Theta Burst Stimulation of Human Primary Motor Cortex Degrades Selective Muscle Activation in the Ipsilateral Arm

Modulation of ipsilateral motor evoked potentials during bimanual coordination tasks

Introduction

Ipsilateral motor evoked potentials (iMEPs) are difficult to obtain in distal upper limb muscles of healthy participants but give a direct insight into the role of ipsilateral motor control.

Methods

We tested a new high-intensity double pulse transcranial magnetic stimulation (TMS) protocol to elicit iMEPs in wrist extensor and flexor muscles during four different bimanual movements (cooperative-asymmetric, cooperative-symmetric, non-cooperative-asymmetric and non-cooperative-symmetric) in 16 participants.

Results

Nine participants showed an iMEP in the wrist extensor in at least 20% of the trials in each of the conditions and were classified as iMEP+ participants. iMEP persistence was greater for cooperative (50.5 ± 28.8%) compared to non-cooperative (31.6 ± 22.1%) tasks but did not differ between asymmetric and symmetric tasks. Area and amplitude of iMEPs were also increased during cooperative (area = 5.41 ± 3.4 mV × ms; amplitude = 1.60 ± 1.09 mV) compared to non-cooperative (area = 3.89 ± 2.0 mV × ms; amplitude = 1.12 ± 0.56 mV) tasks and unaffected by task-symmetry.

Discussion

The upregulation of iMEPs during common-goal cooperative tasks shows a functional relevance of ipsilateral motor control in bimanual movements. The paired-pulse TMS protocol is a reliable method to elicit iMEPs in healthy participants and can give new information about neural control of upper limb movements. With this work we contribute to the research field in two main aspects. First, we describe a reliable method to elicit ipsilateral motor evoked potentials in healthy participants which will be useful in further advancing research in the area of upper limb movements. Second, we add new insight into the motor control of bimanual movements. We were able to show an upregulation of bilateral control represented by increased ipsilateral motor evoked potentials in cooperative, object-oriented movements compared to separate bimanual tasks. This result might also have an impact on neurorehabilitation after stroke.

The distribution of transcallosal inhibition to upper extremity muscles is altered in chronic stroke

OBJECTIVE

To determine if the distribution of transcallosal inhibition (TI) acting on proximal and distal upper extremity muscles is altered in chronic stroke.

METHODS

We examined thirteen healthy controls and sixteen mildly to moderately impaired chronic stroke patients. We used transcranial magnetic stimulation (TMS) to probe TI from the contralesional onto ipsilesional hemisphere (assigned in controls). We recorded the ipsilateral silent period in the paretic biceps (BIC) and first dorsal interosseous (FDI). We measured TI strength, distribution gradient (TI difference between muscles), and motor impairment (Fugl-Meyer Assessment).

RESULTS

Both groups had stronger TI acting on their FDIs than BICs (p < 0.001). However, stroke patients also had stronger TI acting on their BICs than controls (p = 0.034), resulting in a flatter distribution of inhibition (p = 0.028). In patients, stronger FDI inhibition correlated with less hand impairment (p = 0.031); BIC inhibition was not correlated to impairment.

CONCLUSION

TI is more evenly distributed to the paretic FDI and BIC in chronic stroke. The relative increase in proximal inhibition does not relate to better function, as it does distally.

SIGNIFICANCE

The results expand our knowledge about segment-specific neurophysiology and its relevance to impairment after stroke.

Stroke-induced alteration in multi-layer information transmission of cortico-motor system during elbow isometric contraction modulated by myoelectric-controlled interfaces

Objective. Human movement is a complex process requiring information transmission in inter-cortical, cortico-muscular and inter-muscular networks. Though motor deficits after stroke are associated with impaired networks in the cortico-motor system, the mechanisms underlying these networks are to date not fully understood. The purpose of this study is to investigate the changes in information transmission of the inter-cortical, cortico-muscular and inter-muscular networks after stroke and the effect of myoelectric-controlled interface (MCI) dimensionality on such information transmission in each network.Approach. Fifteen healthy control subjects and 11 post-stroke patients were recruited to perform elbow tracking tasks within different dimensional MCIs in this study. Their electromyography (EMG) and functional near-infrared spectroscopy (fNIRS) signals were recorded simultaneously. Transfer entropy was used to analyse the functional connection that represented the information transmission in each network based on the fNIRS and EMG signals.Main results.The results found that post-stroke patients showed the increased inter-cortical connection versus healthy control subjects, which might be attributed to cortical reorganisation to compensate for motor deficits. Compared to healthy control subjects, a lower strength cortico-muscular connection was found in post-stroke patients due to the reduction of information transmission following a stroke. Moreover, the increased MCI dimensionality strengthened inter-cortical, cortico-muscular and inter-muscular connections because of higher visual information processing demands.Significance. These findings not only provide a comprehensive overview to evaluate changes in the cortico-motor system due to stroke, but also suggest that increased MCI dimensionality may serve as a useful rehabilitation tool for boosting information transmission in the cortico-motor system of post-stroke patients.

Corticoreticulospinal tract neurophysiology in an arm and hand muscle in healthy and stroke subjects

KEY POINTS

The corticoreticulospinal tract (CReST) is a descending motor pathway that reorganizes after corticospinal tract (CST) injury in animals. In humans, the pattern of CReST innervation to upper limb muscles has not been carefully examined in healthy individuals or individuals with CST injury. In the present study, we assessed CReST projections to an arm and hand muscle on the same side of the body in healthy and chronic stoke subjects using transcranial magnetic stimulation. We show that CReST connection strength to the muscles differs between healthy and stroke subjects, with stronger connections to the hand than arm in healthy subjects, and stronger connections to the arm than hand in stroke subjects. These results help us better understand CReST innervation patterns in the upper limb, and may point to its role in normal motor function and motor recovery in humans.

ABSTRACT

The corticoreticulospinal tract (CReST) is a major descending motor pathway in many animals, but little is known about its innervation patterns in proximal and distal upper extremity muscles in humans. The contralesional CReST furthermore reorganizes after corticospinal tract (CST) injury in animals, but it is less clear whether CReST innervation changes after stroke in humans. We thus examined CReST functional connectivity, connection strength, and modulation in an arm and hand muscle of healthy (n = 15) and chronic stroke (n = 16) subjects. We delivered transcranial magnetic stimulation to the contralesional hemisphere (assigned in healthy subjects) to elicit ipsilateral motor evoked potentials (iMEPs) from the paretic biceps (BIC) and first dorsal interosseous (FDI) muscle. We operationalized CReST functional connectivity as iMEP presence/absence, CReST projection strength as iMEP size and CReST modulation as change in iMEP size by head rotation. We found comparable CReST functional connectivity to the BICs and FDIs in both subject groups. However, the pattern of CReST connection strength to the muscles diverged between groups, with stronger connections to FDIs than BICs in healthy subjects and stronger connections to BICs than FDIs in stroke subjects. Head rotation modulated only FDI iMEPs of healthy subjects. Our findings indicate that the healthy CReST does not have a proximal innervation bias, and its strong FDI connections may have functional relevance to finger individuation. The reversed CReST innervation pattern in stroke subjects confirms its reorganization after CST injury, and its strong BIC connections may indicate upregulation for particular upper extremity muscles or their functional actions.

Use-dependent directional bias does not transfer to the untrained limb during bimanual contractions

Skills learned through practice with one limb can often be transferred to the untrained limb. In the present report, we sought to determine whether movement direction biases, acquired through repeated movement with one limb, transfer to the untrained limb. In order to do so, we asked participants to perform synchronized bilateral contractions of muscles in both wrists, followed by the unilateral contraction of muscles in one wrist. In four experiments, we manipulated the position of the unilateral target to create use-dependent directional biases; changed the direction of the cursor in relation to the wrist movement to control for attentional biases; and sought to induce directional biases with both right and left unilateral movements. The results showed clear movement-related biases for the wrist that performed unilateral contractions, but no evidence that movement-related bias transferred to the opposite limb during bilateral action. Thus, motor preparation and execution of unilateral contractions does not affect the direction of movement made by the opposite limb during subsequent bilateral contractions.

Anodal Direct Current Stimulation of the Cerebellum Reduces Cerebellar Brain Inhibition but Does Not Influence Afferent Input from the Hand or Face in Healthy Adults

BACKGROUND

The cerebellum controls descending motor commands by outputs to primary motor cortex (M1) and the brainstem in response to sensory feedback. The cerebellum may also modulate afferent input en route to M1 and the brainstem.

OBJECTIVE

The objective of this study is to determine if anodal transcranial direct current stimulation (tDCS) to the cerebellum influences cerebellar brain inhibition (CBI), short afferent inhibition (SAI) and trigeminal reflexes (TRs) in healthy adults.

METHODS

Data from two studies evaluating effects of cerebellar anodal and sham tDCS are presented. The first study used a twin coil transcranial magnetic stimulation (TMS) protocol to investigate CBI and combined TMS and cutaneous stimulation of the digit to assess SAI. The second study evaluated effects on trigemino-cervical and trigemino-masseter reflexes using peripheral nerve stimulation of the face.

RESULTS

Fourteen right-handed healthy adults participated in experiment 1. CBI was observed at baseline and was reduced by anodal cerebellar DCS only (P < 0.01). There was SAI at interstimulus intervals of 25 and 30 ms at baseline (both P < 0.0001), but cerebellar tDCS had no effect. Thirteen right-handed healthy adults participated in experiment 2. Inhibitory reflexes were evoked in the ipsilateral masseter and sternocleidomastoid muscles. There was no effect of cerebellar DCS on either reflex.

CONCLUSIONS

Anodal DCS reduced CBI but did not change SAI or TRs in healthy adults. These results require confirmation in individuals with neurological impairment.

Use of theta-burst stimulation in changing excitability of motor cortex: A systematic review and meta-analysis

Noninvasive brain stimulation has been demonstrated to modulate cortical activity in humans. In particular, theta burst stimulation (TBS) has gained notable attention due to its ability to induce lasting physiological changes after short stimulation durations. The present study aimed to provide a comprehensive meta-analytic review of the efficacy of two TBS paradigms; intermittent (iTBS) and continuous (cTBS), on corticospinal excitability in healthy individuals. Literature searches yielded a total of 87 studies adhering to the inclusion criteria. iTBS yielded moderately large MEP increases lasting up to 30 min with a pooled SMD of 0.71 (p<0.00001). cTBS produced a reduction in MEP amplitudes lasting up to 60 min, with the largest effect size seen at 5 min post stimulation (SMD=-0.9, P<0.00001). The collected studies were of heterogeneous nature, and a series of tests conducted indicated a degree of publication bias. No significant change in SICI and ICF was observed, with exception to decrease in SICI with cTBS at the early time point (SMD=0.42, P=0.00036). The results also highlight several factors contributing to TBS efficacy, including the number of pulses, frequency of stimulation and BDNF polymorphisms. Further research investigating optimal TBS stimulation parameters, particularly for iTBS, is needed in order for these paradigms to be successfully translated into clinical settings.

Are ipsilateral motor evoked potentials subject to intracortical inhibition?

Paired-pulse transcranial magnetic stimulation (TMS) can be used to examine intracortical inhibition in primary motor cortex (M1), termed short-interval intracortical inhibition (SICI). To our knowledge, SICI has only been demonstrated in contralateral motor evoked potentials (MEPs). Ipsilateral MEPs (iMEPs) are assumed to reflect excitability of an uncrossed oligosynaptic pathway, and can sometimes be evoked in proximal upper-limb muscles using high-intensity TMS. We examined whether iMEPs in the biceps brachii (BB) would be suppressed by subthreshold conditioning, therefore demonstrating SICI of iMEPs. TMS was delivered to the dominant M1 to evoke conditioned (C) and nonconditioned (NC) iMEPs in the nondominant BB of healthy participants during weak bilateral elbow flexion. The conditioning stimulus intensities tested were 85%, 100%, and 115% of active motor threshold (AMT), at 2 ms and 4 ms interstimulus intervals (ISI). The iMEP ratio (C/NC) was calculated for each condition to assess the amount of inhibition. Inhibition of iMEPs was present at 2 ms ISI with 100% and 115% AMT (bothP< 0.03), mediated by a reduction in persistence and size (allP< 0.05). To our knowledge, this is the first demonstration of SICI of iMEPs. This technique may be useful as a tool to better understand the role of ipsilateral M1 during functional motor tasks.

Neurophysiological and behavioural effects of dual-hemisphere transcranial direct current stimulation on the proximal upper limb

Dual-hemisphere transcranial direct current stimulation over the primary motor cortex (M1-M1 tDCS) is assumed to modulate neural excitability in a polarity-dependent manner and improve motor performance of the hand. In the proximal upper limb, the neurophysiological and behavioural after-effects of M1-M1 tDCS are not well known. This study investigated the after-effects of M1-M1 tDCS on contralateral, ipsilateral and transcallosal excitability to the proximal upper limb muscle biceps brachii (BB). Circle tracing was used to assess motor performance before and after tDCS as this task requires coordination of proximal and distal musculature. Sixteen healthy right-handed adults participated in the study, each receiving M1-M1 tDCS (1 mA, 15 min) or sham tDCS in separate sessions. The anode was positioned over right M1 and cathode over left M1. M1-M1 tDCS suppressed transcallosal inhibition from the M1 under the cathode (P < 0.045). No other neurophysiologic or behavioural effects were observed (P > 0.6). The study provides important information regarding inconsistent neurophysiological and behavioural changes following tDCS that have implications for future tDCS research on the motor system.

Partial weight support of the arm affects corticomotor selectivity of biceps brachii

BACKGROUND

Weight support of the arm (WS) can be used in stroke rehabilitation to facilitate upper limb therapy, but the neurophysiological effects of this technique are not well understood. While an overall reduction in muscle activity is expected, the mechanism by which WS may alter the expression of muscle synergies has not been examined until now. We explored the neurophysiological effect of WS on the selectivity of biceps brachii (BB) activation in healthy adults.

METHODS

Thirteen participants completed counterbalanced movement tasks in a repeated measures design. Three levels of WS (0, 45, and 90 % of full support) were provided to the arm using a commercial device (Saebo Mobile Arm Support). At each level of WS, participants maintained a flexed shoulder posture while performing rhythmic isometric elbow flexion (BB agonist) or forearm pronation (BB antagonist). Single-pulse transcranial magnetic stimulation of primary motor cortex was used to elicit motor-evoked potentials (MEPs) in BB 100-300 ms before muscle contraction. Baseline muscle activity and MEP amplitude were the primary dependent measures. Effects of movement TASK and SUPPORT LEVEL were statistically analyzed using linear mixed effects models.

RESULTS

As expected, with increased support tonic activity was reduced across all muscles. This effect was greatest in the anti-gravity muscle anterior deltoid, and evident in biceps brachii and pronator teres as well. For BB MEP amplitude, TASK and SUPPORT LEVEL, interacted such that for elbow flexion, MEP amplitudes were smaller with incrementally greater WS whereas, for forearm pronation MEP amplitudes were smaller only at high WS.

CONCLUSIONS

Weight support of the arm influences corticomotor selectivity of biceps brachii. WS may impact coordination independently of a global reduction in muscle activity. The amount of supportive force applied to the arm influences the neuromechanical control profile for the limb. These findings may inform the application of WS in upper limb stroke rehabilitation.

Frontal and frontoparietal injury differentially affect the ipsilateral corticospinal projection from the nonlesioned hemisphere in monkey (Macaca mulatta)

Upper extremity hemiplegia is a common consequence of unilateral cortical stroke. Understanding the role of the unaffected cerebral hemisphere in the motor recovery process has been encouraged, in part, by the presence of ipsilateral corticospinal projections (iCSP). We examined the neuroplastic response of the iCSP from the contralesional primary motor cortex (cM1) hand/arm area to spinal levels C5-T1 after spontaneous long-term recovery from isolated frontal lobe injury and isolated frontoparietal injury. High-resolution tract tracing, stereological, and behavioral methodologies were applied. Recovery from frontal motor injury resulted in enhanced numbers of terminal labeled boutons in the iCSP from cM1 compared with controls. Increases occurred in lamina VIII and the adjacent ventral sectors of lamina VII, which are involved in axial/proximal limb sensorimotor processing. Larger frontal lobe lesions were associated with greater numbers of terminal boutons than smaller frontal lobe lesions. In contrast, frontoparietal injury blocked this response; total bouton number was similar to controls, demonstrating that disruption of somatosensory input to one hemisphere has a suppressive effect on the iCSP from the nonlesioned hemisphere. However, compared with controls, elevated bouton numbers occurred in lamina VIII, at the expense of lamina VII bouton labeling. Lamina IX boutons were also elevated in two frontoparietal lesion cases with extensive cortical injury. Because laminae VIII and IX collectively harbor axial, proximal, and distal motoneurons, therapeutic intervention targeting the ipsilateral corticospinal linkage from cM1 may promote proximal, and possibly distal, upper-limb motor recovery following frontal and frontoparietal injury.

Cooperative hand movements in post-stroke subjects: Neural reorganization

OBJECTIVE

Recent research indicates a task-specific neural coupling controlling cooperative hand movements reflected in bilateral electromyographic reflex responses in arm muscles following unilateral nerve stimulation. Reorganization of this mechanism was explored in post-stroke patients in this study.

METHODS

Electromyographic reflex responses in forearm muscles to unilateral electrical ulnar nerve stimulation were examined during cooperative and non-cooperative hand movements.

RESULTS

Stimulation of the unaffected arm during cooperative hand movements led to electromyographic responses in bilateral forearm muscles, similar to those seen in healthy subjects, while stimulation of the affected side was followed only by ipsilateral responses. No contralateral reflex responses could be evoked in severely affected patients. The presence of contralateral responses correlated with the clinical motor impairment as assessed by the Fugl-Meyer test.

CONCLUSION

The observations suggest that after stroke an impaired processing of afferent input from the affected side leads to a defective neural coupling and is associated with a greater involvement of fiber tracts from the unaffected hemisphere during cooperative hand movements.

SIGNIFICANCE

The mechanism of neural coupling underlying cooperative hand movements is shown to be defective in post-stroke patients. The neural re-organizations observed have consequences for the rehabilitation of hand function.

Afferent inhibition and cortical silent periods in shoulder primary motor cortex and effect of a suprascapular nerve block in people experiencing chronic shoulder pain

OBJECTIVE

To characterise short afferent inhibition (SAI) and the cortical silent period (CSP) in the primary motor cortex representations of the infraspinatus muscle in healthy adults and people experiencing chronic shoulder pain, to determine the impact of a suprascapular nerve block (SSNB).

METHODS

Neurophysiological measures were obtained in 18 controls and 8 patients with chronic shoulder pain, pre and post SSNB and 1 week later. Pain intensity was assessed by a visual analogue scale.

RESULTS

SAI was apparent in controls (all P<0.03) and a CSP was observed which reduced in the presence of SAI (all P<0.0001). Compared to controls, shoulder pain patients demonstrated higher active motor threshold (P=0.046), less SAI (P=0.044), a longer CSP (P=0.048) and less modulation of the CSP by SAI (P=0.045). Higher motor thresholds were related to higher pain scores (P=0.009). The SSNB immediately restored SAI (P=0.013), with a positive relationship between increased SAI and reduced pain (P=0.031). The SSNB further reduced modulation of CSP by SAI at 1 week post injection (P=0.006).

CONCLUSIONS

SAI and the CSP were present and demonstrated robust interaction in controls, which was aberrant in patients. The SSNB transiently restored SAI but had no effect on the CSP; however CSP modulation by SAI was further attenuated 1 week post injection.

SIGNIFICANCE

The current findings improve understanding of the neurophysiology of the shoulder motor cortex and its modulation by chronic pain. The effect of SSNB in shoulder pain patients should be interpreted with caution until proven in a larger population. Interventions that target intracortical inhibition might increase efficacy in people with chronic shoulder pain.

Transcranial direct current stimulation improves ipsilateral selective muscle activation in a frequency dependent manner

Failure to suppress antagonist muscles can lead to movement dysfunction, such as the abnormal muscle synergies often seen in the upper limb after stroke. A neurophysiological surrogate of upper limb synergies, the selectivity ratio (SR), can be determined from the ratio of biceps brachii (BB) motor evoked potentials to transcranial magnetic stimulation prior to forearm pronation versus elbow flexion. Surprisingly, cathodal transcranial direct current stimulation (c-TDCS) over ipsilateral primary motor cortex (M1) reduces (i.e. improves) the SR in healthy adults, and chronic stroke patients. The ability to suppress antagonist muscles may be exacerbated at high movement rates. The aim of the present study was to investigate whether the selective muscle activation of the biceps brachii (BB) is dependent on altering frequency demands, and whether the c-tDCS improvement of SR is dependent on task frequency. Seventeen healthy participants performed repetitive isometric elbow flexion and forearm pronation at three rates, before and after c-tDCS or sham delivered to ipsilateral left M1. Ipsilateral c-tDCS improved the SR in a frequency dependent manner by selectively suppressing BB antagonist excitability. Our findings confirm that c-tDCS is an effective tool for improving selective muscle activation, and provide novel evidence for its efficacy at rates of movement where it is most likely to benefit task performance.

Selective activation of ipsilateral motor pathways in intact humans

It has been proposed that ipsilateral motor pathways play a role in the control of ipsilateral movements and recovery of function after injury. However, the extent to which ipsilateral motor pathways are engaged in voluntary activity in intact humans remains largely unknown. Using transcranial magnetic stimulation over the arm representation of the primary motor cortex, we examined ipsilateral motor-evoked potentials (iMEPs) in a proximal arm muscle during increasing levels of unilateral and bilateral isometric force in a sitting position. We demonstrate that iMEP area and amplitude decreased during bilateral contraction of homonymous (elbow flexor) muscles and increased during bilateral contraction of heteronymous (elbow flexor and extensor) muscles compared with a unilateral contraction, regardless of the level of force tested. To further understand the neuronal inputs involved in the bilateral effects, we examined the contribution from neck afferents projecting onto ipsilateral motor pathways. Medial (away from the muscle tested) and lateral (toward the muscle tested) rotation of the head enhanced bilateral iMEP effects from homonymous and heteronymous muscles, respectively. In contrast, head flexion and extension exerted nonspecific bilateral effects on iMEPs. Intracortical inhibition, in the motor cortex where iMEPs originated, showed modulation compatible with the changes in iMEPs. We conclude that ipsilateral projections to proximal arm muscles can be selectively modulated by voluntary contraction of contralateral arm muscles, likely involving circuits mediating asymmetric tonic neck reflexes acting, at least in part, at the cortical level. The pattern of bilateral actions may represent a strategy to engage ipsilateral motor pathways in a motor behavior.

Dominant vs. nondominant arm advantage in mentally simulated actions in right handers

Although plentiful data are available regarding mental states involving the dominant-right arm, the evidence for the nondominant-left arm is sparse. Here, we investigated whether right-handers can generate accurate predictions with either the right or the left arm. Fifteen adults carried out actual and mental arm movements in two directions with varying inertial resistance (inertial anisotropy phenomenon). We recorded actual and mental movement times and used the degree of their similarity as an indicator for the accuracy of motor imagery/prediction process. We found timing correspondences (isochrony) between actual and mental right arm movements in both rightward (low inertia resistance) and leftward (high inertia resistance) directions. Timing similarities between actual and mental left arm movements existed for the leftward direction (low inertia resistance) but not for the rightward direction (high inertia resistance). We found similar results when participants reaching towards the midline of the workspace, a result that excludes a hemispace effect. Electromyographic analysis during mental movements showed that arm muscles remained inactivate, thus eliminating a muscle activation strategy that could explain intermanual differences. Furthermore, motor-evoked potentials enhancement in both right and left biceps brachii during mental actions indicated that subjects were actively engaged in mental movement simulation and that the disadvantage of the left arm cannot be attributed to the nonactivation of the right motor cortex. Our findings suggest that predictive mechanisms are more robust for the right than the left arm in right-handers. We discussed these findings from the perspective of the internal models theory and the dynamic-dominance hypothesis of laterality.

Ipsilateral motor pathways after stroke: implications for non-invasive brain stimulation

In humans the two cerebral hemispheres have essential roles in controlling the upper limb. The purpose of this article is to draw attention to the potential importance of ipsilateral descending pathways for functional recovery after stroke, and the use of non-invasive brain stimulation (NBS) protocols of the contralesional primary motor cortex (M1). Conventionally NBS is used to suppress contralesional M1, and to attenuate transcallosal inhibition onto the ipsilesional M1. There has been little consideration of the fact that contralesional M1 suppression may also reduce excitability of ipsilateral descending pathways that may be important for paretic upper limb control for some patients. One such ipsilateral pathway is the cortico-reticulo-propriospinal pathway (CRPP). In this review we outline a neurophysiological model to explain how contralesional M1 may gain control of the paretic arm via the CRPP. We conclude that the relative importance of the CRPP for motor control in individual patients must be considered before using NBS to suppress contralesional M1. Neurophysiological, neuroimaging, and clinical assessments can assist this decision making and facilitate the translation of NBS into the clinical setting.

Non-invasive cerebellar stimulation in dystonia

Primary isolated dystonia is a hyperkinetic movement disorder whereby involuntary muscle contractions cause twisted and abnormal postures. Dystonia of the cervical spine and upper limb may present as sustained muscle contractions or task-specific activity when using the hand or upper limb. There is little understanding of the pathophysiology underlying dystonia and this presents a challenge for clinicians and researchers alike. Emerging evidence that the cerebellum is involved in the pathophysiology of dystonia using network models presents the intriguing concept that the cerebellum could provide a novel target for non-invasive brain stimulation. Non-invasive stimulation to increase cerebellar excitability improved aspects of handwriting and circle drawing in a small cohort of people with focal hand and cervical dystonia. Mechanisms underlying the improvement in function are unknown, but putative pathways may involve the red nucleus and/or the cervical propriospinal system. Furthermore, recent understanding that the cerebellum has both motor and cognitive functions suggests that non-invasive cerebellar stimulation may improve both motor and non-motor aspects of dystonia. We propose a combination of motor and non-motor tasks that challenge cerebellar function may be combined with cerebellar non-invasive brain stimulation in the treatment of focal dystonia. Better understanding of how the cerebellum contributes to dystonia may be gained by using network models such as our putative circuits involving red nucleus and/or the cervical propriospinal system. Finally, novel treatment interventions encompassing both motor and non-motor functions of the cerebellum may prove effective for neurological disorders that exhibit cerebellar dysfunction.

Contralesional hemisphere control of the proximal paretic upper limb following stroke

Cathodal transcranial direct current stimulation (c-tDCS) can reduce excitability of neurons in primary motor cortex (M1) and may facilitate motor recovery after stroke. However, little is known about the neurophysiological effects of tDCS on proximal upper limb function. We hypothesized that suppression of contralesional M1 (cM1) excitability would produce neurophysiological effects that depended on the severity of upper limb impairment. Twelve patients with varying upper limb impairment after subcortical stroke were assessed on clinical scales of upper limb spasticity, impairment, and function. Magnetic resonance imaging was used to determine lesion size and fractional anisotropy (FA) within the posterior limbs of the internal capsules indicative of corticospinal tract integrity. Excitability within paretic M1 biceps brachii representation was determined from motor-evoked potentials during selective isometric tasks, after cM1 sham stimulation and after c-tDCS. These neurophysiological data indicate that c-tDCS improved selective proximal upper limb control for mildly impaired patients and worsened it for moderate to severely impaired patients. The direction of the neurophysiological after effects of c-tDCS was strongly related to upper limb spasticity, impairment, function, and FA asymmetry between the posterior limbs of the internal capsules. These results indicate systematic variation of cM1 for proximal upper limb control after stroke and that suppression of cM1 excitability is not a "one size fits all" approach.

Short-term effects of unilateral lesion of the primary motor cortex (M1) on ipsilesional hand dexterity in adult macaque monkeys

Although the arrangement of the corticospinal projection in primates is consistent with a more prominent role of the ipsilateral motor cortex on proximal muscles, rather than on distal muscles involved in manual dexterity, the role played by the primary motor cortex on the control of manual dexterity for the ipsilateral hand remains a matter a debate, either in the normal function or after a lesion. We, therefore, tested the impact of permanent unilateral motor cortex lesion on the manual dexterity of the ipsilateral hand in 11 macaque monkeys, within a time window of 60 days post-lesion. For comparison, unilateral reversible pharmacological inactivation of the motor cortex was produced in an additional monkey. Manual dexterity was assessed quantitatively based on three motor parameters derived from two reach and grasp manual tasks. In contrast to the expected dramatic, complete deficit of manual dexterity of the contralesional hand that persists for several weeks, the impact on the manual dexterity of the ipsilesional hand was generally moderate (but statistically significant) and, when present, lasted less than 20 days. Out of the 11 monkeys, only 3 showed a deficit of the ipsilesional hand for 2 of the 3 motor parameters, and 4 animals had a deficit for only one motor parameter. Four monkeys did not show any deficit. The reversible inactivation experiment yielded results consistent with the permanent lesion data. In conclusion, the primary motor cortex exerts a modest role on ipsilateral manual dexterity, most likely in the form of indirect hand postural control.

Cathodal transcranial direct current stimulation suppresses ipsilateral projections to presumed propriospinal neurons of the proximal upper limb

This study investigated whether cathodal transcranial direct current stimulation (c-tDCS) of left primary motor cortex (M1) modulates excitability of ipsilateral propriospinal premotoneurons (PNs) in healthy humans. Transcranial magnetic stimulation (TMS) of the right motor cortex was used to obtain motor evoked potentials (MEPs) from the left biceps brachii (BB) while participants maintained contraction of the left BB. To examine presumed PN excitability, left BB MEPs were compared with those conditioned by median nerve stimulation (MNS) at the left elbow. Interstimulus intervals between TMS and MNS were set to produce summation at the C3–C4 level of the spinal cord. MNS facilitated BB MEPs elicited at TMS intensities near active motor threshold but inhibited BB MEPs at slightly higher intensities, indicative of putative PN modulation. c-tDCS suppressed the facilitatory and inhibitory effects of MNS. Sham tDCS did not alter either component. There was no effect of c-tDCS and sham tDCS on nonconditioned left BB MEPs or on the ipsilateral silent period of left BB. Right first dorsal interosseous MEPs were suppressed by c-tDCS. These results indicate that M1 c-tDCS can be used to modulate excitability of ipsilateral projections to presumed PNs controlling the proximal arm muscle BB. This technique may hold promise for promoting motor recovery of proximal upper limb function after stroke.

Cathodal transcranial direct current stimulation of the primary motor cortex improves selective muscle activation in the ipsilateral arm

Proximal upper limb muscles are represented bilaterally in primary motor cortex. Goal-directed upper limb movement requires precise control of proximal and distal agonist and antagonist muscles. Failure to suppress antagonist muscles can lead to abnormal movement patterns, such as those commonly experienced in the proximal upper limb after stroke. We examined whether noninvasive brain stimulation of primary motor cortex could be used to improve selective control of the ipsilateral proximal upper limb. Thirteen healthy participants performed isometric left elbow flexion by contracting biceps brachii (BB; agonist) and left forearm pronation (BB antagonist) before and after 20 min of cathodal transcranial direct current stimulation (c-tDCS) or sham tDCS of left M1. During the tasks, motor evoked potentials (MEPs) in left BB were acquired using single-pulse transcranial magnetic stimulation of right M1 150-270 ms before muscle contraction. As expected, left BB MEPs were facilitated before flexion and suppressed before pronation. After c-tDCS, left BB MEP amplitudes were reduced compared with sham stimulation, before pronation but not flexion, indicating that c-tDCS enhanced selective muscle activation of the ipsilateral BB in a task-specific manner. The potential for c-tDCS to improve BB antagonist control correlated with BB MEP amplitude for pronation relative to flexion, expressed as a selectivity ratio. This is the first demonstration that selective muscle activation in the proximal upper limb can be improved after c-tDCS of ipsilateral M1 and that the benefits of c-tDCS for selective muscle activation may be most effective in cases where activation strategies are already suboptimal. These findings may have relevance for the use of tDCS in rehabilitation after stroke.