Perception and action influences on discrete and reciprocal bimanual coordination

Why motor imagery is not really motoric: towards a re-conceptualization in terms of effect-based action control

Overt and imagined action seem inextricably linked. Both have similar timing, activate shared brain circuits, and motor imagery influences overt action and vice versa. Motor imagery is, therefore, often assumed to recruit the same motor processes that govern action execution, and which allow one to play through or simulate actions offline. Here, we advance a very different conceptualization. Accordingly, the links between imagery and overt action do not arise because action imagery is intrinsically motoric, but because action planning is intrinsically imaginistic and occurs in terms of the perceptual effects one want to achieve. Seen like this, the term 'motor imagery' is a misnomer of what is more appropriately portrayed as 'effect imagery'. In this article, we review the long-standing arguments for effect-based accounts of action, which are often ignored in motor imagery research. We show that such views provide a straightforward account of motor imagery. We review the evidence for imagery-execution overlaps through this new lens and argue that they indeed emerge because every action we execute is planned, initiated and controlled through an imagery-like process. We highlight findings that this new view can now explain and point out open questions.

The effect of inherent and incidental constraints on bimanual force control in simulated Martian gravity

The ability to coordinate actions between the limbs is important for many operationally relevant tasks associated with space exploration. A future milestone in space exploration is sending humans to Mars. Therefore, an experiment was designed to examine the influence of inherent and incidental constraints on the stability characteristics associated with the bimanual control of force in simulated Martian gravity. A head-up tilt (HUT)/head-down tilt (HDT) paradigm was used to simulate gravity on Mars (22.3° HUT). Right limb dominant participants (N = 11) were required to rhythmically coordinate patterns of isometric forces in 1:1 in-phase and 1:2 multifrequency patterns by exerting force with their right and left limbs. Lissajous displays were provided to guide task performance. Participants performed 14 twenty-second practice trials at 90° HUT (Earth). Following a 30-min rest period, participants performed 2 test trials for each coordination pattern in both Earth and Mars conditions. Performance during the test trials were compared. Results indicated very effective temporal performance of the goal coordination tasks in both gravity conditions. However, results indicated differences associated with the production of force between Earth and Mars. In general, participants produced less force in simulated Martian gravity than in the Earth condition. In addition, force production was more harmonic in Martian gravity than Earth gravity for both limbs, indicating that less force distortions (adjustments, hesitations, and/or perturbations) occurred in the Mars condition than in the Earth condition. The force coherence analysis indicated significantly higher coherence in the 1:1 task than in the 1:2 task for all force frequency bands, with the highest level of coherence in the 1-4 Hz frequency band for both gravity conditions. High coherence in the 1-4 Hz frequency band is associated with a common neural drive that activates the two arms simultaneously and is consistent with the requirements of the two tasks. The results also support the notion that neural crosstalk stabilizes the performance of the 1:1 in-phase task. In addition, significantly higher coherence in the 8-12 Hz frequency bands were observed for the Earth condition than the Mars condition. Force coherence in the 8-12 Hz bands is associated with the processing of sensorimotor information, suggesting that participants were better at integrating visual, proprioceptive, and/or tactile feedback in Earth than for the Mars condition. Overall, the results indicate less neural interference in Martian gravity; however, participants appear to be more effective at using the Lissajous displays to guide performance under Earth's gravity.

Goal conceptualization has distinct effects on spatial and temporal bimanual coordination after left- and right- hemisphere stroke

Perception of task goal influences motor performance and coordination. In bimanual actions, it is unclear how one's perception of task goals influences bimanual coordination and performance in individuals with unilateral stroke. We characterized inter-limb coordination differences in individuals with chronic right- and left-hemisphere damaged (RCVA: n = 24, LCVA: n = 24) stroke and age-matched neurotypical controls (n = 24) as they completed bimanual reaching tasks under distinct goal conditions. In the dual-goal condition, participants reached to move two virtual bricks (cursors) assigned to each hand toward independent targets. In the common-goal condition, they moved a central common virtual brick representing both hands to a single, central target. Spatial and temporal coordination (cross-correlation coefficients of hand velocity and their time-lag), the redundant axis deviations (the hand deviations in the axis orthogonal to the axis along the cursor-target direction), and the contribution ratio of the paretic hand were measured. Compared to the dual-goal condition, reaching actions to the common-goal demonstrated better spatial bimanual coordination in all three participant groups. Temporal coordination was better during common-goal than dual-goal actions only for the LCVA group. Additionally, and novel to this field, sex, as a biological variable, differently influenced movement time and redundant axis deviation in participants with stroke under the common-goal condition. Specifically, female stroke survivors showed larger movements in the redundant axes and, consequently, longer movement times, which was more prominent in the LCVA group. Our results indicate that perception of task goals influences bimanual coordination, with common goal improving spatial coordination in neurotypical individuals and individuals with unilateral stroke and providing additional advantage for temporal coordination in those with LCVA. Sex influences bimanual performance in stroke survivors and needs to be considered in future investigations.

Restart errors reaction time of a two-step inhibition process account for the violation of the race model's independence in multi-effector selective stop signal task

Goal-oriented actions often require the coordinated movement of two or more effectors. Sometimes multi-effector movements need to be adjusted according to a continuously changing environment, requiring stopping an effector without interrupting the movement of the others. This form of control has been investigated by the selective Stop Signal Task (SST), requiring the inhibition of an effector of a multicomponent action. This form of selective inhibition has been hypothesized to act through a two-step process, where a temporary global inhibition deactivating all the ongoing motor responses is followed by a restarting process that reactivates only the moving effector. When this form of inhibition takes place, the reaction time (RT) of the moving effector pays the cost of the previous global inhibition. However, it is poorly investigated if and how this cost delays the RT of the effector that was required to be stopped but was erroneously moved (Stop Error trials). Here we measure the Stop Error RT in a group of participants instructed to simultaneously rotate the wrist and lift the foot when a Go Signal occurred, and interrupt both movements (non-selective Stop version) or only one of them (selective Stop version) when a Stop Signal was presented. We presented this task in two experimental conditions to evaluate how different contexts can influence a possible proactive inhibition on the RT of the moving effector in the selective Stop versions. In one context, we provided the foreknowledge of the effector to be inhibited by presenting the same selective or non-selective Stop versions in the same block of trials. In a different context, while providing no foreknowledge of the effector(s) to be stopped, the selective and non-selective Stop versions were intermingled, and the information on the effector to be stopped was delivered at the time of the Stop Signal presentation. We detected a cost in both Correct and Error selective Stop RTs that was influenced by the different task conditions. Results are discussed within the framework of the race model related to the SST, and its relationship with a restart model developed for selective versions of this paradigm.

Investigating the role of contextual cues and interhemispheric inhibitory mechanisms in response-selective stopping: a TMS study

Response-selective stopping requires cancellation of only one component of a multicomponent action. While research has investigated how delays to the continuing action components ("stopping interference") can be attenuated by way of contextual cues of the specific stopping demands ("foreknowledge"), little is known of the underlying neural mechanisms. Twenty-seven, healthy, young adults undertook a multicomponent stop-signal task. For two thirds of trials, participants responded to an imperative (go) stimulus (IS) with simultaneous button presses using their left and right index fingers. For the remaining one third of trials, the IS was followed by a stop-signal requiring cancellation of only the left, or right, response. To manipulate foreknowledge of stopping demands, a cue preceded the IS that informed participants which hand might be required to stop (proactive) or provided no such information (reactive). Transcranial magnetic stimulation (TMS) assessed corticospinal excitability (CSE) as well as short- and long-interval interhemispheric inhibition (SIHI, LIHI) between the primary motor cortices. Proactive cues reduced, but did not eliminate, stopping interference relative to the reactive condition. Relative to TMS measures at cue onset, decreases in CSE (both hands and both cue conditions) and LIHI (both hands, proactive condition only) were observed during movement preparation. During movement cancellation, LIHI reduction in the continuing hand was greater than that in the stopping hand and greater than LIHI reductions in both hands during execution of multicomponent responses. Our results indicate that foreknowledge attenuates stopping interference and provide evidence for a novel role of LIHI, mediated via prefrontal regions, in facilitating continuing action components.

The influence of distal and proximal muscle activation on neural crosstalk

Previous research has indicated that neural crosstalk is asymmetric, with the dominant effector exerting a stronger influence on the non-dominant effector than vice versa. Recently, it has been hypothesized that this influence is more substantial for proximal than distal effectors. The current investigation was designed to determine the effects of distal ((First Dorsal Interosseous (FDI)) and proximal (triceps brachii (TBI)) muscle activation on neural crosstalk. Twelve right-limb dominant participants (mean age = 21.9) were required to rhythmically coordinate a 1:2 pattern of isometric force guided by Lissajous displays. Participants performed 10, 30 s trials with both distal and proximal effectors. Coherence between the two effector groups were calculated using EMG-EMG wavelet coherence. The results indicated that participants could effectively coordinate the goal coordination pattern regardless of the effectors used. However, spatiotemporal performance was more accurate when performing the task with distal than proximal effectors. Force distortion, quantified by harmonicity, indicated that more perturbations occurred in the non-dominant effector than in the dominant effector. The results also indicated significantly lower harmonicity for the non-dominant proximal effector compared to the distal effectors. The current results support the notion that neural crosstalk is asymmetric in nature and is greater for proximal than distal effectors. Additionally, the EMG-EMG coherence results indicated significant neural crosstalk was occurring in the Alpha bands (5-13 Hz), with higher values observed in the proximal condition. Significant coherence in the Alpha bands suggest that the influence of neural crosstalk is occurring at a subcortical level.

Bimanual coordination during reach-to-grasp actions is sensitive to task goal with distinctions between left- and right-hemispheric stroke

The perceptual feature of a task such as how a task goal is perceived influences performance and coordination of bimanual actions in neurotypical adults. To assess how bimanual task goal modifies paretic and non-paretic arm performance and bimanual coordination in individuals with stroke affecting left and right hemispheres, 30 participants with hemispheric stroke (15 right-hemisphere damage-RHD); 15 left-hemisphere damage-LHD) and 10 age-matched controls performed reach-to-grasp and pick-up actions under bimanual common-goal (i.e., two physically coupled dowels), bimanual independent-goal (two physically uncoupled dowels), and unimanual conditions. Reach-to-grasp time and peak grasp aperture indexed motor performance, while time lags between peak reach velocities, peak grasp apertures, and peak pick-up velocities of the two hands characterized reach, grasp, and pick-up coordination, respectively. Compared to unimanual actions, bimanual actions significantly slowed non-paretic arm speed to match paretic arm speed, thus affording no benefit to paretic arm performance. Detriments in non-paretic arm performance during bimanual actions was more pronounced in the RHD group. Under common-goal conditions, movements were faster with smaller peak grasp apertures compared to independent-goal conditions for all groups. Compared to controls, individuals with stroke demonstrated poor grasp and pick-up coordination. Of the patient groups, patients with LHD showed more pronounced deficits in grasp coordination between hands. Finally, grasp coordination deficits related to paretic arm motor deficits (upper extremity Fugl-Meyer score) for LHD group, and to Trail-Making Test performance for RHD group. Findings suggest that task goal and distinct clinical deficits influence bimanual performance and coordination in patients with left- and right-hemispheric stroke.

Instructions on Task Constraints Mediate Perceptual-Motor Search and How Movement Variability Relates to Performance Outcome

Movement variability has been postulated to afford perception of the perceptual motor workspace and to be directly linked to improved performance. Here, we investigated how instructions mediate the search process and the relation between performance outcome and movement variability. We used a novel bimanual force tracking task where zero error was achieved via proportional output between the hands. Participants were either instructed or not as to how to coordinate their force output to achieve this goal, but the goal to minimize error was explained to all participants. The provision of instructions restricted the overall area of the task space that was searched. Moreover, the time dependent properties of the search were influenced; where instructions increased the likelihood that participants would produce a higher force level over practice. Multiple regression revealed that variability was positively correlated with performance outcome, but the strength of this relation was dependent on instructions and individual search strategies. The findings are consistent with the view that information through instructions shapes individual emergent perceptual-motor search strategies that in turn mediate how movement variability relates to performance outcome.

The Influence of Altered-Gravity on Bimanual Coordination: Retention and Transfer

Many of the activities associated with spaceflight require individuals to coordinate actions between the limbs (e.g., controlling a rover, landing a spacecraft). However, research investigating the influence of gravity on bimanual coordination has been limited. The current experiment was designed to determine an individual's ability to adapt to altered-gravity when performing a complex bimanual force coordination task, and to identify constraints that influence coordination dynamics in altered-gravity. A tilt table was used to simulate gravity on Earth [90° head-up tilt (HUT)] and microgravity [6° head-down tilt (HDT)]. Right limb dominant participants (N = 12) were required to produce 1:1 in-phase and 1:2 multi-frequency force patterns. Lissajous information was provided to guide performance. Participants performed 14, 20 s trials at 90° HUT (Earth). Following a 30-min rest period, participants performed, for each coordination pattern, two retention trials (Earth) followed by two transfer trials in simulated microgravity (6° HDT). Results indicated that participants were able to transfer their training performance during the Earth condition to the microgravity condition with no additional training. No differences between gravity conditions for measures associated with timing (interpeak interval ratio, phase angle slope ratio) were observed. However, despite the effective timing of the force pulses, there were differences in measures associated with force production (peak force, STD of peak force mean force). The results of this study suggest that Lissajous displays may help counteract manual control decrements observed during microgravity. Future work should continue to explore constraints that can facilitate or interfere with bimanual control performance in altered-gravity environments.

Decoupling countermands nonselective response inhibition during selective stopping

Response inhibition is essential for goal-directed behavior within dynamic environments. Selective stopping is a complex form of response inhibition where only part of a multi-effector response must be cancelled. A substantial response delay emerges on unstopped effectors when a cued effector is successfully stopped. This stopping-interference effect is indicative of nonselective response inhibition during selective stopping which may, in-part, be a consequence of functional coupling. The present study examined selective stopping of (de)coupled bimanual responses in healthy human participants of either sex. Participants performed synchronous and asynchronous versions of an anticipatory stop-signal paradigm across two sessions while mu (µ) and beta (β) rhythm were measured with electroencephalography. Results showed that responses were behaviorally decoupled during asynchronous go trials and the extent of response asynchrony was associated with lateralized sensorimotor µ and β desynchronization during response preparation. Selective stopping produced a stopping-interference effect and was marked by a nonselective increase and subsequent rebound in prefrontal and sensorimotor β. In support of the coupling account, stopping-interference was smaller during selective stopping of asynchronous responses, and negatively associated with the magnitude of decoupling. However, the increase in sensorimotor β during selective stopping was equivalent between the stopped and unstopped hand irrespective of response synchrony. Overall, the findings demonstrate that decoupling facilitates selective stopping after a global pause process and emphasizes the importance of considering the influence of both the go and stop context when investigating response inhibition.

The effect of inherent and incidental constraints on bimanual and social coordination

The current investigation was designed to examine the influence of inherent and incidental constraints on the stability characteristics associated with bimanual and social coordination. Individual participants (N = 9) and pairs of participants (N = 18, 9 pairs) were required to rhythmically coordinate patterns of isometric forces in 1:1 in-phase and 1:2 multi-frequency patterns by exerting force with their right and left limbs. Lissajous information was provided to guide performance. Participants performed 13 practice trials and 1 test trial per pattern. On the test trial, muscle activity from the triceps brachii muscles of each arm was recorded. EMG-EMG coherence between the two EMG signals was calculated using wavelet coherence. The behavioral data indicated that individual participants performed the 1:1 in-phase pattern more accurately and with less variability than paired participants. The EMG coherence analysis indicated significantly higher coherence for individual participants than for the paired participants during the 1:1 in-phase pattern, whereas no differences were observed between groups for the 1:2 coordination pattern. The results of the current investigation support the notion that neural crosstalk can stabilize 1:1 in-phase coordination when contralateral and ipsilateral signals are integrated via the neuromuscular linkage between two effectors.

Bimanual coordination associated with left- and right-hand dominance: testing the limb assignment and limb dominance hypothesis

In an experiment conducted by Kennedy et al. (Exp Brain Res 233:181–195, 2016), dominant right-handed individuals were required to produce a rhythm of isometric forces in a 2:1 or 1:2 bimanual coordination pattern. In the 2:1 pattern, the left limb performed the faster rhythm, while in the 1:2 pattern, the right limb produced the faster pattern. In the 1:2 pattern, interference occurred in the limb which had to produce the slower rhythm of forces. However, in the 2:1 condition, interference occurred in both limbs. The conclusion was that interference was not only influenced by movement frequency, but also influenced by limb dominance. The present experiment was designed to replicate these findings in dynamic bimanual 1:2 and 2:1 tasks where performers had to move one wrist faster than the other, and to determine the influence of limb dominance. Dominant left-handed (N = 10; LQ = − 89.81) and dominant right-handed (N = 14; LQ = 91.25) participants were required to perform a 2:1 and a 1:2 coordination pattern using Lissajous feedback. The harmonicity value was calculated to quantify the interference in the trial-time series. The analysis demonstrated that regardless of limb dominance, harmonicity was always lower in the slower moving limb than in the faster moving limb. The present results indicated that for dominant left- and dominant right-handers the faster moving limb influenced the slower moving limb. This is in accordance with the assumption that movement frequency has a higher impact on limb control in bimanual 2:1 and 1:2 coordination tasks than handedness.

The influence of accuracy constraints on bimanual and unimanual sequence learning

An experiment was designed to determine whether accuracy constraints can influence how unimanual and bimanual motor sequences are produced and learned. The accuracy requirements of the task were manipulated using principles derived from Fitts' Law to create relatively low (ID = 3) and high (ID = 5) accuracy demands. Right-limb dominant participants (N = 28, age = 21.9 yrs; 15 females and 13 males) were required to produce unimanual left, unimanual right or bimanual movement sequences using elbow extension and flexion movements to hit a series of illuminated targets. The targets were illuminated in a repeating sequence of 16 elements. Participants performed 20 practice trials. Thirty minutes following the practice trials participants performed a retention test. Element duration (time interval between target hits) and segment harmonicity (hesitations/adjustments in movement pattern) were calculated. The results indicate longer element duration and lower harmonicity values (more adjustments) when the task required higher accuracy demands (ID = 5) compared to low accuracy demands (ID = 3). Element duration was shorter and harmonicity was higher at ID = 5 for both unimanual groups than the bimanual group. However, element duration was shorter and harmonicity was higher at ID = 3 for the bimanual group than for both unimanual groups. These results indicate that the accuracy demands of the task can influence both performance and learning of motor sequences and suggest differences between unimanual and bimanual motor sequence learning. It appears there is a bimanual advantage for tasks with lower accuracy demands whereas performance is more accurate with unimanual performance, regardless of limb, with higher accuracy demands. These results are consistent with recent research indicating that accuracy requirements change the control processes for bimanual performance differently than for unimanual tasks.

Effects of practice on a mechanical horse with an online feedback on performing a sitting postural coordination

The present research aims at quantifying the impact of practicing a new coordination pattern with an online visual feedback on the postural coordination performed on a mechanical horse. Forty-four voluntary participants were recruited in this study. They were randomly assigned to four practice groups based on i) with or without feedback (i.e., group 1, control, did not receive the feedback; group 2, 3 and 4 received an online feedback during practice) and ii) the specific trunk/horse coordination to target during practice (group 1, target coordination = 180° (without feedback); group 2, target coordination = 0°; group 3, target coordination = 90°; group 4, target coordination = 180°). All participants performed pre-, practice, post- and retention sessions. The pre-, post- and retention sessions consisted of four trials, with one trial corresponding to one specific target coordination to maintain between their own oscillations and the horse oscillations (spontaneous, 0°, 90°, and 180°). The practice phase was composed of three different sessions during which participants received an online feedback about the coordination between their own oscillations and the horse oscillations. Results showed a significant change with practice in the trunk/horse coordination patterns which persisted even after one month (retention-test). However, all the groups did not show the same nature of change, evidenced by a high postural variability during post-test for 0° and 90° target coordination groups, in opposition to the 180° and spontaneous groups who showed a decrease in coordination variability for the 180° group. The coordination in anti-phase was characterized as spontaneously adopted by participants on the mechanical horse, explaining the ease of performing this coordination (compared to the 0° and 90° target coordination). The effect of online visual feedback appeared not only on the coordination pattern itself, but most importantly on its variability during practice, including concerning initially stable coordination patterns.

Towards an Understanding of Control of Complex Rhythmical "Wavelike" Coordination in Humans

How does the human neurophysiological system self-organize to achieve optimal phase relationships among joints and limbs, such as in the composite rhythms of butterfly and front crawl swimming, drumming, or dancing? We conducted a systematic review of literature relating to central nervous system (CNS) control of phase among joint/limbs in continuous rhythmic activities. SCOPUS and Web of Science were searched using keywords “Phase AND Rhythm AND Coordination”. This yielded 1039 matches from which 23 papers were extracted for inclusion based on screening criteria. The empirical evidence arising from in-vivo, fictive, in-vitro, and modelling of neural control in humans, other species, and robots indicates that the control of movement is facilitated and simplified by innervating muscle synergies by way of spinal central pattern generators (CPGs). These typically behave like oscillators enabling stable repetition across cycles of movements. This approach provides a foundation to guide the design of empirical research in human swimming and other limb independent activities. For example, future research could be conducted to explore whether the Saltiel two-layer CPG model to explain locomotion in cats might also explain the complex relationships among the cyclical motions in human swimming.

Stability of bimanual finger tapping coordination is constrained by salient phases

In bimanual cyclical continuous movements, the relative timing of the most salient movement phase in each movement is a predominant constraint. This is the case for coordination when both movements have a single most salient phase (the relative-salience hypothesis). We tested whether the relative-salience hypothesis could explain results obtained for repetitive discrete movements, utilizing finger tapping. In experiment 1, participants performed unimanual alternate two-finger tapping with the metronome beat (i.e., one finger taps on the beat and the other finger taps off the beat). The stability of the tapping timing relative to the beat, which reflects the extent of salience, was higher in the index finger than the middle finger, and was lower in the ring finger than the middle finger. In experiment 2, participants performed four conditions of repetitive bimanual four-finger tapping (i.e., alternate two-finger tapping in each hand) without external pacing signals. Under all four conditions, a more stable pattern occurred when the timing of the more salient tapping in each hand was simultaneous rather than alternate, regardless of relative direction in the external space or movement coupling of the homologous fingers. The results indicated that bimanual four-finger tapping could be explained by the relative-salience hypothesis.

Accessing interpersonal and intrapersonal coordination dynamics

Both intrapersonal and interpersonal coordination dynamics have traditionally been investigated using relative phase patterns of in-phase (ϕ = 0°) and/or anti-phase (ϕ = 180°). Numerous investigations have demonstrated that coordination tasks that require other relative phase patterns (e.g., 90°) are difficult or near impossible to perform without extended practice. Recent findings, however, have demonstrated that an individual can produce a wide range of intrapersonal bimanual patterns within a few minutes of practice when provided integrated feedback. The present experiment was designed to directly compare intra- and interpersonal coordination performance and variability when provided Lissajous feedback or pacing metronome. Single participants (N = 12) and pairs of participants (N = 24, 12 pairs) were required to produce relative phase patterns between 0° and 180° in 30° increments using either pacing metronomes or Lissajous displays. The Lissajous displays involved a goal template and a cursor providing integrated feedback regarding the position of the two effectors. The results indicated both single and pairs of participants could effectively produce a large range of coordination patterns that typically act as repellers after only 6 min of practice when provided integrated feedback. However, single participants performed the in-phase coordination pattern more accurately and with less variability than paired participants, regardless of the feedback condition. These results suggest an advantage for intrapersonal coordination when performing in-phase coordination, possibly due to the stabilizing effect occurring via the neuro-muscular linkage between effectors.

Bimanual control strategies

Two tasks (A and B) were designed which required participants to sequentially move through four target positions in a Lissajous display. Task A was designed so that participants could complete the task using either unimanual or bimanual control strategies. Task B was designed so that participants could complete the task using relatively simple or more complex bimanual control strategies. The purpose of this study was to determine which control strategy the participant utilises to complete the two tasks when Lissajous displays are provided and to determine the degree to which the size of the targets influences the control strategy chosen under these conditions. The movement amplitude between two adjacent targets and the target size resulted in an Index of Difficulty (ID) of 2 and 4 for each task. For both tasks, participants practised 15 trials (30 s per trial) for each ID and then was administered a test trial. The results for both Tasks A and B indicated that the ID2 condition resulted in a circular path, whereas the ID4 condition resulted in a straight-line path on the Lissajous plot. This suggests that at the low ID condition participants produced a continuous 1:1 with 90° phase offset bimanual coordination pattern. At the high ID condition, the participants consistently chose to switch to a more stable unimanual left and right movements in Task A and to transition between in-phase and anti-phase bimanual coordination patterns in Task B. In addition, both limbs' movements were more harmonic in the low ID condition than in the high ID condition.

Does motor expertise facilitate amplitude differentiation of lower limb-movements in an asymmetrical bipedal coordination task?

The motor system's natural tendency is to move the limbs over equal amplitudes, for example in walking. However, in many situations in which people must perform complex movements, a certain degree of amplitude differentiation of the limbs is required. Visual and haptic feedback have recently been shown to facilitate such independence of limb movements. However, it is unknown whether motor expertise moderates the extent to which individuals are able to differentiate the amplitudes of their limb-movements while being supported with visual and haptic feedback. To answer this question 14 pre-professional dancers were compared to 14 non-dancers on simultaneously generating a small displacement with one foot, and a larger one with the other foot, in four different feedback conditions. In two conditions, haptic guidance was offered, either in a passive or active mode. In the other two conditions, veridical and enhanced visual feedback were provided. Surprisingly, no group differences were found regarding the degree to which the visual or haptic feedback assisted the generation of the different target amplitudes of the feet (mean amplitude difference between the feet). The correlation between the displacements of the feet and the standard deviation of the continuous relative phase between the feet, reflecting the degree of independence of the feet movements, also failed to show between-group differences. Sample entropy measures, indicating the predictability of the foot movements, did show a group difference. In the haptically-assisted conditions, the dancers demonstrated more predictable coordination patterns than the non-dancers as reflected by lower sample entropy values whereas the reverse was true in the visual-feedback conditions. The results demonstrate that motor expertise does not moderate the extent to which haptic tracking facilitates the differentiation of the amplitudes of the lower limb movements in an asymmetrical bipedal coordination task.

Bimanual coordination: A missing piece of arm rehabilitation after stroke

Inability to use the arm in daily actions significantly lowers quality of life after stroke. Most contemporary post-stroke arm rehabilitation strategies that aspire to re-engage the weaker arm in functional activities have been greatly limited in their effectiveness. Most actions of daily life engage the two arms in a highly coordinated manner. In contrast, most rehabilitation approaches predominantly focus on restitution of the impairments and unilateral practice of the weaker hand alone. We present a perspective that this misalignment between real world requirements and intervention strategies may limit the transfer of unimanual capability to spontaneous arm use and functional recovery. We propose that if improving spontaneous engagement and use of the weaker arm in real life is the goal, arm rehabilitation research and treatment need to address the coordinated interaction between arms in targeted theory-guided interventions. Current narrow focus on unimanual deficits alone, difficulty in quantifying bimanual coordination in real-world actions and limited theory-guided focus on control and remediation of different coordination modes are some of the biggest obstacles to successful implementation of effective interventions to improve bimanual coordination in the real world. We present a theory-guided taxonomy of bimanual actions that will facilitate quantification of coordination for different real-world tasks and provide treatment targets for addressing coordination deficits. We then present evidence in the literature that points to bimanual coordination deficits in stroke survivors and demonstrate how current rehabilitation approaches are limited in their impact on bimanual coordination. Importantly, we suggest theory-based areas of future investigation that may assist quantification, identification of neural mechanisms and scientifically-based training/remediation approaches for bimanual coordination deficits post-stroke. Advancing the science and practice of arm rehabilitation to incorporate bimanual coordination will lead to a more complete functional recovery of the weaker arm, thus improving the effectiveness of rehabilitation interventions and augmenting quality of life after stroke.

Effect of the loci of visual cues on ability of polyrhythmic bimanual movement

The present study investigated whether the deviation of the performed movement cycle from the required cycle during polyrhythmic bimanual (BM) movement depends on the loci of the visual cues that guide the rhythm of finger movements. Twelve healthy right-handed males rhythmically abducted and adducted the index finger or index fingers with the rhythm of the visual cues. During UM movement, the visual cue guiding the rhythm of finger movement was provided in the left or right visual hemifield. During 2:3 polyrhythmic BM movement, two visual cues, one guiding the rhythm of the left finger movement and another guiding the rhythm of the right finger movement, were provided in a single visual hemifield, or each visual cue guiding each finger movement was provided in each visual hemifield. During polyrhythmic BM movement, the cycle duration of the slower side of the movement guided by the rhythm of the visual cues provided in one visual hemifield was shorter than the required cycle duration, and the magnitude of the shortage in this condition was greater than that guided by each visual cue provided in each visual hemifield. Slower side of the movement is more precisely performed by each visual cue guiding each finger movement in each visual hemifield rather than that guided by visual cues provided in one visual hemifield during polyrhythmic BM movement. This may be explained by bottle-neck model in which visual information overflows the processing capacity when two visual processes are simultaneously provided in a single visual cortex.

The simplest acquisition protocol is sometimes the best protocol: performing and learning a 1:2 bimanual coordination task

An experiment was conducted to determine if the performance and learning of a multi-frequency (1:2) coordination pattern between the limbs are enhanced when a model is provided prior to each acquisition trial. Research has indicated very effective performance of a wide variety of bimanual coordination tasks when Lissajous plots with goal templates are provided, but this research has also found that participants become dependent on this information and perform quite poorly when it is withdrawn. The present experiment was designed to test three forms of modeling (Lissajous with template, Lissajous without template, and limb model), but in each situations, the model was presented prior to practice and not available during the performance of the task. This was done to decrease dependency on the model and increase the development of an internal reference of correctness that could be applied on test trials. A control condition was also collected, where a metronome was used to guide the movement. Following less than 7 min of practice, participants in the three modeling conditions performed the first test block very effectively; however, performance of the control condition was quite poor. Note that Test 1 was performed under the same conditions as used during acquisition. Test 2 was conducted with no augmented information provided prior to or during the performance of the task. Only participants in the limb model condition were able to maintain performance on Test 2. The findings suggest that a very simple intuitive display can provide the necessary information to form an effective internal representation of the coordination pattern which can be used guide performance when the augmented display is withdrawn.

Abstract spatial, but not body-related, visual information guides bimanual coordination

Visual spatial information is paramount in guiding bimanual coordination, but anatomical factors, too, modulate performance in bimanual tasks. Vision conveys not only abstract spatial information, but also informs about body-related aspects such as posture. Here, we asked whether, accordingly, visual information induces body-related, or merely abstract, perceptual-spatial constraints in bimanual movement guidance. Human participants made rhythmic, symmetrical and parallel, bimanual index finger movements with the hands held in the same or different orientations. Performance was more accurate for symmetrical than parallel movements in all postures, but additionally when homologous muscles were concurrently active, such as when parallel movements were performed with differently rather than identically oriented hands. Thus, both perceptual and anatomical constraints were evident. We manipulated visual feedback with a mirror between the hands, replacing the image of the right with that of the left hand and creating the visual impression of bimanual symmetry independent of the right hand’s true movement. Symmetrical mirror feedback impaired parallel, but improved symmetrical bimanual performance compared with regular hand view. Critically, these modulations were independent of hand posture and muscle homology. Thus, visual feedback appears to contribute exclusively to spatial, but not to body-related, anatomical movement coding in the guidance of bimanual coordination.

Intentional Switching Between Bimanual Coordination Patterns

Previous theoretical and empirical work indicates that intentional changes in a bimanual coordination pattern depends on the stability of the bimanual coordination pattern (Kelso, Schotz, & Schöner, 1988; Scholz & Kelso, 1990). The present experiments retest this notion when online Lissajous displays are provided. Switching to and from in-phase and antiphase and to and from 90° and 270° were tested in Experiment 1. Participants were able to very effectively produce the 180°, 90°, and 270° coordination patterns although performance of the in-phase coordination task was even more stable. The data indicated that switching to in-phase from antiphase was more rapid than vice versa and that switching times between 90° to 270° were similar. Experiment 2 investigated switching between 1:2 and 2:1 bimanual coordination patterns. The results indicated that switching time was similar between the 2:1 and 1:2 coordination tasks and that increases in stability over practice resulted in additional decreases in switching times. This provides additional evidence that the attractor landscape is fundamentally different dependent on the type of information provided the performer. What remains to be done is to reconcile these results with the various theories/perspectives currently used to describe and explain bimanual coordination.

Advantages of melodic over rhythmic movement sonification in bimanual motor skill learning

An important question for skill acquisition is whether and how augmented feedback can be designed to improve the learning of complex skills. Auditory information triggered by learners' actions, movement sonification, can enhance learning of a complex bimanual coordination skill, specifically polyrhythmic bimanual shape tracing. However, it is not clear whether the coordination of polyrhythmic sequenced movements is enhanced by auditory-specified timing information alone or whether more complex sound mappings, such as melodic sonification, are necessary. Furthermore, while short-term retention of bimanual coordination performance has been shown with movement sonification training, longer term retention has yet to be demonstrated. In the present experiment, participants learned to trace a diamond shape with one hand while simultaneously tracing a triangle with the other to produce a sequenced 4:3 polyrhythmic timing pattern. Two groups of participants received real-time auditory feedback during training: melodic sonification (individual movements triggered a separate note of a melody) and rhythmic sonification (each movement triggered a percussive sound), while a third control group received no augmented feedback. Task acquisition and performance in immediate retention were superior in the melodic sonification group as compared to the rhythmic sonification and control group. In a 24-h retention phase, a decline in performance in the melodic sonification group was reversed by brief playback of the target pattern melody. These results show that melodic sonification of movement can provide advantages over augmented feedback which only provides timing information by better structuring the sequencing of timed actions, and also allow recovery of complex target patterns of movement after training. These findings have important implications for understanding the role of augmented perceptual information in skill learning, as well as its application to real-world training or rehabilitation scenarios.

Effects of visual and auditory guidance on bimanual coordination complexity

Perceptual guidance of movement with simple visual or temporal information can facilitate performance of difficult coordination patterns. Guidance may override coordination constraints that usually limit stability of bimanual coordination to only in-phase and anti-phase. Movement dynamics, however, might not have the same characteristics with and without perceptual guidance. Do visual and auditory guidance produce qualitatively different dynamical organization of movement? An anti-phase wrist flexion and extension coordination task was performed under no specific perceptual guidance, under temporal guidance with a metronome, and under visual guidance with a Lissajous plot. For the time series of amplitudes, periods and relative phases, temporal correlations were measured with Detrended Fluctuation Analysis and complexity levels were measured with multiscale entropy. Temporal correlations of amplitudes and relative phases deviated from the typical 1/f variation towards more random variation under visual guidance. The same was observed for the series of periods under temporal guidance. Complexity levels for all time series were lower in visual guidance, but higher for periods under temporal guidance. Perceptual simplification of the task's goal may produce enhancement of performance, but it is accompanied by changes in the details of movement organization that may be relevant to explain dependence and poor retention after practice under guidance.

Life span changes: Performing a continuous 1:2 bimanual coordination task

The experiment was conducted to determine the influence of mirror movements in bimanual coordination during life span. Children, young adults, and older adults were instructed to perform a continuous 1:2 bimanual coordination task by performing flexion-extension wrist movements over 30s where symmetrical and non-symmetrical coordination patterns alternate throughout the trial. The vision of the wrists was covered and Lissajous-feedback was provided online. All age groups had to perform 10 trials under three different load conditions (0kg, .5kg, 1.0kg: order counterbalanced). Load was manipulated to determine if increased load increases the likelihood of mirror movements. The data indicated that the performance of the young adults was superior compared to the children and older adults. Children and older adults showed a stronger tendency to develop mirror movements and had particular difficulty in performing the non-symmetrical mode. This type of influence may be attributed to neural crosstalk.

Effects of integrated feedback on discrete bimanual movements in choice reaction time

The ability to coordinate the simultaneous movements of our arms is limited by a coalition of constraints. Some of these constraints can be overcome when the task conceptualisation is improved. The present study investigated how the movement preparation of bimanual reaching movements was affected by integrated visual feedback of the responses. Previous research has shown that the preparation of bimanual asymmetric movements takes longer than bimanual symmetric movements. The goal of the present study was to determine whether integrated, Lissajous feedback could eliminate this bimanual asymmetric cost. Fifteen participants made unimanual and bimanual symmetric and asymmetric reaches with separate feedback, where there was a cursor and a target for each hand. Participants also made bimanual symmetric and asymmetric movements with integrated feedback; a single cursor and a single target represented the locations and goals of both arms in this condition. The results showed a bimanual asymmetric cost with separate feedback, and that this cost persisted with integrated feedback. We suggest that integrated feedback improved continuous and discrete bimanual movements in other experiments by facilitating error detection and correction processes. We hypothesise that the bimanual asymmetric cost persisted in the present experiment because the uncertainty associated with choice reaction time prevented the facilitated error processing from improving the preparation of the next trial.

Bimanual coordination and the intermittency of visual information in isometric force tracking

The effect of the intermittency of visual information in the bimanual coordination of an isometric force coordination task was investigated as a function of criterion force level. Eight levels of visual information intermittency (.2-25.6 Hz) were used in blocked fashion at each force level. Participants were required to produce a constant force output matching as accurately as possible the criterion force target. The results showed that performance improved as the intermittency of visual information was reduced-this effect being a function of force level. The distribution of the relative phase through the trial revealed a preference for the two hands to be coupled together (in-phase) at the slower rates of visual presentation (~.2 Hz). However, as the rate of visual feedback was increased (up to ~25.6 Hz), there was a transition to predominantly a negative correlation pattern (anti-phase). The pattern of bimanual coordination in this isometric tracking task is driven by the availability of information for error correction and the interactive influence of perceptual-motor constraints.