Bimanual coordination and interhemispheric interaction

Cross-hemispheric communication: Insights on lateralized brain functions

On the surface, the two hemispheres of vertebrate brains look almost perfectly symmetrical, but several motor, sensory, and cognitive systems show a deeply lateralized organization. Importantly, the two hemispheres are connected by various commissures, white matter tracts that cross the brain's midline and enable cross-hemispheric communication. Cross-hemispheric communication has been suggested to play an important role in the emergence of lateralized brain functions. Here, we review current advances in understanding cross-hemispheric communication that have been made using modern neuroscientific tools in rodents and other model species, such as genetic labeling, large-scale recordings of neuronal activity, spatiotemporally precise perturbation, and quantitative behavior analyses. These findings suggest that the emergence of lateralized brain functions cannot be fully explained by largely static factors such as genetic variation and differences in structural brain asymmetries. In addition, learning-dependent asymmetric interactions between the left and right hemispheres shape lateralized brain functions.

Altered excitation-inhibition balance in the primary sensorimotor cortex to proprioceptive hand stimulation in cerebral palsy

OBJECTIVE

Our objective was to clarify the primary sensorimotor (SM1) cortex excitatory and inhibitory alterations in hemiplegic (HP) and diplegic (DP) cerebral palsy (CP) by quantifying SM1 cortex beta power suppression and rebound with magnetoencephalography (MEG).

METHODS

MEG was recorded from 16 HP and 12 DP adolescents, and their 32 healthy controls during proprioceptive stimulation of the index fingers evoked by a movement actuator. The related beta power changes were computed with Temporal Spectral Evolution (TSE). Peak strengths of beta suppression and rebound were determined from representative channels over the SM1 cortex.

RESULTS

Beta suppression was stronger contralateral to the stimulus and rebound was weaker ipsilateral to the stimulation in DP compared to controls. Beta modulation strengths did not differ significantly between HP and the control group.

CONCLUSIONS

The emphasized beta suppression in DP suggests less efficient proprioceptive processing in the SM1 contralateral to the stimulation. Their weak rebound further indicates reduced intra- and/or interhemispheric cortical inhibition, which is a potential neuronal mechanism for their bilateral motor impairments.

SIGNIFICANCE

The excitation-inhibition balance of the SM1 cortex related to proprioception is impaired in diplegic CP. Therefore, the cortical and behavioral proprioceptive deficits should be better diagnosed and considered to better target individualized effective rehabilitation in CP.

A table tennis serve versus rally hit elicits differential hemispheric electrocortical power fluctuations

Human visuomotor control requires coordinated interhemispheric interactions to exploit the brain's functional lateralization. In right-handed individuals, the left hemisphere (right arm) is better for dynamic control and the right hemisphere (left arm) is better for impedance control. Table tennis is a game that requires precise movements of the paddle, whole body coordination, and cognitive engagement, providing an ecologically valid way to study visuomotor integration. The sport has many different types of strokes (e.g., serve, return, and rally shots), which should provide unique cortical dynamics given differences in the sensorimotor demands. The goal of this study was to determine the hemispheric specialization of table tennis serving - a sequential, self-paced, bimanual maneuver. We used time-frequency analysis, event-related potentials, and functional connectivity measures of source-localized electrocortical clusters and compared serves with other types of shots, which varied in the types of movement required, attentional focus, and other task demands. We found greater alpha (8-12 Hz) and beta (13-30 Hz) power in the right sensorimotor cortex than in the left sensorimotor cortex, and we found a greater magnitude of spectral power fluctuations in the right sensorimotor cortex for serve hits than return or rally hits, in all right-handed participants. Surprisingly, we did not find a difference in interhemispheric functional connectivity between a table tennis serve and return or rally hits, even though a serve could arguably be a more complex maneuver. Studying real-world brain dynamics of table tennis provides insight into bilateral sensorimotor integration.NEW & NOTEWORTHY We found different spectral power fluctuations in the left and right sensorimotor cortices during table tennis serves, returns, and rallies. Our findings contribute to the basic science understanding of hemispheric specialization in a real-world context.

The power of multivariate approach in identifying EEG correlates of interlimb coupling

Interlimb coupling refers to the interaction between movements of one limb and movements of other limbs. Understanding mechanisms underlying this effect is important to real life because it reflects the level of interdependence between the limbs that plays a role in daily activities including tool use, cooking, or playing musical instruments. Interlimb coupling involves multiple brain regions working together, including coordination of neural activity in sensory and motor regions across the two hemispheres. Traditional neuroscience research took a univariate approach to identify neural features that correspond to behavioural coupling measures. Yet, this approach reduces the complexity of the neural activity during interlimb tasks to one value. In this brief research report, we argue that identifying neural correlates of interlimb coupling would benefit from a multivariate approach in which full patterns from multiple sources are used to predict behavioural coupling. We demonstrate the feasibility of this approach in an exploratory EEG study where participants (n = 10) completed 240 trials of a well-established drawing paradigm that involves interlimb coupling. Using artificial neural network (ANN), we show that multivariate representation of the EEG signal significantly captures the interlimb coupling during bimanual drawing whereas univariate analyses failed to identify such correlates. Our findings demonstrate that analysing distributed patterns of multiple EEG channels is more sensitive than single-value techniques in uncovering subtle differences between multiple neural signals. Using such techniques can improve identification of neural correlates of complex motor behaviours.

Neuromodulation-induced prehabilitation to leverage neuroplasticity before brain tumor surgery: a single-cohort feasibility trial protocol

Introduction

Neurosurgery for brain tumors needs to find a complex balance between the effective removal of targeted tissue and the preservation of surrounding brain areas. Neuromodulation-induced cortical prehabilitation (NICP) is a promising strategy that combines temporary inhibition of critical areas (virtual lesion) with intensive behavioral training to foster the activation of alternative brain resources. By progressively reducing the functional relevance of targeted areas, the goal is to facilitate resection with reduced risks of neurological sequelae. However, it is still unclear which modality (invasive vs. non-invasive neuromodulation) and volume of therapy (behavioral training) may be optimal in terms of feasibility and efficacy.

Methods and analysis

Patients undertake between 10 and 20 daily sessions consisting of neuromodulation coupled with intensive task training, individualized based on the target site and neurological functions at risk of being compromised. The primary outcome of the proposed pilot, single-cohort trial is to investigate the feasibility and potential effectiveness of a non-invasive NICP protocol on neuroplasticity and post-surgical outcomes. Secondary outcomes investigating longitudinal changes (neuroimaging, neurophysiology, and clinical) are measured pre-NICP, post-NICP, and post-surgery.

Ethics and dissemination

Ethics approval was obtained from the Research Ethical Committee of Fundació Unió Catalana d'Hospitals (approval number: CEI 21/65, version 1, 13/07/2021). The results of the study will be submitted to a peer-reviewed journal and presented at scientific congresses.

Clinical trial registration

ClinicalTrials.gov, identifier NCT05844605.

The embodied typist: Bimanual actions are modulated by words' implied motility and number of evoked limbs

The planning and execution of manual actions can be influenced by concomitant processing of manual action verbs. However, this phenomenon manifests in varied ways throughout the literature, ranging from facilitation to interference effects. Suggestively, stimuli across studies vary randomly in two potentially relevant variables: verb motility and effector quantity (i.e., the amount of movement and the number of hands implied by the word, respectively). Here we examine the role of these factors during keyboard typing, a strategic bimanual task validated in previous works. Forty-one participants read and typed high and low motility items from four categories: bimanual, unimanual, and non-manual action verbs, as well as minimally motoric verbs. Motor planning and execution were captured by first-letter lag (the lapse between word presentation and first keystroke) and whole-word lag (the lapse between the first and last keystroke). We found that verb motility modulated action planning and execution, both stages being delayed by high (relative to low) motility verbs. Effector quantity also influenced both stages, which were facilitated by bimanual verbs relative to unimanual verbs and non-manual verbs (this effect being confined to high motility items during action execution). Accordingly, motor-language coupling effects seem sensitive to words' implied motility and number of evoked limbs. These findings refine our understanding of how semantics influences bodily movement.

Sensory and motor cortical excitability changes induced by rTMS and sensory stimulation in stroke: A randomized clinical trial

Background

The ability to produce coordinated movement is dependent on dynamic interactions through transcallosal fibers between the two cerebral hemispheres of the brain. Although typically unilateral, stroke induces changes in functional and effective connectivity across hemispheres, which are related to sensorimotor impairment and stroke recovery. Previous studies have focused almost exclusively on interhemispheric interactions in the primary motor cortex (M1).

Objective

To identify the presence of interhemispheric asymmetry (ASY) of somatosensory cortex (S1) excitability and to investigate whether S1 repetitive transcranial magnetic stimulation (rTMS) combined with sensory stimulation (SS) changes excitability in S1 and M1, as well as S1 ASY, in individuals with subacute stroke.

Methods

A randomized clinical trial. Participants with a single episode of stroke, in the subacute phase, between 35 and 75 years old, were allocated, randomly and equally balanced, to four groups: rTMS/sham SS, sham rTMS/SS, rTMS/SS, and sham rTMS/Sham SS. Participants underwent 10 sessions of S1 rTMS of the lesioned hemisphere (10 Hz, 1,500 pulses) followed by SS. SS was applied to the paretic upper limb (UL) (active SS) or non-paretic UL (sham SS). TMS-induced motor evoked potentials (MEPs) of the paretic UL and somatosensory evoked potential (SSEP) of both ULs assessed M1 and S1 cortical excitability, respectively. The S1 ASY index was measured before and after intervention. Evaluator, participants and the statistician were blinded.

Results

Thirty-six participants divided equally into groups (nine participants per group). Seven patients were excluded from MEP analysis because of failure to produce consistent MEP. One participant was excluded in the SSEP analysis because no SSEP was detected. All somatosensory stimulation groups had decreased S1 ASY except for the sham rTMS/Sham SS group. When compared with baseline, M1 excitability increased only in the rTMS/SS group.

Conclusion

S1 rTMS and SS alone or in combination changed S1 excitability and decreased ASY, but it was only their combination that increased M1 excitability.

Clinical trial registration

clinicaltrials.gov, identifier (NCT03329807).

Beta rhythmicity in human motor cortex reflects neural population coupling that modulates subsequent finger coordination stability

Human behavior is not performed completely as desired, but is influenced by the inherent rhythmicity of the brain. Here we show that anti-phase bimanual coordination stability is regulated by the dynamics of pre-movement neural oscillations in bi-hemispheric primary motor cortices (M1) and supplementary motor area (SMA). In experiment 1, pre-movement bi-hemispheric M1 phase synchrony in beta-band (M1-M1 phase synchrony) was online estimated from 129-channel scalp electroencephalograms. Anti-phase bimanual tapping preceded by lower M1-M1 phase synchrony exhibited significantly longer duration than tapping preceded by higher M1-M1 phase synchrony. Further, the inter-individual variability of duration was explained by the interaction of pre-movement activities within the motor network; lower M1-M1 phase synchrony and spectral power at SMA were associated with longer duration. The necessity of cortical interaction for anti-phase maintenance was revealed by sham-controlled repetitive transcranial magnetic stimulation over SMA in another experiment. Our results demonstrate that pre-movement cortical oscillatory coupling within the motor network unknowingly influences bimanual coordination performance in humans after consolidation, suggesting the feasibility of augmenting human motor ability by covertly monitoring preparatory neural dynamics.

Neurological and Functional Outcomes after Pediatric Stroke

Pediatric stroke results in life-long morbidity for many patients, but the outcomes can vary depending on factors such as age of injury, or mechanism, size, and location of stroke. In this review, we summarize the current understanding of outcomes in different neurological domains (eg, motor, cognitive, language) for children with stroke of different mechanisms (ie, arterial ischemic stroke, cerebral sinus venous thrombosis, and hemorrhagic stroke), but with a focus on World Health Organization International Classification for Functioning, Disability, and Health (ICF-CY) framework for measuring health and disability for children and youth. We describe outcomes for the population as a whole and certain factors that may further refine prognostication.

Bilateral Sensorimotor Cortical Communication Modulated by Multiple Hand Training in Stroke Participants: A Single Training Session Pilot Study

Bi-manual therapy (BT), mirror therapy (MT), and robot-assisted rehabilitation have been conducted in hand training in a wide range of stages in stroke patients; however, the mechanisms of action during training remain unclear. In the present study, participants performed hand tasks under different intervention conditions to study bilateral sensorimotor cortical communication, and EEG was recorded. A multifactorial design of the experiment was used with the factors of manipulating objects (O), robot-assisted bimanual training (RT), and MT. The sum of spectral coherence was applied to analyze the C3 and C4 signals to measure the level of bilateral corticocortical communication. We included stroke patients with onset <6 months (n = 6), between 6 months and 1 year (n = 14), and onset >1 year (n = 20), and their Brunnstrom recovery stage ranged from 2 to 4. The results showed that stroke duration might influence the effects of hand rehabilitation in bilateral cortical corticocortical communication with significant main effects under different conditions in the alpha and beta bands. Therefore, stroke duration may influence the effects of hand rehabilitation on interhemispheric coherence.

Cross-attractor repertoire provides new perspective on structure-function relationship in the brain

The brain exhibits complex intrinsic dynamics, i.e., spontaneously arising activity patterns without any external inputs or tasks. Such intrinsic dynamics and their alteration are thought to play crucial roles in typical as well as atypical cognitive functioning. Linking the ever-changing intrinsic dynamics to the rather static anatomy is a challenging endeavor. Dynamical systems models are important tools for understanding how structure and function are linked in the brain. Here, we provide a novel modeling framework to examine how functional connectivity depends on structural connectivity in the brain. Existing modeling frameworks typically focus on noise-driven (or stochastic) dynamics near a single attractor. Complementing existing approaches, we examine deterministic features of the distribution of attractors, in particular, how regional states are correlated across all attractors - cross-attractor coordination. We found that cross-attractor coordination between brain regions better predicts human functional connectivity than noise-driven single-attractor dynamics. Importantly, cross-attractor coordination better accounts for the nonlinear dependency of functional connectivity on structural connectivity. Our findings suggest that functional connectivity patterns in the brain may reflect transitions between attractors, which impose an energy cost. The framework may be used to predict transitions and energy costs associated with experimental or clinical interventions.

Free will strikes back: Steady-state movement-related cortical potentials are modulated by cognitive control

In psychology and neuroscience, opposition to free will has asserted that any degree of perceived self-control or choice is a mere epiphenomenon which provides no meaningful influence on action. The present research tested the validity of this conclusion by designing a paradigm in which the potential effect of self-monitoring on motor output could be investigated. Using a repetitive finger tapping task that evokes automatic patterns in participants tapping responses, we have obtained evidence that (1) participants may voluntarily reduce the predictability of their tapping patterns (2) by exercising cognitive control that (3) modulates response-locked steady-state movement-related potentials over primary and supplementary motor areas. These findings challenge the most radical accounts of the nonexistence of free will and instead provide support for a more balanced model of human behaviour in which cognitive control may constrain automatic response tendencies in response preparation and action execution.

The central medial thalamic nucleus facilitates bilateral movement execution in rats

Intralaminar thalamic nuclei, including the central medial nucleus (CMT), have been classically implicated in the control of attentional functional states such as sleep-wake transitions. In rodents, the CMT innervates large cortical and subcortical areas bilaterally, including sensorimotor regions of the cortex and striatum, but its contribution to motor function, which regularly develops in faster temporal scales than attentional states, is still far from being completely understood. Here, by using a novel behavioral protocol to evaluate bilateral coordination in rats, combined with electrophysiological recordings and optogenetic manipulations, we studied the contribution of the CMT to motor control and coordination. We found that optogenetic stimulation of the central region of the CMT produced bilateral recruitment of neural activity in the sensorimotor cortex and striatum. The same type of stimulations produced a significant increase in bilateral movement coordination of the forelimbs accompanied by a decrease in movement trajectory variability. Optogenetic inactivation of the CMT did not affect motor execution but significantly increased execution times, suggesting less interest in the task. Altogether, our results indicate that brief CMT activations create windows of synchronized bilateral cortico-striatal activity, suitable to facilitate motor coordination in temporal scales relevant for motor execution. Significance Statement The central medial thalamic nucleus (CMT) has been classically implicated in attentional processes. However, it also innervates large motor cortico-striatal regions, but its participation in motor control and coordination is still not well understood. Here, by combining a novel behavioral protocol with optogenetic manipulations, we have found that brief CMT activations create windows of synchronized bilateral cortico-striatal activity, suitable to facilitate motor coordination in temporal scales relevant for motor execution.

Investigating Different Levels of Bimanual Interaction With a Novel Motor Learning Task: A Behavioural and Transcranial Alternating Current Stimulation Study

Many tasks require the skilled interaction of both hands, such as eating with knife and fork or keyboard typing. However, our understanding of the behavioural and neurophysiological mechanisms underpinning bimanual motor learning is still sparse. Here, we aimed to address this by first characterising learning-related changes of different levels of bimanual interaction and second investigating how beta tACS modulates these learning-related changes. To explore early bimanual motor learning, we designed a novel bimanual motor learning task. In the task, a force grip device held in each hand (controlling x- and y-axis separately) was used to move a cursor along a path of streets at different angles (0°, 22.5°, 45°, 67.5°, and 90°). Each street corresponded to specific force ratios between hands, which resulted in different levels of hand interaction, i.e., unimanual (Uni, i.e., 0°, 90°), bimanual with equal force (Bi eq , 45°), and bimanual with unequal force (Bi uneq 22.5°, 67.5°). In experiment 1, 40 healthy participants performed the task for 45 min with a minimum of 100 trials. We found that the novel task induced improvements in movement time and error, with no trade-off between movement time and error, and with distinct patterns for the three levels of bimanual interaction. In experiment 2, we performed a between-subjects, double-blind study in 54 healthy participants to explore the effect of phase synchrony between both sensorimotor cortices using tACS at the individual's beta peak frequency. The individual's beta peak frequency was quantified using electroencephalography. 20 min of 2 mA peak-to-peak amplitude tACS was applied during task performance (40 min). Participants either received in-phase (0° phase shift), out-of-phase (90° phase shift), or sham (3 s of stimulation) tACS. We replicated the behavioural results of experiment 1, however, beta tACS did not modulate motor learning. Overall, the novel bimanual motor task allows to characterise bimanual motor learning with different levels of bimanual interaction. This should pave the way for future neuroimaging studies to further investigate the underlying mechanism of bimanual motor learning.

The functional characterization of callosal connections

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The functional characterization of callosal connections is informed by anatomical data.

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Callosal connections play a conditional driving role depending on the brain state and behavioral demands.

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Callosal connections play a modulatory function, in addition to a driving role.

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The corpus callosum participates in learning and interhemispheric transfer of sensorimotor habits.

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The corpus callosum contributes to language processing and cognitive functions.

The brain operates through the synaptic interaction of distant neurons within flexible, often heterogeneous, distributed systems. Histological studies have detailed the connections between distant neurons, but their functional characterization deserves further exploration. Studies performed on the corpus callosum in animals and humans are unique in that they capitalize on results obtained from several neuroscience disciplines. Such data inspire a new interpretation of the function of callosal connections and delineate a novel road map, thus paving the way toward a general theory of cortico-cortical connectivity. Here we suggest that callosal axons can drive their post-synaptic targets preferentially when coupled to other inputs endowing the cortical network with a high degree of conditionality. This might depend on several factors, such as their pattern of convergence-divergence, the excitatory and inhibitory operation mode, the range of conduction velocities, the variety of homotopic and heterotopic projections and, finally, the state-dependency of their firing. We propose that, in addition to direct stimulation of post-synaptic targets, callosal axons often play a conditional driving or modulatory role, which depends on task contingencies, as documented by several recent studies.

GABA levels are differentially associated with bimanual motor performance in older as compared to young adults

Although gamma aminobutyric acid (GABA) is of particular importance for efficient motor functioning, very little is known about the relationship between regional GABA levels and motor performance. Some studies suggest this relation to be subject to age-related differences even though literature is scarce. To clarify this matter, we employed a comprehensive approach and investigated GABA levels within young and older adults across multiple motor tasks as well as multiple brain regions. Specifically, 30 young and 30 older adults completed a task battery of three different bimanual tasks. Furthermore, GABA levels were obtained within bilateral primary sensorimotor cortex (SM1), bilateral dorsal premotor cortex, the supplementary motor area and bilateral dorsolateral prefrontal cortex (DLPFC) using magnetic resonance spectroscopy. Results indicated that older adults, as compared to their younger counterparts, performed worse on all bimanual tasks and exhibited lower GABA levels in bilateral SM1 only. Moreover, GABA levels across the motor network and DLPFC were differentially associated with performance in young as opposed to older adults on a manual dexterity and bimanual coordination task but not a finger tapping task. Specifically, whereas higher GABA levels related to better manual dexterity within older adults, higher GABA levels predicted poorer bimanual coordination performance in young adults. By determining a task-specific and age-dependent association between GABA levels across the cortical motor network and performance on distinct bimanual tasks, the current study advances insights in the role of GABA for motor performance in the context of aging.

The role of interhemispheric communication during complete and partial cancellation of bimanual responses

Precise control of upper limb movements in response to external stimuli is vital to effectively interact with the environment. Accurate execution of bimanual movement is known to rely on finely orchestrated interhemispheric communication between the primary motor cortices (M1s). However, relatively little is known about the role of interhemispheric communication during sudden cancellation of prepared bimanual movement. The current study investigated the role of interhemispheric interactions during complete and partial cancellation of bimanual movement. In two experiments, healthy young human participants received transcranial magnetic stimulation to both M1s during a bimanual response inhibition task. The increased corticomotor excitability in anticipation of bimanual movement was accompanied by a release of inhibition from both M1s. After a stop cue, inhibition was reengaged onto both hemispheres to successfully cancel the complete bimanual response. However, when the stop cue signaled partial cancellation (stopping of one digit only), inhibition was reengaged with regard to the cancelled digit, but the responding digit representation was facilitated. This bifurcation in interhemispheric communication between M1s occurred 75 ms later in the more difficult condition when the nondominant, as opposed to dominant, hand was still responding. Our results demonstrate that interhemispheric communication is integral to response inhibition once a bimanual response has been prepared. Interestingly, M1-M1 interhemispheric circuitry does not appear to be responsible for the nonselective suppression of all movement components that has been observed during partial cancellation. Instead such interhemispheric communication enables uncoupling of bimanual response components and facilitates the selective initiation of just the required unimanual movement.NEW & NOTEWORTHY We provide the first evidence that interhemispheric communication plays an important role during sudden movement cancellation of two-handed responses. Simultaneously increased inhibition onto both hemispheres assists with two-handed movement cancellation. However, this network is not responsible for the widespread suppression of motor activity observed when only one of the two hands is cancelled. Instead, communication between hemispheres enables the separation of motor activity for the two hands and helps to execute the required one-handed response.

Altered task-induced cerebral blood flow and oxygen metabolism underlies motor impairment in multiple sclerosis

The neural mechanisms underlying motor impairment in multiple sclerosis (MS) remain unknown. Motor cortex dysfunction is implicated in blood-oxygen-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) studies, but the role of neural-vascular coupling underlying BOLD changes remains unknown. We sought to independently measure the physiologic factors (i.e., cerebral blood flow (ΔCBF), cerebral metabolic rate of oxygen (ΔCMRO2), and flow-metabolism coupling (ΔCBF/ΔCMRO2), utilizing dual-echo calibrated fMRI (cfMRI) during a bilateral finger-tapping task. We utilized cfMRI to measure physiologic responses in 17 healthy volunteers and 32 MS patients (MSP) with and without motor impairment during a thumb-button-press task in thumb-related (task-central) and surrounding primary motor cortex (task-surround) regions of interest (ROIs). We observed significant ΔCBF and ΔCMRO2 increases in all MSP compared to healthy volunteers in the task-central ROI and increased flow-metabolism coupling (ΔCBF/ΔCMRO2) in the MSP without motor impairment. In the task-surround ROI, we observed decreases in ΔCBF and ΔCMRO2 in MSP with motor impairment. Additionally, ΔCBF and ΔCMRO2 responses in the task-surround ROI were associated with motor function and white matter damage in MSP. These results suggest an important role for task-surround recruitment in the primary motor cortex to maintain motor dexterity and its dependence on intact white matter microstructure and neural-vascular coupling.

The effect of sequence learning on sensorimotor adaptation

Motor skill learning involves both sensorimotor adaptation (calibrating the response to task dynamics and kinematics), and sequence learning (executing task elements in the correct order at the necessary speed). These processes typically occur together in natural behavior and share much in common, such as working memory demands, development, and possibly neural substrates. However, sensorimotor and sequence learning are usually studied in isolation in research settings, for example as force field adaptation or serial reaction time tasks (SRTT), respectively. It is therefore unclear whether having predictive sequence information during sensorimotor adaptation would facilitate performance, perhaps by improving sensorimotor planning, or if it would impair performance, perhaps by occupying neural resources needed for sensorimotor adaptation. Here we evaluated adaptation to a position-dependent force field in two different SRTT contexts: In Experiment 1, 28 subjects reached between 4 targets in a sequenced or random order. In Experiment 2, 40 subjects reached to one target, but 3 force field directions were applied in a sequenced or random order. No consistent influence of target position sequence on force field adaptation was observed in Experiment 1. However, sequencing of force field directions facilitated sensorimotor adaptation and retention in Experiment 2. This is inconsistent with the idea that sensorimotor and sequence learning share neural resources in any mutually exclusive fashion. These findings indicate that under certain conditions, sequence learning interacts with sensorimotor adaptation in a facilitatory manner. Future research will be needed to determine what circumstances and features of sequence learning are facilitatory to sensorimotor adaptation.

Musical mirror-symmetrical movement tasks: comparison of rhythm versus melody-playing

Bimanual mirror-symmetrical movement (MSM) is relatively easy to control movement. Different MSM tasks may have different activations and interhemispheric interactions. The purpose of this study is to compare anatomo-physiological features such as hemispheric activations and dominance of two different MSMs, namely melody-playing and rhythm. We examined functional MRI (fMRI) recordings in a group of fifteen right-handed pianists performing two separate tasks: bimanual rhythm and bimanual melody-playing on two different keyboards with standard key order for right hand and reversed for left hand, which allows homolog fingers' movements. Activations and laterality indices on fMRI were examined. The results show that significant cerebellar activations (especially in anterior cerebellum) in both groups. Significant primary sensorimotor cortical activations are observed in the melody-playing group. While there are also bilaterally symmetric activations, and laterality indices suggest overall lateralization towards the left hemisphere in both groups. Activations in the left fronto-parietal cortex, left putamen and left thalamus in conjunction with right cerebellar activations suggest that the left cortico-thalamo-cerebellar loop may be a dominant loop. Dynamic causal modeling (DCM) indicates the presence of causal influences from the left to the right cerebral cortex. In conclusion, melody-playing with bimanual MSM is a complex in-phase task and may help activate the bilateral cortical areas, and left hemisphere is dominant according to laterality indices and DCM results. On the other hand, bimanual rhythm is a simpler in-phase task and may help activate subcortical areas, which might be independent of the voluntary cortical task.

Neurophysiological Correlates of Adaptation and Interference during Asymmetrical Bimanual Movements

In this study, we investigated brain dynamics during interference between hands during bimanual movements. Participants performed a bimanual center-out reaching task in which a visuomotor rotation was applied to the right hand while the left hand did not receive visual feedback of its movements. This manipulation resulted in interference from the adapting right hand to the kinesthetically guided left hand. Electroencephalography (EEG) recordings during the task showed that spectral power in the high and low beta frequency bands was elevated early in exposure, but decreased throughout learning. This may be representative of error-based updating of internal models of movement. Additionally, coherence, a measure of neural functional connectivity, was elevated both within and between hemispheres in the beta frequencies during the initial presentation of the visuomotor rotation, and then decreased throughout adaptation. This suggests that beta oscillatory neural activity may be marker for transmission of conflicting motor information between hemispheres, which manifests in interference between the hands during asymmetrical bimanual movements.

Bimanual Skill Learning after Transcranial Direct Current Stimulation in Children with Unilateral Cerebral Palsy: A Brief Report

Bimanual skills are important for goal-oriented activities. Children with unilateral cerebral palsy (UCP) have deficits in unimanual and bimanual motor control and learning. The application of non-invasive brain stimulation with existing motor training may further promote motor learning; however, the effects of stimulation on bimanual learning have not been examined. Here, we assessed the performance of a novel bimanual skill (modified Speed Stacks task) in eight children with UCP before, during, and after a combined motor training and brain stimulation intervention. Participants received 10 days (120 min/day) of goal-oriented bimanual therapy combined initially with transcranial direct current stimulation (tDCS, 20 min/day). Results showed task improvement tapered (p < 0.001) during and after the intervention and task variability decreased in 6/8 participants, indicating the potential impact of novel rehabilitation to improve skill learning in children with UCP. Future work is required to understand how both tDCS and bimanual training contribute to learning bimanual tasks.

Assessment of Somatosensory Reorganization by Functional Magnetic Resonance Imaging After Hand Replantation

INTRODUCTION

Amputation of the hand is a rare and extremely intense trauma. Replanting and allografting after this type of injury require a major reorganization of the brain. Brain plasticity, though better known in the context of disorders of the central nervous system, is just as indispensable when the extremities are damaged.

MATERIALS AND METHODS

A 17-year-old patient underwent replantation of the nondominant hand after transmetaphyseal amputation after traumatic injury. After 18 days in hospital and subsequent treatment in a physical rehabilitation center, the patient attended clinical and radiology follow-up sessions over the next 2 years.

RESULTS

The management of this patient led to an excellent functional outcome in conjunction with successful social and professional reintegration. Electromyography at 18 months confirmed nerve regrowth. Functional magnetic resonance imaging was done at 2 years to evaluate cerebral plasticity. Motor function, largely dependent on the primary motor area, is aided by the addition of secondary and accessory motor areas for both simple and complex movements. A change in sensory information is stimulation in its own right hemisphere and increases solicitation of the contralateral precentral and postcentral gyrus.

CONCLUSIONS

There seems to be a real reversible dynamic plasticity under the balance of inhibitory and excitatory influences exerted on the cortical neurons. Any disruption of this balance requires the brain to adapt to the new circumstances to reestablish the hand as a functioning part of the body.

Hand forces and placement are modulated and covary during anticipatory control of bimanual manipulation

Dexterous object manipulation relies on the feedforward and feedback control of kinetics (forces) and kinematics (hand shaping and digit placement). Lifting objects with an uneven mass distribution involves the generation of compensatory moments at object lift-off to counter object torques. This is accomplished through the modulation and covariation of digit forces and placement, which has been shown to be a general feature of unimanual manipulation. These feedforward anticipatory processes occur before performance-specific feedback. Whether this adaptation is a feature unique to unimanual dexterous manipulation or general across unimanual and bimanual manipulation is not known. We investigated the generation of compensatory moments through hand placement and force modulation during bimanual manipulation of an object with variable center of mass. Participants were instructed to prevent object roll during the lift. Similar to unimanual grasping, we found modulation and covariation of hand forces and placement for successful performance. Thus this motor adaptation of the anticipatory control of compensatory moment is a general feature across unimanual and bimanual effectors. Our results highlight the involvement of high-level representation of manipulation goals and underscore a sensorimotor circuitry for anticipatory control through a continuum of force and placement modulation of object manipulation across a range of effectors.

NEW & NOTEWORTHY This is the first study, to our knowledge, to show that successful bimanual manipulation of objects with asymmetrical centers of mass is performed through the modulation and covariation of hand forces and placements to generate compensatory moments. Digit force-to-placement modulation is thus a general phenomenon across multiple effectors, such as the fingers of one hand, and both hands. This adds to our understanding of integrating low-level internal representations of object properties into high-level task representations.

EEG coherence as a diagnostic tool to measure the initial stages of Parkinson Disease

Although Parkinson Disease was described a long time ago by James Parkinson and several biomarkers were used to predict the symptoms of PD, there is no accepted tool to distinguish the initial stages of this pathology. The present hypothesis discusses the Coherence Function, an Electroencephalography measure which could be used as a simple, and low-cost tool to describe the onset of cardinal signals of PD. Our hypothesis is based on three factors: beta frequency related to movement, motor action over particular cortical regions, and cortical coupling between cortical areas involved in the execution of voluntary movement. We believe that these factors support our hypothesis pointing out coherence function as an interesting measure to detect initial stages of PD.

The Basal Ganglia: More than just a switching device

The basal ganglia consist of a variety of subcortical nuclei engaged in motor control and executive functions, such as motor learning, behavioral control, and emotion. The striatum, a major basal ganglia component, is particularly useful for cognitive planning of purposive motor acts owing to its structural features and the neuronal circuitry established with the cerebral cortex. Recent data indicate emergent functions played by the striatum. Indeed, cortico-striatal circuits carrying motor information are paralleled by circuits originating from associative and limbic territories, which are functionally integrated in the striatum. Functional integration between brain areas is achieved through patterns of coherent activity. Coherence belonging to cortico-basal ganglia circuits is also present in Parkinson's disease patients. Excessive synchronization occurring in this pathology is reduced by dopaminergic therapies. The mechanisms through which the dopaminergic effects may be addressed are the object of several ongoing investigations. Overall, the bulk of data reported in recent years has provided new vistas concerning basal ganglia role in the organization and control of movement and behavior, both in physiological and pathological conditions. In this review, basal ganglia functions involved in the organization of main movement categories and behaviors are critically discussed. Comparatively, the multiplicity of Parkinson's disease symptomatology is also revised.

Cooperative hand movements: task-dependent modulation of ipsi- and contralateral cortical control

Cooperative hand movements are known to be controlled by a task-specific neural coupling associated with an involvement of the respective ipsilateral hemispheres. The aim of this study was to explore in how far this neural control applies to and is modulated during various, fine and gross, cooperative hand movements required during activities of daily living. Somatosensory evoked potentials and contralateral electromyographic reflex responses to unilateral ulnar nerve stimulation were simultaneously recorded in healthy participants during three different cooperative hand movement tasks and a resting condition. Amplitude ratio (ipsi-/contralateral) of the somatosensory evoked potentials, which is a measure for the involvement of the ipsilateral hemisphere in movement control, was higher in all three movement tasks compared to resting. This ratio was highest during the fine cooperative movement studied here. Contralateral reflex responses, as a measure for the functional coupling of the arms, were elicited following stimulation of both arms during gross cooperative movements. However, such a response could only be elicited in the dominant arm during fine movement. It is concluded that the neural coupling and thus enhancement of ipsilateral cortical control is preserved through different cooperative hand movement tasks, independently whether fine or gross motor tasks are performed. However, modulation of cortical control can be observed as ipsilateral cortical control is stronger during fine movements and functional coupling of the arms more focused to the dominant hand compared to gross cooperative tasks.

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.

Gait Asymmetry in People With Parkinson's Disease Is Linked to Reduced Integrity of Callosal Sensorimotor Regions

Background

Individuals with Parkinson’s disease (PD) often manifest significant temporal and spatial asymmetries of the lower extremities during gait, which significantly contribute to mobility impairments. While the neural mechanisms underlying mobility asymmetries within this population remain poorly understood, recent evidence points to altered microstructural integrity of white matter fiber tracts within the corpus callosum as potentially playing a substantial role.

Objectives

The purpose of this study was to quantify spatial and temporal gait asymmetries as well as transcallosal microstructural integrity of white matter fiber tracts connecting the primary and secondary sensorimotor cortices in people with PD and age-matched control participants.

Methods

Spatial and temporal gait asymmetry in the levodopa off state was assessed using an instrumented walkway. On the next day, diffusion-weighted images were collected to assess white matter microstructural integrity in transcallosal fibers connecting the homologous sensorimotor cortical regions.

Results

People with PD exhibited significantly more temporal and spatial gait asymmetry than healthy control subjects. Furthermore, people with PD had significantly reduced white matter microstructural integrity of transcallosal fibers connecting homologous regions of the pre-supplementary motor and supplementary motor areas (SMAs), but not the primary motor or somatosensory cortices. Finally, reduced transcallosal fiber tract integrity of the pre-SMA and S1 was associated with greater step length asymmetry in people with PD.

Conclusion

People with PD showed increased step length asymmetries and decreased microstructural integrity of callosal white matter tracts connecting the higher-order sensorimotor cortices (pre-SMA and SMA). The strong association between gait asymmetries and corpus collosum integrity, supports the hypothesis that reduced transcallosal structural connectivity is a significant mechanism underlying gait asymmetries in people with PD.

Coherence and interlimb force control: Effects of visual gain

Neural coupling across hemispheres and homologous muscles often appears during bimanual motor control. Force coupling in a specific frequency domain may indicate specific bimanual force coordination patterns. This study investigated coherence on pairs of bimanual isometric index finger force while manipulating visual gain and task asymmetry conditions. We used two visual gain conditions (low and high gain = 8 and 512 pixels/N), and created task asymmetry by manipulating coefficient ratios imposed on the left and right index finger forces (0.4:1.6; 1:1; 1.6:0.4, respectively). Unequal coefficient ratios required different contributions from each hand to the bimanual force task resulting in force asymmetry. Fourteen healthy young adults performed bimanual isometric force control at 20% of their maximal level of the summed force of both fingers. We quantified peak coherence and relative phase angle between hands at 0-4, 4-8, and 8-12 Hz, and estimated a signal-to-noise ratio of bimanual forces. The findings revealed higher peak coherence and relative phase angle at 0-4 Hz than at 4-8 and 8-12 Hz for both visual gain conditions. Further, peak coherence and relative phase angle values at 0-4 Hz were larger at the high gain than at the low gain. At the high gain, higher peak coherence at 0-4 Hz collapsed across task asymmetry conditions significantly predicted greater signal-to-noise ratio. These findings indicate that a greater level of visual information facilitates bimanual force coupling at a specific frequency range related to sensorimotor processing.

Age-Dependent Modulations of Resting State Connectivity Following Motor Practice

Recent work in young adults has demonstrated that motor learning can modulate resting state functional connectivity. However, evidence for older adults is scarce. Here, we investigated whether learning a bimanual tracking task modulates resting state functional connectivity of both inter- and intra-hemispheric regions differentially in young and older individuals, and whether this has behavioral relevance. Both age groups learned a set of complex bimanual tracking task variants over a 2-week training period. Resting-state and task-related functional magnetic resonance imaging scans were collected before and after training. Our analyses revealed that both young and older adults reached considerable performance gains. Older adults even obtained larger training-induced improvements relative to baseline, but their overall performance levels were lower than in young adults. Short-term practice resulted in a modulation of resting state functional connectivity, leading to connectivity increases in young adults, but connectivity decreases in older adults. This pattern of age differences occurred for both inter- and intra-hemispheric connections related to the motor network. Additionally, long-term training-induced increases were observed in intra-hemispheric connectivity in the right hemisphere across both age groups. Overall, at the individual level, the long-term changes in inter-hemispheric connectivity correlated with training-induced motor improvement. Our findings confirm that short-term task practice shapes spontaneous brain activity differentially in young and older individuals. Importantly, the association between changes in resting state functional connectivity and improvements in motor performance at the individual level may be indicative of how training shapes the short-term functional reorganization of the resting state motor network for improvement of behavioral performance.

Interhemispheric motor interactions in hemiparetic children with perinatal stroke: Clinical correlates and effects of neuromodulation therapy

OBJECTIVE

Brain stimulation and constraint therapy may enhance function after perinatal stroke but mechanisms are unknown. We characterized interhemispheric interactions (IHI) in hemiparetic children and explored their relationship to motor function and neuromodulation.

METHODS

Forty-five hemiparetic perinatal stroke subjects aged 6-19 years completed a clinical trial of repetitive-transcranial magnetic stimulation (rTMS) and constraint therapy. Paired-pulse TMS measured IHI in cases and normal controls. Suprathreshold conditioning stimuli preceded contralateral test stimuli bidirectionally: stroke to non-stroke (SNS) and non-stroke to stroke (NSS). Primary outcome was the interhemispheric ratio (IHR) between conditioned and test only MEP amplitudes X100 (<100 implied inhibition). Motor outcomes at baseline and post-intervention were compared to IHR.

RESULTS

Procedures were well tolerated. IHI occurred bidirectionally in controls. Eighteen stroke participants had complete data. IHR were increased in stroke participants in both directions. SNS IHR was >100 (facilitation) in 39% of measurements and correlated with better motor function. NSS IHR correlated with poorer motor function. Intervention-induced clinical change was not associated with IHR.

CONCLUSIONS

Interhemispheric interactions are altered and related to clinical function, but not necessarily neuromodulation, in children with perinatal stroke.

SIGNIFICANCE

Adding interhemispheric interactions to evolving models of developmental plasticity following early injury may advance neuromodulation strategies.

Task-relevant brain networks identified with simultaneous PET/MR imaging of metabolism and connectivity

Except for task-specific functional MRI, the vast majority of imaging studies assessed human brain function at resting conditions. However, tracking task-specific neuronal activity yields important insight how the brain responds to stimulation. We specifically investigated changes in glucose metabolism, functional connectivity and white matter microstructure during task performance using several recent methodological advancements. Opening the eyes and right finger tapping had elicited an increased glucose metabolism in primary visual and motor cortices, respectively. Furthermore, a decreased metabolism was observed in the regions of the default mode network, which allowed absolute quantification of commonly described deactivations during cognitive tasks. These brain regions showed widespread task-specific changes in functional connectivity, which stretched beyond their primary resting-state networks and presumably reflected the level of recruitment of certain brain regions for each task. Finally, the corresponding white matter fiber pathways exhibited changes in axial and radial diffusivity during the tasks, which were regionally distinctive for certain tract groups. These results highlight that even simple task performance leads to substantial changes of entire brain networks. Exploiting the complementary nature of the different imaging modalities may reveal novel insights how the brain processes external stimuli and which networks are involved in certain tasks.

Musical training increases functional connectivity, but does not enhance mu suppression

Musical training provides an ideal platform for investigating action representation for sound. Learning to play an instrument requires integration of sensory and motor perception-action processes. Functional neuroimaging studies have indicated that listening to trained music can result in the activity in premotor areas, even after a short period of training. These studies suggest that action representation systems are heavily dependent on specific sensorimotor experience. However, others suggest that because humans naturally move to music, sensorimotor training is not necessary and there is a more general action representation for music. We previously demonstrated that EEG mu suppression, commonly implemented to demonstrate mirror-neuron-like action representation while observing movements, can also index action representations for sounds in pianists. The current study extends these findings to a group of non-musicians who learned to play randomised sequences on a piano, in order to acquire specific sound-action mappings for the five fingers of their right hand. We investigated training-related changes in neural dynamics as indexed by mu suppression and task-related coherence measures. To test the specificity of training effects, we included sounds similar to those encountered in the training and additionally rhythm sequences. We found no effect of training on mu suppression between pre- and post-training EEG recordings. However, task-related coherence indexing functional connectivity between electrodes over audiomotor areas increased after training. These results suggest that long-term training in musicians and short-term training in novices may be associated with different stages of audiomotor integration that can be reflected in different EEG measures. Furthermore, the changes in functional connectivity were specifically found for piano tones, and were not apparent when participants listened to rhythms, indicating some degree of specificity related to training.

No effects of transcranial DLPFC stimulation on implicit task sequence learning and consolidation

Neurostimulation of the dorsolateral prefrontal cortex (DLPFC) can modulate performance in cognitive tasks. In a recent study, however, transcranial direct current stimulation (tDCS) of the DLPFC did not affect implicit task sequence learning and consolidation in a paradigm that involved bimanual responses. Because bimanual performance increases the coupling between homologous cortical areas of the hemispheres and left and right DLPFC were stimulated separately the null findings may have been due to the bimanual setup. The aim of the present study was to test the effect of neuro-stimulation on sequence learning in a uni-manual setup. For this purpose two experiments were conducted. In Experiment 1, the DLPFC was stimulated with tDCS. In Experiment 2 the DLPFC was stimulated with transcranial magnetic stimulation (TMS). In both experiments, consolidation was measured 24 hours later. The results showed that sequence learning was present in all conditions and sessions, but it was not influenced by stimulation. Likewise, consolidation of sequence learning was robust across sessions, but it was not influenced by stimulation. These results replicate and extend previous findings. They indicate that established tDCS and TMS protocols on the DLPFC do not influence implicit task sequence learning and consolidation.

Electroencephalography Pattern Variations During Motor Skill Acquisition

The present study examined how motor skill acquisition affects electroencephalography patterns and compared short- and long-term electroencephalography variations. For this purpose, 17 volunteers with no history of disease, aged 18 to 22 years, attended seven training sessions every other day to practice a pursuit tracking motor skill. Electroencephalography brainwaves were recorded and analyzed on the first and last days within pre- and post-training intervals. The results showed a significant decrease in performance error and variability with practice over time. This progress slowed at the end of training, and there was no significant improvement in individual performance at the last session. In accordance with performance variations, some changes occurred in brainwaves. Specifically, θ power at Fz and α power at Cz increased on the last test day, compared with the first, while the coherence of α at Fz-T3 and Fz-Cz decreased. β Coherence between Fz-Cz was significantly reduced from pre- to posttest. Based on these results, power changes seem to be more affected by long-term training, whereas coherence changes are sensitive to both short- and long-term training. Specifically, β coherence at Fz-Cz was more influenced by short-term effects of training, whereas θ power at Fz, α power at Cz, and α coherence at Fz-T3 and Fz-Cz were affected by longer training.

Bimanual tapping of a syncopated rhythm reveals hemispheric preferences for relative movement frequencies

In bimanual multifrequency tapping, right-handers commonly use the right hand to tap the relatively higher rate and the left hand to tap the relatively lower rate. This could be due to hemispheric specializations for the processing of relative frequencies. An extension of the double-filtering-by-frequency theory to motor control proposes a left hemispheric specialization for the control of relatively high and a right hemispheric specialization for the control of relatively low tapping rates. We investigated timing variability and rhythmic accentuation in right handers tapping mono- and multifrequent bimanual rhythms to test the predictions of the double-filtering-by-frequency theory. Yet, hemispheric specializations for the processing of relative tapping rates could be masked by a left hemispheric dominance for the control of known sequences. Tapping was thus either performed in an overlearned quadruple meter (tap of the slow rhythm on the first auditory beat) or in a syncopated quadruple meter (tap of the slow rhythm on the fourth auditory beat). Independent of syncopation, the right hand outperformed the left hand in timing accuracy for fast tapping. A left hand timing benefit for slow tapping rates as predicted by the double-filtering-by-frequency theory was only found in the syncopated tapping group. This suggests a right hemisphere preference for the control of slow tapping rates when rhythms are not overlearned. Error rates indicate that overlearned rhythms represent hierarchically structured meters that are controlled by a single timer that could potentially reside in the left hemisphere.

Learned control of inter-hemispheric connectivity: Effects on bimanual motor performance

Bimanual movements involve the interactions between both primary motor cortices. These interactions are assumed to involve phase-locked oscillatory brain activity referred to as inter-hemispheric functional coupling. So far, inter-hemispheric functional coupling has been investigated as a function of motor performance. These studies report mostly a negative correlation between the performance in motor tasks and the strength of functional coupling. However, correlation might not reflect a causal relationship. To overcome this limitation, we opted for an alternative approach by manipulating the strength of inter-hemispheric functional coupling and assessing bimanual motor performance as a dependent variable. We hypothesize that an increase/decrease of functional coupling deteriorates/facilitates motor performance in an out-of-phase bimanual finger-tapping task. Healthy individuals were trained to volitionally regulate functional coupling in an operant conditioning paradigm using real-time magnetoencephalography neurofeedback. During operant conditioning, two discriminative stimuli were associated with upregulation and downregulation of functional coupling. Effects of training were assessed by comparing motor performance prior to (pre-test) and after the training (post-test). Participants receiving contingent feedback learned to upregulate and downregulate functional coupling. Comparing motor performance, as indexed by the ratio of tapping speed for upregulation versus downregulation trials, no change was found in the control group between pre- and post-test. In contrast, the group receiving contingent feedback evidenced a significant decrease of the ratio implicating lower tapping speed with stronger functional coupling. Results point toward a causal role of inter-hemispheric functional coupling for the performance in bimanual tasks. Hum Brain Mapp 38:4353-4369, 2017. © 2017 Wiley Periodicals, Inc.

Vision of the body improves inter-hemispheric integration of tactile-motor responses

Sensory input from and motor output to the two sides of the body needs to be continuously integrated between the two cerebral hemispheres. This integration can be measured through its cost in terms of processing speed. In simple detection tasks, reaction times (RTs) are faster when stimuli are presented to the side of the body ipsilateral to the body part used to respond. This advantage - the contralateral-ipsilateral difference (also known as the crossed-uncrossed difference: CUD) - is thought to reflect inter-hemispheric interactions needed for sensorimotor information to be integrated between the two hemispheres. Several studies have shown that non-informative vision of the body enhances performance in tactile tasks. However, it is unknown whether the CUD can be similarly affected by vision. Here, we investigated whether the CUD is modulated by vision of the body (i.e., the stimulated hand) by presenting tactile stimuli unpredictably on the middle fingers when one hand was visible (i.e., either the right or left hand). Participants detected the stimulus and responded as fast as possible using either their left or right foot. Consistent with previous results, a clear CUD (5.8ms) was apparent on the unseen hand. Critically, however, no such effect was found on the hand that was visible (-2.2ms). Thus, when touch is delivered to a seen hand, the usual cost in processing speed of responding with a contralateral effector is eliminated. This result suggests that vision of the body improves the interhemispheric integration of tactile-motor responses.

The Role of Visual and Haptic Feedback During Dynamically Coupled Bimanual Manipulation

The mechanisms that underlie the control of bimanual actions in which the two hands act separately to manipulate different objects (uncoupled independent control) has been well studied. In contrast, much less is known about how the central nervous system controls bimanual actions that require the two hands act cooperatively to manipulate a single object (dynamically coupled control). Furthermore, there is scant research into the manual lateralization and role assignment in the processing of visual and haptic feedback during dynamically coupled bimanual tasks. In this experiment, we examined the role of the dominant and non-dominant hands during a dynamically coupled bimanual task in which visual and haptic feedback regarding object penetration were manipulated. Twelve subjects performed a bimanual grasp and reach task towards different target locations in the workspace by using two identical wrist robotic devices. Results showed haptic feedback is necessary for task completion, and that hand specialization plays a fundamental role in spatial and temporal coordination between the two limbs.

Age-Related Changes in Frontal Network Structural and Functional Connectivity in Relation to Bimanual Movement Control

Changes in both brain structure and neurophysiological function regulating homotopic as well as heterotopic interhemispheric interactions (IHIs) are assumed to be responsible for the bimanual performance deficits in older adults. However, how the structural and functional networks regulating bimanual performance decline in older adults, as well as the interplay between brain structure and function remain largely unclear. Using a dual-site transcranial magnetic stimulation paradigm, we examined the age-related changes in the interhemispheric effects from the dorsolateral prefrontal cortex and dorsal premotor cortex onto the contralateral primary motor cortex (M1) during the preparation of a complex bimanual coordination task in human. Structural properties of these interactions were assessed with diffusion-based fiber tractography. Compared with young adults, older adults showed performance declines in the more difficult bimanual conditions, less optimal brain white matter (WM) microstructure, and a decreased ability to regulate the interaction between dorsolateral prefrontal cortex and M1. Importantly, we found that WM microstructure, neurophysiological function, and bimanual performance were interrelated in older adults, whereas only the task-related changes in IHI predicted bimanual performance in young adults. These results reflect unique interactions between structure and function in the aging brain, such that declines in WM microstructural organization likely lead to dysfunctional regulation of IHI, ultimately accounting for bimanual performance deficits.

SIGNIFICANCE STATEMENT

The structural and functional changes in the aging brain are associated with a decline in movement control, compromising functional independence. We used MRI and noninvasive brain stimulation techniques to investigate white matter microstructural organization and neurophysiological function in the aging brain, in relation to bimanual movement control. We found that less optimal brain microstructural organization and task-related modulations in neurophysiological function resulted in poor bimanual performance in older adults. By interrelating brain structure, neurophysiological function, and behavior, the current study provides a comprehensive picture of biological alterations in the aging brain that underlie declines in bimanual performance.

Goal conceptualization and symmetry of arm movements affect bimanual coordination in individuals after stroke

Coordination during goal-directed movements emerges from an interaction of task and individual constraints. It is not known how individuals with unilateral stroke and age-matched controls coordinate their arms when performing symmetric and asymmetric movements to accomplish common task goals compared to independent task goals. Eleven individuals with chronic stroke and ten age-matched controls executed a bimanual task under virtual conditions that allowed systematic manipulation of symmetry and goal conditions. Spatial and temporal bimanual coordination was characterized using the cross-correlation coefficients and time lag between the tangential velocities between the two hands. While task conditions had little effect on coordination of control participants, individuals with stroke were less coordinated in space and time during common-goal bimanual actions employing asymmetric arm movements. Further, patients demonstrated lesser contribution of their paretic arm compared to their non-paretic arm during common-goal conditions. These findings indicate that conceptualization of task goals (common vs. independent) and symmetry of arm movements influence coordination and contribution of the two hands during bimanual tasks in patients with stroke.

Motor learning and cross-limb transfer rely upon distinct neural adaptation processes

Performance benefits conferred in the untrained limb after unilateral motor practice are termed cross-limb transfer. Although the effect is robust, the neural mechanisms remain incompletely understood. In this study we used noninvasive brain stimulation to reveal that the neural adaptations that mediate motor learning in the trained limb are distinct from those that underlie cross-limb transfer to the opposite limb. Thirty-six participants practiced a ballistic motor task with their right index finger (150 trials), followed by intermittent theta-burst stimulation (iTBS) applied to the trained (contralateral) primary motor cortex (cM1 group), the untrained (ipsilateral) M1 (iM1 group), or the vertex (sham group). After stimulation, another 150 training trials were undertaken. Motor performance and corticospinal excitability were assessed before motor training, pre- and post-iTBS, and after the second training bout. For all groups, training significantly increased performance and excitability of the trained hand, and performance, but not excitability, of the untrained hand, indicating transfer at the level of task performance. The typical facilitatory effect of iTBS on MEPs was reversed for cM1, suggesting homeostatic metaplasticity, and prior performance gains in the trained hand were degraded, suggesting that iTBS interfered with learning. In stark contrast, iM1 iTBS facilitated both performance and excitability for the untrained hand. Importantly, the effects of cM1 and iM1 iTBS on behavior were exclusive to the hand contralateral to stimulation, suggesting that adaptations within the untrained M1 contribute to cross-limb transfer. However, the neural processes that mediate learning in the trained hemisphere vs. transfer in the untrained hemisphere appear distinct.

Task-Dependent Bimanual Coordination After Stroke: Relationship With Sensorimotor Impairments

OBJECTIVES

To determine (1) bimanual coordination deficits in patients with stroke using 3-dimensional kinematic analyses as they perform naturalistic tasks requiring collaborative interaction of the 2 arms; and (2) whether bimanual coordination deficits are related to clinical measures of sensorimotor impairments and unimanual performance of the paretic arm.

DESIGN

Case-control study.

SETTING

Rehabilitation hospital research institute.

PARTICIPANTS

Participants (N=24) were patients with unilateral chronic stroke (n=14) and age-matched controls (n=10).

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Temporal coordination between the 2 hands as participants performed (1) a symmetric task: reach to pick up a box using both hands; and (2) an asymmetric task: open a drawer with 1 hand to press a button inside with the other hand.

RESULTS

During the symmetric task, patients and controls showed preserved temporal coupling while transporting the hands to the box. However, on reaching the box, patients demonstrated an impaired ability to cooperatively interact their 2 arms for an efficient pickup. This led to significantly longer pickup times compared with controls. Pickup time positively correlated with proprioceptive deficits of the paretic arm. During the asymmetric task, patients had a longer time delay between drawer opening and button pressing movements than controls. The deficits in asymmetric coordination did not significantly correlate with sensorimotor impairments or unimanual paretic arm performance.

CONCLUSIONS

Bimanual coordination was impaired in patients poststroke during symmetric and asymmetric bimanual tasks that required cooperative interaction between the 2 arms. While the proprioceptive system contributes to symmetric cooperative coordination, commonly tested measures of paretic arm impairment or performance, or both, do not strongly predict deficits in bimanual coordination.