Demand on skillfulness modulates interhemispheric inhibition of motor cortices

Gray matter volume of functionally relevant primary motor cortex is causally related to learning a hand motor task

Variability in brain structure is associated with the capacity for behavioral change. However, a causal link between specific brain areas and behavioral change (such as motor learning) has not been demonstrated. We hypothesized that greater gray matter volume of a primary motor cortex (M1) area active during a hand motor learning task is positively correlated with subsequent learning of the task, and that the disruption of this area blocks learning of the task. Healthy participants underwent structural MRI before learning a skilled hand motor task. Next, participants performed this learning task during fMRI to determine M1 areas functionally active during this task. This functional ROI was anatomically constrained with M1 boundaries to create a group-level "Active-M1" ROI used to measure gray matter volume in each participant. Greater gray matter volume in the left hemisphere Active-M1 ROI was related to greater motor learning in the corresponding right hand. When M1 hand area was disrupted with repetitive transcranial stimulation (rTMS), learning of the motor task was blocked, confirming its causal link to motor learning. Our combined imaging and rTMS approach revealed greater cortical volume in a task-relevant M1 area is causally related to learning of a hand motor task in healthy humans.

Corticospinal excitability reductions during action preparation and action stopping in humans: Different sides of the same inhibitory coin?

Motor functions and cognitive processes are closely associated with each other. In humans, this linkage is reflected in motor system state changes both when an action must be prepared and stopped. Single-pulse transcranial magnetic stimulation showed that both action preparation and action stopping are accompanied by a reduction of corticospinal excitability, referred to as preparatory and response inhibition, respectively. While previous efforts have been made to describe both phenomena extensively, an updated and comprehensive comparison of the two phenomena is lacking. To ameliorate such deficit, this review focuses on the role and interpretation of single-coil (single-pulse and paired-pulse) and dual-coil TMS outcome measures during action preparation and action stopping in humans. To that effect, it aims to identify commonalities and differences, detailing how TMS-based outcome measures are affected by states, traits, and psychopathologies in both processes. Eventually, findings will be compared, and open questions will be addressed to aid future research.

Task-related brain activity in upper limb dystonia revealed by simultaneous fNIRS and EEG

OBJECTIVE

The aim of this study was to explore differences in brain activity and connectivity using simultaneous electroencephalography and near-infrared spectroscopy in patients with focal dystonia during handwriting and finger-tapping tasks.

METHODS

Patients with idiopathic right upper limb focal dystonia and controls were assessed by simultaneous near-infrared spectroscopy and electroencephalography during the writing and finger-tapping tasks in terms of the mu-alpha, mu-beta, beta and low gamma power and effective connectivity, as well as relative changes in oxyhemoglobin (oxy-Hb) and deoxyhemoglobin using a channel-wise approach with a mixed-effect model.

RESULTS

Patients exhibited higher oxy-Hb levels in the right and left motor cortex and supplementary motor area during writing, but lower oxy-Hb levels in the left sensorimotor and bilateral somatosensory area during finger-tapping compared to controls. During writing, patients showed increased low gamma power in the bilateral sensorimotor cortex and less mu-beta and beta attenuation compared to controls. Additionally, patients had reduced connectivity between the supplementary motor area and the left sensorimotor cortex during writing. No differences were observed in terms of effective connectivity in either task. Finally, patients failed to attenuate the mu-alpha, mu-beta, and beta rhythms during the finger-tapping task.

CONCLUSIONS

Cortical blood flow and EEG spectral power differ between controls and dystonia patients, depending on the task. Writing increased blood flow and altered connectivity in dystonia patients, and it also decreased slow-band attenuation. Finger-tapping decreased blood flow and slow-band attenuation.

SIGNIFICANCE

Simultaneous fNIRS and EEG may show relevant information regarding brain dynamics in movement disorders patients in unconstrained environments.

Roles of Handedness and Hemispheric Lateralization: Implications for Rehabilitation of the Central and Peripheral Nervous Systems: A Rapid Review

IMPORTANCE

Handedness and motor asymmetry are important features of occupational performance. With an increased understanding of the basic neural mechanisms surrounding handedness, clinicians will be better able to implement targeted, evidence-based neurorehabilitation interventions to promote functional independence.

OBJECTIVE

To review the basic neural mechanisms behind handedness and their implications for central and peripheral nervous system injury.

DATA SOURCES

Relevant published literature obtained via MEDLINE.

FINDINGS

Handedness, along with performance asymmetries observed between the dominant and nondominant hands, may be due to hemispheric specializations for motor control. These specializations contribute to predictable motor control deficits that are dependent on which hemisphere or limb has been affected. Clinical practice recommendations for occupational therapists and other rehabilitation specialists are presented.

CONCLUSIONS AND RELEVANCE

It is vital that occupational therapists and other rehabilitation specialists consider handedness and hemispheric lateralization during evaluation and treatment. With an increased understanding of the basic neural mechanisms surrounding handedness, clinicians will be better able to implement targeted, evidence-based neurorehabilitation interventions to promote functional independence. Plain-Language Summary: The goal of this narrative review is to increase clinicians' understanding of the basic neural mechanisms related to handedness (the tendency to select one hand over the other for specific tasks) and their implications for central and peripheral nervous system injury and rehabilitation. An enhanced understanding of these mechanisms may allow clinicians to better tailor neurorehabilitation interventions to address motor deficits and promote functional independence.

Induced neural phase precession through exogenous electric fields

The gradual shifting of preferred neural spiking relative to local field potentials (LFPs), known as phase precession, plays a prominent role in neural coding. Correlations between the phase precession and behavior have been observed throughout various brain regions. As such, phase precession is suggested to be a global neural mechanism that promotes local neuroplasticity. However, causal evidence and neuroplastic mechanisms of phase precession are lacking so far. Here we show a causal link between LFP dynamics and phase precession. In three experiments, we modulated LFPs in humans, a non-human primate, and computational models using alternating current stimulation. We show that continuous stimulation of motor cortex oscillations in humans lead to a gradual phase shift of maximal corticospinal excitability by ~90°. Further, exogenous alternating current stimulation induced phase precession in a subset of entrained neurons (~30%) in the non-human primate. Multiscale modeling of realistic neural circuits suggests that alternating current stimulation-induced phase precession is driven by NMDA-mediated synaptic plasticity. Altogether, the three experiments provide mechanistic and causal evidence for phase precession as a global neocortical process. Alternating current-induced phase precession and consequently synaptic plasticity is crucial for the development of novel therapeutic neuromodulation methods.

The phase of sensorimotor mu and beta oscillations has the opposite effect on corticospinal excitability

BACKGROUND

Neural oscillations in the primary motor cortex (M1) shape corticospinal excitability. Power and phase of ongoing mu (8-13 Hz) and beta (14-30 Hz) activity may mediate motor cortical output. However, the functional dynamics of both mu and beta phase and power relationships and their interaction, are largely unknown.

OBJECTIVE

Here, we employ recently developed real-time targeting of the mu and beta rhythm, to apply phase-specific brain stimulation and probe motor corticospinal excitability non-invasively. For this, we used instantaneous read-out and analysis of ongoing oscillations, targeting four different phases (0°, 90°, 180°, and 270°) of mu and beta rhythms with suprathreshold single-pulse transcranial magnetic stimulation (TMS) to M1. Ensuing motor evoked potentials (MEPs) in the right first dorsal interossei muscle were recorded. Twenty healthy adults took part in this double-blind randomized crossover study.

RESULTS

Mixed model regression analyses showed significant phase-dependent modulation of corticospinal output by both mu and beta rhythm. Strikingly, these modulations exhibit a double dissociation. MEPs are larger at the mu trough and rising phase and smaller at the peak and falling phase. For the beta rhythm we found the opposite behavior. Also, mu power, but not beta power, was positively correlated with corticospinal output. Power and phase effects did not interact for either rhythm, suggesting independence between these aspects of oscillations.

CONCLUSION

Our results provide insights into real-time motor cortical oscillation dynamics, which offers the opportunity to improve the effectiveness of TMS by specifically targeting different frequency bands.

Evaluating the Abnormality of Bilateral Motor Cortex Activity in Subacute Stroke Patients Executing a Unimanual Motor Task With Increasing Demand on Precision

Abnormal contralesional M1 activity is consistently reported in patients with compromised upper limb and hand function after stroke. The underlying mechanisms and functional implications of this activity are not clear, which hampers the development of treatment strategies targeting this brain area. The goal of the present study was to determine the extent to which contralesional M1 activity can be explained by the demand of a motor task, given recent evidence for increasing ipsilateral M1 activity with increasing demand in healthy age-matched controls. We hypothesized that higher activity in contralesional M1 is related to greater demand on precision in a hand motor task. fMRI data were collected from 19 patients with ischemic stroke affecting hand function in the subacute recovery phase and 31 healthy, right-handed, age-matched controls. The hand motor task was designed to parametrically modulate the demand on movement precision. Electromyography data confirmed strictly unilateral task performance by all participants. Patients showed significant impairment relative to controls in their ability to perform the task in the fMRI scanner. However, patients and controls responded similarly to an increase in demand for precision, with better performance for larger targets and poorer performance for smaller targets. Patients did not show evidence of elevated ipsilesional or contralesional M1 blood oxygenation level-dependent (BOLD) activation relative to healthy controls and mean BOLD activation levels were not elevated for patients with poorer performance relative to patients with better task performance. While both patients and healthy controls showed demand-dependent increases in BOLD activation in both ipsilesional/contralateral and contralesional/ipsilateral hemispheres, patients with stroke were less likely to show evidence of a linear relationship between the demand on precision and BOLD activation in contralesional M1 than healthy controls. Taken together, the findings suggest that task demand affects the BOLD response in contralesional M1 in patients with stroke, though perhaps less strongly than in healthy controls. This has implications for the interpretation of reported abnormal bilateral M1 activation in patients with stroke because in addition to contralesional M1 reorganization processes it could be partially related to a response to the relatively higher demand of a motor task when completed by patients rather than by healthy controls.

Long-Term Wu Qin Xi Exercise on Response Inhibition and Cortical Connectivity in Parkinson's Disease: Design and Implementation of a Randomized Controlled Clinical Trial

Background: Motor symptom disorders in patients with Parkinson disease (PD) are closely related to reduced inhibitory ability. Although exercise has been shown to improve this ability in patients with PD, its effects on proactive and reactive inhibition have not been determined. Most previous studies of inhibitory control disorder in people with PD have been behavioral, and little attention has been paid to functional cortical connectivity. Wu Qin Xi, a low–medium-intensity qigong exercise that is safe and easy to do for elderly individuals, can support physical well-being and help prevent and alleviate disease. In this study, our aims were to explore the effects of a long-term Wu Qin Xi intervention on response inhibition and to examine how improved inhibition control relates to cortical connectivity using dual-site paired-pulse transcranial magnetic stimulation (ppTMS), in patients with mild–moderate PD.

Methods: A single-blind randomized controlled trial will be conducted. A total of 90 elderly subjects will be recruited and allocated randomly to Wu Qin Xi, balance exercise, and healthy control groups. The exercise interventions will be implemented in three 90-min sessions per week for 24 weeks; the healthy control group will receive no intervention. The primary assessments will be response inhibition metrics and task-based ppTMS. The secondary outcomes will include motor symptom severity, mobility, balance, emotional state, and quality of life. Assessments will be conducted at baseline, at the conclusion of the intervention period (week 24), and a few months after the intervention (week 36 follow-up).

Discussion: This study is designed to provide insights into the effects of practicing Wu Qin Xi on response inhibition function in people with PD. The results will provide evidence on the value of traditional Chinese exercise as a therapeutic rehabilitation option for these patients. They will also provide data addressing how brain function–related cortical connectivity is related to reactive vs. proactive inhibition in people with PD participating in an exercise intervention.

Clinical Trial Registration: This study has been registered prospectively in the Chinese Clinical Trial Registry (ChiCTR2000038517, 18 January 2021).

Exploring cortico-cortical interactions during action preparation by means of dual-coil transcranial magnetic stimulation: A systematic review

Action preparation is characterized by a set of complex and distributed processes that occur in multiple brain areas. Interestingly, dual-coil transcranial magnetic stimulation (TMS) is a relevant technique to probe effective connectivity between cortical areas, with a high temporal resolution. In the current systematic review, we aimed at providing a detailed picture of the cortico-cortical interactions underlying action preparation focusing on dual-coil TMS studies. We considered four theoretical processes (impulse control, action selection, movement initiation and action reprogramming) and one task modulator (movement complexity). The main findings highlight 1) the interplay between primary motor cortex (M1) and premotor, prefrontal and parietal cortices during action preparation, 2) the varying (facilitatory or inhibitory) cortico-cortical influence depending on the theoretical processes and the TMS timing, and 3) the key role of the supplementary motor area-M1 interactions that shape the preparation of simple and complex movements. These findings are of particular interest for clinical perspectives, with a need to better characterize functional connectivity deficiency in clinical population with altered action preparation.

Motor Cortex Activation During Writing in Focal Upper-Limb Dystonia: An fNIRS Study

BACKGROUND

Functional imaging studies have associated dystonia with abnormal activation in motor and sensory brain regions. Commonly used techniques such as functional magnetic resonance imaging impose physical constraints, limiting the experimental paradigms. Functional near-infrared spectroscopy (fNIRS) offers a new noninvasive possibility for investigating cortical areas and the neural correlates of complex motor behaviors in unconstrained settings.

METHODS

We compared the cortical brain activation of patients with focal upper-limb dystonia and controls during the writing task under naturalistic conditions using fNIRS. The primary motor cortex (M1), the primary somatosensory cortex (S1), and the supplementary motor area were chosen as regions of interest (ROIs) to assess differences in changes in both oxyhemoglobin (oxy-Hb) and deoxyhemoglobin (deoxy-Hb) between groups.

RESULTS

Group average activation maps revealed an expected pattern of contralateral recruitment of motor and somatosensory cortices in the control group and a more bilateral pattern of activation in the dystonia group. Between-group comparisons focused on specific ROIs revealed an increased activation of the contralateral M1 and S1 cortices and also of the ipsilateral M1 cortex in patients.

CONCLUSIONS

Overactivity of contralateral M1 and S1 in dystonia suggest a reduced specificity of the task-related cortical areas, whereas ipsilateral activation possibly indicates a primary disorder of the motor cortex or an endophenotypic pattern. To our knowledge, this is the first study using fNIRS to assess cortical activity in dystonia during the writing task under natural settings, outlining the potential of this technique for monitoring sensory and motor retraining in dystonia rehabilitation.

Interhemispheric Parietal-Frontal Connectivity Predicts the Ability to Acquire a Nondominant Hand Skill

Introduction: After chronic impairment of the right dominant hand, some individuals are able to compensate with increased performance with the intact left nondominant hand. This process may depend on the nondominant (right) hemisphere's ability to access dominant (left) hemisphere mechanisms. To predict or modulate patients' ability to compensate with the left hand, we must understand the neural mechanisms and connections that underpin this process. Methods: We studied 17 right-handed healthy adults who underwent resting-state functional connectivity (FC) magnetic resonance imaging scans before 10 days of training on a left-hand precision drawing task. We sought to identify right-hemisphere areas where FC from left-hemisphere seeds (primary motor cortex, intraparietal sulcus [IPS], inferior parietal lobule) would predict left-hand skill learning or magnitude. Results: Left-hand skill learning was predicted by convergent FC from left primary motor cortex and left IPS onto the same small region (0.31 cm3) in the right superior parietal lobule (SPL). Discussion: For patients who must compensate with the left hand, the right SPL may play a key role in integrating left-hemisphere mechanisms that typically control the right hand. Our study provides the first model of how interhemispheric functional connections in the human brain may support compensation after chronic injury to the right hand.

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

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

NMDA Receptor-Mediated Motor Cortex Plasticity After 20 Hz Transcranial Alternating Current Stimulation

Transcranial alternating current stimulation (tACS) has been shown to modulate neural oscillations and excitability levels in the primary motor cortex (M1). These effects can last for more than an hour and an involvement of N-methyl-d-aspartate receptor (NMDAR) mediated synaptic plasticity has been suggested. However, to date the cortical mechanisms underlying tACS after-effects have not been explored. Here, we applied 20 Hz beta tACS to M1 while participants received either the NMDAR antagonist dextromethorphan or a placebo and the effects on cortical beta oscillations and excitability were explored. When a placebo medication was administered, beta tACS was found to increase cortical excitability and beta oscillations for at least 60 min, whereas when dextromethorphan was administered, these effects were completely abolished. These results provide the first direct evidence that tACS can induce NMDAR-mediated plasticity in the motor cortex, which contributes to our understanding of tACS-induced influences on human motor cortex physiology.

Primary motor cortical activity during unimanual movements with increasing demand on precision

In functional magnetic resonance imaging (fMRI) studies, performance of unilateral hand movements is associated with primary motor cortex activity ipsilateral to the moving hand (M1_ipsi), in addition to contralateral activity (M1_contra). The magnitude of M1_ipsi activity increases with the demand on precision of the task. However, it is unclear how demand-dependent increases in M1_ipsi recruitment relate to the control of hand movements. To address this question, we used fMRI to measure blood oxygenation level-dependent (BOLD) activity during performance of a task that varied in demand on precision. Participants (n = 23) manipulated an MRI-compatible joystick with their right or left hand to move a cursor into targets of different sizes (small, medium, large, extra large). Performance accuracy, movement time, and number of velocity peaks scaled with target size, whereas reaction time, maximum velocity, and initial direction error did not. In the univariate analysis, BOLD activation in M1_contra and M1_ipsi was higher for movements to smaller targets. Representational similarity analysis, corrected for mean activity differences, revealed multivoxel BOLD activity patterns during movements to small targets were most similar to those for medium targets and least similar to those for extra-large targets. Only models that varied with demand (target size, performance accuracy, and number of velocity peaks) correlated with the BOLD dissimilarity patterns, though differently for right and left hands. Across individuals, M1_contra and M1_ipsi similarity patterns correlated with each other. Together, these results suggest that increasing demand on precision in a unimanual motor task increases M1 activity and modulates M1 activity patterns.NEW & NOTEWORTHY Contralateral primary motor cortex (M1) predominantly controls unilateral hand movements, but the role of ipsilateral M1 is unclear. We used functional magnetic resonance imaging (fMRI) to investigate how M1 activity is modulated by unimanual movements at different levels of demand on precision. Our results show that task characteristics related to demand on precision influence bilateral M1 activity, suggesting that in addition to contralateral M1, ipsilateral M1 plays a key role in controlling hand movements to meet performance precision requirements.

Individual differences in motor development during early childhood: An MEG study

In a previous study, we reported the first measurements of pre-movement and sensorimotor cortex activity in preschool age children (ages 3-5 years) using a customized pediatric magnetoencephalographic system. Movement-related activity in the sensorimotor cortex differed from that typically observed in adults, suggesting that maturation of cortical motor networks was still incomplete by late preschool age. Here we compare these earlier results to a group of school age children (ages 6-8 years) including seven children from the original study measured again two years later, and a group of adults (mean age 31.1 years) performing the same task. Differences in movement-related brain activity were observed both longitudinally within children in which repeated measurements were made, and cross-sectionally between preschool age children, school age children, and adults. Movement-related mu (8-12 Hz) and beta (15-30 Hz) oscillations demonstrated linear increases in amplitude and mean frequency with age. In contrast, movement-evoked gamma synchronization demonstrated a step-like transition from low (30-50 Hz) to high (70-90 Hz) narrow-band oscillations, and this occurred at different ages in different children. Notably, pre-movement activity ('readiness fields') observed in adults was absent in even the oldest children. These are the first direct observations of brain activity accompanying motor responses throughout early childhood, confirming that maturation of this activity is still incomplete by mid-childhood. In addition, individual children demonstrated markedly different developmental trajectories in movement-related brain activity, suggesting that individual differences need to be taken into account when studying motor development across age groups.

Functional plasticity of the ipsilateral primary sensorimotor cortex in an elite long jumper with below-knee amputation

Functional plasticity of the sensorimotor cortex occurs following motor practice, as well as after limb amputation. However, the joint effect of limb amputation and intensive, long-term motor practice on cortical plasticity remains unclear. Here, we recorded brain activity during unilateral contraction of the hip, knee, and ankle joint muscles from a long jump Paralympic gold medalist with a unilateral below-knee amputation (Amputee Long Jumper, ALJ). He used the amputated leg with a prosthesis for take-off. Under similar conditions to the ALJ, we also recorded brain activity from healthy long jumpers (HLJ) and non-athletes with a below-knee amputation. During a rhythmic isometric contraction of knee extensor muscles with the take-off/prosthetic leg, the ALJ activated not only the contralateral primary sensorimotor cortex (M1/S1), but also the ipsilateral M1/S1. In addition, this ipsilateral M1/S1 activation was significantly greater than that seen in the HLJ. However, we did not find any significant differences between the ALJ and HLJ in M1/S1 activation during knee muscle contraction in the non-take-off/intact leg, nor during hip muscle contraction on either side. Region of interest analysis revealed that the ALJ exhibited a greater difference in M1/S1 activity and activated areas ipsilateral to the movement side between the take-off/prosthetic and non-take-off/intact legs during knee muscle contraction compared with the other two groups. However, difference in activity in M1/S1 contralateral to the movement side did not differ across groups. These results suggest that a combination of below-knee amputation and intensive, prolonged long jump training using a prosthesis (i.e. fine knee joint control) induced an expansion of the functional representation of the take-off/prosthetic leg in the ipsilateral M1/S1 in a muscle-specific manner. These results provide novel insights into the potential for substantial cortical plasticity with an extensive motor rehabilitation program.

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A Paralympic gold medalist with a unilateral below-knee amputation was recruited.

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Brain activity during hip, knee, and ankle movements was recorded.

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Brain activity was compared with healthy athletes and non-athletes with amputation.

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Greater ipsilateral M1/S1 activity during knee movement was observed in the medalist.

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Intensive motor practice and limb amputation would induce drastic neural plasticity.

Brain networks and their relevance for stroke rehabilitation

Stroke has long been regarded as focal disease with circumscribed damage leading to neurological deficits. However, advances in methods for assessing the human brain and in statistics have enabled new tools for the examination of the consequences of stroke on brain structure and function. Thereby, it has become evident that stroke has impact on the entire brain and its network properties and can therefore be considered as a network disease. The present review first gives an overview of current methodological opportunities and pitfalls for assessing stroke-induced changes and reorganization in the human brain. We then summarize principles of plasticity after stroke that have emerged from the assessment of networks. Thereby, it is shown that neurological deficits do not only arise from focal tissue damage but also from local and remote changes in white-matter tracts and in neural interactions among wide-spread networks. Similarly, plasticity and clinical improvements are associated with specific compensatory structural and functional patterns of neural network interactions. Innovative treatment approaches have started to target such network patterns to enhance recovery. Network assessments to predict treatment response and to individualize rehabilitation is a promising way to enhance specific treatment effects and overall outcome after stroke.

The supplementary motor area modulates interhemispheric interactions during movement preparation

The execution of coordinated hand movements requires complex interactions between premotor and primary motor areas in the two hemispheres. The supplementary motor area (SMA) is involved in movement preparation and bimanual coordination. How the SMA controls bimanual coordination remains unclear, although there is evidence suggesting that the SMA could modulate interhemispheric interactions. With a delayed-response task, we investigated interhemispheric interactions underlying normal movement preparation and the role of the SMA in these interactions during the delay period of unimanual or bimanual hand movements. We used functional MRI and transcranial magnetic stimulation in 22 healthy volunteers (HVs), and then in two models of SMA dysfunction: (a) in the same group of HVs after transient disruption of the right SMA proper by continuous transcranial magnetic theta-burst stimulation; (b) in a group of 22 patients with congenital mirror movements (CMM), whose inability to produce asymmetric hand movements is associated with SMA dysfunction. In HVs, interhemispheric connectivity during the delay period was modulated according to whether or not hand coordination was required for the forthcoming movement. In HVs following SMA disruption and in CMM patients, interhemispheric connectivity was modified during the delay period and the interhemispheric inhibition was decreased. Using two models of SMA dysfunction, we showed that the SMA modulates interhemispheric interactions during movement preparation. This unveils a new role for the SMA and highlights its importance in coordinated movement preparation.

The effects of five sessions of continuous theta burst stimulation over contralesional sensorimotor cortex paired with paretic skilled motor practice in people with chronic stroke

BACKGROUND

In individuals with chronic stroke, impairment of the paretic arm may be exacerbated by increased contralesional transcallosal inhibition (TCI). Continuous theta burst stimulation (cTBS) can decrease primary motor cortex (M1) excitability and TCI. However, contralesional cTBS shows inconsistent effects after stroke. Variable effects of cTBS could stem from failure to pair stimulation with skilled motor practice or a focus of applying cTBS over M1.

OBJECTIVE

Here, we investigated the effects of pairing cTBS with skilled practice on motor learning and arm function. We considered the differential effects of stimulation over two different brain regions: contralesional M1 (M1c) or contralesional primary somatosensory cortex (S1c).

METHODS

37 individuals with chronic stroke participated in five sessions of cTBS and paretic arm skilled practice of a serial targeting task (STT); participants received either cTBS over M1c or S1c or sham before STT practice. Changes in STT performance and Wolf Motor Function Test (WMFT) were assessed as primary outcomes. Assessment of bilateral corticospinal, intracortical excitability and TCI were secondary outcomes.

RESULTS

cTBS over sensorimotor cortex did not improve STT performance and paretic WMFT-rate beyond sham cTBS. TCI was reduced bi-directionally following the intervention, regardless of stimulation group. In addition, we observed an association between STT performance change and paretic WMFT-rate change in the M1c stimulation group only.

CONCLUSIONS

Multiple sessions of STT practice can improve paretic arm function and decrease TCI bilaterally, with no additional benefit of prior cTBS. Our results suggest that improvement in STT practice following M1c cTBS scaled with change in paretic arm function in some individuals. Our results highlight the need for a better understanding of the mechanisms of cTBS to effectively identify who may benefit from this form of brain stimulation.

Influences of 12-Week Physical Activity Interventions on TMS Measures of Cortical Network Inhibition and Upper Extremity Motor Performance in Older Adults-A Feasibility Study

Objective: Data from previous cross-sectional studies have shown that an increased level of physical fitness is associated with improved motor dexterity across the lifespan. In addition, physical fitness is positively associated with increased laterality of cortical function during unimanual tasks; indicating that sedentary aging is associated with a loss of interhemispheric inhibition affecting motor performance. The present study employed exercise interventions in previously sedentary older adults to compare motor dexterity and measure of interhemispheric inhibition using transcranial magnetic stimulation (TMS) after the interventions. Methods: Twenty-one community-dwelling, reportedly sedentary older adults were recruited, randomized and enrolled to a 12-week aerobic exercise group or a 12-week non-aerobic exercise balance condition. The aerobic condition was comprised of an interval-based cycling "spin" activity, while the non-aerobic "balance" exercise condition involved balance and stretching activities. Participants completed upper extremity dexterity batteries and estimates of VO2max in addition to undergoing single (ipsilateral silent period-iSP) and paired-pulse interhemispheric inhibition (ppIHI) in separate assessment sessions before and after study interventions. After each intervention during which heart rate was continuously recorded to measure exertion level (load), participants crossed over into the alternate arm of the study for an additional 12-week intervention period in an AB/BA design with no washout period. Results: After the interventions, regardless of intervention order, participants in the aerobic spin condition showed higher estimated VO2max levels after the 12-week intervention as compared to estimated VO2max in the non-aerobic balance intervention. After controlling for carryover effects due to the study design, participants in the spin condition showed longer iSP duration than the balance condition. Heart rate load was more strongly correlated with silent period duration after the Spin condition than estimated VO2. Conclusions: Aging-related changes in cortical inhibition may be influenced by 12-week physical activity interventions when assessed with the iSP. Although inhibitory signaling is mediates both ppIHI and iSP measures each TMS modality likely employs distinct inhibitory networks, potentially differentially affected by aging. Changes in inhibitory function after physical activity interventions may be associated with improved dexterity and motor control at least as evidence from this feasibility study show.

Physiological Markers of Motor Inhibition during Human Behavior

Transcranial magnetic stimulation (TMS) studies in humans have shown that many behaviors engage processes that suppress excitability within the corticospinal tract. Inhibition of the motor output pathway has been extensively studied in the context of action stopping, where a planned movement needs to be abruptly aborted. Recent TMS work has also revealed markers of motor inhibition during the preparation of movement. Here, we review the evidence for motor inhibition during action stopping and action preparation, focusing on studies that have used TMS to monitor changes in the excitability of the corticospinal pathway. We discuss how these physiological results have motivated theoretical models of how the brain selects actions, regulates movement initiation and execution, and switches from one state to another.