Test-retest reliability of intrahemispheric dorsal premotor and primary motor cortex dual-site TMS connectivity measures

OBJECTIVE

Investigating the optimal interstimulus interval (ISI) and the 24-hour test-retest reliability for intrahemispheric dorsal premotor cortex (PMd) - primary motor cortex (M1) connectivity using dual-site transcranial magnetic stimulation (dsTMS).

METHODS

In 21 right-handed adults, left intrahemispheric PMd-M1 connectivity has been investigated with a stacked-coil dsTMS setup (conditioning stimulus: 75% of resting motor threshold; test stimulus: eliciting MEPs of 1-1.5 mV) at ISIs of 3, 5-8, and 10 ms. Additionally, M1-M1 short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) were investigated to assess comparability to standard paired-pulse setups.

RESULTS

Conditioning PMd led to significant inhibition of M1 output at ISIs of 3 and 5 ms, whereas 10 ms resulted in facilitation (all, p < 0.001), with a fair test-retest reliability for 3 (ICC: 0.47) and 6 ms (ICC: 0.44) ISIs. Replication of SICI (p < 0.001) and ICF (p = 0.017) was successful, with excellent test-retest reliability for SICI (ICC: 0.81).

CONCLUSION

This dsTMS setup can probe the inhibitory and facilitatory PMd-M1 connections, as well as reliably replicate SICI and ICF paradigms.

SIGNIFICANCE

The stacked-coil dsTMS setup for investigating intrahemispheric PMd-M1 connectivity offers promising possibilities to better understand motor control.

Neurophysiological markers of motor compensatory mechanisms in early Parkinson's disease

Compensatory mechanisms in Parkinson's disease are defined as the changes that the brain uses to adapt to neurodegeneration and progressive dopamine reduction. Motor compensation in early Parkinson's disease could, in part, be responsible for a unilateral onset of clinical motor signs despite the presence of bilateral nigrostriatal degeneration. Although several mechanisms have been proposed for compensatory adaptations in Parkinson's disease, the underlying pathophysiology is unclear. Here, we investigate motor compensation in Parkinson's disease by investigating the relationship between clinical signs, dopamine transporter imaging data and neurophysiological measures of the primary motor cortex (M1), using transcranial magnetic stimulation in presymptomatic and symptomatic hemispheres of patients. In this cross-sectional, multicentre study, we screened 82 individuals with Parkinson's disease. Patients were evaluated clinically in their medication OFF state using standardized scales. Sixteen Parkinson's disease patients with bilateral dopamine transporter deficit in the putamina but unilateral symptoms were included. Twenty-eight sex- and age-matched healthy controls were also investigated. In all participants, we tested cortical excitability using single- and paired-pulse techniques, interhemispheric inhibition and cortical plasticity with paired associative stimulation. Data were analysed with ANOVAs, multiple linear regression and logistic regression models. Individual coefficients of motor compensation were defined in patients based on clinical and imaging data, i.e. the motor compensation coefficient. The motor compensation coefficient includes an asymmetry score to balance motor and dopamine transporter data between the two hemispheres, in addition to a hemispheric ratio accounting for the relative mismatch between the magnitude of motor signs and dopaminergic deficit. In patients, corticospinal excitability and plasticity were higher in the presymptomatic compared with the symptomatic M1. Also, interhemispheric inhibition from the presymptomatic to the symptomatic M1 was reduced. Lower putamen binding was associated with higher plasticity and reduced interhemispheric inhibition in the presymptomatic hemisphere. The motor compensation coefficient distinguished the presymptomatic from the symptomatic hemisphere. Finally, in the presymptomatic hemisphere, a higher motor compensation coefficient was associated with lower corticospinal excitability and interhemispheric inhibition and with higher plasticity. In conclusion, the present study suggests that motor compensation involves M1-striatal networks and intercortical connections becoming more effective with progressive loss of dopaminergic terminals in the putamen. The balance between these motor networks seems to be driven by cortical plasticity.

Plasticity-Induced Effects of Theta Burst Transcranial Ultrasound Stimulation in Parkinson's Disease

BACKGROUND

Low-intensity transcranial ultrasound stimulation (TUS) is a noninvasive brain stimulation (NIBS) technique with high spatial specificity. Previous studies showed that TUS delivered in a theta burst pattern (tbTUS) increased motor cortex (MI) excitability up to 30 minutes due to long-term potentiation (LTP)-like plasticity. Studies using other forms of NIBS suggested that cortical plasticity may be impaired in patients with Parkinson's disease (PD).

OBJECTIVE

The aim was to investigate the neurophysiological effects of tbTUS in PD patients off and on dopaminergic medications compared to healthy controls.

METHODS

We studied 20 moderately affected PD patients in on and off dopaminergic medication states (7 with and 13 without dyskinesia) and 17 age-matched healthy controls in a case-controlled study. tbTUS was applied for 80 seconds to the MI. Motor-evoked potentials (MEP), short-interval intracortical inhibition (SICI), and short-interval intracortical facilitation (SICF) were recorded at baseline, and at 5 minutes (T5), T30, and T60 after tbTUS. Motor Unified Parkinson's Disease Rating Scale (mUPDRS) was measured at baseline and T60.

RESULTS

tbTUS significantly increased MEP amplitude at T30 compared to baseline in controls and in PD patients on but not in PD patients off medications. SICI was reduced in PD off medications compared to controls. tbTUS did not change in SICI or SICF. The bradykinesia subscore of mUPDRS was reduced at T60 compared to baseline in PD on but not in the off medication state. The presence of dyskinesia did not affect tbTUS-induced plasticity.

CONCLUSIONS

tbTUS-induced LTP plasticity is impaired in PD patients off medications and is restored by dopaminergic medications. © 2024 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Neurophysiological profiles of patients with bipolar disorders as probed with transcranial magnetic stimulation: A systematic review

AIM

Bipolar disorder (BD) has a significant impact on global health, yet its neurophysiological basis remains poorly understood. Conventional treatments have limitations, highlighting the need for a better understanding of the neurophysiology of BD for early diagnosis and novel therapeutic strategies.

DESIGN

Employing a systematic review approach of the PRISMA guidelines, this study assessed the usefulness and validity of transcranial magnetic stimulation (TMS) neurophysiology in patients with BD.

METHODS

Databases searched included PubMed, MEDLINE, Embase, and PsycINFO, covering studies from January 1985 to January 2024.

RESULTS

Out of 6597 articles screened, nine studies met the inclusion criteria, providing neurophysiological insights into the pathophysiological basis of BD using TMS-electromyography and TMS-electroencephalography methods. Findings revealed significant neurophysiological impairments in patients with BD compared to healthy controls, specifically in cortical inhibition and excitability. In particular, short-interval cortical inhibition (SICI) was consistently diminished in BD across the studies, which suggests a fundamental impairment of cortical inhibitory function in BD. This systematic review corroborates the potential utility of TMS neurophysiology in elucidating the pathophysiological basis of BD. Specifically, the reduced cortical inhibition in the SICI paradigm observed in patients with BD suggests gamma-aminobutyric acid (GABA)-A receptor-mediated dysfunction, but results from other TMS paradigms have been inconsistent. Thus, complex neurophysiological processes may be involved in the pathological basis underlying BD. This study demonstrated that BD has a neural basis involving impaired GABAergic function, and it is highly expected that further research on TMS neurophysiology will further elucidate the pathophysiological basis of BD.

Effects of the motor cortical theta-burst transcranial-focused ultrasound stimulation on the contralateral motor cortex

Theta-burst transcranial ultrasound stimulation (tbTUS) increases primary motor cortex (M1) excitability for at least 30 min. However, the remote effects of focal M1 tbTUS on the excitability of other cortical areas are unknown. Here, we examined the effects of left M1 tbTUS on right M1 excitability. An 80 s train of active or sham tbTUS was delivered to the left M1 in 20 healthy subjects. Before and after the tbTUS, we measured: (1) corticospinal excitability using motor-evoked potential (MEP) amplitudes from single-pulse transcranial magnetic stimulation (TMS) of left and right M1; (2) interhemispheric inhibition (IHI) from left to right M1 and from right to left M1 using a dual-site paired-pulse TMS paradigm; and (3) intracortical circuits of the right M1 with short-interval intracortical inhibition and intracortical facilitation (ICF) using paired-pulse TMS. Left M1 tbTUS decreased right M1 excitability as shown by decreased MEP amplitudes, increased right M1 ICF and decreased short-interval IHI from left to right hemisphere at interstimulus interval (ISI) of 10 ms but not long-interval IHI at interstimulus interval of 40 ms. The study showed that left M1 tbTUS can change the excitability of remote cortical areas with decreased right M1 excitability and interhemispheric inhibition. The remote effects of tbTUS should be considered when it is used in neuroscience research and as a potential neuromodulation treatment for brain disorders. KEY POINTS: Transcranial ultrasound stimulation (TUS) is a novel non-invasive brain stimulation technique for neuromodulation with the advantages of being able to achieve high spatial resolution and target deep brain structures. A repetitive TUS protocol, with an 80 s train of theta burst patterned TUS (tbTUS), has been shown to increase primary motor cortex (M1) excitability, as well as increase alpha and beta movement-related spectral power in distinct brain regions. In this study, we examined on the effects of the motor cortical tbTUS on the excitability of contralateral M1 measured with MEPs elicited by transcranial magnetic stimulation. We showed that left M1 tbTUS decreased right M1 excitability and left-to-right M1 interhemispheric inhibition, and increased intracortical facilitation of right M1. These results lead to better understand the effects of tbTUS and can help the development of tbTUS for the treatment of neurological and psychiatric disorders and in neuroscience research.

Table tennis experience enhances motor control in older adults: Insights into sensorimotor-related cortical connectivity

Background

Motor control declines with age and requires effective connectivity between the sensorimotor cortex and the primary motor cortex (M1). Despite research indicating that physical exercise can improve motor control in older individuals the effect of physical exercise on neural connectivity in older adults remains to be elucidated.

Methods

Older adults with experience in table tennis and fit aerobics and individuals without such experience for comparison were recruited for the study. Differences in motor control were assessed using the stop-signal task. The impact of exercise experience on DLPFC-M1 and pre-SMA-M1 neural connectivity was assessed with transcranial magnetic stimulation. Varied time intervals (short and long term) and stimulus intensities (subthreshold and suprathreshold) were used to explore neural connectivity across pathways.

Results

The present study showed that behavioral iexpression of the table tennis group was significantly better than the other two groups;The facilitatory regulation of preSMA-M1 in all groups is negatively correlated with SSRT. Regulatory efficiency was highest in the table tennis group. Only the neural network regulatory ability of the Table Tennis group showed a negative correlation with SSRT; Inhibitory regulation of DLPFC-M1 was positively correlated with SSRT; this effect was most robust in the table tennis group.

Conclusion

The preliminary findings of this study suggest that table tennis exercise may enhance the motor system regulated by neural networks and stabilize inhibitory regulation of DLPFC-M1, thereby affecting motor control in older adults.

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.

Elevated Intraocular Pressure Moderated Brain Morphometry in High-tension Glaucoma: a Structural MRI Study

High-tension glaucoma (HTG) is one of the most common forms of primary open angle glaucoma. The purpose of this study was to assess in HTG brain, whether the elevated intraocular pressure (IOP) had an effect on the brain morphological alterations via structural MRI. We acquired T1WI structural MRI images from 56 subjects including 36 HTG patients and 20 healthy controls. We tested whether the brain morphometry was associated with the mean IOP in HTG patients. Moreover, we conducted moderation analysis to assess the interactions between subject type (HTG - healthy controls) and IOP. In HTG group, cortical thickness was negatively correlated with the mean IOP in the left rostral middle frontal gyrus, left pars triangularis, right precentral gyrus, left postcentral gyrus, left superior temporal gyrus (p < 0.05, FDR corrected). Four of the five regions negatively correlated with mean IOP showed reduced cortical thickness in HTG group compared with healthy controls, which were the left rostral middle frontal gyrus, left pars triangularis, left postcentral gyrus and left superior temporal gyrus (p < 0.05, FDR corrected). IOP moderated the interaction between subject type and cortical thickness of the left rostral middle frontal gyrus (p = 0.0017), left pars triangularis (p = 0.0011), left postcentral gyrus (p = 0.0040) and left superior temporal gyrus (p = 0.0066). Elevated IOP may result brain morphometry alterations such as cortical thinning. The relationship between IOP and brain morphometry underlines the importance of the IOP regulation for HTG patients.

Functional and structural abnormalities of the speech disorders: a multimodal activation likelihood estimation meta-analysis

Speech disorders are associated with different degrees of functional and structural abnormalities. However, the abnormalities associated with specific disorders, and the common abnormalities shown by all disorders, remain unclear. Herein, a meta-analysis was conducted to integrate the results of 70 studies that compared 1843 speech disorder patients (dysarthria, dysphonia, stuttering, and aphasia) to 1950 healthy controls in terms of brain activity, functional connectivity, gray matter, and white matter fractional anisotropy. The analysis revealed that compared to controls, the dysarthria group showed higher activity in the left superior temporal gyrus and lower activity in the left postcentral gyrus. The dysphonia group had higher activity in the right precentral and postcentral gyrus. The stuttering group had higher activity in the right inferior frontal gyrus and lower activity in the left inferior frontal gyrus. The aphasia group showed lower activity in the bilateral anterior cingulate gyrus and left superior frontal gyrus. Across the four disorders, there were concurrent lower activity, gray matter, and fractional anisotropy in motor and auditory cortices, and stronger connectivity between the default mode network and frontoparietal network. These findings enhance our understanding of the neural basis of speech disorders, potentially aiding clinical diagnosis and intervention.

Interhemispheric inhibition between dorsal premotor and primary motor cortices is released during preparation of unimanual but not bimanual movements

Previous research applying transcranial magnetic stimulation during unimanual reaction time tasks indicates a transient change in the inhibitory influence of the dorsal premotor cortex over the contralateral primary motor cortex shortly after the presentation of an imperative stimulus. The degree of interhemispheric inhibition from the dorsal premotor cortex to the contralateral primary motor cortex shifts depending on whether the targeted effector representation in the primary motor cortex is selected for movement. Further, the timing of changes in inhibition covaries with the selection demands of the reaction time task. Less is known about modulation of dorsal premotor to primary motor cortex interhemispheric inhibition during the preparation of bimanual movements. In this study, we used a dual coil transcranial magnetic stimulation to measure dorsal premotor to primary motor cortex interhemispheric inhibition between both hemispheres during unimanual and bimanual simple reaction time trials. Interhemispheric inhibition was measured early and late in the 'pre-movement period' (defined as the period immediately after the onset of the imperative stimulus and before the beginning of voluntary muscle activity). We discovered that interhemispheric inhibition was more facilitatory early in the pre-movement period compared with late in the pre-movement period during unimanual reaction time trials. In contrast, interhemispheric inhibition was unchanged throughout the pre-movement period during symmetrical bimanual reaction time trials. These results suggest that there is greater interaction between the dorsal premotor cortex and contralateral primary motor cortex during the preparation of unimanual actions compared to bimanual actions.

Probing intrahemispheric interactions with a novel dual-site TMS setup

OBJECTIVE

Using dual-site transcranial magnetic stimulation (dsTMS), the effective connectivity between the primary motor cortex (M1) and adjacent brain areas such as the dorsal premotor cortex (PMd) can be investigated. However, stimulating two brain regions in close proximity (e.g., ±2.3 cm for intrahemispheric PMd-M1) is subject to considerable spatial restrictions that potentially can be overcome by combining two standard figure-of-eight coils in a novel dsTMS setup.

METHODS

After a technical evaluation of its induced electric fields, the dsTMS setup was tested in vivo (n = 23) by applying a short-interval intracortical inhibition (SICI) protocol. Additionally, the intrahemispheric PMd-M1 interaction was probed. E-field modelling was performed using SimNIBS.

RESULTS

The technical evaluation yielded no major alterations of the induced electric fields due to coil overlap. In vivo, the setup reliably elicited SICI. Investigating intrahemispheric PMd-M1 interactions was feasible (inter-stimulus interval 6 ms), resulting in modulation of M1 output.

CONCLUSIONS

The presented dsTMS setup provides a novel way to stimulate two adjacent brain regions with fewer technical and spatial limitations than previous attempts.

SIGNIFICANCE

This dsTMS setup enables more accurate and repeatable targeting of brain regions in close proximity and can facilitate innovation in the field of effective connectivity.

Interhemispheric imbalance and bradykinesia features in Parkinson's disease

In patients with Parkinson's disease, the connectivity between the two primary motor cortices may be altered. However, the correlation between asymmetries of abnormal interhemispheric connections and bradykinesia features has not been investigated. Furthermore, the potential effects of dopaminergic medications on this issue remain largely unclear. The aim of the present study is to investigate the interhemispheric connections in Parkinson's disease by transcranial magnetic stimulation and explore the potential relationship between interhemispheric inhibition and bradykinesia feature asymmetry in patients. Additionally, we examined the impact of dopaminergic therapy on neurophysiological and motor characteristics. Short- and long-latency interhemispheric inhibition was measured in 18 Parkinson's disease patients and 18 healthy controls, bilaterally. We also assessed the corticospinal and intracortical excitability of both primary motor cortices. We conducted an objective analysis of finger-tapping from both hands. Correlation analyses were performed to explore potential relationships among clinical, transcranial magnetic stimulation and kinematic data in patients. We found that short- and long-latency interhemispheric inhibition was reduced (less inhibition) from both hemispheres in patients than controls. Compared to controls, finger-tapping movements in patients were slower, more irregular, of smaller amplitudes and characterized by a progressive amplitude reduction during movement repetition (sequence effect). Among Parkinson's disease patients, the degree of short-latency interhemispheric inhibition imbalance towards the less affected primary motor cortex correlated with the global clinical motor scores, as well as with the sequence effect on the most affected hand. The greater the interhemispheric inhibition imbalance towards the less affected hemisphere (i.e. less inhibition from the less to the most affected primary motor cortex than that measured from the most to the less affected primary motor cortex), the more severe the bradykinesia in patients. In conclusion, the inhibitory connections between the two primary motor cortices in Parkinson's disease are reduced. The interhemispheric disinhibition of the primary motor cortex may have a role in the pathophysiology of specific bradykinesia features in patients, i.e. the sequence effect.

Interhemispheric Inhibition Between Primary Motor Cortices is Not Altered in Individuals With Chronic Lateral Epicondylalgia

Lateral epicondylalgia (LE), commonly referred to as tennis elbow, is a musculoskeletal condition characterized by pain and sensorimotor dysfunction. In some individuals with chronic unilateral LE, sensorimotor symptoms develop on the unaffected side despite no evidence of tissue damage. Altered interhemispheric inhibition (IHI) is one mechanism that could underpin this phenomenon. The aim of this cross-sectional study was to examine IHI between the primary motor cortices (M1) in individuals with chronic LE and healthy controls. In 20 individuals with chronic LE and 20 healthy participants, transcranial magnetic stimulation was used to assess 1) short and long-latency IHI from the affected (corresponding to the injured side) to the unaffected M1 and 2) corticomotor excitability of the affected and unaffected M1. Sensorimotor function was evaluated bilaterally at the extensor carpi radialis brevis muscle using pressure pain threshold, grip strength, 2-point discrimination, and temporal summation tests. Short- and long-latency IHI from the affected to the unaffected M1 and corticomotor excitability of the affected and unaffected M1 were not altered in individuals with LE compared with healthy participants. No differences in sensorimotor function were observed for the affected or unaffected extensor carpi radialis brevis muscles when individuals with LE were compared with healthy participants. IHI is not altered in individuals with chronic LE. Further studies are required to determine the mechanisms that underpin the development of bilateral sensorimotor symptoms in unilateral LE. PERSPECTIVE: IHI is unaltered from the affected M1 (corresponding to the painful muscle) to unaffected M1 in individuals with LE compared to healthy controls. The absence of bilateral sensorimotor dysfunction and low pain severity in this cohort of individuals with LE may explain this finding.

Connecting the dots: harnessing dual-site transcranial magnetic stimulation to quantify the causal influence of medial frontal areas on the motor cortex

Dual-site transcranial magnetic stimulation has been widely employed to investigate the influence of cortical structures on the primary motor cortex. Here, we leveraged this technique to probe the causal influence of two key areas of the medial frontal cortex, namely the supplementary motor area and the medial orbitofrontal cortex, on primary motor cortex. We show that supplementary motor area stimulation facilitates primary motor cortex activity across short (6 and 8 ms) and long (12 ms) inter-stimulation intervals, putatively recruiting cortico-cortical and cortico-subcortico-cortical circuits, respectively. Crucially, magnetic resonance imaging revealed that this facilitatory effect depended on a key morphometric feature of supplementary motor area: individuals with larger supplementary motor area volumes exhibited more facilitation from supplementary motor area to primary motor cortex for both short and long inter-stimulation intervals. Notably, we also provide evidence that the facilitatory effect of supplementary motor area stimulation at short intervals is unlikely to arise from spinal interactions of volleys descending simultaneously from supplementary motor area and primary motor cortex. On the other hand, medial orbitofrontal cortex stimulation moderately suppressed primary motor cortex activity at both short and long intervals, irrespective of medial orbitofrontal cortex volume. These results suggest that dual-site transcranial magnetic stimulation is a fruitful approach to investigate the differential influence of supplementary motor area and medial orbitofrontal cortex on primary motor cortex activity, paving the way for the multimodal assessment of these fronto-motor circuits in health and disease.

PMd and action preparation: bridging insights between TMS and single neuron research

Transcranial magnetic stimulation (TMS) research has furthered understanding of human dorsal premotor cortex (PMd) function due to its unrivalled ability to measure the inhibitory and facilitatory influences of PMd over the primary motor cortex (M1) in a temporally precise manner. TMS research indicates that PMd transiently modulates inhibitory output to effector representations within M1 during motor preparation, with the direction of modulation depending on which effectors are selected for response, and the timing of modulations co-varying with task selection demands. In this review, we critically assess this literature in the context of a dynamical systems approach used to model nonhuman primate (NHP) PMd/M1 single-neuron recordings during action preparation. Through this process, we identify gaps in the literature and propose future experiments.

Clinical diagnostic utility of transcranial magnetic stimulation in neurological disorders. Updated report of an IFCN committee

The review provides a comprehensive update (previous report: Chen R, Cros D, Curra A, Di Lazzaro V, Lefaucheur JP, Magistris MR, et al. The clinical diagnostic utility of transcranial magnetic stimulation: report of an IFCN committee. Clin Neurophysiol 2008;119(3):504-32) on clinical diagnostic utility of transcranial magnetic stimulation (TMS) in neurological diseases. Most TMS measures rely on stimulation of motor cortex and recording of motor evoked potentials. Paired-pulse TMS techniques, incorporating conventional amplitude-based and threshold tracking, have established clinical utility in neurodegenerative, movement, episodic (epilepsy, migraines), chronic pain and functional diseases. Cortical hyperexcitability has emerged as a diagnostic aid in amyotrophic lateral sclerosis. Single-pulse TMS measures are of utility in stroke, and myelopathy even in the absence of radiological changes. Short-latency afferent inhibition, related to central cholinergic transmission, is reduced in Alzheimer's disease. The triple stimulation technique (TST) may enhance diagnostic utility of conventional TMS measures to detect upper motor neuron involvement. The recording of motor evoked potentials can be used to perform functional mapping of the motor cortex or in preoperative assessment of eloquent brain regions before surgical resection of brain tumors. TMS exhibits utility in assessing lumbosacral/cervical nerve root function, especially in demyelinating neuropathies, and may be of utility in localizing the site of facial nerve palsies. TMS measures also have high sensitivity in detecting subclinical corticospinal lesions in multiple sclerosis. Abnormalities in central motor conduction time or TST correlate with motor impairment and disability in MS. Cerebellar stimulation may detect lesions in the cerebellum or cerebello-dentato-thalamo-motor cortical pathways. Combining TMS with electroencephalography, provides a novel method to measure parameters altered in neurological disorders, including cortical excitability, effective connectivity, and response complexity.

Interhemispheric interplay between the left and right premotor cortex during grasping as assessed by dynamic causal modelling

Research on the contribution of the ipsilateral hemisphere to unilateral movements, and how it is mediated by transcallosal connections, has so far provided contradictory findings. By using dynamic causal modelling (DCM) and Parametric Empirical Bayes analyses applied to fMRI data, we sought to describe effective connectivity during pantomimed and imagined right-hand grasping within the grasping network, namely the anterior intraparietal sulcus, ventral and dorsal (PMd) premotor cortex, supplementary motor area and primary motor cortex (M1). The two-fold aim of the present work was to explore a) whether right and left parieto-frontal areas show similar connectivity couplings, and b) the interhemispheric dynamics between these regions across the two hemispheres. We detected a network architecture comparable across hemispheres during executed but not imagined grasping movements. Furthermore, during pantomimed grasping the interhemispheric crosstalk was mainly driven by premotor areas: we found an inhibitory influence from the right PMd toward the left premotor and motor areas and excitatory couplings between homologous ventral premotor and supplementary motor regions. Overall, our results support the view that dissociable components of unilateral grasping execution are encoded by a non-lateralized set of brain areas complexly intertwined by interhemispheric dynamics, whereas motor imagery obeys different principles.

Recovery of Patients With Upper Limb Paralysis Due to Stroke Who Underwent Intervention Using Low-Frequency Repetitive Transcranial Magnetic Stimulation Combined With Occupational Therapy: A Retrospective Cohort Study

OBJECTIVES

The combination of repetitive transcranial magnetic stimulation (rTMS) and motor practice is based on the theory of neuromodulation and use-dependent plasticity. Predictive planning of occupational therapy (OT) is important for patients with rTMS conditioning. Recovery characteristics based on the severity of pretreatment upper extremity paralysis can guide the patient's practice plan for using the paretic hand. Therefore, we evaluated the recovery of patients with upper limb paralysis due to stroke who underwent a novel intervention of rTMS combined with OT (NEURO) according to the severity of upper limb paralysis based on the scores of the Fugl-Meyer assessment for upper extremity (FMA-UE) with recovery in proximal upper extremity, wrist, hand, and coordination.

MATERIALS AND METHODS

In this multicenter retrospective cohort study, the recovery of 1397 patients with upper limb paralysis was analyzed by severity at six hospitals that were accredited by the Japanese Stimulation Therapy Society for treatment. The delta values of the FMA-UE scores before and after NEURO were compared among the groups with severe, moderate, and mild paralysis using the generalized linear model.

RESULTS

NEURO significantly improved the FMA-UE total score according to the severity of paralysis (severe = 5.3, moderate = 6.0, and mild = 2.9). However, when the FMA-UE subscores were analyzed separately, the results indicated specific improvements in shoulder/elbow, wrist, fingers, and coordination movements, depending on the severity.

CONCLUSIONS

This study had enough patients who were divided according to severity and stratified by lesion location and handedness parameters. Our results suggest that independently of these factors, the extent of recovery of upper limb motor parts after NEURO varies according to the severity of paralysis.

No Polarity-specific Modulation of Prefrontal-to-M1 Interhemispheric Inhibition by Transcranial Direct Current Stimulation Over the Lateral Prefrontal Cortex

The lateral prefrontal cortex (PFC) plays a variety of crucial roles in higher-order cognitive functions. Previous works have attempted to modulate lateral PFC function by applying non-invasive transcranial direct current stimulation (tDCS) and demonstrated positive effects on performance of tasks involving cognitive processes. The neurophysiological underpinning of the stimulation effects, however, remain poorly understood. Here, we explored the neurophysiological after-effects of tDCS over the lateral PFC by assessing changes in the magnitude of interhemispheric inhibition from the lateral PFC to the contralateral primary motor cortex (PFC-M1 IHI). Using a dual-site transcranial magnetic stimulation paradigm, we assessed PFC-M1 IHI before and after the application of tDCS over the right lateral PFC. We conducted a double-blinded, crossover, and counterbalanced design where 15 healthy volunteers participated in three sessions during which they received either anodal, cathodal, and sham tDCS. In order to determine whether PFC-M1 IHI could be modulated at all, we completed the same assessment on a separate group of 15 participants as they performed visuo-motor reaction tasks that likely engage the lateral PFC. The results showed that tDCS over the right lateral PFC did not modulate the magnitude of PFC-M1 IHI, whereas connectivity changed when Go/NoGo decisions were implemented in reactions during the motor tasks. Although PFC-M1 IHI is sensitive enough to be modulated by behavioral manipulations, tDCS over the lateral PFC does not have substantial modulatory effects on PFC to M1 functional connectivity, or at least not to the degree that can be detected with this measure.

Dual-site TMS as a tool to probe effective interactions within the motor network: a review

Dual-site transcranial magnetic stimulation (ds-TMS) is well suited to investigate the causal effect of distant brain regions on the primary motor cortex, both at rest and during motor performance and learning. However, given the broad set of stimulation parameters, clarity about which parameters are most effective for identifying particular interactions is lacking. Here, evidence describing inter- and intra-hemispheric interactions during rest and in the context of motor tasks is reviewed. Our aims are threefold: (1) provide a detailed overview of ds-TMS literature regarding inter- and intra-hemispheric connectivity; (2) describe the applicability and contributions of these interactions to motor control, and; (3) discuss the practical implications and future directions. Of the 3659 studies screened, 109 were included and discussed. Overall, there is remarkable variability in the experimental context for assessing ds-TMS interactions, as well as in the use and reporting of stimulation parameters, hindering a quantitative comparison of results across studies. Further studies examining ds-TMS interactions in a systematic manner, and in which all critical parameters are carefully reported, are needed.

Investigating the role of contextual cues and interhemispheric inhibitory mechanisms in response-selective stopping: a TMS study

Response-selective stopping requires cancellation of only one component of a multicomponent action. While research has investigated how delays to the continuing action components ("stopping interference") can be attenuated by way of contextual cues of the specific stopping demands ("foreknowledge"), little is known of the underlying neural mechanisms. Twenty-seven, healthy, young adults undertook a multicomponent stop-signal task. For two thirds of trials, participants responded to an imperative (go) stimulus (IS) with simultaneous button presses using their left and right index fingers. For the remaining one third of trials, the IS was followed by a stop-signal requiring cancellation of only the left, or right, response. To manipulate foreknowledge of stopping demands, a cue preceded the IS that informed participants which hand might be required to stop (proactive) or provided no such information (reactive). Transcranial magnetic stimulation (TMS) assessed corticospinal excitability (CSE) as well as short- and long-interval interhemispheric inhibition (SIHI, LIHI) between the primary motor cortices. Proactive cues reduced, but did not eliminate, stopping interference relative to the reactive condition. Relative to TMS measures at cue onset, decreases in CSE (both hands and both cue conditions) and LIHI (both hands, proactive condition only) were observed during movement preparation. During movement cancellation, LIHI reduction in the continuing hand was greater than that in the stopping hand and greater than LIHI reductions in both hands during execution of multicomponent responses. Our results indicate that foreknowledge attenuates stopping interference and provide evidence for a novel role of LIHI, mediated via prefrontal regions, in facilitating continuing action components.

Altered frontoparietal activity in acoustic startle priming tasks during reticulospinal tract facilitation: An fNIRS study

Background

Because it is one of the important pathways for promoting motor recovery after cortical injury, the function of the reticulospinal tract (RST) has received increasing attention in recent years. However, the central regulatory mechanism of RST facilitation and reduction of apparent response time is not well understood.

Objectives

To explore the potential role of RST facilitation in the acoustic startle priming (ASP) paradigm and observe the cortical changes induced by ASP reaching tasks.

Methods

Twenty healthy participants were included in this study. The reaching tasks were performed with their left and right hands. Participants were instructed to get ready after the warning cue and complete the reach as soon as they heard the Go cue. Half of the testing trials were set as control trials with an 80-dB Go cue. The other half of the trials had the Go cue replaced with 114-dB white noise to evoke the StartleReact effect, inducing reticulospinal tract facilitation. The response of the bilateral sternocleidomastoid muscle (SCM) and the anterior deltoid was recorded via surface electromyography. Startle trials were labeled as exhibiting a positive or negative StartleReact effect, according to whether the SCM was activated early (30-130 ms after the Go cue) or late, respectively. Functional near-infrared spectroscopy was used to synchronously record the oxyhemoglobin and deoxyhemoglobin fluctuations in bilateral motor-related cortical regions. The β values representing cortical responses were estimated via the statistical parametric mapping technique and included in the final analyses.

Results

Separate analyses of data from movements of the left or right side revealed significant activation of the right dorsolateral prefrontal cortex during RST facilitation. Moreover, left frontopolar cortex activation was greater in positive startle trials than in control or negative startle trials during left-side movements. Furthermore, decreased activity of the ipsilateral primary motor cortex in positive startle trials during ASP reaching tasks was observed.

Conclusion

The right dorsolateral prefrontal cortex and the frontoparietal network to which it belongs may be the regulatory center for the StartleReact effect and RST facilitation. In addition, the ascending reticular activating system may be involved. The decreased activity of the ipsilateral primary motor cortex suggests enhanced inhibition of the non-moving side during the ASP reaching task. These findings provide further insight into the SE and into RST facilitation.

Connectivity from ipsilateral and contralateral dorsolateral prefrontal cortex to the active primary motor cortex during approaching-avoiding behavior

Automatic action tendency is reflected by a fast reaction to approach positive stimulus and to avoid negative stimulus (automatic behaviors), while a slow reaction to approach negative stimulus and avoid positive stimulus (controlled behaviors). The dorsolateral prefrontal cortex (DLPFC) is involved in the modulation of the automatic action tendency; however, it remains unclear whether DLPFC modulates the behavior through motor inhibition or excitation, as well as the exact timing of the modulation. We used paired-pulse, dual-site TMS protocols to investigate the connectivity between left/right DLPFC and the left primary motor cortex (M1) during the manikin task performed with the right hand. For the behavioral data, the results from reaction time (RT) and premotor time (PMT), which represents the beginning of finger movements, of the approaching-avoiding behavior in both experiments showed a shorter duration for automatic behavior compared to the controlled behavior. There was stronger facilitation of the left DLPFC-left M1 connectivity at interstimulus-interval of 25 ms in controlled behavior compared to automatic behavior (positive-approaching vs. positive-avoiding: P = .002; negative-avoiding vs. negative-approaching: P = .017). The right DLPFC-left M1 connectivity did not change with the task. The present study confirmed the automatic action tendency from both reaction time and the premotor time. During the right-handed task, the DLPFC contralateral but not ipsilateral to the effector could facilitate the left M1 to speed up the execution of the controlled behavior through a polysynaptic pathway.

Clinical neurophysiology of Parkinson's disease and parkinsonism

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This review is part of the series on the clinical neurophysiology of movement disorders and focuses on Parkinson’s disease and parkinsonism.

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The pathophysiology of cardinal parkinsonian motor symptoms and myoclonus are reviewed.

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The recordings from microelectrode and deep brain stimulation electrodes are reported in detail.

This review is part of the series on the clinical neurophysiology of movement disorders. It focuses on Parkinson’s disease and parkinsonism. The topics covered include the pathophysiology of tremor, rigidity and bradykinesia, balance and gait disturbance and myoclonus in Parkinson’s disease. The use of electroencephalography, electromyography, long latency reflexes, cutaneous silent period, studies of cortical excitability with single and paired transcranial magnetic stimulation, studies of plasticity, intraoperative microelectrode recordings and recording of local field potentials from deep brain stimulation, and electrocorticography are also reviewed. In addition to advancing knowledge of pathophysiology, neurophysiological studies can be useful in refining the diagnosis, localization of surgical targets, and help to develop novel therapies for Parkinson’s disease.

Spatially bivariate EEG-neurofeedback can manipulate interhemispheric inhibition

Human behavior requires inter-regional crosstalk to employ the sensorimotor processes in the brain. Although external neuromodulation techniques have been used to manipulate interhemispheric sensorimotor activity, a central controversy concerns whether this activity can be volitionally controlled. Experimental tools lack the power to up- or down-regulate the state of the targeted hemisphere over a large dynamic range and, therefore, cannot evaluate the possible volitional control of the activity. We addressed this difficulty by using the recently developed method of spatially bivariate electroencephalography (EEG)-neurofeedback to systematically enable the participants to modulate their bilateral sensorimotor activities. Here, we report that participants learn to up- and down-regulate the ipsilateral excitability to the imagined hand while maintaining constant contralateral excitability; this modulates the magnitude of interhemispheric inhibition (IHI) assessed by the paired-pulse transcranial magnetic stimulation (TMS) paradigm. Further physiological analyses revealed that the manipulation capability of IHI magnitude reflected interhemispheric connectivity in EEG and TMS, which was accompanied by intrinsic bilateral cortical oscillatory activities. Our results show an interesting approach for neuromodulation, which might identify new treatment opportunities, e.g., in patients suffering from a stroke.

Plasticity changes in dorsolateral prefrontal cortex associated with procedural sequence learning are hemisphere-specific

Corticocortical neuroplastic changes from higher-order cortices to primary motor cortex (M1) have been described for procedural sequence learning. The dorsolateral prefrontal cortex (DLPFC) plays critical roles in cognition, including in motor learning and memory. However, neuroplastic changes in the DLPFC and their influence on M1 and on motor learning are not well understood. The present study examined bilateral DLPFC-M1 changes in plasticity induced by procedural motor sequence learning in a serial reaction time task. DLPFC plasticity induced by procedural sequence learning was examined by comparing before vs. after training assessments of ipsilateral/contralateral DLPFC-M1 interactions between sequence order and random order trials performed using either the left or right hand. Intra-hemispheric (inter-stimulus interval [ISI] = 10 ms) and inter-hemispheric (ISI = 10 or 50 ms) DLPFC-M1 interactions and single-pulse motor-evoked potentials (MEPs) were measured with transcranial magnetic stimulation (TMS). The reaction times of participants measured during motor training were faster for sequence learning than for random learning with either hand. Paired-pulse TMS induced DLPFC-M1 interactions that were disinhibited after motor sequence learning, especially for left DLPFC-left M1 interactions with right hand task performance and for left DLPFC-right M1 interactions with left hand task performance. These findings indicate that motor sequence learning induces neuroplastic changes to enhance DLPFC-M1 interactions. This manifestation of plasticity showed hemispheric specificity, favoring the left DLPFC. DLPFC plasticity may be a useful index of DLPFC function and may be a treatment target for enhancing DLPFC function and motor learning.

Hyper-connectivity between the left motor cortex and prefrontal cortex is associated with the severity of dysfunction of the descending pain modulatory system in fibromyalgia

INTRODUCTION

The association between descending pain modulatory system (DPMS) dysfunction and fibromyalgia has been previously described, but more studies are required on its relationship with aberrant functional connectivity (FC) between the motor and prefrontal cortices.

OBJECTIVES

The objective of this cross-sectional observational study was to compare the intra- and interhemispheric FC between the bilateral motor and prefrontal cortices in women with fibromyalgia, comparing responders and nonresponders to the conditioned pain modulation (CPM) test.

METHODS

A cross-sectional sample of 37 women (23 responders and 14 nonresponders to the CPM test) with fibromyalgia diagnosed according to the American College of Rheumatology criteria underwent a standardized clinical assessment and an FC analysis using functional near-infrared spectroscopy. DPMS function was inferred through responses to the CPM test, which were induced by hand immersion in cold water (0-1°C). A multivariate analysis of covariance for main effects between responders and nonresponders was conducted using the diagnosis of multiple psychiatric disorders and the use of opioid and nonopioid analgesics as covariates. In addition, we analyzed the interaction between the CPM test response and the presence of multiple psychiatric diagnoses.

RESULTS

Nonresponders showed increased FC between the left motor cortex (lMC) and the left prefrontal cortex (lPFC) (t = -2.476, p = 0.01) and right prefrontal cortex (rPFC) (t = -2.363, p = 0.02), even when both were considered as covariates in the regression analysis (lMC-lPFC: β = -0.127, t = -2.425, p = 0.021; lMC-rPFC: β = -0.122, t = -2.222, p = 0.033). Regarding main effects, a significant difference was only observed for lMC-lPFC (p = 0.035). A significant interaction was observed between the psychiatric disorders and nonresponse to the CPM test in lMC-lPFC (β = -0.222, t = -2.275, p = 0.03) and lMC-rPFC (β = -0.211, t = -2.2, p = 0.035). Additionally, a significant interaction was observed between the CPM test and FC in these two region-of-interest combinations, despite the psychiatric diagnoses (lMC-lPFC: β = -0.516, t = -2.447, p = 0.02; lMC-rPFC: β = -0.582, t = -2.805, p = 0.008).

CONCLUSIONS

Higher FC between the lMC and the bilateral PFC may be a neural marker of DPMS dysfunction in women with fibromyalgia, although its interplay with psychiatric diagnoses also seems to influence this association.

Impact of interhemispheric inhibition on bimanual movement control in young and old

Interhemispheric interactions demonstrate a crucial role for directing bimanual movement control. In humans, a well-established paired-pulse transcranial magnetic stimulation paradigm enables to assess these interactions by means of interhemispheric inhibition (IHI). Previous studies have examined changes in IHI from the active to the resting primary motor cortex during unilateral muscle contractions; however, behavioral relevance of such changes is still inconclusive. In the present study, we evaluated two bimanual tasks, i.e., mirror activity and bimanual anti-phase tapping, to examine behavioral relevance of IHI for bimanual movement control within this behavioral framework. Two age groups (young and older) were evaluated as bimanual movement control demonstrates evident behavioral decline in older adults. Two types of IHI with differential underlying mechanisms were measured; IHI was tested at rest and during a motor task from the active to the resting primary motor cortex. Results demonstrate an association between behavior and short-latency IHI in the young group: larger short-latency IHI correlated with better bimanual movement control (i.e., less mirror activity and better bimanual anti-phase tapping). These results support the view that short-latency IHI represents a neurophysiological marker for the ability to suppress activity of the contralateral side, likely contributing to efficient bimanual movement control. This association was not observed in the older group, suggesting age-related functional changes of IHI. To determine underlying mechanisms of impaired bimanual movement control due to neurological disorders, it is crucial to have an in-depth understanding of age-related mechanisms to disentangle disorder-related mechanisms of impaired bimanual movement control from age-related ones.

Anatomical and fMRI-network comparison of multiple DLPFC targeting strategies for repetitive transcranial magnetic stimulation treatment of depression

BACKGROUND

The efficacy of repetitive transcranial magnetic stimulation (rTMS) for depression may vary depending on the subregion stimulated within the dorsolateral prefrontal cortex (DLPFC). Clinical TMS typically uses scalp-based landmarks for DLPFC targeting, rather than individualized MRI guidance.

OBJECTIVE

In rTMS patients, determine the brain systems targeted by multiple DLPFC stimulation rules by computing several surrogate measures: underlying brain targets labeled with connectivity-based atlases, subgenual cingulate anticorrelation strength, and functionally connected networks.

METHODS

Forty-nine patients in a randomized controlled trial of rTMS therapy for treatment resistant major depression underwent structural and functional MRI. DLPFC rules were applied virtually using MR-image guidance. Underlying cortical regions were labeled, and connectivity with the subgenual cingulate and whole-brain computed.

RESULTS

Scalp-targeting rules applied post hoc to these MRIs that adjusted for head size, including Beam F3, were comparably precise, successful in directly targeting classical DLPFC and frontal networks, and anticorrelated with the subgenual cingulate. In contrast, all rules involving fixed distances introduced variability in regions and networks targeted. The 5 cm rule targeted a transitional DLPFC region with a different connectivity profile from the adjusted rules. Seed-based connectivity analyses identified multiple regions, such as posterior cingulate and inferior parietal lobe, that warrant further study in order to understand their potential contribution to clinical response.

CONCLUSION

EEG-based rules consistently targeted DLPFC brain regions with resting-state fMRI features known to be associated with depression response. These results provide a bridge from lab to clinic by enabling clinicians to relate scalp-targeting rules to functionally connected brain systems.

Response bias reveals the role of interhemispheric inhibitory networks in movement preparation and execution

Human movement is influenced by various cognitive processes, such as bias, that dynamically shape competing movement representations. However, the neurophysiological mechanisms underlying the effects of bias on movement selection across the lifespan remains poorly understood. Healthy young (n = 21) and older (n = 20) adults completed a choice reaction-time task necessitating left- or right-hand responses to imperative stimuli (IS). Response bias was manipulated via a cue that informed participants a particular response was 70% likely (i.e., the IS was either congruent, or incongruent, with the cue); biasing was either fixed for blocks of trials (block-wise bias) or varied from trial-to-trial (trial-wise bias). As well as assessing the behavioural manifestations of bias, we used transcranial magnetic stimulation to determine changes in corticospinal excitability (CSE) and short- and long-interval interhemispheric inhibition (SIHI, LIHI) during movement preparation and execution. Participants responded more quickly, and accurately, in congruent compared to incongruent trials. CSE decreases occurred in both hands following the cue, consistent with the 'inhibition for impulse control' hypothesis of preparatory inhibition. In contrast, IHI modulations occurred in a hand-specific manner. Greater SIHI was observed during movement preparation in the hand biased away from, compared to the hand biased towards, the cue; furthermore, greater SIHI was observed during movement execution in the hand biased towards the cue when it was not required to respond (i.e., incongruent trial) compared to when it was required to respond (congruent trial). Additionally, during the movement preparation period, the LIHI ratio of the hand biased towards, compared to the hand biased away from, the cue was greatest when the cue varied trial-by-trial. Overall, the IHI results provide support for the 'inhibition for competition resolution' hypothesis, with hand specific modulation of inhibition during movement preparation and execution.

Neural mechanisms mediating cross education: With additional considerations for the ageing brain

CALVERT, G.H.M., and CARSON, R.G. Neural mechanisms mediating cross education: With additional considerations for the ageing brain. NEUROSCI BIOBEHAV REV 21(1) XXX-XXX, 2021. - Cross education (CE) is the process whereby a regimen of unilateral limb training engenders bilateral improvements in motor function. The contralateral gains thus derived may impart therapeutic benefits for patients with unilateral deficits arising from orthopaedic injury or stroke. Despite this prospective therapeutic utility, there is little consensus concerning its mechanistic basis. The precise means through which the neuroanatomical structures and cellular processes that mediate CE may be influenced by age-related neurodegeneration are also almost entirely unknown. Notwithstanding the increased incidence of unilateral impairment in later life, age-related variations in the expression of CE have been examined only infrequently. In this narrative review, we consider several mechanisms which may mediate the expression of CE with specific reference to the ageing CNS. We focus on the adaptive potential of cellular processes that are subserved by a specific set of neuroanatomical pathways including: the corticospinal tract, corticoreticulospinal projections, transcallosal fibres, and thalamocortical radiations. This analysis may inform the development of interventions that exploit the therapeutic utility of CE training in older persons.

Neurophysiological modulations in the (pre)motor-motor network underlying age-related increases in reaction time and the role of GABA levels - a bimodal TMS-MRS study

It has been argued that age-related changes in the neurochemical and neurophysiological properties of the GABAergic system may underlie increases in reaction time (RT) in older adults. However, the role of GABA levels within the sensorimotor cortices (SMC) in mediating interhemispheric interactions (IHi) during the processing stage of a fast motor response, as well as how both properties explain interindividual differences in RT, are not yet fully understood. In this study, edited magnetic resonance spectroscopy (MRS) was combined with dual-site transcranial magnetic stimulation (dsTMS) for probing GABA+ levels in bilateral SMC and task-related neurophysiological modulations in corticospinal excitability (CSE), and primary motor cortex (M1)-M1 and dorsal premotor cortex (PMd)-M1 IHi, respectively. Both CSE and IHi were assessed during the preparatory and premotor period of a delayed choice RT task. Data were collected from 25 young (aged 18-33 years) and 28 older (aged 60-74 years) healthy adults. Our results demonstrated that older as compared to younger adults exhibited a reduced bilateral CSE suppression, as well as a reduced magnitude of long latency M1-M1 and PMd-M1 disinhibition during the preparatory period, irrespective of the direction of the IHi. Importantly, in older adults, the GABA+ levels in bilateral SMC partially accounted for task-related neurophysiological modulations as well as individual differences in RT. In contrast, in young adults, neither task-related neurophysiological modulations, nor individual differences in RT were associated with SMC GABA+ levels. In conclusion, this study contributes to a comprehensive initial understanding of how age-related differences in neurochemical properties and neurophysiological processes are related to increases in RT.

Time course of the effects of low-intensity transcranial ultrasound on the excitability of ipsilateral and contralateral human primary motor cortex

Low-intensity transcranial ultrasound stimulation (TUS) is a promising non-invasive brain stimulation technique that can modulate the excitability of cortical and deep brain structures with a high degree of focality. Previous human studies showed that TUS decreases motor cortex (M1) excitability measured by transcranial magnetic stimulation (TMS), but whether the effects appear beyond sonication and whether TUS affects the excitability of other interconnected cortical areas is not known. The time course of M1 TUS on ipsilateral and contralateral M1 excitability was investigated in 22 healthy human subjects via TMS-induced motor-evoked potentials. With sonication duration of 500 ms, we found suppression of M1 excitability from 10 ms before to 20 ms after the end of sonication, and the effects were stronger with blocked design compared to interleaved design. There was no significant effect on contralateral M1 excitability. Using ex-vivo measurements, we showed that the ultrasound transducer did not affect the magnitude or time course of the TMS-induced electromagnetic field. We conclude that the online-suppressive effects of TUS on ipsilateral M1 cortical excitability slightly outlast the sonication but did not produce long-lasting effects. The absence of contralateral effects may suggest that there are little tonic interhemispheric interactions in the resting state, or the intensity of TUS was too low to induce transcallosal inhibition.

Paradoxical facilitation alongside interhemispheric inhibition

Neurophysiological experiments using transcranial magnetic stimulation (TMS) have sought to probe the function of the motor division of the corpus callosum. Primary motor cortex sends projections via the corpus callosum with a net inhibitory influence on the homologous region of the opposite hemisphere. Interhemispheric inhibition (IHI) experiments probe this inhibitory pathway. A test stimulus (TS) delivered to the motor cortex in one hemisphere elicits motor evoked potentials (MEPs) in a target muscle, while a conditioning stimulus (CS) applied to the homologous region of the opposite hemisphere modulates the effect of the TS. We predicted that large CS MEPs would be associated with increased IHI since they should be a reliable index of how effectively contralateral motor cortex was stimulated and therefore of the magnitude of interhemispheric inhibition. However, we observed a strong tendency for larger CS MEPs to be associated with reduced interhemispheric inhibition which in the extreme lead to a net effect of facilitation. This surprising effect was large, systematic, and observed in nearly all participants. We outline several hypotheses for mechanisms which may underlie this phenomenon to guide future research.

Existence of Interhemispheric Inhibition between Foot Sections of Human Primary Motor Cortices: Evidence from Negative Blood Oxygenation-Level Dependent Signal

Interhemispheric inhibition (IHI) between the left and right primary motor cortices (M1) plays an important role when people perform an isolated unilateral limb movement. Moreover, negative blood oxygenation-level dependent signal (deactivation) obtained from the M1 ipsilateral to the limb could be a surrogate IHI marker. Studies have reported deactivation in the hand section of the ipsilateral M1 during simple unilateral hand movement. However, deactivation in the foot section during unilateral foot movement has not been reported. Therefore, IHI between the foot sections of the bilateral M1s has been considered very weak or absent. Thirty-seven healthy adults performed active control of the right foot and also passively received vibration to the tendon of the tibialis anterior muscle of the right foot, which activates the foot section of the contralateral M1, with brain activity being examined through functional magnetic resonance imaging. The vibration and active tasks significantly and non-significantly, respectively, deactivated the foot section of the ipsilateral M1, with a corresponding 86% and 60% of the participants showing decreased activity. Thus, there could be IHI between the foot sections of the bilateral M1s. Further, our findings demonstrate between-task differences and similarities in cross-somatotopic deactivation.

Two Distinct Systems Represent Contralateral and Ipsilateral Sensorimotor Processes in the Human Premotor Cortex: A Dense TMS Mapping Study

Animal brains contain behaviorally committed representations of the surrounding world, which integrate sensory and motor information. In primates, sensorimotor mechanisms reside in part in the premotor cortex (PM), where sensorimotor neurons are topographically clustered according to functional specialization. Detailed functional cartography of the human PM is still under investigation. We explored the topographic distribution of spatially dependent sensorimotor functions in healthy volunteers performing left or right, hand or foot, responses to visual cues presented in the left or right hemispace, thus combining independently stimulus side, effector side, and effector type. Event-related transcranial magnetic stimulation was applied to single spots of a dense grid of 10 points on the participants' left hemiscalp, covering the whole PM. Results showed: (1) spatially segregated hand and foot representations, (2) focal representations of contralateral cues and movements in the dorsal PM, and (3) distributed representations of ipsilateral cues and movements in the ventral and dorso-medial PM. The present novel causal information indicates that (1) the human PM is somatotopically organized and (2) the left PM contains sensory-motor representations of both hemispaces and of both hemibodies, but the hemispace and hemibody contralateral to the PM are mapped on a distinct, nonoverlapping cortical region compared to the ipsilateral ones.

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.

Functional role of dorsolateral prefrontal cortex in the modulation of cognitive bias

Human cognition is often biased. It is a fundamental question in psychology how cognitive bias is modulated in the human brain. Automatic action tendency is a typical cognitive bias. The dorsolateral prefrontal cortex (DLPFC) is a crucial area for processing various behavioral tasks. We investigated the functional role of DLPFC in the modulation of cognitive bias by testing the automatic action tendency during automatic and regulated behavioral tasks. Unilateral intermittent or continuous theta burst stimulation (excitatory iTBS or inhibitory cTBS) was used to manipulate the left or right DLPFC excitability and assess the changes in automatic action tendency during a manikin task. An approaching behavior with positive stimulus and avoiding behavior with negative stimulus were performed in an automatic task. An approaching behavior with negative stimulus and avoiding behavior with positive stimulus were performed in a regulated task. Reaction time was measured. We confirmed the automatic action tendency that reaction time for performing an automatic task was shorter than that for performing a regulated task. The automatic action tendency was enhanced after left DLPFC excitatory iTBS and was abolished after left DLPFC inhibitory cTBS stimulation. On the other hand, right DLPFC excitatory iTBS accelerated the avoiding behaviors and right DLPFC inhibitory cTBS accelerated approaching behaviors. The results suggest that left DLPFC modulates the automatic action tendency while the right DLPFC modulates the direction of behavioral tasks. We conclude that left DLPFC and right DLPFC are key nodes in modulating the cognitive bias while their functional roles are different.

Asymmetry of Interhemispheric Connectivity during Rapid Movements of Right and Left Hands: A TMS-EEG Study

The interhemispheric signal propagation (ISP) obtained by electroencephalography during transcranial magnetic stimulation (TMS) allows for the assessment of the interhemispheric connectivity involved in inhibitory processes. To investigate the functional asymmetry of hemispheres during rapid movement, we compared ISP in the left and right hemispheres during rapid hand movements. In 11 healthy right-handed adults, we delivered TMS to the M1 and recorded ISP from the M1 to the contralateral hemisphere. We found that ISP from the left to right hemisphere during right-hand rapid movement was higher than ISP from the right to left hemisphere during the left-hand rapid movement. These results indicate that the left M1 strongly inhibits the right M1, and that the left hemisphere is dominant for rapid movements as well as sequential movements.

Transcranial Magnetic Stimulation to Assess Exercise-Induced Neuroplasticity

Aerobic exercise facilitates neuroplasticity and has been linked to improvements in cognitive and motor function. Transcranial magnetic stimulation (TMS) is a non-invasive technique that can be used to quantify changes in neurophysiology induced by exercise. The present review summarizes the single- and paired-pulse TMS paradigms that can be used to probe exercise-induced neuroplasticity, the optimal stimulation parameters and the current understanding of the neurophysiology underlying each paradigm. Further, this review amalgamates previous research exploring the modulation of these paradigms with exercise-induced neuroplasticity in healthy and clinical populations and highlights important considerations for future TMS-exercise research.

Contribution of TMS and TMS-EEG to the Understanding of Mechanisms Underlying Physiological Brain Aging

In the human brain, aging is characterized by progressive neuronal loss, leading to disruption of synapses and to a degree of failure in neurotransmission. However, there is increasing evidence to support the notion that the aged brain has a remarkable ability to reorganize itself, with the aim of preserving its physiological activity. It is important to develop objective markers able to characterize the biological processes underlying brain aging in the intact human, and to distinguish them from brain degeneration associated with many neurological diseases. Transcranial magnetic stimulation (TMS), coupled with electromyography or electroencephalography (EEG), is particularly suited to this aim, due to the functional nature of the information provided, and thanks to the ease with which it can be integrated with behavioral manipulation. In this review, we aimed to provide up to date information about the role of TMS and TMS-EEG in the investigation of brain aging. In particular, we focused on data about cortical excitability, connectivity and plasticity, obtained by using readouts such as motor evoked potentials and transcranial evoked potentials. Overall, findings in the literature support an important potential contribution of TMS to the understanding of the mechanisms underlying normal brain aging. Further studies are needed to expand the current body of information and to assess the applicability of TMS findings in the clinical setting.

The modulation of short and long-latency interhemispheric inhibition during bimanually coordinated movements

Bimanual coordination is essential for the performance of many everyday tasks. There are several types of bimanually coordinated movements, classified according to whether the arms are acting to achieve a single goal (cooperative) or separate goals (independent), and whether the arms are moving symmetrically or asymmetrically. Symmetric bimanual movements are thought to facilitate corticomotor excitability (CME), while asymmetric bimanual movements are thought to recruit interhemispheric inhibition to reduce functional coupling between the motor cortices. The influences of movement symmetry and goal conceptualisation on interhemispheric interactions have not been studied together, and not during bimanually active dynamic tasks. The present study used transcranial magnetic stimulation (TMS) to investigate the modulation of CME and short- and long-latency interhemispheric inhibition (SIHI and LIHI, respectively) during bimanually active dynamic tasks requiring different types of bimanual coordination. Twenty healthy right-handed adults performed four bimanual tasks in which they held a dumbbell in each hand (independent) or a custom device between both hands (cooperative) while rhythmically flexing and extending their wrists symmetrically or asymmetrically. Motor-evoked potentials were recorded from the right extensor carpi ulnaris. We found CME was greater during asymmetric tasks than symmetric tasks, and movement symmetry did not modulate SIHI or LIHI. There was no effect of goal conceptualisation nor any interaction with movement symmetry for CME, SIHI or LIHI. Based on these results, movement symmetry and goal conceptualisation may not modulate interhemispheric inhibition during dynamic bimanual tasks. These findings contradict prevailing thinking about the roles of CME and interhemispheric inhibition in bimanual coordination.

Transcranial static magnetic stimulation over the motor cortex can facilitate the contralateral cortical excitability in human

Transcranial static magnetic stimulation (tSMS) has been focused as a new non-invasive brain stimulation, which can suppress the human cortical excitability just below the magnet. However, the non-regional effects of tSMS via brain network have been rarely studied so far. We investigated whether tSMS over the left primary motor cortex (M1) can facilitate the right M1 in healthy subjects, based on the hypothesis that the functional suppression of M1 can cause the paradoxical functional facilitation of the contralateral M1 via the reduction of interhemispheric inhibition (IHI) between the bilateral M1. This study was double-blind crossover trial. We measured the corticospinal excitability in both M1 and IHI from the left to right M1 by recording motor evoked potentials from first dorsal interosseous muscles using single-pulse and paired-pulse transcranial magnetic stimulation before and after the tSMS intervention for 30 min. We found that the corticospinal excitability of the left M1 decreased, while that of the right M1 increased after tSMS. Moreover, the evaluation of IHI revealed the reduced inhibition from the left to the right M1. Our findings provide new insights on the mechanistic understanding of neuromodulatory effects of tSMS in human.

Interhemispheric interactions between the right angular gyrus and the left motor cortex: a transcranial magnetic stimulation study

The interconnection of the angular gyrus of right posterior parietal cortex (PPC) and the left motor cortex (LM1) is essential for goal-directed hand movements. Previous work with transcranial magnetic stimulation (TMS) showed that right PPC stimulation increases LM1 excitability, but right PPC followed by left PPC-LM1 stimulation (LPPC-LM1) inhibits LM1 corticospinal output compared with LPPC-LM1 alone. It is not clear if right PPC-mediated inhibition of LPPC-LM1 is due to inhibition of left PPC or to combined effects of right and left PPC stimulation on LM1 excitability. We used paired-pulse TMS to study the extent to which combined right and left PPC stimulation, targeting the angular gyri, influences LM1 excitability. We tested 16 healthy subjects in five paired-pulsed TMS experiments using MRI-guided neuronavigation to target the angular gyri within PPC. We tested the effects of different right angular gyrus (RAG) and LM1 stimulation intensities on the influence of RAG on LM1 and on influence of left angular gyrus (LAG) on LM1 (LAG-LM1). We then tested the effects of RAG and LAG stimulation on LM1 short-interval intracortical facilitation (SICF), short-interval intracortical inhibition (SICI), and long-interval intracortical inhibition (LICI). The results revealed that RAG facilitated LM1, inhibited SICF, and inhibited LAG-LM1. Combined RAG-LAG stimulation did not affect SICI but increased LICI. These experiments suggest that RAG-mediated inhibition of LAG-LM1 is related to inhibition of early indirect (I)-wave activity and enhancement of GABA_B receptor-mediated inhibition in LM1. The influence of RAG on LM1 likely involves ipsilateral connections from LAG to LM1 and heterotopic connections from RAG to LM1.NEW & NOTEWORTHY Goal-directed hand movements rely on the right and left angular gyri (RAG and LAG) and motor cortex (M1), yet how these brain areas functionally interact is unclear. Here, we show that RAG stimulation facilitated right hand motor output from the left M1 but inhibited indirect (I)-waves in M1. Combined RAG and LAG stimulation increased GABA_B, but not GABA_A, receptor-mediated inhibition in left M1. These findings highlight unique brain interactions between the RAG and left M1.

Dissociating the causal role of left and right dorsal premotor cortices in planning and executing bimanual movements - A neuro-navigated rTMS study

BACKGROUND

The dorsal premotor cortex (PMd) is a key region in bimanual coordination. However, causal evidence linking PMd functionality during motor planning and execution to movement quality is lacking.

OBJECTIVE

We investigated how left (PMd_L) and right PMd (PMd_R) are causally involved in planning and executing bimanual movements, using short-train repetitive transcranial magnetic stimulation (rTMS). Additionally, we explored to what extent the observed rTMS-induced modulation of performance could be explained by rTMS-induced modulation of PMd-M1 interhemispheric interactions (IHI).

METHODS

Twenty healthy adults (mean age ± SD = 22.85 ± 3.73 years) participated in two sessions, in which either PMd_L or PMd_R was targeted with rTMS (10 Hz) in a pseudo-randomized design. PMd functionality was transiently modulated during the planning or execution of a complex bimanual task, whereby the participant was asked to track a moving dot by controlling two dials. The effect of rTMS on several performance measures was investigated. Concurrently, rTMS-induced modulation of PMd-M1 IHI was measured using a dual-coil paradigm, and associated with the rTMS-induced performance modulation.

RESULTS

rTMS over PMd_L during planning increased bilateral hand movement speed (p = 0.03), thereby improving movement accuracy (p = 0.02). In contrast, rTMS over PMd_R during both planning and execution induced deterioration of movement stability (p = 0.04). rTMS-induced modulation of PMd-M1 IHI during planning did not predict rTMS-induced performance modulation.

CONCLUSION

The current findings support the growing evidence on PMd_L dominance during motor planning, as PMd_L was crucially involved in planning the speed of each hand, subserving bimanual coordination accuracy. Moreover, the current results suggest that PMd_R fulfills a role in continuous adjustment processes of movement.

Sensorimotor Functional Connectivity: A Neurophysiological Factor Related to BCI Performance

Brain-Computer Interfaces (BCIs) are systems that allow users to control devices using brain activity alone. However, the ability of participants to command BCIs varies from subject to subject. About 20% of potential users of sensorimotor BCIs do not gain reliable control of the system. The inefficiency to decode user's intentions requires the identification of neurophysiological factors determining “good” and “poor” BCI performers. One of the important neurophysiological aspects in BCI research is that the neuronal oscillations, used to control these systems, show a rich repertoire of spatial sensorimotor interactions. Considering this, we hypothesized that neuronal connectivity in sensorimotor areas would define BCI performance. Analyses for this study were performed on a large dataset of 80 inexperienced participants. They took part in a calibration and an online feedback session recorded on the same day. Undirected functional connectivity was computed over sensorimotor areas by means of the imaginary part of coherency. The results show that post- as well as pre-stimulus connectivity in the calibration recording is significantly correlated to online feedback performance in μ and feedback frequency bands. Importantly, the significance of the correlation between connectivity and BCI feedback accuracy was not due to the signal-to-noise ratio of the oscillations in the corresponding post and pre-stimulus intervals. Thus, this study demonstrates that BCI performance is not only dependent on the amplitude of sensorimotor oscillations as shown previously, but that it also relates to sensorimotor connectivity measured during the preceding training session. The presence of such connectivity between motor and somatosensory systems is likely to facilitate motor imagery, which in turn is associated with the generation of a more pronounced modulation of sensorimotor oscillations (manifested in ERD/ERS) required for the adequate BCI performance. We also discuss strategies for the up-regulation of such connectivity in order to enhance BCI performance.

Supplementary motor area contributes to carrying previous movement information over to current movement

The purpose of the present study was to determine the cortical areas contributing to the influence of the previous movement on the current movement. Right-handed healthy human participants abducted and then adducted the left index finger in response to a start cue. Twenty consecutive trials with 10 s intertrial intervals were performed in each trial block. An odd-numbered trial was considered to be the previous trial, and a trial immediately after the previous trial (even-numbered trial) was the current trial. In each trial block, transcranial magnetic stimulation (TMS) was given over one of the seven TMS sites with the start cue in the previous trial. The TMS site was over the supplementary motor area (SMA), right dorsolateral prefrontal cortex, right dorsal premotor cortex, right or left posterior parietal cortex or right primary sensory cortex. Sham TMS, producing magnetic stimulation with the coil tilting 90 degrees off the scalp, was delivered over the Cz. In the current trial, TMS was not delivered. The correlation coefficient of the reaction time between the previous and current trials was positive and significant in the sham TMS trial block. This indicates that the current movement is partially dependent on the previous movement. The correlation coefficient of the reaction time between the previous and current movements in the SMA trial block was significantly different from that in the sham TMS trial block, indicating that the SMA contributes to the influence of the previous movement on the current movement. The SMA contributes to carrying the responsiveness level in the previous movement over to the current movement.