no spatial distortion of the circles drawn by the vibrated arm, but the phase relation between the two arms was still shifted as if vision were completely unavailable. It was concluded that, in bimanual movements such as these, the spatial and temporal characteristics of movement are controlled independently. Whereas the spatial characteristics of hand movement seem to be controlled unilaterally, the temporal characteristics of interlimb coupling appear to be controlled by proprioceptive information from both limbs, possibly by a proprioceptive triggering mechanism.

The stability of pen-joint and interjoint coordination in loop writing

This study is concerned with pen-joint and interjoint coordination in handwriting. In particular, it focuses on the stability of coordination as a means to find the locus of coordination control in the arm-pen effector system. Twelve subjects generated loop sequences of varying length at various positions on the baseline of writing. Joint excursions and pen-tip displacements were recorded by means of a 3D-motion tracking system. The coordination stability of 15 pairs of 6 mechanical degrees of freedom (d.f.s) of the arm-pen effector system was investigated by means of relative phase analyses. Pen-joint coordination between horizontal pen-tip displacements and wrist excursions was found to be most stable; that between vertical pen-tip displacements and finger excursions was considerably less stable. Interjoint coordination was generally less stable than pen-joint coordination, and most stable between the wrist and the elbow. Sequence length and its position on the line differentially affected the coordination stability of the d.f. combinations. The results are discussed in relation to assumptions about joint coordination in writing as expressed by computational handwriting models.

Interactive processes during interlimb coordination: combining movement patterns with different frequency ratios

The present study examined the formation of a movement pattern that was added to an ongoing coordinative regime across different limb combinations. It was hypothesized that the addition of the secondary mode would perturb the ongoing primary mode by adding rhythmic complexity to the task requirements. Furthermore, the formation of the secondary mode was predicted to be affected by the ongoing coordination pattern. In Exp. 1, a primary multifrequency mode (2:1 ratio) was performed while a secondary isofrequency mode (1:1 ratio) was initiated midway into the trials, whereas the reversed dual-pattern conditions were examined in Exp. 2. The results from both experiments showed that the multifrequency mode deteriorated across limb combination under dual-pattern as compared to single-pattern conditions. The isofrequency mode was also affected under combined pattern conditions, but its degradation was a function of the limb combination under consideration. In particular, the non-homologous limbs, which demonstrated less stable behavior than the homologous limbs under single-pattern conditions, were affected most strongly when confronted with the simultaneous production of the multifrequency mode. In addition, anti-phase movements deteriorated more than in-phase movements, supporting indirectly the contention that afferent feedback monitoring complexity differs for the two movement configurations. The findings of this study suggest that manipulation of task requirements can be used to examine pattern durability and formation in view of dynamical perturbations.

Hierarchical control of different elbow-wrist coordination patterns

The present paper focused on the role of mechanical factors arising from the multijoint structure of the musculoskeletal system and their use in the control of different patterns of cyclical elbow-wrist movements. Across five levels of cycling frequency (from 0.45 Hz up to 3.05 Hz), three movement patterns were analyzed: (1) unidirectional, including rotations at the elbow and wrist in the same direction; (2) bidirectional, with rotation at the joints in opposite directions, and (3) free-wrist pattern, which is characterized by alternating flexions and extensions at the elbow with the wrist relaxed. Angular position of both joints and electromyographic activity of biceps, triceps, the wrist flexor, and the wrist extensor were recorded. It was demonstrated that control at the elbow was principally different from control at the wrist. Elbow control in all three patterns was similar to that typically observed during single-joint movements: elbow accelerations-decelerations resulted from alternating activity of the elbow flexor and extensor and were largely independent of wrist motion at all frequency plateaus. The elbow muscles were responsible not only for the elbow movement, but also for the generation of interactive torques that played an important role in wrist control. There were two types of interactive torques exerted at the wrist: inertial torque arising from elbow motion and restraining torque arising from physical limits imposed on wrist rotation. These interactive torques were the primary source of wrist motion, whereas the main function of wrist-muscle activity was to intervene with the interactive effects and to adjust the wrist movement to comply with the required coordination pattern. The unidirectional pattern was more in agreement with interactive effects than the bidirectional pattern, thus causing their differential difficulty at moderate cycle frequencies. When cycling frequency was further increased, both the unidirectional and bidirectional movements lost their individual features and acquired features of the free-wrist pattern. The deterioration of the controlled patterns at high cycling frequencies suggests a crucial role for proprioceptive information in wrist control. These results are supportive of a hierarchical organization of control with respect to elbow-wrist coordination, during which the functions of control at the elbow and wrist are principally different: the elbow muscles generate movement of the whole linkage and the wrist muscles produce corrections of the movement necessary to fulfill the task.

Load compensation during homologous and non-homologous coordination

Two-limb coordination of homologous and non-homologous effectors was examined during isofrequency (1:1) and multifrequency (2:1) conditions. The coordination patterns involved flexion and extension movements in the sagittal plane and were performed under unloaded and single-limb (right arm) loaded conditions. Previous studies suggested that the lower degree of 1:1 synchronization observed during nonhomologous as compared to homologous coordination results from natural differences in biophysical (inertial) properties. Elaborating on this idea, adding weight to the right arm was hypothesized to modulate its inertial characteristics, rendering homologous limbs more dissimilar and nonhomologous limbs more similar by enhancing and decreasing their inertial differences, respectively. Therefore, the observations made during unloaded conditions were predicted to be completely reversed during loaded conditions. Findings revealed that during 1:1 coordination (experiment 1) single-limb loading resulted in a decreased relative phase stability, whereas relative phase accuracy depended upon the limb combination. In particular, phase-locking was more accurately maintained for loaded homologous than for nonhomologous limbs, whereas loading the nonhomologous limbs resulted in a deterioration of the quality of synchronization. These findings suggest that there is an additional explanation of differential coordination capabilities among limb combinations. It is hypothesized that the neural networks subserving the control centers of the homologous limbs are more tightly connected than those of the nonhomologous effectors, allowing 1:1 synchronization to be more successfully preserved in the face of (load) perturbations. During 2:1 coordination (experiment 2), the loading procedure disturbed the coordination dynamics across all limb combinations. That no differential effect of loading on effector combination was observed is possibly a result of the fact that only an initial level of practice was studied in which optimal relative phase dynamics are still being explored for both homologous and nonhomologous limbs.

AGE-RELATED DEFICITS IN MOTOR LEARNING AND DIFFERENCES IN FEEDBACK PROCESSING DURING THE PRODUCTION OF A BIMANUAL COORDINATION PATTERN

Learning and transfer of a new bimanual coordination pattern were investigated in a group of adolescents and elderly subjects. The pattern consisted of continuous horizontal flexionextension movements with a 90 phase offset between the upper limbs. All subjects practised the task under augmented feedback conditions, involving a real-time orthogonal display of both limb movements. Three different transfer test conditions were administered at regular intervals during practice, i.e. blindfolded, with normal vision, and with augmented visual feedback. Findings showed that the performance levels of the elderly group were lower than the group of adolescents and their rate of improvement was also smaller. The observed learning deficits in the elderly are hypothesised to be a consequence of a decreased capability to overcome the preferred coordination modes, as required for developing new coordination modes. This reduced capability to suppress prepotent response tendencies may reflect an age-related decrease in the efficiency of inhibitory processes in the central nervous system and may be associated with changes in frontal lobe functioning.

Representation of wrist joint kinematics by the ensemble of muscle spindles from synergistic muscles

Proprioceptive information about movement is transmitted to the central nervous system by a variety of receptor types, which are widely distributed among the muscles, joints, and skin. Muscle spindles are known to be an important and reliable source of information for the perception of movement kinematics. Previous studies that focused on the characteristics of single muscle spindle firing patterns have left the impression that each receptor fires in relation to a number of kinematic variables, leaving the following question unanswered: what role is played by the ensemble of muscle spindles within the same muscle or within synergistic muscles? The study described in this paper addressed whether the perception of joint position and velocity is based on the net input of muscle spindles residing in all synergistic muscles crossing a joint. Normal human adults performed a motor coordination task that required perception of joint velocity and dynamic position at the wrist. The task was to open the left hand briskly as the right wrist was passively rotated in the flexion direction through a prescribed target angle. In randomly occurring trials, the tendons to three muscles [extensor carpi radialis (ECR), extensor carpi ulnaris (ECU), and extensor digitorum (ED)] were vibrated either individually or in different combinations during the performance of the motor task. Tendon vibration is known to distort muscle spindle firing patterns, and consequently, kinesthesia. By comparing performance errors with and without tendon vibration, the relative influences of muscle spindles residing in ECR, ECU, and ED were quantified. Vibration of the individual ECR, ECU, or ED tendons produced systematic undershoot errors in performance, consistent with the misperception of wrist velocity and dynamic position. Performance errors were larger when combinations of, rather than individual, muscle tendons were vibrated. The error resulting from simultaneous vibration of ECR and ECU was roughly equal to the sum of the errors produced by vibration of the individual tendons. These effects of vibrating synergistic tendons at the wrist suggest that kinesthesia is derived from the integrated input of muscle spindles from all synergistic muscles.

Coping with systematic bias during bilateral movement

The present studies examined the nature of kinematic interlimb interference during bilateral elbow movements of 1:1, 2:1 and 3:1 frequency ratios and the manner in which subjects cope with coordination bias. Analysis of movement trajectories in the first experiment indicated progressively greater angular velocity assimilation across 2:1 and 3:1 conditions. The desired temporal relationship was maintained by slowing or pausing the low-frequency movement at peak extension while the high-frequency arm produced intervening cycles. An increase in amplitude was also evident for concurrent, homologous cycles. Movement smoothness was emphasized and additional practice was provided in a second experiment. This resulted in dissociated peak angular velocity between limbs and eliminated hesitations and amplitude effects. Bias was still evident, however, as an intermittent approach toward a 1:1 ratio within each cycle. This systematic tendency was somewhat greater at the lower of two absolute frequency combinations but was not influenced by the role of each arm in producing the higher or lower frequency movement. The findings from the first experiment suggest that subjects initially accommodate interlimb kinematic assimilation, while producing the intended timing ratio, by intermittently slowing or pausing the lower-frequency movement. This attenuates the need for bilaterally-disparate movement parameters and provides additional time for organizing residual kinematic differences, perhaps reducing "transient coupling." Evidence from the second experiment indicates that subtle relative motion preferences are still evident following sufficient practice to perform the movements smoothly. The within-cycle locations of the points of greatest interlimb bias for the 2:1 rhythms were positively displaced from those previously observed for 1:1 oscillations. The persistent coordination tendencies noted in both experiments perhaps reflect an assimilation/compensation cycle and constitute one potential source of the systematic error that often emerges during the acquisition of complex skills.

Exploring interlimb constraints during bimanual graphic performance: effects of muscle grouping and direction

Past studies on bimanual coordination have revealed a general preference to move the limbs in a symmetrical fashion, also denoted as the in-phase mode. Its counterpart, the asymmetrical or anti-phase mode, is performed with lower degrees of accuracy and stability. This ubiquitous tendency to activate the homologous muscle groups is referred to as the muscle grouping constraint (egocentric constraint). The present study confirmed the generalizability of this constraint across various coordination patterns, performed in the horizontal plane. In addition, evidence was generated that movement direction in extrinsic space also constrains bimanual coordination (allocentric constraint). Overall, the present observations suggest that direction is an important movement parameter that is encoded in the central nervous system and that is subject to interactions between the neural specifications of both limbs.

Interlimb coordination deficits in patients with Parkinson's disease during the production of two-joint oscillations in the sagittal plane

Two-limb coordination patterns involving cyclical flexion-extension movements, performed in the same or in different directions, were studied in patients with Parkinson's disease and a group of elderly subjects. The three patterns referred to the homologous (both arms or legs), homolateral (right or left arm and leg), and heterolateral (right arm and left leg or vice versa) limb segment combinations that were performed in the sagittal plane from a seated position. Findings revealed that interlimb coordination deficits were evident in patients with Parkinson's disease. Moreover, mean cycle duration and its variability were increased, particularly during the production of nonhomologous limb movements in different directions. These temporal findings suggest that movement slowness was not a primary consequence of an intrinsic inability to move the limb segments at the required speed but rather reflected an intentional strategy to cope with the complexity of the coordination pattern. Finally, movement amplitude was substantially smaller and more variable in patients with Parkinson's disease, suggestive of hypometria during the production of these cyclical tasks.

Learning a new bimanual coordination pattern: reciprocal influences of intrinsic and to-be-learned patterns

According to dynamic pattern theory, intrinsically stable bimanual coordination patterns affect, and are affected by, the acquisition of a new coordination pattern. In Experiment 1, subjects practiced either a 45 degrees or a 135 degrees relative phase pattern for 4 days; in Experiment 2, they practiced a 90 degrees relative phase pattern over 6 days. Retention tests were conducted 4 weeks after the last practice session in both experiments. Performance on both the in-phase (0 degree) and anti-phase (180 degrees) patterns was also measured on each day. Contrary to predictions, the experiments revealed that reciprocal effects between the intrinsic patterns and the new pattern were only temporary, and did not affect learning in any permanent way. As well, learning a new pattern was not differentially affected by its relation to an intrinsic pattern.

Interlimb coordination in patients with Parkinson's disease: motor learning deficits and the importance of augmented information feedback

The basal ganglia have traditionally been associated with motor control functions and this view has prevailed since the late nineteenth century. Recent experimental studies suggest that this neuroanatomical system is also critically involved in motor learning. In the present study, motor learning/transfer capabilities were compared between patients with Parkinson's disease and a group of normal elderly people. Subjects practiced a bimanual coordination task that required continuous flexion-extension movements in the transverse plane with a 90 degrees phase offset between the forearms. During acquisition, augmented visual feedback of the relative motions was provided in real time. The findings revealed improvements in the bimanual coordination pattern across practice in both groups when the augmented concurrent feedback was present. However, when transferred to performance conditions in which the augmented information was withheld, performance deteriorated (relative to the augmented condition) and this effect was more prevalent in the Parkinson patients. More specifically, no improvement in interlimb coordination was observed under nonaugmented feedback conditions across practice. Instead, a drift toward the preferred in-phase and antiphase coordination patterns was evident. The present findings suggest that Parkinson patients can improve their performance on a new motor task, but they remain strongly dependent on augmented visual information to guide these newly acquired movements. The apparent adoption of a closed-loop control mode is accompanied with decreases in movement speed in order to use the feedback to ensure accuracy. When the augmented feedback is withheld and the movement pattern is to be controlled by means of intrinsic information feedback sources, performance is severely hampered. The findings are hypothesized to indicate that learning/transfer is affected in Parkinson patients who apparently prefer some constancy in the environmental contingencies under which practice takes place. The present findings are consistent with the notion that the basal ganglia form a critical neuroanatomical substrate for motor learning.

Between-limb asynchronies during bimanual coordination: effects of manual dominance and attentional cueing

Whereas previous studies on interlimb coordination have mainly underscored the ubiquitous tendency to synchronize the motions of the limbs, the present experiment revealed a small, but distinct, interlimb asynchrony or phase offset, i.e. the dominant limb led the non-dominant limb during the production of bimanual circle drawing. This asynchrony was clearly evident in the majority of right-handers, but not in left-handers. Moreover, attentional cueing affected the size of the asynchrony. Instructions to visually monitor the dominant limb or non-dominant limb strengthened and weakened the phase offset, respectively. A multifactorial neural account is proposed to underly the temporal asynchrony.

The organization of patterns of multilimb coordination as revealed through reaction time measures

Simple visual reaction time (RT) during the performance of sagittal movements of the upper and/or lower limbs was investigated. Experiment 1 demonstrated that RTs increased when more limbs were to be moved simultaneously. This effect was more apparent for the upper than for the lower limbs. Experiment 2 allowed a separation of RT into premotor time (PMT) and motor time (MOT) components through analysis of electromyographic activity, and showed that these longer response delays were associated with increased PMTs. This suggests that the time required for the central organization of movements increased as more limbs were to be controlled simultaneously. Compared to single-limb performance conditions, the increases in RT were much larger in the upper limbs (up to 16%) than in the lower limbs (up to 5%) when limb segments were added. During single-limb conditions, RTs in the upper limbs tended to be smaller than in the lower limbs, in accordance with efferent nerve conduction time estimates. Conversely, the lower limb(s) was (were) initiated before the upper limb(s) when both effector types were moved simultaneously. This pattern of activation is reminiscent of the organization of postural control during upright standing, where goal-directed arm activity is preceded by (bilateral) leg activity to anticipate for the upcoming postural destabilization. Finally, hemifield manipulations in experiment 2 revealed faster RTs and PMTs for stimuli presented in the right visual field in comparison with the left field. This advantage was evident for ipsilateral as well as contralateral responses and supports the pre-eminence of the left hemisphere in the complex organization of gross motor responses.

Acquiring bimanual skills: contrasting forms of information feedback for interlimb decoupling

The present experiments addressed the learner's capability to perform different upper-limb actions simultaneously with the help of various sources of information feedback. An elbow flexion movement was made in the left limb together with a flexion-extension-flexion movement in the right limb. Interlimb interactions were assessed at the structural as well as the metrical level of movement specification during acquisition and retention. Despite a strong initial tendency for the limbs to be synchronized, findings revealed that Ss became gradually more successful in interlimb decoupling as a result of practice with augmented feedback. However, detailed knowledge of movement kinematics was no more effective than global outcome information for interlimb decoupling, indicating that knowledge of results may have more potential for acquiring multiple degree-of-freedom tasks than previously believed. Finally, the data support the general notion that learning new coordination tasks involves the suppression of preexisting preferred coordination tendencies, which is often a prerequisite for building new coordination modes.

Acquiring bimanual skills: contrasting forms of information feedback for interlimb decoupling

The present experiments addressed the learner's capability to perform different upper-limb actions simultaneously with the help of various sources of information feedback. An elbow flexion movement was made in the left limb together with a flexion-extension-flexion movement in the right limb. Interlimb interactions were assessed at the structural as well as the metrical level of movement specification during acquisition and retention. Despite a strong initial tendency for the limbs to be synchronized, findings revealed that Ss became gradually more successful in interlimb decoupling as a result of practice with augmented feedback. However, detailed knowledge of movement kinematics was no more effective than global outcome information for interlimb decoupling, indicating that knowledge of results may have more potential for acquiring multiple degree-of-freedom tasks than previously believed. Finally, the data support the general notion that learning new coordination tasks involves the suppression of preexisting preferred coordination tendencies, which is often a prerequisite for building new coordination modes.

Adaptive Tuning of Interlimb Attraction to Facilitate Bimanual Decoupling

Motor skills that require limbs to concurrently produce different spatiotemporal patterns are often quite difficult to learn. This article outlines a general strategy for training subjects to perform skills that require such disparate limb movements. The strategy is based on the notion that certain preferred movement patterns naturally emerge through the dynamics of the perceptual-motor system, even when quite different movements are intended. The training strategy proposes that the acquisition of relative motion patterns that diverge from preferred patterns can be facilitated by initially "tuning" system dynamics to reduce interlimb attraction. The schedule for the dynamical tuning is adopted from the adaptive training method previously applied to tracking tasks. Preliminary evidence is provided in support of this strategy for learning a bimanual task requiring both structural and metrical interlimb decoupling.

Control of asymmetrical bimanual movements

When movements are performed together in the upper-limbs, a strong tendency emerges to synchronize the patterns of motor output. This is most apparent when trying to do different things at the same time. The present experiment explored the simultaneous organization and control of spatiotemporally different movements. There were two practice conditions: symmetrical and asymmetrical. In the symmetrical condition, subjects performed a series of unidirectional elbow flexion movements, followed by a series of elbow flexion-extension-flexion (reversal) movements in both limbs simultaneously. In the asymmetrical practice condition, subjects performed the unidirectional movement in the left limb together with the reversal movement in the right limb. Findings revealed a tendency for each limb movement to assimilate the features of its counterpart under the latter condition. This effect was "asymmetrical" in that the unidirectional movement was more attracted to the reversal movement than vice versa. Nevertheless, subjects were able to partly suppress this synchronization tendency as was evident from the moderate cross correlations between the angular acceleration patterns of both limb movements and from an increasingly successful differentiation of the activity levels in the right and left limb muscles. All together, these findings provide evidence for some degree of parallel control of spatiotemporally different actions. The data are discussed in view of the possible suppression of a bilaterally distributed motor control system, that is mainly held responsible for activiting proximal limb musculature.

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.

Asymmetric interlimb interference during the performance of a dynamic bimanual task

The control of a dynamic bimanual task was examined by manipulating two independent factors that potentially influence interlimb interference. Subjects attempted to perform a unidirectional movement with either their preferred or nonpreferred arm while concurrently producing a sequential movement with the contralateral arm. The magnitude of force required to produce the more complex, sequential action was manipulated in addition to the arm with which it was performed. The degree of interlimb interference was determined through an analysis of limb kinematics. A clear performance asymmetry was noted, with greater interference evident when the sequential action was generated by the nonpreferred left arm than by the preferred right arm. The level of force needed to produce the sequential movement also directly influenced interlimb interference, but this effect was bilaterally symmetrical. The findings are generally consistent with a hierarchical view of movement organization comprising lateralized hemispheric specialization for the organization of time-domain characteristics of sequential actions, followed by nonlateralized metrical scaling of force parameters. Implications of the findings for "dynamical" descriptions of bimanual actions are also discussed.

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 nonhomologous 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.