Attentional loads associated with interlimb interactions underlying rhythmic bimanual coordination

The effect of elastic and viscous force fields on bimanual coordination

Bimanual in-phase and anti-phase coordination modes represent two basic movement patterns with distinct characteristics-homologous muscle contraction and non-homologous muscle contraction, respectively. A method to understand the contribution of each limb to the overall coordination pattern involves detuning (Δω) the natural eigenfrequency of each limb. In the present experiment, we experimentally broke the symmetry between the two upper limbs by adding elastic and viscous force fields using a Kinarm robot exoskeleton. We measured the effect of this symmetry breaking on coordination stability as participants performed bimanual in-phase and anti-phase movements using their left and right hand in 1:1 frequency locking mode. Differences between uncoupled frequencies were manipulated via the application of viscous & elastic force fields and using fast and slow oscillation frequencies with a custom task developed using the Kinarm robotic exoskeleton. The effects of manipulating the asymmetry between the limbs were measured through the mean and variability of relative phase (ϕ) from the intended modes of 0 ° or 180 °. In general, participants deviated less from intended phase irrespective of coordination mode in all matched conditions, except for when elastic loads are applied to both arms in the anti-phase coordination. Second, we found that when force fields were mismatched participants exhibited a larger deviation from the intended phase. Overall, there was increased phase deviation during anti-phase coordination. Finally, participants exhibited higher variability in relative phase in mismatched force conditions compared to matched force conditions, with overall higher variability during anti-phase coordination mode. We extend previous research by demonstrating that symmetry breaking caused by force differences between the limbs disrupts stability in each coordination mode.

Unveiling Intermittency in the Control of Quiet Upright Standing: Beyond Automatic Behavior

The control of posture, as in quiet upright standing, is distributed among postural reflexes and higher (cortical) centers. According to the theory of “intermittent control,” the control of posture involves a rapid succession of brief periods of postural stability, during which the body dwells relatively motionless in a particular posture, and postural instability, during which the body rapidly transits to a new stable point. This theory assumes a combination of stiffness control, keeping the body in the same position, and top-down ballistic control, moving the body to a new reference position. We tested the prediction that exerting ballistic control consumes more attention, relative to stiffness control, using variations in reaction time as our index of attention load. Slower reactions to external stimulus events were expected if these events happen to coincide with ballistic control regimes compared to stiffness regimes, as unveiled from local features of the posturogram. Thirty-two participants stood on a force plate, and were instructed to press a hand-held button as soon as they heard a stimulus tone. About 40 stimuli were presented at random instances during a 3-min trial. Postural control regimes were characterized using sway-density analysis for each stimulus-response interval, by computing local dwell times from the corresponding center-of-pressure samples. We correlated stimulus-response durations with the corresponding local dwell times, and also with local velocity and local eccentricity (distance from the origin). As predicted, an overall negative correlation was observed, meaning that shorter dwell times are associated with longer stimulus-response intervals, as well as a positive correlation with local center-of-pressure velocity. The correlation between reaction times and local eccentricity was not significant. Thus, by mapping stimulus-response intervals to local center-of-pressure features we demonstrated attentional fluctuations in the control of quiet upright standing, thereby validating a core assumption underlying the notion of intermittent postural control.

Age-Related Changes in Bimanual Instrument Playing with Rhythmic Cueing

Deficits in bimanual coordination of older adults have been demonstrated to significantly limit their functioning in daily life. As a bimanual sensorimotor task, instrument playing has great potential for motor and cognitive training in advanced age. While the process of matching a person’s repetitive movements to auditory rhythmic cueing during instrument playing was documented to involve motor and attentional control, investigation into whether the level of cognitive functioning influences the ability to rhythmically coordinate movement to an external beat in older populations is relatively limited. Therefore, the current study aimed to examine how timing accuracy during bimanual instrument playing with rhythmic cueing differed depending on the degree of participants’ cognitive aging. Twenty one young adults, 20 healthy older adults, and 17 older adults with mild dementia participated in this study. Each participant tapped an electronic drum in time to the rhythmic cueing provided using both hands simultaneously and in alternation. During bimanual instrument playing with rhythmic cueing, mean and variability of synchronization errors were measured and compared across the groups and the tempo of cueing during each type of tapping task. Correlations of such timing parameters with cognitive measures were also analyzed. The results showed that the group factor resulted in significant differences in the synchronization errors-related parameters. During bimanual tapping tasks, cognitive decline resulted in differences in synchronization errors between younger adults and older adults with mild dimentia. Also, in terms of variability of synchronization errors, younger adults showed significant differences in maintaining timing performance from older adults with and without mild dementia, which may be attributed to decreased processing time for bimanual coordination due to aging. Significant correlations were observed between variability of synchronization errors and performance of cognitive tasks involving executive control and cognitive flexibility when asked for bimanual coordination in response to external timing cues at adjusted tempi. Also, significant correlations with cognitive measures were more prevalent in variability of synchronization errors during alternative tapping compared to simultaneous tapping. The current study supports that bimanual tapping may be predictive of cognitive processing of older adults. Also, tempo and type of movement required for instrument playing both involve cognitive and motor loads at different levels, and such variables could be important factors for determining the complexity of the task and the involved task requirements for interventions using instrument playing.

Auditory rhythmic cueing in movement rehabilitation: findings and possible mechanisms

Moving to music is intuitive and spontaneous, and music is widely used to support movement, most commonly during exercise. Auditory cues are increasingly also used in the rehabilitation of disordered movement, by aligning actions to sounds such as a metronome or music. Here, the effect of rhythmic auditory cueing on movement is discussed and representative findings of cued movement rehabilitation are considered for several movement disorders, specifically post-stroke motor impairment, Parkinson's disease and Huntington's disease. There are multiple explanations for the efficacy of cued movement practice. Potentially relevant, non-mutually exclusive mechanisms include the acceleration of learning; qualitatively different motor learning owing to an auditory context; effects of increased temporal skills through rhythmic practices and motivational aspects of musical rhythm. Further considerations of rehabilitation paradigm efficacy focus on specific movement disorders, intervention methods and complexity of the auditory cues. Although clinical interventions using rhythmic auditory cueing do not show consistently positive results, it is argued that internal mechanisms of temporal prediction and tracking are crucial, and further research may inform rehabilitation practice to increase intervention efficacy.

The dopaminergic system in upper limb motor blocks (ULMB) investigated during bimanual coordination in Parkinson's disease (PD)

Upper limb motor blocks (ULMB) (inability to initiate or sudden discontinue in voluntary movements) have been identified in both unimanual and bimanual tasks in individuals with Parkinson's disease (PD). In particular, ULMB have been observed during rhythmic bimanual coordination when switching between phase patterns which is required (e.g. between in-phase and anti-phase). While sensory-perceptual mechanisms have recently been suggested to be involved in lower limb freezing, there has been no consensus on the mechanism that evokes ULMB or whether motor blocks respond to dopamine replacement like other motor symptoms of PD. The current study investigated the occurrence of ULMB in PD participants without ('off') and with ('on') dopamine replacement using bimanual wrist flexion-extension with external auditory cues. In Experiment 1, coordination was performed in either in-phase (simultaneous flexion and extension) or anti-phase (asymmetrical flexion and extension between the limbs) in one of three sensory conditions: no vision, normal vision or augmented vision. Cycle frequency was increased within each trial across seven cycle frequencies (0.75-2 Hz). In Experiment 2, coordination was initiated in either phase pattern and participants were cued to make an intentional switch between phases in the middle of trials. Trials were performed at one of two cycle frequencies (1 or 2 Hz) and one of two sensory conditions: no vision or normal vision. Healthy age-matched control participants were also investigated in both experiments for the occurrence of motor blocks that were measured using automated detection from a computer algorithm. The results from Experiment 1 indicated that increasing cycle frequency resulted in more ULMB in individuals with PD during continuous coordinated movement, regardless of dopaminergic status, phase pattern or sensory condition. Experiment 2 also confirmed an increased occurrence of ULMB with increased cycle frequency. Furthermore, a large amount of ULMB were observed when initiating anti-phase coordination at 2 Hz, as well as after both externally-cued switches and in 'catch trials' with distracting auditory cues when no switch was required. Dopamine replacement was not found to influence the frequency of ULMB in either experiment. Therefore, ULMB likely result from non-hypodopaminergic impairments associated with PD. Specifically, ULMB may be caused by an inability to shift attentional control under increased cognitive demand that could be associated with hypoactivation in motor and prefrontal areas.

Is DOPA-Responsive Hypokinesia Responsible for Bimanual Coordination Deficits in Parkinson's Disease?

Bradykinesia is a well-documented DOPA-responsive clinical feature of Parkinson’s disease (PD). While amplitude deficits (hypokinesia) are a key component of this slowness, it is important to consider how dopamine influences both the amplitude (hypokinesia) and frequency components of bradykinesia when a bimanually coordinated movement is required. Based on the notion that the basal ganglia are associated with sensory deficits, the influence of dopaminergic replacement on sensory feedback conditions during bimanual coordination was also evaluated. Bimanual movements were examined in PD and healthy comparisons in an unconstrained three-dimensional coordination task. PD were tested “off” (overnight withdrawal of dopaminergic treatment) and “on” (peak dose of dopaminergic treatment), while the healthy group was evaluated for practice effects across two sessions. Required cycle frequency (increased within each trial from 0.75 to 2 Hz), type of visual feedback (no vision, normal vision, and augmented vision), and coordination pattern (symmetrical in-phase and non-symmetrical anti-phase) were all manipulated. Overall, coordination (mean accuracy and standard deviation of relative phase) and amplitude deficits during bimanual coordination were confirmed in PD participants. In addition, significant correlations were identified between severity of motor symptoms as well as bradykinesia to greater coordination deficits (accuracy and stability) in PD “off” group. However, even though amplitude deficits (hypokinesia) improved with dopaminergic replacement, it did not improve bimanual coordination performance (accuracy or stability) in PD patients from “off” to “on.” Interestingly, while coordination performance in both groups suffered in the augmented vision condition, the amplitude of the more affected limb of PD was notably influenced. It can be concluded that DOPA-responsive hypokinesia contributes to, but is not directly responsible for bimanual coordination impairments in PD. It is likely that bimanual coordination deficits in PD are caused by the combination of dopaminergic system dysfunction as well as other neural impairments that may be DOPA-resistant or related to non-dopaminergic pathways.

Evaluating dopaminergic system contributions to cued pattern switching during bimanual coordination

Switching between different coordinated movements has been shown to be slow, with delayed responses and even freezing deficits in individuals with Parkinson's disease (PD). While it is well accepted that the dopaminergic system responds to dopamine replacement to ameliorate overall slowness (bradykinesia) and other motor symptoms of PD, it is unknown whether the dopaminergic system can influence overall coordination between limbs and if this may be impacted by the availability of sensory feedback. In the current study, PD and healthy age-matched control participants performed a rhythmic coordination task that required a cued voluntary switch between movement patterns (in-phase and anti-phase). PD participants performed the task first after overnight withdrawal ('off'), and subsequently after administration ('on') of dopamine replacement. Coordinated movements were performed while paced by an auditory metronome in two sensory conditions: 'no vision' or 'normal vision'. Measures of voluntary switch time and delayed responses revealed that PD 'off' required significantly more time than healthy participants to switch between movement patterns. Interestingly, PD 'off' demonstrated disrupted coordination, as revealed by mean (accuracy) and standard deviation (stability) of absolute error of relative phase. Dopamine replacement improved the time needed to switch and amount of delayed responses in PD participants, but had no influence on coordination itself. It is concluded that although modulation of the dopaminergic system improves the slowness during switching, coordination deficits may be the result of secondary impairments (possibly attention-related) that cannot be improved with dopamine replacement.

Frequency-induced changes in interlimb interactions: increasing manifestations of closed-loop control

In bimanual coordination, interactions between the limbs result in attraction to in-phase and antiphase coordination. Increasing movement frequency leads to decreasing stability of antiphase coordination, often resulting in a transition to the more stable in-phase pattern. It is unknown, however, how this frequency-induced loss of stability is engendered in terms of the interlimb interactions underwriting bimanual coordination. The present study was conducted to help resolve this issue. Using an established method (based on comparison of various unimanual and bimanual tasks involving both passive and active movements), three sources of interlimb interaction were dissociated: (1) integrated timing of feedforward signals, (2) afference-based correction of relative phase errors, and (3) phase entrainment by contralateral afference. Results indicated that phase entrainment strength remained unaffected by frequency and that the stabilizing effects of error correction and integrated timing decreased with increasing frequency. Their contributions, however, reflected an interesting interplay as frequency increased. For moderate frequencies coordinative stability was predominantly secured by integrated timing processes. However, at high frequencies, the stabilization of the antiphase pattern required combined contributions of both integrated timing and error correction. In sum, increasing frequency was found to induce a shift from predominantly open-loop control to more closed-loop control. The results may be accounted for by means of an internal forward model for sensorimotor integration in which the sensory signals are compared to values predicted on the basis of efference copies.

Covariation of attentional cost and stability provides further evidence for two routes to learning new coordination patterns

This study investigated how learning a new bimanual coordination pattern affects the attentional resources allotted by the CNS to maintain it throughout the acquisition process. The repertoire of the existing stable coordination patterns was individually evaluated before and after practice in order to detect expected changes with learning. Bistable participants, who initially exhibited stable and accurate coordination patterns at 0 degrees and 180 degrees of relative phase, practiced a 90 degrees pattern, whereas multistable participants, who already mastered the 90 degrees pattern, practiced 135 degrees pattern instead. In a typical dual-task paradigm, all participants had to simultaneously perform a reaction time task that assessed the associated attentional cost. Beyond an overall increase in accuracy, the results revealed a significant decrease in the attentional cost for bistable participants, accompanying the stabilization of the 90 degrees pattern with learning, but not for multistable participants, as the 135 degrees pattern barely stabilized. Pattern stability and attentional cost co-evolve during learning and the process follows two different routes depending on the interplay between the task and the learner's coordination abilities before practice.