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.     

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. Received: 15 October 1997 / Accepted: 6 January 1998     

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.     

Visual perception of the relative phasing of human limb movements

Studies of bimanual coordination have found that only two stable relative phases (0 degree and 180 degrees) are produced when a participant rhythmically moves two joints in different limbs at the same frequency. Increasing the frequency of oscillation causes an increase in relative phase variability in both of these phase modes. However, relative phasing at 180 degrees is more variable than relative phasing at 0 degree, and when the frequency of oscillation reaches a critical frequency, a transition to 0 degree occurs. These results have been replicated when 2 people have coordinated their respective limb movements using vision. This inspired us to investigate the visual perception of relative phase. In Experiment 1, recordings of human interlimb oscillations exhibiting different frequencies, mean relative phases, and different amounts of phase variability were used to generate computer displays of spheres oscillating either side to side in a frontoparallel plane or in depth. Participants judged the stability of relative phase. Judgments covaried with phase variability only when the mean phase was 0 degree or 180 degrees. Otherwise, judgments covaried with mean relative phase, even after extensive instruction and demonstration. In Experiment 2, mean relative phase and phase variability were manipulated independently via simulations, and participants were trained to perceive phase variability in testing sessions in which mean phase was held constant. The results of Experiment 1 were replicated. The HKB model was fitted to mean judgment standard deviations.     

Interlimb coordination: Learning and transfer under different feedback conditions

The role of intrinsic and extrinsic information feedback in learning a new bimanual coordination pattern was investigated. The pattern required continuous flexion-extension movements of the upper limbs with a 90 ° phase offset. Separate groups practiced the task under one of the following visual feedback conditions: (a) blindfolded (reduced FB group), (b) with normal vision (normal FB group), or (c) with concurrent relative motion information (enhanced FB group). All groups were subjected to three different transfer test conditions at regular intervals during practice. These tests included reduced, normal vision, and enhanced vision conditions. Experiment 1 showed that the group receiving augmented information feedback about its relative motions in real-time produced the required coordination pattern more successfully than the remaining two groups, irrespective of the transfer conditions under which performance was evaluated. Experiment 2 replicated and extended the superiority of the enhanced feedback group during acquisition and retention. Experiment 3 demonstrated that successful transfer to various transfer test conditions was not a result of test-trial effects. Overall, the data suggest that the conditions that optimized performance of the coordination pattern during acquisition also optimized transfer performance.     

Perception of the symmetrical patterning of human gait by infants

Two experiments tested 3- and 5-month-old infants' sensitivity to properties of point-light displays of human gait. In Experiment 1, infants were tested for discrimination of point-light displays of a walker and a runner, which, although they differed in many ways, were equivalent with regard to the phasing of limb movements. Results revealed that 3-month-old, but not 5-month-old, infants discriminated these displays. In Experiment 2, the symmetrical phase-patterning of the runner display was perturbed by advancing two of its limbs by 25% of the gait cycle. Both 3- and 5-month-old infants discriminated the walker display from this new phase-shifted runner display. These findings suggest that 3-month-old infants respond to the absolute and relative motions within a single limb, whereas 5-month-old infants respond primarily to the relations between limbs and, in particular, to the bilateral symmetry between the limbs.     

Interaction of directional,neuromuscular and egocentric constraints on the stability of preferred bimanual coordination patterns

We investigated how the relative direction of limb movements in external space (iso- and non-isodirectionality), muscular constraints (the relative timing of homologous muscle activation) and the egocentric frame of reference (moving simultaneously toward/away the longitudinal axis of the body) contribute to the stability of coordinated movements. In the first experiment, we attempted to determine the respective stability of isodirectional and non-isodirectional movements in between-persons coordination. In a second experiment, we determined the effect of the relative direction in external space, and of muscular constraints, on pattern stability during a within-person bimanual coordination task. In the third experiment we dissociated the effects on pattern stability of the muscular constraints, relative direction and egocentric frame of reference. The results showed that (1) simultaneous activation of homologous muscles resulted in more stable performance than simultaneous activation of non-homologous muscles during within-subject coordination, and that (2) isodirectional movements were more stable than non-isodirectional movements during between-persons coordination, confirming the role of the relative direction of the moving limbs in the stability of bimanual coordination. Moreover, the egocentric constraint was to some extent found distinguishable from the effect of the relative direction of the moving limbs in external space, and from the effect of the relative timing of muscle activation. In summary, the present study showed that relative direction of the moving limbs in external space and muscular constraints may interact either to stabilize or destabilize coordination patterns.     

Dynamics of 1:2 Coordination: Generalizing Relative Phase to n:m Rhythms

Interlimb rhythmic movements can be modeled as coupled oscillators, with stable performance characterized by the relative phase between the limbs. In the present study, that modeling strategy, verified previously for 1:1 coordination, was generalized to 1:2 coordination with a view to n:m coordination. The generalized model predicted interactions between coordination (specifically, 1:1 vs. 1:2) and the frequency asymmetry between the limbs determining mean relative phase and its variability. The predicted interactions were evaluated with bimanual 1:2 and 1:1 rhythmic tasks in which participants (N = 8) oscillated hand-held pendulums whose uncoupled frequencies could be adjusted so that different interlimb asymmetries were produced. The authors needed new analytic procedures to verify stable 1:2 coordination and to resolve stochastic and deterministic sources of variability in the component oscillations. The major expectations from the generalized model were confirmed, and the implications of additional but unpredicted findings for the modeling of multifrequency behavior are discussed.     

Torso movement constraint in stability of bimanual coordination

This study investigated the relation between postural movement and upper-limb coordination stability. Adults produced bimanual circles using in-phase and anti-phase coordination patterns in time to an increasing rate metronome (i.e., movement-time instruction) in the horizontal (e.g., tabletop) and vertical (e.g., "wall" perpendicular to body) planes. All participants produced the instructed in- and anti-phase patterns. Coordination stability (i.e., SD of relative phase) was larger for anti-phase than in-phase patterns in both planes; however, anti-phase coordination stability was lower in the vertical plane than in the horizontal plane. Torso movement was larger during anti-phase coordination patterns in the horizontal plane, whereas it was larger during in-phase coordination patterns in the vertical plane. These results indicate that different orientations of the same task can produce different results for stability of coordination. This information may be important for performing and learning complex motor-coordination movements (e.g., playing musical instruments).     

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.     

The coordinated movement of the spine and pelvis during running

Previous research into running has demonstrated consistent patterns in pelvic, lumbar and thoracic motions between different human runners. However, to date, there has been limited attempt to explain why observed coordination patterns emerge and how they may relate to centre of mass (CoM) motion. In this study, kinematic data were collected from the thorax, lumbar spine, pelvis and lower limbs during over ground running in n = 28 participants. These data was subsequently used to develop a theoretical understanding of the coordination of the spine and pelvis in all three body planes during the stance phase of running. In the sagittal plane, there appeared to be an antiphase coordinate pattern which may function to increase femoral inclination at toe off whilst minimising anterior–posterior accelerations of the CoM. In the medio-lateral direction, CoM motion appears to facilitate transition to the contralateral foot. However, an antiphase coordination pattern was also observed, most likely to minimise unnecessary accelerations of the CoM. In the transverse plane, motion of the pelvis was observed to lag slightly behind that of the thorax. However, it is possible that the close coupling between these two segments facilitates the thoracic rotation required to passively drive arm motion. This is the first study to provide a full biomechanical rationale for the coordination of the spine and pelvis during human running. This insight should help clinicians develop an improved understanding of how spinal and pelvic motions may contribute to, or result from, common running injuries.     

Blocking Perturbations and Multilimb Control in Rapid-Aiming Movements

The effect of an unexpected mechanical block on the control of multilimbaiming movements was studied in two experiments. In the first experiment, subjects (N = 10) attempted to push two hand levers 9 cm forward in 200 ms without vision. In the second experiment, subjects (N = 9) attempted to push hand levers and foot pedals forward 9 cm in 200 ms. After a practice period, five attempts at blocking the limb movements were made on the left lever and the right lever (Experiment 1) and on both levers (Experiment 2 only) during randomly selected trials. When one hand was blocked in Experiment 1, the other hand undershot the target on the first blocked trial, with slight reductions in movement time. When one hand was blocked in Experiment 2, the contralateral limb undershot the target on all blocked trials, but this had little effect on the lower limbs. The lower limbs undershot the target when both up- per limbs were blocked. Discrete movement corrections were made on more of the blocked trials relative to the unblocked control trials. Interlimb correlations decreased following the block, suggesting that subjects dissociated the limbs in an effort to minimize the effect of the block.     

Mass perturbation of a body segment: 2. Effects on interlimb coordination

The shifts in relative phase that are observed when rhythmically coordinated limbs are submitted to asymmetric mass perturbations have typically been attributed to the induced eigenfrequency difference (delta omega) between limbs. Modeling the moving limbs as forced linear oscillators, however, reveals that asymmetric mass perturbations may induce a difference not only in eigenfrequency (i.e., delta omega not equal 0) but also in the covarying low-frequency control gains (i.e., delta k not equal 0). Because the inverse of the low-frequency control gain (k) reflects the level of muscular torque (input) required for a particular displacement from equilibrium (output), asymmetric mass perturbations may result in an imbalance in the muscular torques required for task performance (related to delta k not equal 0). Thus, it is possible that the effects attributed to delta omega were in fact mediated by delta k. In 2 experiments, the authors manipulated delta k and delta omega separately by applying mass perturbations to the lower legs of 9 participants. The relative phasing between the legs was not affected by delta k, but manipulation of delta omega (while delta k remained approximately 0) induced systematic relative phase shifts that were more pronounced for antiphase than for in-phase coordination. That indication that the coordination dynamics is indeed influenced by an imbalance in eigenfrequency is discussed vis-a-vis the question of how such a merely peripheral property may affect the underlying coordination process.     

Age-related differences in the integration of sensory information during the execution of a bimanual coordination task

Young (n = 7) and elderly (n = 7) subjects performed bimanual coordination patterns in the transverse plane according to the in-phase or antiphase mode. Sensory information was manipulated through visual (with or without vision of the limbs) and proprioceptive input (with or without vibratory stimuli on one limb). Movement patterns with vibrations showed higher deviations from the intended relative phase than did those without vibrations. This finding suggests that the proprioceptive information induced by the vibrations and the movement interfered, leading to a disruption of the coordination patterns. In addition, as compared with the elderly, the young subjects performed more stable movements under normal circumstances but were more strongly affected by vibratory stimuli during the performance of in-phase movements. During antiphase movements, both age groups experienced a decrease of pattern stability. Furthermore, the absence or presence of visual feedback influenced the performance of the young subjects more than that of the elderly. The presence of vision led to stable in-phase movements, whereas a decrease of pattern stability was observed for antiphase movements. In general, these results demonstrate that manipulation of feedback sources affects young subjects more than elderly ones, and this can be related to a reduced sensory sensitivity as a function of aging.     

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

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

The Timing of Intralimb Coordination

The authors examined the manner in which self-selected movement frequencies are impacted upon by repeated engagement in an intralimb coordination task and by alterations in the inertial characteristics of the limb. Twelve healthy adult volunteers rhythmically flexed and extended their elbow and wrist joints at a comfortable self-established frequency in 1 of 2 modes of coordination (in-phase and antiphase), while grasping 1 of 3 weighted dowels (no-weight condition [0.03 kg], light weight condition [0.5 kg], heavy weight condition [1.0 kg]). The movement frequencies adopted by subjects on the 3rd of 3 weekly sessions, following more than 120 experimental trials, were appreciably higher than those obtained during an initial session. The addition of mass to the system had an inconsistent influence upon the preferred frequency of movement. When subjects' limbs were loaded with what was deemed to be a light weight (0.5 kg), the movement frequencies that were adopted were indistinguishable from those selected when there was no (0.03 kg) loading of the limbs. In contrast, when subjects' limbs were loaded with a relatively heavy weight (1 kg), the resulting self-selected movement frequencies were reliably lower than when there was no loading of the limbs. The adopted frequency of movement was also influenced in a reliable fashion by the mode of coordination in which the movements were prepared. Those results are discussed with reference to mechanical and neuromuscular constraints on coordination dynamics.     

Symmetry constraints mediate the learning and transfer of bimanual coordination patterns across planes of motion

The authors investigated whether neuromuscular and directional constraints are dissociable limitations that affect learning and transfer of a bimanual coordination pattern. Participants (N = 9) practiced a 45 degrees muscular relative phasing pattern in the transverse plane over 4 days. The corresponding to-be-learned spatial relative phasing was 225 degrees. Before, during, and following practice, the authors administered probe tests in the sagittal plane to assess transfer of learning. In the probe tests, participants performed various patterns characterized by different muscular and spatial relative phasing (45 degrees, 45 degrees, 45 degrees, 225 degrees, 225 degrees, 45 degrees, and 225 degrees, 225 degrees). The acquisition of the to-be-learned pattern in the transverse plane resulted in spontaneous positive transfer of learning only to coordination patterns having 45 degrees of spatial relative phase, irrespective of muscular phasing. Moreover, transfer occurred in the sagittal plane to coordination patterns that had symmetry properties similar to those of the to-be-learned pattern. The authors conclude that learning and transfer of spatial features of coordination patterns from the transverse to the sagittal plane of motion are mediated by mirror-symmetry constraints.     

Mass Perturbation of a Body Segment: 2. Effects on Interlimb Coordination

The shifts in relative phase that are observed when rhythmically coordinated limbs are submitted to asymmetric mass perturbations have typically been attributed to the induced eigenfrequency difference ($DL$oM) between the limbs. Modeling the moving limbs as forced linear oscillators, however, reveals that asymmetric mass perturbations may induce a difference not only in eigenfrequency (i.e., $DL$oM $$ 0) but also in the covarying low-frequency control gains (i.e., $DLk $$ 0). Because the inverse of the lowfrequency control gain (k) reflects the level of muscular torque (input) required for a particular displacement from equilibrium (output), asymmetric mass perturbations may result in an imbalance in the muscular torques required for task performance (related to $DLk $$ 0). Thus, it is possible that the effects attributed to $DL$oM were in fact mediated by $DLk. In 2 experiments, the authors manipulated $DLk and $DL$oM separately by applying mass perturbations to the lower legs of 9 participants. The relative phasing between the legs was not affected by $DLk, but manipulation of $DL$oM (while $DLk remained approximately 0) induced systematic relative phase shifts that were more pronounced for antiphase than for in-phase coordination. That indication that the coordination dynamics is indeed influenced by an imbalance in eigenfrequency is discussed vis-à-vis the question of how such a merely peripheral property may affect the underlying coordination process.     

Asymmetries in Coupling Dynamics of Perception and Action

The influence of information-based dynamics on coordination dynamics of rhythmic movement was examined with special reference to the expression of asymmetries. In Experiment 1, right-handed subjects performed unimanual, rhythmical movements in coordination with either a discrete or continuous visual display. The right hand-visual display system defined a more stable perception-action collective than the left, particularly when continuous visual information was available. In Experiment 2, the same subjects performed rhythmic bimanual movements in coordination with a continuous visual display. The action collective was inherently more stable than the perception-action collective, although similar patterns were observed at both levels. Importantly, the dynamics of the perception-action collective impinged upon the dynamics of the action collective in terms of stability. Asymmetries remained evident between limbs in the bimanual preparations, with the left hand exhibiting greater limit-cycle variability and also a tendency to more often effect transitions at the action couple. Features of dynamical models that capture characteristics of manual asymmetries are discussed.     

Auditory clicks distort perceived velocity but only when the system has to rely on extraretinal signals

Previous work has found that repetitive auditory stimulation (click trains) increases the subjective velocity of subsequently presented moving stimuli. We ask whether the effect of click trains is stronger for retinal velocity signals (produced when the target moves across the retina) or for extraretinal velocity signals (produced during smooth pursuit eye movements, when target motion across the retina is limited). In Experiment 1, participants viewed leftward or rightward moving single dot targets, travelling at speeds from 7.5 to 17.5 deg/s. They estimated velocity at the end of each trial. Prior presentation of auditory click trains increased estimated velocity, but only in the pursuit condition, where estimates were based on extraretinal velocity signals. Experiment 2 generalized this result to vertical motion. Experiment 3 found that the effect of clicks during pursuit disappeared when participants tracked across a visually textured background that provided strong local motion cues. Together these results suggest that auditory click trains selectively affect extraretinal velocity signals. This novel finding suggests that the cross-modal integration required for auditory click trains to influence subjective velocity operates at later stages of processing.