Neural pathways mediating bilateral interactions between the upper limbs

The Cross-Education Phenomenon: Brain and Beyond

Objectives: Unilateral resistance training produces strength gains in the untrained homologous muscle group, an effect termed “cross-education.” The observed strength transfer has traditionally been considered a phenomenon of the nervous system, with few studies examining the contribution of factors beyond the brain and spinal cord. In this hypothesis and theory article, we aim to discuss further evidence for structural and functional adaptations occurring within the nervous, muscle, and endocrine systems in response to unilateral resistance training. The limitations of existing cross-education studies will be explored, and novel potential stakeholders that may contribute to the cross-education effect will be identified.

Design: Critical review of the literature.

Method: Search of online databases.

Results: Studies have provided evidence that functional reorganization of the motor cortex facilitates, at least in part, the effects of cross-education. Cross-activation of the “untrained” motor cortex, ipsilateral to the trained limb, plays an important role. While many studies report little or no gains in muscle mass in the untrained limb, most experimental designs have not allowed for sensitive or comprehensive investigation of structural changes in the muscle.

Conclusions: Increased neural drive originating from the “untrained” motor cortex contributes to the cross-education effect. Adaptive changes within the muscle fiber, as well as systemic and hormonal factors require further investigation. An increased understanding of the physiological mechanisms contributing to cross-education will enable to more effectively explore its effects and potential applications in rehabilitation of unilateral movement disorders or injury.

Consecutive learning of opposing unimanual motor tasks using the right arm followed by the left arm causes intermanual interference

Intermanual transfer (motor memory generalization across arms) and motor memory interference (impairment of retest performance in consecutive motor learning) are well-investigated motor learning phenomena. However, the interplay of these phenomena remains elusive, i.e., whether intermanual interference occurs when two unimanual tasks are consecutively learned using different arms. Here, we examine intermanual interference when subjects consecutively adapt their right and left arm movements to novel dynamics. We considered two force field tasks A and B which were of the same structure but mirrored orientation (B = -A). The first test group (ABA-group) consecutively learned task A using their right arm and task B using their left arm before being retested for task A with their right arm. Another test group (AAA-group) learned only task A in the same right-left-right arm schedule. Control subjects learned task A using their right arm without intermediate left arm learning. All groups were able to adapt their right arm movements to force field A and both test groups showed significant intermanual transfer of this initial learning to the contralateral left arm of 21.9% (ABA-group) and 27.6% (AAA-group). Consecutively, both test groups adapted their left arm movements to force field B (ABA-group) or force field A (AAA-group). For the ABA-group, left arm learning caused significant intermanual interference of the initially learned right arm task (68.3% performance decrease). The performance decrease of the AAA-group (10.2%) did not differ from controls (15.5%). These findings suggest that motor control and learning of right and left arm movements involve partly similar neural networks or underlie a vital interhemispheric connectivity. Moreover, our results suggest a preferred internal task representation in extrinsic Cartesian-based coordinates rather than in intrinsic joint-based coordinates because interference was absent when learning was performed in extrinsically equivalent fashion (AAA-group) but interference occurred when learning was performed in intrinsically equivalent fashion (ABA-group).

High-definition transcranial direct current stimulation to both primary motor cortices improves unimanual and bimanual dexterity

While most research on brain stimulation with transcranial direct current stimulation (tDCS) targets unimanual motor tasks, little is known about its effects on bimanual motor performance. This study aims to investigate the effects of tDCS on unimanual as well as bimanual motor dexterity. We examined the effects of bihemispheric anodal high-definition tDCS (HD-atDCS) on both primary motor cortices (M1) applied concurrent with unimanual and bimanual motor training. We then measured the effects with the Purdue Pegboard Test (PPT) and compared them to a sham stimulation. Between a pretest and posttest, 31 healthy, right-handed participants practiced the PPT on three consecutive days and received - simultaneous to motor practice - either HD-atDCS over the left and right M1 (STIM, n=16) or a sham stimulation (SHAM, n=15). Five to seven days after the posttest, a follow-up test was conducted. Two-way ANOVAs with repeated measures showed significantly increased performance for all PPT-scores (p<0.001) in both groups. The scores for the right hand, both hands, and overall showed significant TIME x GROUP interactions (p<.05) with more improved performance for the STIM group, while left hand performance was not significantly altered. These effects were most pronounced in the follow-up test. Thus, we can conclude that a bihemispheric HD-atDCS of both M1's improves performance of unimanual and bimanual dexterity. The strength of the effects, however, depends on which hand is used in the unimanual task and the type of bimanual task performed.

Structural and Functional Cortical Connectivity Mediating Cross Education of Motor Function

Cross-education (CE) is the process whereby training with one limb leads to subsequent improvement in performance by the opposite untrained limb. We used multimodal neuroimaging in humans to investigate the mediating neural mechanisms by relating quantitative estimates of functional and structural cortical connectivity to individual levels of interlimb transfer. Resting-state (rs)-fMRI and diffusion weighted imaging (DWI) scans were undertaken before unilateral ballistic wrist flexion training. The rs-fMRI sequence was repeated immediately afterward. The increase in performance of the untrained limb was 83.6% of that observed for the trained limb and significantly greater than that of a control group who undertook no training. Functional connectivity in the resting motor network between right and left supplementary motor areas (SMA) was elevated after training. These changes were not, however, correlated with individual levels of transfer. Analysis of the DWI data using constrained spherical deconvolution-based tractography indicated that fractional anisotropy and apparent fiber density in tracts connecting bilateral SMA were negatively correlated with and predictive of transfer. The findings suggest that interhemispheric interactions between bilateral SMA play an instrumental role in CE and that the structural integrity of the connecting white matter pathways influences the level of transfer.

SIGNIFICANCE STATEMENT Strength or skill training with one limb also brings about improvements in the performance of the opposite, untrained limb. This phenomenon, termed cross-education (CE), has obvious potential for the rehabilitation of functional capacity that has been lost through brain insult or musculoskeletal injury. The neural mechanisms that give rise to CE are, however, poorly understood. We used a combination of neuroimaging methods to investigate the pathways in the human brain that mediate CE. We determined that the supplementary motor area (SMA) plays an important role in the interlimb transfer of performance gains and demonstrate that the quality of the white matter fibers connecting right and left SMA predicts the benefit that an individual derives from CE.

Spontaneous alterations of regional brain activity in patients with adult generalized anxiety disorder

OBJECTIVE

We aimed to examine how spontaneous brain activity might be related to the pathophysiology of generalized anxiety disorder (GAD).

PATIENTS AND METHODS

Using resting-state functional MRI, we examined spontaneous regional brain activity in 31 GAD patients (mean age, 36.87±9.16 years) and 36 healthy control participants (mean age, 39.53±8.83 years) matched for age, education, and sex from December 2014 to October 2015. We performed a two-sample t-test on the voxel-based analysis of the regional homogeneity (ReHo) maps. We used Pearson correlation analysis to compare scores from the Hamilton Anxiety Rating Scale, Hamilton Depression Rating Scale, State-Trait Anxiety Scale-Trait Scale, and mean ReHo values.

RESULTS

We found abnormal spontaneous activity in multiple regions of brain in GAD patients, especially in the sensorimotor cortex and emotional regions. GAD patients showed decreased ReHo values in the right orbital middle frontal gyrus, left anterior cingulate cortex, right middle frontal gyrus, and bilateral supplementary motor areas, with increased ReHo values in the left middle temporal gyrus, left superior temporal gyrus, and right superior occipital gyrus. The ReHo value of the left middle temporal gyrus correlated positively with the Hamilton Anxiety Rating Scale scores.

CONCLUSION

These results suggest that altered local synchronization of spontaneous brain activity may be related to the pathophysiology of GAD.

High-frequency neuromuscular electrical stimulation modulates interhemispheric inhibition in healthy humans

High-frequency neuromuscular electrical stimulation (HF NMES) induces muscular contractions through neural mechanisms that partially match physiological motor control. Indeed, a portion of the contraction arises from central mechanisms, whereby spinal motoneurons are recruited through the evoked sensory volley. However, the involvement of supraspinal centers of motor control during such stimulation remains poorly understood. Therefore, we tested whether a single HF NMES session applied to the upper limb influences interhemispheric inhibition (IHI) from left to right motor cortex (M1). Using noninvasive electrophysiology and transcranial magnetic stimulation, we evaluated the effects of a 10-min HF NMES session applied to a right wrist flexor on spinal and corticospinal excitability of both arms, as well as IHI, in healthy subjects. HF NMES induced a rapid decline in spinal excitability on the right stimulated side that closely matched the modulation of evoked force during the protocol. More importantly, IHI was significantly increased by HF NMES, and this increase was correlated to the electromyographic activity within the contralateral homologous muscle. Our study highlights a new neurophysiological mechanism, suggesting that HF NMES has an effect on the excitability of the transcallosal pathway probably to regulate the lateralization of the motor output. The data suggest that HF NMES can modify the hemispheric balance between both M1 areas. These findings provide important novel perspectives for the implementation of HF NMES in sport training and neurorehabilitation.

NEW & NOTEWORTHY

High-frequency neuromuscular electrical stimulation (HF NMES) induces muscular contractions that partially match physiological motor control. Here, we tested whether HF NMES applied to the upper limb influences interhemispheric inhibition. Our results show that interhemispheric inhibition was increased after HF NMES and that this increase was correlated to the electromyographic activity within the contralateral homologous muscle. This opens up original perspectives for the implementation of HF NMES in sport training and neurorehabilitation.

Task-oriented robot-assisted stroke therapy of paretic limb improves control in a unilateral and bilateral functional drink task: a case study

The purpose of this paper is to evaluate the functional, temporal and spatial effect of a unilateral task-oriented, robot-assisted training on unilateral and bilateral task performance of a drinking task using a real object. Two chronic stroke survivors experienced task-oriented robot assisted therapy, in which the paretic arm was trained using reaching and grasping tasks over 4 weeks. Both subjects experienced improvement in motor control as measured by Fugl-Meyer. The paretic arm was evaluated using movement smoothness (MS) and time to completion (TCT) measures before and after therapy. From the results, we found that the unilateral robotassisted training improved paretic arm control in the unilateral and the bilateral drink task. However, the influence of the non-paretic movement on the temporal and spatial paretic arm control was evident both pre and post therapy suggesting inter-limb coupling aids in the transfer of unilateral improvements in motor control to improvements in bilateral motor control.

Neural coupling of cooperative hand movements after stroke: role of ipsilateral afference

We investigated the role of ipsilateral ascending pathways in the neural coupling underlying cooperative hand movements of stroke subjects. Ipsi‐ and contralateral somatosensory evoked potentials (SSEP) were recorded following ulnar nerve stimulation during cooperative and non‐cooperative hand movements. The amplitude ratio, that is, ipsilateral divided by contralateral amplitude, was highest during the cooperative task when the affected arm was stimulated, reflecting an enhanced afferent volley to the unaffected hemisphere. The presence of ipsilateral SSEP from the paretic arm was closely related with the patients' hand function. This shows for the first time a laterality in ascending pathways after unilateral stroke and implies an involvement of the unaffected hemisphere in the control of paretic hand movements.

Transcallosal connectivity of the human cortical motor network

The organisational and architectural configuration of white matter pathways connecting brain regions has ramifications for all facets of the human condition, including manifestations of incipient neurodegeneration. Although diffusion tensor imaging (DTI) has been used extensively to visualise white matter connectivity, due to the widespread presence of crossing fibres, the lateral projections of the corpus callosum are not normally detected using this methodology. Detailed knowledge of the transcallosal connectivity of the human cortical motor network has, therefore, remained elusive. We employed constrained spherical deconvolution (CSD) tractography—an approach that is much less susceptible to the influence of crossing fibres, in order to derive complete in vivo characterizations of white matter pathways connecting specific motor cortical regions to their counterparts and other loci in the opposite hemisphere. The revealed patterns of connectivity closely resemble those derived from anatomical tracing in primates. It was established that dorsal premotor cortex (PMd) and supplementary motor area (SMA) have extensive interhemispheric connectivity—exhibiting both dense homologous projections, and widespread structural relations with every other region in the contralateral motor network. Through this in vivo portrayal, the importance of non-primary motor regions for interhemispheric communication is emphasised. Additionally, distinct connectivity profiles were detected for the anterior and posterior subdivisions of primary motor cortex. The present findings provide a comprehensive representation of transcallosal white matter projections in humans, and have the potential to inform the development of models and hypotheses relating structural and functional brain connectivity.

Neural Adaptations Associated with Interlimb Transfer in a Ballistic Wrist Flexion Task

Cross education is the process whereby training of one limb gives rise to increases in the subsequent performance of its opposite counterpart. The execution of many unilateral tasks is associated with increased excitability of corticospinal projections from primary motor cortex (M1) to the opposite limb. It has been proposed that these effects are causally related. Our aim was to establish whether changes in corticospinal excitability (CSE) arising from prior training of the opposite limb determine levels of interlimb transfer. We used three vision conditions shown previously to modulate the excitability of corticospinal projections to the inactive (right) limb during wrist flexion movements performed by the training (left) limb. These were: (1) mirrored visual feedback of the training limb; (2) no visual feedback of either limb; and (3) visual feedback of the inactive limb. Training comprised 300 discrete, ballistic wrist flexion movements executed as rapidly as possible. Performance of the right limb on the same task was assessed prior to, at the mid point of, and following left limb training. There was no evidence that variations in the excitability of corticospinal projections (assessed by transcranial magnetic stimulation (TMS)) to the inactive limb were associated with, or predictive of, the extent of interlimb transfer that was expressed. There were however associations between alterations in muscle activation dynamics observed for the untrained limb, and the degree of positive transfer that arose from training of the opposite limb. The results suggest that the acute adaptations that mediate the bilateral performance gains realized through unilateral practice of this ballistic wrist flexion task are mediated by neural elements other than those within M1 that are recruited at rest by single-pulse TMS.

Change in Movement-Related Cortical Potentials Following Constraint-Induced Movement Therapy (CIMT) After Stroke

Abstract. Patients with chronic stroke were given Constraint-Induced Movement Therapy (CIMT) over an intensive two-week course of treatment. The intervention resulted in a large improvement in use of the more-affected upper extremity in the laboratory and in the real-world environment. High-resolution electroencephalography (EEG) showed that the treatment produced marked changes in cortical activity that correlated with the significant rehabilitative effects. Repetitive unilateral self-paced voluntary movements showed a large increase after treatment in the amplitudes of the late components of the Bereitschaftspotential (BP) both in the hemisphere contralateral to the more-affected arm and in the ipsilateral hemisphere. Simultaneous electromyographic recordings (EMG) and other aspects of the data indicate that the emergence of the movement-related neural source in the healthy hemisphere was not due to mirror movements of the non-test hand and that the increase in BP amplitudes was not the result of an increase in the force or effort of the response pre- to post-treatment. The results are consistent with the rehabilitation treatment having produced a use-dependent cortical reorganization and is a case where the physiological data interdigitates with and provides additional credibility to the clinical data.

Corticospinal Modulations during Bimanual Movement with Different Relative Phases

The purpose of this study was to investigate corticospinal modulation of bimanual (BM) movement with different relative phases (RPs). The participants rhythmically abducted and adducted the right index finger (unimanual (UM) movement) or both index fingers (BM movement) with a cyclic duration of 1 s. The RP of BM movement, defined as the time difference between one hand movement and the other hand movement, was 0°, 90°, or 180°. Motor evoked potentials (MEPs) in the right flexor dorsal interosseous muscle elicited by transcranial magnetic stimulation (TMS) were obtained during UM or BM movement. Corticospinal excitability in the first dorsal interosseous muscle during BM movement with 90° RP was higher than that during UM movement or BM movement with 0° or 180° RP. The correlation between muscle activity level and corticospinal excitability during BM movement with 90° RP was smaller than that during UM movement or BM movement with 0° or 180° RP. The higher corticospinal excitability during BM movement with 90° RP may be caused by the greater effort expended to execute a difficult task, the involvement of interhemispheric interaction, a motor binding process, or task acquisition. The lower dependency of corticospinal excitability on the muscle activity level during BM movement with 90° RP may reflect the minor corticospinal contribution to BM movement with an RP that is not in the attractor state.

Longitudinal Follow-Up of Mirror Movements after Stroke: A Case Study

Mirror movement (MM), or visible involuntary movements of a relaxed hand during voluntary fine finger movements of an activated opposite hand, can be observed in the hand that is on the unaffected side of patients with stroke. In the present study, we longitudinally examined the relationship between voluntary movement of the affected hand and MM in the unaffected hand in a single case. We report a 73-year-old woman with a right pontine infarct and left moderate hemiparesis. MM was observed as an extension movement of the unaffected right index finger during extension movement of the affected left index finger. The affected right index movement was found to increase, while MM of the unaffected left index finger was observed to decrease with time. These results indicate that the assessment of MM might be useful for studying the process of motor recovery in patients with stroke.

Bimanual motor coordination controlled by cooperative interactions in intrinsic and extrinsic coordinates

Although strong motor coordination in intrinsic muscle coordinates has frequently been reported for bimanual movements, coordination in extrinsic visual coordinates is also crucial in various bimanual tasks. To explore the bimanual coordination mechanisms in terms of the frame of reference, here we characterized implicit bilateral interactions in visuomotor tasks. Visual perturbations (finger-cursor gain change) were applied while participants performed a rhythmic tracking task with both index fingers under an in-phase or anti-phase relationship in extrinsic coordinates. When they corrected the right finger's amplitude, the left finger's amplitude unintentionally also changed [motor interference (MI)], despite the instruction to keep its amplitude constant. Notably, we observed two specificities: one was large MI and low relative-phase variability (PV) under the intrinsic in-phase condition, and the other was large MI and high PV under the extrinsic in-phase condition. Additionally, using a multiple-interaction model, we successfully decomposed MI into intrinsic components caused by motor correction and extrinsic components caused by visual-cursor mismatch of the right finger's movements. This analysis revealed that the central nervous system facilitates MI by combining intrinsic and extrinsic components in the condition with in-phases in both intrinsic and extrinsic coordinates, and that under-additivity of the effects is explained by the brain's preference for the intrinsic interaction over extrinsic interaction. In contrast, the PV was significantly correlated with the intrinsic component, suggesting that the intrinsic interaction dominantly contributed to bimanual movement stabilization. The inconsistent features of MI and PV suggest that the central nervous system regulates multiple levels of bilateral interactions for various bimanual tasks.

Interhemispheric connectivity during bimanual isometric force generation

Interhemispheric interactions through the corpus callosum play an important role in the control of bimanual forces. However, the extent to which physiological connections between primary motor cortices are modulated during increasing levels of bimanual force generation in intact humans remains poorly understood. Here we studied coherence between electroencephalographic (EEG) signals and the ipsilateral cortical silent period (iSP), two well-known measures of interhemispheric connectivity between motor cortices, during unilateral and bilateral 10%, 40%, and 70% of maximal isometric voluntary contraction (MVC) into index finger abduction. We found that EEG-EEG coherence in the alpha frequency band decreased while the iSP area increased during bilateral compared with unilateral 40% and 70% but not 10% of MVC. Decreases in coherence in the alpha frequency band correlated with increases in the iSP area, and subjects who showed this inverse relation were able to maintain more steady bilateral muscle contractions. To further examine the relationship between the iSP and coherence we electrically stimulated the ulnar nerve at the wrist at the alpha frequency. Electrical stimulation increased coherence in the alpha frequency band and decreased the iSP area during bilateral 70% of MVC. Altogether, our findings demonstrate an inverse relation between alpha oscillations and the iSP during strong levels of bimanual force generation. We suggest that interactions between neural pathways mediating alpha oscillatory activity and transcallosal inhibition between motor cortices might contribute to the steadiness of strong bilateral isometric muscle contractions in intact humans.

Ipsilateral cortical motor desynchronisation is reduced in Benign Epilepsy with Centro-Temporal Spikes

OBJECTIVE

Magnetoencephalography (MEG) and a simple motor paradigm were used to study induced sensorimotor responses and their relationship to motor skills in children diagnosed with Benign Epilepsy with Centro-Temporal Spikes (BECTS).

METHODS

Twenty-one children with BECTS and 15 age-matched controls completed a finger abduction task in MEG; movement-related oscillatory responses were derived and contrasted between groups. A subset of children also completed psycho-behavioural assessments. Regression analyses explored the relationship of MEG responses to manual dexterity performance, and dependence upon clinical characteristics.

RESULTS

In children with BECTS, manual dexterity was below the population mean (p=.002) and three showed severe impairment. Our main significant finding was of reduced ipsilateral movement related beta desynchrony (MRBDi) in BECTS relative to the control group (p=.03) and predicted by epileptic seizure recency (p=.02), but not age, medication status, or duration of epilepsy. Laterality scores across the entire cohort indicated that less lateralised MRBD predicted better manual dexterity (p=.04).

CONCLUSIONS

Altered movement-related oscillatory responses in ipsilateral motor cortex were associated with motor skill deficits in children with BECTS. These changes were more marked in those with more recent seizures.

SIGNIFICANCE

These findings may reflect differences in inter-hemispheric interactions during motor control in BECTS.

Effects of Computer-Aided Interlimb Force Coupling Training on Paretic Hand and Arm Motor Control following Chronic Stroke: A Randomized Controlled Trial

Objective

We investigated the training effects of interlimb force coupling training on paretic upper extremity outcomes in patients with chronic stroke and analyzed the relationship between motor recovery of the paretic hand, arm and functional performances on paretic upper limb.

Design

A randomized controlled trial with outcome assessment at baseline and after 4 weeks of intervention.

Setting

Taipei Veterans General Hospital, National Yang-Ming University.

Participants

Thirty-three subjects with chronic stroke were recruited and randomly assigned to training (n = 16) and control groups (n = 17).

Interventions

The computer-aided interlimb force coupling training task with visual feedback included different grip force generation methods on both hands.

Main Outcome Measures

The Barthel Index (BI), the upper extremity motor control Fugl-Meyer Assessment (FMA-UE), the Motor Assessment Score (MAS), and the Wolf Motor Function Test (WMFT). All assessments were executed by a blinded evaluator, and data management and statistical analysis were also conducted by a blinded researcher.

Results

The training group demonstrated greater improvement on the FMA-UE (p<.001), WMFT (p<.001), MAS (p = .004) and BI (p = .037) than the control group after 4 weeks of intervention. In addition, a moderate correlation was found between the improvement of scores for hand scales of the FMA and other portions of the FMA UE (r = .528, p = .018) or MAS (r = .596, p = .015) in the training group.

Conclusion

Computer-aided interlimb force coupling training improves the motor recovery of a paretic hand, and facilitates motor control and enhances functional performance in the paretic upper extremity of people with chronic stroke.

Trial Registration

ClinicalTrials.gov NCT02247674.

One hand clapping: lateralization of motor control

Lateralization of motor control refers to the ability to produce pure unilateral or asymmetric movements. It is required for a variety of coordinated activities, including skilled bimanual tasks and locomotion. Here we discuss the neuroanatomical substrates and pathophysiological underpinnings of lateralized motor outputs. Significant breakthroughs have been made in the past few years by studying the two known conditions characterized by the inability to properly produce unilateral or asymmetric movements, namely human patients with congenital “mirror movements” and model rodents with a “hopping gait”. Whereas mirror movements are associated with altered interhemispheric connectivity and abnormal corticospinal projections, abnormal spinal cord interneurons trajectory is responsible for the “hopping gait”. Proper commissural axon guidance is a critical requirement for these mechanisms. Interestingly, the analysis of these two conditions reveals that the production of asymmetric movements involves similar anatomical and functional requirements but in two different structures: (i) lateralized activation of the brain or spinal cord through contralateral silencing by cross-midline inhibition; and (ii) unilateral transmission of this activation, resulting in lateralized motor output.

Force control in chronic stroke

Force control deficits are common dysfunctions after a stroke. This review concentrates on various force control variables associated with motor impairments and suggests new approaches to quantifying force control production and modulation. Moreover, related neurophysiological mechanisms were addressed to determine variables that affect force control capabilities. Typically, post stroke force control impairments include: (a) decreased force magnitude and asymmetrical forces between hands, (b) higher task error, (c) greater force variability, (d) increased force regularity, and (e) greater time-lag between muscular forces. Recent advances in force control analyses post stroke indicated less bimanual motor synergies and impaired low-force frequency structure. Brain imaging studies demonstrate possible neurophysiological mechanisms underlying force control impairments: (a) decreased activation in motor areas of the ipsilesional hemisphere, (b) increased activation in secondary motor areas between hemispheres, (c) cerebellum involvement, and (d) relatively greater interhemispheric inhibition from the contralesional hemisphere. Consistent with identifying neurophysiological mechanisms, analyzing bimanual motor synergies as well as low-force frequency structure will advance our understanding of post stroke force control.

Lateralization of brain activity pattern during unilateral movement in Parkinson's disease

We investigated the lateralization of brain activity pattern during performance of unilateral movement in drug-naïve Parkinson's disease (PD) patients with only right hemiparkinsonian symptoms. Functional MRI was obtained when the subjects performed strictly unilateral right hand movement. A laterality index was calculated to examine the lateralization. Patients had decreased activity in the left putamen and left supplementary motor area, but had increased activity in the right primary motor cortex, right premotor cortex, left postcentral gyrus, and bilateral cerebellum. The laterality index was significantly decreased in PD patients compared with controls (0.41 ± 0.14 vs. 0.84 ± 0.09). The connectivity from the left putamen to cortical motor regions and cerebellum was decreased, while the interactions between the cortical motor regions, cerebellum, and right putamen were increased. Our study demonstrates that in early PD, the lateralization of brain activity during unilateral movement is significantly reduced. The dysfunction of the striatum-cortical circuit, decreased transcallosal inhibition, and compensatory efforts from cortical motor regions, cerebellum, and the less affected striatum are likely reasons contributing to the reduced motor lateralization. The disruption of the lateralized brain activity pattern might be a reason underlying some motor deficits in PD, like mirror movements or impaired bilateral motor coordination.

Control of discrete bimanual movements: how each hand benefits from the other

Lateralized sensorimotor hand functions are often investigated separately or sequentially for each hand, e.g., in matching tasks, but rarely under more ecological circumstances where both hands move simultaneously. Using a novel bimanual paradigm in 21 young, healthy participants, this study addresses how postulated lateralized control processes of one hand influence control of the other hand across modalities. More specifically, in this paradigm one hand operates under visuomotor conditions, while the other hand receives no visual feedback and operates predominantly under kinesthetic control. Performance of the hand that does not receive visual feedback is compared between when moving alone (unimanual condition) and when moving together with the contralateral visually controlled hand (bimanual condition). Results suggest that during concurrent bimanual movements the 'invisible' hand benefits from specific control proficiencies of the 'visible' hand, indicating crossmodal and interhemispheric sharing of information that complements each hand's own strengths. These findings lend further support to a more differentiated view of functional lateralization of handedness.

Task-specific role of ipsilateral pathways: somatosensory evoked potentials during cooperative hand movements

Task-specific neural coupling during cooperative hand movements has been described in healthy volunteers, manifested by bilateral reflex electromyographic responses in forearm muscles following unilateral ulnar nerve stimulation and by task-specific activation of secondary somatosensory cortical areas (S2) in functional MRI. The aim of this study was to investigate the role of sensory input to the ipsilateral and contralateral cortex during a cooperative task. Somatosensory evoked potentials from the ulnar nerve were recorded over the ipsilateral and contralateral cortex during resting and during cooperative and noncooperative hand movements. Ipsilateral potentials with smaller amplitude were present under all conditions in almost all participants. In relation to the resting condition, the amplitudes of both the ipsilateral and the contralateral potential were reduced during the cooperative and the noncooperative tasks. Nevertheless, the reduction in amplitude was similar for the ipsilateral and the contralateral potentials in the noncooperative task, but less on the ipsilateral compared with the contralateral side during the cooperative task. The ratio of ipsilateral/contralateral somatosensory evoked potential amplitude was thus significantly larger during the cooperative task compared with the control task and the resting condition. This indicates a functional role of ipsilateral pathways connecting the cervical spinal cord with the cortex during the cooperative task. These observations favor the idea of a task-specific mediation of sensory input from both hands to the ipsilateral and contralateral hemispheres as the basis of neuronal coupling.

Interhemispheric inhibition of corticospinal projections to forearm muscles

OBJECTIVE

Interhemispheric inhibition (IHI) is typically examined via responses elicited in intrinsic hand muscles. As the cortical representations of proximal and distal muscles in the upper limb are distinguished in terms of their inter-hemispheric projections, we sought to determine whether the IHI parameters established for the hand apply more generally.

METHODS

We investigated IHI at 5 different conditioning stimulus (CS) intensities and a range of short-latency inter-stimulus intervals (ISIs) in healthy participants. Conditioning and test stimuli were delivered over the M1 representation of the right and left flexor carpi radialis respectively.

RESULTS

IHI increased as a function of CS intensity, and was present for ISIs between 7 and 15ms. Inhibition was most pronounced for the 10ms ISI at all CS intensities.

CONCLUSIONS

The range of parameters for which IHI is elicited in projections to the forearm is similar to that reported for the hand. The specific utility lies in delineation of stimulus parameters that permit both potentiation and attenuation of IHI to be assessed.

SIGNIFICANCE

In light of evidence that there is a greater density of callosal projections between cortical areas that represent proximal muscles than between those corresponding to distal limb muscles, and in view of the assumption that variations in functional connectivity to which such differences give rise may have important implications for motor behavior, it is critical to determine whether processes mediating the expression of IHI depend on the effector that is studied. This issue is of further broad significance given the practical utility of movements generated by muscles proximal to the wrist in the context of upper limb rehabilitation.

The pattern of coupling dynamics between postural motion, isotonic hand movements and physiological tremor

This study was designed to examine differences in the coupling dynamics between upper limb motion, physiological tremor and whole body postural sway in young healthy adults. Acceleration of the hand and fingers, forearm EMG activity and postural sway data were recorded. Estimation of the degree of bilateral and limb motion-postural sway coupling was determined by cross correlation, coherence and Cross-ApEn analyses. The results revealed that, under postural tremor conditions, there was no significant coupling between limbs, muscles or sway across all metrics of coupling. In contrast, performing a rapid alternating flexion/extension movement about the wrist joint (with one or both limbs) resulted in stronger coupling between limb motion and postural sway. These results support the view that, for physiological tremor responses, the control of postural sway is maintained independent to tremor in the upper limb. However, increasing the level of movement about a distal segment of one arm (or both) leads to increased coupling throughout the body. The basis for this increased coupling would appear to be related to the enhanced neural drive to task-specific muscles within the upper limb.

Fractional anisotropy in corpus callosum is associated with facilitation of motor representation during ipsilateral hand movements

BACKGROUND

Coactivation of primary motor cortex ipsilateral to a unilateral movement (M1(ipsilateral)) has been observed, and the magnitude of activation is influenced by the contracting muscles. It has been suggested that the microstructural integrity of the callosal motor fibers (CMFs) connecting M1 regions may reflect the observed response. However, the association between the structural connectivity of CMFs and functional changes in M1(ipsilateral) remains unclear. The purpose of this study was to investigate the relationship between functional changes within M1(ipsilateral) during unilateral arm or leg movements and the microstructure of the CMFs connecting both homotopic representations (arm or leg).

METHODS

Transcranial magnetic stimulation was used to assess changes in motor evoked potentials (MEP) in an arm muscle during unilateral movements compared to rest in fifteen healthy adults. Functional magnetic resonance imaging was then used to identify regions of M1 associated with either arm or leg movements. Diffusion-weighted imaging data was acquired to generate CMFs for arm and leg areas using the areas of activation from the functional imaging as seed masks. Individual values of regional fractional anisotropy (FA) of arm and leg CMFs was then calculated by examining the overlap between CMFs and a standard atlas of corpus callosum.

RESULTS

The change in the MEP was significantly larger in the arm movement compared to the leg movement. Additionally, regression analysis revealed that FA in the arm CMFs was positively correlated with the change in MEP during arm movement, whereas a negative correlation was observed during the leg movement. However, there was no significant relationship between FA in the leg CMF and the change in MEP during the movements.

CONCLUSIONS

These findings suggest that individual differences in interhemispheric structural connectivity may be used to explain a homologous muscle-dominant effect within M1(ipsilateral) hand representation during unilateral movement with topographical specificity.

Mirror therapy enhances motor performance in the paretic upper limb after stroke: a pilot randomized controlled trial

OBJECTIVE

To investigate the effectiveness of mirror therapy (MT) combined with bilateral arm training and graded activities to improve motor performance in the paretic upper limb after stroke.

DESIGN

Randomized, controlled, assessor-blinded study.

SETTING

Inpatient stroke rehabilitation center of a tertiary care teaching hospital.

PARTICIPANTS

Patients with first-time ischemic or hemorrhagic stroke (N=20), confined to the territory of the middle cerebral artery, occurring <6 months before the commencement of the study.

INTERVENTION

The MT and control group participants underwent a patient-specific multidisciplinary rehabilitation program including conventional occupational therapy, physical therapy, and speech therapy for 5 d/wk, 6 h/d, over 3 weeks. The participants in the MT group received 1 hour of MT in addition to the conventional stroke rehabilitation.

MAIN OUTCOME MEASURES

The Upper Extremity Fugl-Meyer Assessment for motor recovery, Brunnstrom stages of motor recovery for the arm and hand, Box and Block Test for gross manual hand dexterity, and modified Ashworth scale to assess the spasticity.

RESULTS

After 3 weeks of MT, mean change scores were significantly greater in the MT group than in the control group for the Fugl-Meyer Assessment (P=.008), Brunnstrom stages of motor recovery for the arm (P=.003) and hand (P=.003), and the Box and Block Test (P=.022). No significant difference was found between the groups for modified Ashworth scale (P=.647).

CONCLUSIONS

MT when combined with bilateral arm training and graded activities was effective in improving motor performance of the paretic upper limb after stroke compared with conventional therapy without MT.

Unilateral and bilateral upper-limb training interventions after stroke have similar effects on bimanual coupling strength

BACKGROUND

Bilateral training in poststroke upper-limb rehabilitation is based on the premise that simultaneous movements of the nonparetic upper limb facilitate performance and recovery of paretic upper-limb function through neural coupling effects.

OBJECTIVE

To determine whether the degree of coupling between both hands is higher after bilateral than after unilateral training and control treatment.

METHODS

In a single-blinded randomized controlled trial, we investigated rhythmic interlimb coordination after unilateral (mCIMT) and bilateral (mBATRAC) upper-limb training and a dose-matched control treatment (DMCT) in 60 patients suffering from stroke. To this end, we used a series of tasks to discern intended and unintended coupling effects between the hands. In addition, we investigated the control over the paretic hand as reflected by movement harmonicity and amplitude. All tasks were performed before and after a 6-week intervention period and at follow-up 6 weeks later.

RESULTS

There were no significant between-group differences in change scores from baseline to postintervention and from postintervention to follow-up with regard to interlimb coupling. However, the mBATRAC group showed greater movement harmonicity and larger amplitudes with the paretic hand after training than the mCIMT and DMCT groups.

CONCLUSIONS

The degree of coupling between both hands was not significantly higher after bilateral than after unilateral training and control treatment. Although improvements in movement harmonicity and amplitude following mBATRAC may indicate a beneficial influence of the interlimb coupling, those effects were more likely due to the particular type of limb movements employed during this training protocol.

A survey on bimanual haptic interaction

When interacting with virtual objects through haptic devices, most of the time only one hand is involved. However, the increase of computational power, along with the decrease of device costs, allow more and more the use of dual haptic devices. The field which encompasses all studies of the haptic interaction with either remote or virtual environments using both hands of the same person is referred to as bimanual haptics. It differs from the common unimanual haptic field notably due to specificities of the human bimanual haptic system, e.g., the dominance of the hands, their differences in perception and their interactions at a cognitive level. These specificities call for adapted solutions in terms of hardware and software when applying the use of two hands to computer haptics. This paper reviews the state of the art on bimanual haptics, encompassing the human factors in bimanual haptic interaction, the currently available bimanual haptic devices, the software solutions for two-handed haptic interaction, and the existing interaction techniques.

Interactions between brain structure and behavior: the corpus callosum and bimanual coordination

Bimanual coordination skills are required for countless everyday activities, such as typing, preparing food, and driving. The corpus callosum (CC) is the major collection of white matter bundles connecting both hemispheres that enables the coordination between the two sides of the body. Principal evidence for this brain-behavior relationship in humans was first provided by research on callosotomy patients, showing that sectioning (parts of) the CC affected interactions between both hands directly. Later, new noninvasive in vivo imaging techniques, such as diffusion tensor imaging, have energized the study of the link between microstructural properties of the CC and bimanual performance in normal volunteers. Here we discuss the principal factors (such as age, pathology and training) that mediate the relationship between specific bimanual functions and distinct anatomical CC subdivisions. More specifically, the question is whether different bimanual task characteristics can be mapped onto functionally distinct CC subregions. We review the current status of this mapping endeavor, and propose future perspectives to inspire research on this unique link between brain structure and behavior.

The elbow flex-ex: a new sign to detect unilateral upper extremity non-organic paresis

OBJECTIVE

To examine a new neurological sign that uses synergistic oppositional movements of the arms to evaluate for non-organic upper extremity weakness.

METHODS

Patients with unilateral arm weakness were tested in a standing or sitting position with the elbows flexed at 30°. The examiner held both forearms near the wrists while asking the patient to flex or extend the normal arm at the elbow and simultaneously feeling for flexion or extension of the contralateral (paretic) arm. In patients with organic paresis, there was not a significant detectable force of contralateral opposition of the paretic limb. Patients with non-organic arm weakness had detectable strength of contralateral opposition in the paretic arm when the normal arm was tested.

RESULTS

The test was first performed on 23 patients with no complaint of arm weakness. Then, 31 patients with unilateral arm weakness were tested (10 with non-organic weakness and 21 with organic weakness). The elbow flex-ex sign correctly identified the cause of weakness in all cases.

CONCLUSIONS

The elbow flex-ex sign is useful in differentiating between functional and organic arm paresis.

Nonparetic arm force does not overinhibit the paretic arm in chronic poststroke hemiparesis

OBJECTIVE

To determine whether nonparetic arm force overinhibits the paretic arm in patients with chronic unilateral poststroke hemiparesis.

DESIGN

Case-control neurophysiological and behavioral study of patients with chronic stroke.

SETTING

Research institution.

PARTICIPANTS

Eighty-six referred patients were screened to enroll 9 participants (N=9) with a >6 month history of 1 unilateral ischemic infarct that resulted in arm hemiparesis with residual ability to produce 1Nm of wrist flexion torque and without contraindication to transcranial magnetic stimulation. Eight age- and handedness-matched healthy volunteers without neurologic diagnosis were studied for comparison.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURE

Change in interhemispheric inhibition targeting the ipsilesional primary motor cortex (M1) during nonparetic arm force. We hypothesized that interhemispheric inhibition would increase more in healthy controls than in patients with hemiparesis.

RESULTS

Healthy age-matched controls had significantly greater increases in inhibition from their active to resting M1 than patients with stroke from their active contralesional to resting ipsilesional M1 in the same scenario (20%±7% vs -1%±4%, F1,12=6.61, P=.025). Patients with greater increases in contralesional to ipsilesional inhibition were better performers on the 9-hole peg test of paretic arm function.

CONCLUSIONS

Our findings reveal that producing force with the nonparetic arm does not necessarily overinhibit the paretic arm. Though our study is limited in generalizability by the small sample size, we found that greater active contralesional to resting ipsilesional M1 inhibition was related with better recovery in this subset of patients with chronic poststroke.

Two-dimensional representation of action and arm-use sequences in the presupplementary and supplementary motor areas

The medial frontal cortex has been thought to be crucially involved in temporal structuring of behavior in monkeys and humans. We examined neuronal activity in the supplementary and presupplementary motor areas of monkeys to investigate how the nervous system deals with the coding of 16 motor sequences resulting from multiple actions involving bilateral use of the arms. We first found in both areas that this behavioral demand resulted in attribute-based representation of individual motor acts, reflecting functional (action) or anatomical (right/left arm) attributes. Actions were frequently represented according to a body-axis-centered reference frame (supination or pronation) regardless of the arm to be used. Moreover, behavioral sequences were primarily represented with respect to the action- or arm-use sequence rather than the sequence of individual movements. We propose that the two-dimensional attribute-based sequence representation provides a robust and efficient means of processing multiple behavioral sequences.

Modulation between bilateral legs and within unilateral muscle synergists of postural muscle activity changes with development and aging

The effect of development and aging on common modulation between bilateral plantarflexors (i.e., the right and left soleus, and the right and left medial gastrocnemius) (bilateral comodulation) and within plantarflexors in one leg (i.e., the right soleus and the right medial gastrocnemius) (unilateral comodulation) was investigated during bipedal quiet standing by comparing electromyography-electromyography (EMG) coherence among three age groups: adult (23-35 years), child (6-8 years), and elderly (60-80 years). The results demonstrate that there was significant coherence between bilateral plantarflexors and within plantarflexors in one leg in the 0- to 4-Hz frequency region in all three age groups. Coherence in this frequency region was stronger in the elderly group than in the adult group, while no difference was found between the adult and child groups. Of particular interest was the finding of significant coherence in bilateral and unilateral EMG recordings in the 8- to 12-Hz frequency region in some subjects in the elderly group, whereas it was not observed in the adult and child groups. These results suggest that aging affects the organization of bilateral and unilateral postural muscle activities (i.e., bilateral and unilateral comodulation) in the plantarflexors during quiet standing.

Force control improvements in chronic stroke: bimanual coordination and motor synergy evidence after coupled bimanual movement training

Bimanual movement disorders are common dysfunctions post stroke. This stroke study investigated bimanual force control capabilities to determine the effect of coupled bimanual movement training on bimanual coordination and motor synergy. Stroke participants (N = 11) completed three bimanual force control tasks at 5, 25, and 50 % of maximum voluntary contraction before and after coupled bimanual movement training. Root mean square error (RMSE), approximate entropy, correlation, and bimanual motor synergy were analyzed in two-way completely within-subjects ANOVAs (Test Session × Force Level: 2 × 3). Multiple linear regression analysis determined the relationship between RMSE and other force control measures. The analyses revealed three important findings: (1) RMSE decreased from baseline to posttest (2) negative correlation (e.g., error compensation) and bimanual motor synergy increased at 25 and 50 % after rehabilitation, and (3) increased bimanual motor synergy was strongly associated with decreased RMSE after training. The findings indicate that coupled bimanual movement training improved force control performance, bimanual coordination, and motor synergies. Indeed, the present findings extend bimanual motor synergies as a meaningful indicator for estimating task performance improvements. Finally, bimanual force control is a valid outcome measure in quantifying progress toward motor recovery post stroke.