Non-invasive brain stimulation as a tool to study cerebellar-M1 interactions in humans

The Effect of taVNS on the Cerebello-Thalamo-Cortical Pathway: a TMS Study

fMRI studies show activation of cerebellum during transcutaneous auricular vagal nerve stimulation (taVNS); however, there is no evidence whether taVNS induced activation of the cerebellum translates to the cerebellar closed loops involved in motor functions. We assessed the propensity of taVNS at 25 Hz (taVNS25) and 100 Hz (taVNS100) to modulate cerebello-thalamo-cortical pathways using transcranial magnetic stimulation. In our double blind within-subjects study thirty-two participants completed one visit during which cerebellar brain inhibition (CBI) was assessed at baseline (no stimulation) and in a randomized order during taVNS100, taVNS25, and sham taVNS (xVNS). Generalized linear mixed models with gamma distribution were built to assess the effect of taVNS on CBI. The estimated marginal means of linear trends during each taVNS condition were computed and compared in a pairwise fashion with Benjamini-Hochberg correction for multiple comparisons. CBI significantly increased during taVNS100 compared to taVNS25 and xVNS (p = 0.0003 and p = 0.0465, respectively). The taVNS current intensity and CBI conditioning stimulus intensity had no significant effect on CBI. taVNS has a frequency dependent propensity to modulate the cerebello-thalamo-cortical pathway. The cerebellum participates in closed-loop circuits involved in motor, cognitive, and affective operations and may serve as an entry for modulating effects of taVNS.

A meta-analysis of non-invasive brain stimulation (NIBS) effects on cerebellar-associated cognitive processes

Non-invasive brain stimulation (NIBS) techniques, including transcranial magnetic stimulation (TMS) and transcranial electrical stimulation (tES), have provided valuable insights into the role of the cerebellum in cognitive processes. However, replicating findings from studies involving cerebellar stimulation poses challenges. This meta-analysis investigates the impact of NIBS on cognitive processes associated with the cerebellum. We conducted a systematic search and analyzed 66 studies and 91 experiments involving healthy adults who underwent either TMS or transcranial direct current stimulation (tDCS) targeting the cerebellum. The results indicate that anodal tDCS applied to the medial cerebellum enhances cognitive performance. In contrast, high-frequency TMS disrupts cognitive performance when targeting the lateral cerebellar hemispheres or when employed in online protocols. Similarly, low-frequency TMS and continuous theta burst stimulation (cTBS) diminish performance in offline protocols. Moreover, high-frequency TMS impairs accuracy. By identifying consistent effects and moderators of modulation, this meta-analysis contributes to improving the replicability of studies using NIBS on the cerebellum and provides guidance for future research aimed at developing effective NIBS interventions targeting the cerebellum.

Physiology of dystonia: Human studies

In this chapter, we discuss neurophysiological techniques that have been used in the study of dystonia. We examine traditional disease models such as inhibition and excessive plasticity and review the evidence that these play a causal role in pathophysiology. We then review the evidence for sensory and peripheral influences within pathophysiology and look at an emergent literature that tries to probe how oscillatory brain activity may be linked to dystonia pathophysiology.

Cerebellar TMS Induces Motor Responses Mediating Modulation of Spinal Excitability: A Literature Review

Since individuals with cerebellar lesions often exhibit hypotonia, the cerebellum may contribute to the regulation of muscle tone and spinal motoneuron pool excitability. Neurophysiological methods using transcranial magnetic stimulation (TMS) of the cerebellum have been recently proposed for testing the role of the cerebellum in spinal excitability. Under specific conditions, single-pulse TMS administered to the cerebellar hemisphere or vermis elicits a long-latency motor response in the upper or lower limb muscles and facilitates the H-reflex of the soleus muscle, indicating increased excitability of the spinal motoneuron pool. This literature review examined the methods and mechanisms by which cerebellar TMS modulates spinal excitability.

Targeting Cerebellum with Non-Invasive Transcranial Magnetic or Current Stimulation after Cerebral Hemispheric Stroke-Insights for Corticocerebellar Network Reorganization: A Comprehensive Review

Non-invasive brain stimulation (NIBS) has emerged as one of the methods implemented in stroke rehabilitation. Cerebellar stimulation has gained research interest as an alternative strategy to cortical stimulation, based on the role of the cerebellum and corticocerebellar tracts in different motor and cognitive functions. This review investigates the role of the cerebellum in motor and cognitive rehabilitation following cerebral stroke using NIBS techniques combined with other therapies (e.g., speech or physical therapy). Fifteen randomized clinical trials were included. The majority of the literature findings point towards the cerebellum as a promising neurostimulation target following stroke of the cerebral cortex. Findings concern mostly rehabilitation of gait and balance, where cathodal transcranial direct current stimulation (tDCS) and intermittent theta-burst stimulation (iTBS) of the contralesional cerebellar hemisphere produce, in the presented clinical sample, improved performance and plasticity changes in the corticocerebellar network, combined with other rehabilitation methods. Data regarding aphasia rehabilitation are scarce, with right cerebellar tDCS exercising some impact in individual linguistic functions combined with language therapy. Based on recent data concerning cerebellar functions and corticocerebellar networks, along with the development of clinical protocols regarding non-invasive cerebellar (NICS) application, the cerebellum can prove a crucial intervention target in rehabilitation following stroke.

Cerebellar repetitive transcranial magnetic stimulation modulates the motor learning of visually guided voluntary postural control task

We investigated whether vermal cerebellar low-frequency repetitive transcranial magnetic stimulation (crTMS) affects motor learning of visually guided postural tracking training (VTT) using foot center of pressure (COP) as well as the stability and sensory contribution of upright standing. Twenty-one healthy volunteers participated (10 in the sham-crTMS group and 11 in the active-crTMS group). For VTT, participants stood on the force plate 1.5 m from the monitor on which the COP and target moved in a circle. Participants tracked the target with their own COP for 1 min, and 10 VTT sessions were conducted. The tracking error (TE) was compared between trials. Active- or sham-crTMS sessions were conducted prior to VTT. At baseline (before crTMS), pre-VTT (after crTMS), and post-VTT, the COP trajectory during upright static standing under four conditions (eyes, open/closed; surface, hard/rubber) was recorded. Comparison of the length of the COP trajectory or path and sensory-contribution-rate showed no significant difference between baseline and pre- and post-VTT. There was a significant decrease in TE in the sham-crTMS but not in the active-crTMS group. VTT and crTMS did not immediately affect the stability and sensory contribution of upright standing; however, crTMS immediately affected motor learning. The vermal cerebellum may contribute to motor learning of voluntary postural control.

Identifying novel biomarkers with TMS-EEG - Methodological possibilities and challenges

Biomarkers are essential for understanding the underlying pathologies in brain disorders and for developing effective treatments. Combined transcranial magnetic stimulation and electroencephalography (TMS-EEG) is an emerging neurophysiological tool that can be used for biomarker development. This method can identify biomarkers associated with the function and dynamics of the inhibitory and excitatory neurotransmitter systems and effective connectivity between brain areas. In this review, we outline the current state of the TMS-EEG biomarker field by summarizing the existing protocols and the possibilities and challenges associated with this methodology.

Cerebellar noninvasive neuromodulation influences the reactivity of the contralateral primary motor cortex and surrounding areas: a TMS-EMG-EEG study

Understanding cerebellar-cortical physiological interactions is of fundamental importance to advance the efficacy of neurorehabilitation strategies for patients with cerebellar damage. Previous works have aimed to modulate this pathway by applying transcranial electrical or magnetic stimulation (TMS) over the cerebellum and probing the resulting changes in the primary motor cortex (M1) excitability with motor-evoked potentials (MEPs). While these protocols produce changes in cerebellar excitability, their ability to modulate MEPs has produced inconsistent results, mainly due to the MEP being a highly variable outcome measure that is susceptible to fluctuations in the excitability of M1 neurons and spinal interneurons. To overcome this limitation, we combined TMS with electroencephalography (EEG) to directly record TMS-evoked potentials (TEPs) and oscillations from the scalp. In three sessions, we applied intermittent theta-burst stimulation (iTBS), cathodal direct current stimulation (c-DC) or sham stimulation to modulate cerebellar activity. To assess the effects on M1 and nearby cortex, we recorded TMS-EEG and MEPs before, immediately after (T1) and 15 min (T2) following cerebellar neuromodulation. We found that cerebellar iTBS immediately increased TMS-induced alpha oscillations and produced lasting facilitatory effects on TEPs, whereas c-DC immediately decreased TMS-induced alpha oscillations and reduced TEPs. We also found increased MEP following iTBS but not after c-DC. All of the TMS-EEG measures showed high test-retest repeatability. Overall, this work importantly shows that cerebellar neuromodulation influences both cortical and corticospinal physiological measures; however, they are more pronounced and detailed when utilizing TMS-EEG outcome measures. These findings highlight the advantage of using TMS-EEG over MEPs when assessing the effects of neuromodulation.

Cerebellar tDCS as Therapy for Cerebellar Ataxias

In recent years, a growing body of literature has investigated the use of non-invasive brain stimulation (NIBS) techniques to influence cerebellar activity and the effects of cerebellar stimulation on other brain regions through its multiple complex projections. From the early 1990s, with the discovery of the so-called cerebellar inhibition (CBI), several studies have focused their attention on the use of cerebellar NIBS as treatment for different motor disorders. Cerebellar ataxias (CAs) represent the prototypical clinical manifestation of cerebellar alterations, but other movement disorders, such as Parkinson's disease, essential tremor, and dystonia have also been associated with alterations of networks which include the cerebellum, or of the cerebellum itself. Cerebellar transcranial direct current stimulation (ctDCS) could indeed represent an economical, non-invasive therapeutic tool with minimal side effects, thus improving the clinical management of patients and their quality of life. Studies show that ctDCS is effective as a therapeutic option for motor symptoms in patients with CAs, and especially in those with less severe forms, suggesting that ctDCS efficacy could result from augmented neuronal compensation, which itself relies on preserved cerebellar volume. Evidence for the efficacy of ctDCS is less conclusive for the other aforementioned motor disorders, although preliminary results are promising. Future studies should adopt more rigorous methods (e.g., larger sample sizes, double blinding, better characterization of the sample, reliable biomarkers), in order to allow the scientific community to derive higher-quality evidence on the efficacy of ctDCS as a therapeutic option for motor disorders.

Consensus Paper: Novel Directions and Next Steps of Non-invasive Brain Stimulation of the Cerebellum in Health and Disease

The cerebellum is involved in multiple closed-loops circuitry which connect the cerebellar modules with the motor cortex, prefrontal, temporal, and parietal cortical areas, and contribute to motor control, cognitive processes, emotional processing, and behavior. Among them, the cerebello-thalamo-cortical pathway represents the anatomical substratum of cerebellum-motor cortex inhibition (CBI). However, the cerebellum is also connected with basal ganglia by disynaptic pathways, and cerebellar involvement in disorders commonly associated with basal ganglia dysfunction (e.g., Parkinson's disease and dystonia) has been suggested. Lately, cerebellar activity has been targeted by non-invasive brain stimulation (NIBS) techniques including transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) to indirectly affect and tune dysfunctional circuitry in the brain. Although the results are promising, several questions remain still unsolved. Here, a panel of experts from different specialties (neurophysiology, neurology, neurosurgery, neuropsychology) reviews the current results on cerebellar NIBS with the aim to derive the future steps and directions needed. We discuss the effects of TMS in the field of cerebellar neurophysiology, the potentials of cerebellar tDCS, the role of animal models in cerebellar NIBS applications, and the possible application of cerebellar NIBS in motor learning, stroke recovery, speech and language functions, neuropsychiatric and movement disorders.

Feasibility of Cerebellar Transcranial Direct Current Stimulation to Facilitate Goal-Directed Weight Shifting in Chronic Post-Stroke Hemiplegics

Neurological disorder such as stroke can adversely affect one's weight-bearing symmetry leading to dysfunctional postural control. Recovery after stroke is facilitated through functionally-relevant neuroplastic modulation. Functionally-relevant cerebellum coordinates voluntary movements. Specifically, the dentate nuclei and lower limb representations (lobules VII-IX) of the cerebellum are involved in error-correction, crucial for postural control. It is postulated that cerebellar transcranial direct current stimulation (ctDCS) of the dentate nuclei and lobules VII-IX can modulate postural control in chronic stroke survivors. The objectives of this work were to (1) present a refined Virtual Reality (VR)-based balance training platform (VBaT) that can measure Center of Pressure (CoP) and (2) carry out a study to understand the implication of ctDCS stimulating the dentate nuclei (Phase_D) and lobules VII-IX (Phase_L) on the postural control of chronic stroke patients when they interacted with VBaT. Also, we investigated whether hemiplegic patients (with intact cerebellum) having Basal Ganglia (BG) infarction had any differential abilities to correct postural sway from those with no BG infarction (while shifting weight to the Affected side). Results of a single-session single-blind crossover study on randomized Phase_D and Phase_L stimulation (with an intermediate resting state bipolar bilateral ctDCS) on 12 chronic hemiplegic patients on separate days indicated differentiated findings (post stimulation) on CoP-related indices. We observed an incremental effect on one's postural control during Phase_D and inhibitory effect on the dentate nuclei during Phase_L. Clustering analysis showed that those with BG infarction demonstrated poor postural control and deficit in error correction ability irrespective of the ctDCS Phase.

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.

Interactions Between Cerebellum and the Intracortical Excitatory Circuits of Motor Cortex: a Mini-Review

Interactions between cerebellum (CB) and primary motor cortex (M1) are critical for effective motor function. Although the associated neurophysiological processes are yet to be fully characterised, a growing body of work using non-invasive brain stimulation (NIBS) techniques has significantly progressed our current understanding. In particular, recent developments with both transcranial magnetic (TMS) and direct current (tDCS) stimulation suggest that CB modulates the activity of local excitatory interneuronal circuits within M1. These circuits are known to be important both physiologically and functionally, and understanding the nature of their connectivity with CB therefore has the potential to provide important insight for NIBS applications. Consequently, this mini-review provides an overview of the emerging literature that has investigated interactions between CB and the intracortical excitatory circuits of M1.

A narrative review on non-invasive stimulation of the cerebellum in neurological diseases

IMPORTANCE

The cerebellum plays an important role in motor, cognitive, and affective functions owing to its dense interconnections with basal ganglia and cerebral cortex. This review aimed at summarizing the non-invasive cerebellar stimulation (NICS) approaches used to modulate cerebellar output and treat cerebellar dysfunction in the motor domain.

OBSERVATION

The utility of NICS in the treatment of cerebellar and non-cerebellar neurological diseases (including Parkinson's disease, dementia, cerebellar ataxia, and stroke) is discussed. NICS induces meaningful clinical effects from repeated sessions alone in both cerebellar and non-cerebellar diseases. However, there are no conclusive data on this issue and several concerns need to be still addressed before NICS could be considered a valuable, standard therapeutic tool.

CONCLUSIONS AND RELEVANCE

Even though some challenges must be overcome to adopt NICS in a wider clinical setting, this tool might become a useful strategy to help patients with lesions in the cerebellum and cerebral areas that are connected with the cerebellum whether one could enhance cerebellar activity with the intention of facilitating the cerebellum and the entire, related network, rather than attempting to facilitate a partially damaged cortical region or inhibiting the homologs' contralateral area. The different outcome of each approach would depend on the residual functional reserve of the cerebellum, which is confirmed as a critical element to be probed preliminary in order to define the best patient-tailored NICS.

Cerebellar rTMS and PAS effectively induce cerebellar plasticity

Non-invasive brain stimulation techniques including repetitive transcranial magnetic stimulation (rTMS), continuous theta-burst stimulation (cTBS), paired associative stimulation (PAS), and transcranial direct current stimulation (tDCS) have been applied over the cerebellum to induce plasticity and gain insights into the interaction of the cerebellum with neo-cortical structures including the motor cortex. We compared the effects of 1 Hz rTMS, cTBS, PAS and tDCS given over the cerebellum on motor cortical excitability and interactions between the cerebellum and dorsal premotor cortex / primary motor cortex in two within subject designs in healthy controls. In experiment 1, rTMS, cTBS, PAS, and tDCS were applied over the cerebellum in 20 healthy subjects. In experiment 2, rTMS and PAS were compared to sham conditions in another group of 20 healthy subjects. In experiment 1, PAS reduced cortical excitability determined by motor evoked potentials (MEP) amplitudes, whereas rTMS increased motor thresholds and facilitated dorsal premotor-motor and cerebellum-motor cortex interactions. TDCS and cTBS had no significant effects. In experiment 2, MEP amplitudes increased after rTMS and motor thresholds following PAS. Analysis of all participants who received rTMS and PAS showed that MEP amplitudes were reduced after PAS and increased following rTMS. rTMS also caused facilitation of dorsal premotor-motor cortex and cerebellum-motor cortex interactions. In summary, cerebellar 1 Hz rTMS and PAS can effectively induce plasticity in cerebello-(premotor)-motor pathways provided larger samples are studied.

Frequency-dependent modulation of cerebellar excitability during the application of non-invasive alternating current stimulation

Background

it is well-known that the cerebellum is critical for the integrity of motor and cognitive actions. Applying non-invasive brain stimulation techniques over this region results in neurophysiological and behavioural changes, which have been associated with the modulation of cerebellar-cerebral cortex connectivity. Here, we investigated whether online application of cerebellar transcranial alternating current stimulation (tACS) results in changes to this pathway.

Methods

thirteen healthy individuals participated in two sessions of cerebellar tACS delivered at different frequencies (5Hz and 50Hz). We used transcranial magnetic stimulation to measure cerebellar-motor cortex (M1) inhibition (CBI), short-intracortical inhibition (SICI) and short-afferent inhibition (SAI) before, during and after the application of tACS.

Results

we found that CBI was specifically strengthened during the application of 5Hz cerebellar tACS. No changes were detected immediately following the application of 5Hz stimulation, nor at any time point with 50Hz stimulation. We also found no changes to M1 intracortical circuits (i.e. SICI) or sensorimotor interaction (i.e. SAI), indicating that the effects of 5Hz tACS over the cerebellum are site-specific.

Conclusions

cerebellar tACS can modulate cerebellar excitability in a time- and frequency-dependent manner. Additionally, cerebellar tACS does not appear to induce any long-lasting effects (i.e. plasticity), suggesting that stimulation enhances oscillations within the cerebellum only throughout the stimulation period. As such, cerebellar tACS may have significant implications for diseases manifesting with abnormal cerebellar oscillatory activity and also for future behavioural studies.

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Cerebellar tACS increases the inhibitory tone that the cerebellum exerts over M1 (CBI).

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CBI changes were found only during the online application of 5Hz tACS and not immediately following stimulation.

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The effects are specific to the cerebellum, as no changes were found in intracortical measures (e.g. SICI and SAI).

Cerebral Cortical Activity Following Non-invasive Cerebellar Stimulation-a Systematic Review of Combined TMS and EEG Studies

The cerebellum sends dense projections to both motor and non-motor regions of the cerebral cortex via the cerebellarthalamocortical tract. The integrity of this tract is crucial for healthy motor and cognitive function. This systematic review examines research using transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) to the cerebellum with combined cortical electroencephalography (EEG) to explore the temporal features of cerebellar-cortical connectivity. A detailed discussion of the outcomes and limitations of the studies meeting review criteria is presented. Databases were searched between 1 December 2017 and 6 December 2017, with Scopus alerts current as of 23 July 2019. Of the 407 studies initially identified, 10 met review criteria. Findings suggested that cerebellar-cortical assessment is suited to combined TMS and EEG, although work is required to ensure experimental procedures are optimal for eliciting a reliable cerebellar response from stimulation. A distinct variation in methodologies and outcome measures employed across studies, and small sample sizes limited the conclusions that could be drawn regarding the electrophysiological signatures of cerebellar-cortical communication. This review highlights the need for stringent protocols and methodologies for cerebellar-cortical assessments via combined TMS and EEG. With these in place, combined TMS and EEG will provide a valuable means for exploring cerebellar connectivity with a wide range of cortical sites. Assessments have the potential to aid in the understanding of motor and cognitive function in both healthy and clinical groups, and provide insights into long-range neural communication generally.

Can neuromodulation techniques optimally exploit cerebello-thalamo-cortical circuit properties to enhance motor learning post-stroke?

Individuals post-stroke sustain motor deficits years after the stroke. Despite recent advancements in the applications of non-invasive brain stimulation techniques and Deep Brain Stimulation in humans, there is a lack of evidence supporting their use for rehabilitation after brain lesions. Non-invasive brain stimulation is already in use for treating motor deficits in individuals with Parkinson's disease and post-stroke. Deep Brain Stimulation has become an established treatment for individuals with movement disorders, such as Parkinson's disease, essential tremor, epilepsy, cerebral palsy and dystonia. It has also been utilized for the treatment of Tourette's syndrome, Alzheimer's disease and neuropsychiatric conditions such as obsessive-compulsive disorder, major depression and anorexia nervosa. There exists growing scientific knowledge from animal studies supporting the use of Deep Brain Stimulation to enhance motor recovery after brain damage. Nevertheless, these results are currently not applicable to humans. This review details the current literature supporting the use of these techniques to enhance motor recovery, both from human and animal studies, aiming to encourage development in this domain.

Cerebellar Repetitive Transcranial Magnetic Stimulation and Noisy Galvanic Vestibular Stimulation Change Vestibulospinal Function

Background

The cerebellum strongly contributes to vestibulospinal function, and the modulation of vestibulospinal function is important for rehabilitation. As transcranial magnetic stimulation (TMS) and electrical stimulation may induce functional changes in neural systems, we investigated whether cerebellar repetitive TMS (crTMS) and noisy galvanic vestibular stimulation (nGVS) could modulate vestibulospinal response excitability. We also sought to determine whether crTMS could influence the effect of nGVS.

Methods

Fifty-nine healthy adults were recruited; 28 were randomly allocated to a real-crTMS group and 31 to a sham-crTMS group. The crTMS was conducted using 900 pulses at 1 Hz, while the participants were in a static position. After the crTMS, each participant was allocated to either a real-nGVS group or sham-nGVS group, and nGVS was delivered (15 min., 1 mA; 0.1–640 Hz) while patients were in a static position. The H-reflex ratio (with/without bilateral bipolar square wave pulse GVS), which reflects vestibulospinal excitability, was measured at pre-crTMS, post-crTMS, and post-nGVS.

Results

We found that crTMS alone and nGVS alone have no effect on H-reflex ratio but that the effect of nGVS was obtained after crTMS.

Conclusion

crTMS and nGVS appear to act as neuromodulators of vestibulospinal function.

Cerebellar transcranial magnetic stimulation: The role of coil type from distinct manufacturers

BACKGROUND

Stimulating the cerebellum with transcranial magnetic stimulation is often perceived as uncomfortable. No study has systematically tested which coil design can effectively trigger a cerebellar response with the least discomfort.

OBJECTIVE

To determine the relationship between perceived discomfort and effectiveness of cerebellar stimulation using different coils: MagStim (70 mm, 110 mm-coated, 110-uncoated), MagVenture and Deymed.

METHODS

Using the cerebellar-brain inhibition (CBI) protocol, we conducted a CBI recruitment curve with respect to each participant's maximum tolerated-stimulus intensity (MTI) to assess how effective each coil was at activating the cerebellum.

RESULTS

Only the Deymed double-cone coil elicited CBI at low intensities (-20% MTI). At the MTI, the MagStim (110 mm coated/uncoated) and Deymed coils produced reliable CBI, whereas no CBI was found with the MagVenture coil.

CONCLUSION

s: The Deymed double-cone coil was most effective at cerebellar stimulation at tolerable intensities. These results can guide coil selection and stimulation parameters when designing cerebellar TMS studies.

New Horizons in Early Dementia Diagnosis: Can Cerebellar Stimulation Untangle the Knot?

Differentiating Mild Cognitive Impairment (MCI) from dementia and estimating the risk of MCI-to-dementia conversion (MDC) are challenging tasks. Thus, objective tools are mandatory to get early diagnosis and prognosis. About that, there is a growing interest on the role of cerebellum-cerebrum connectivity (CCC). The aim of this study was to differentiate patients with an early diagnosis of dementia and MCI depending on the effects of a transcranial magnetic stimulation protocol (intermittent theta-burst stimulation -iTBS) delivered on the cerebellum able to modify cortico-cortical connectivity. Indeed, the risk of MDC is related to the response to iTBS, being higher in non-responder individuals. All patients with MCI, but eight (labelled as MCI-), showed preserved iTBS aftereffect. Contrariwise, none of the patients with dementia showed iTBS aftereffects. None of the patients showed EEG aftereffects following a sham TBS protocol. Five among the MCI- patients converted to dementia at 6-month follow-up. Our data suggest that cerebellar stimulation by means of iTBS may support the differential diagnosis between MCI and dementia and potentially identify the individuals with MCI who may be at risk of MDC. These findings may help clinicians to adopt a better prevention/follow-up strategy in such patients.

Effects of two different protocols of cerebellar transcranial direct current stimulation combined with transcutaneous spinal direct current stimulation on robot-assisted gait training in patients with chronic supratentorial stroke: A single blind, randomized controlled trial

BACKGROUND

The neural organization of locomotion involves motor patterns generated by spinal interneuronal networks and supraspinal structures, which are approachable by noninvasive stimulation techniques. Recent evidences supported the hypothesis that transcranial direct current stimulation (combined with transcutaneous spinal direct current stimulation) may actually enhance the effects of robot-assisted gait training in chronic stroke patients. The cerebellum has many connections to interact with neocortical areas and may provide some peculiar plasticity mechanisms. So, it has been proposed as "non-lesioned entry" to the motor or cognitive system for the application of noninvasive stimulation techniques in patients with supratentorial stroke.

OBJECTIVE

To compare the effects of two different protocols of cerebellar transcranial direct current stimulation combined with transcutaneous spinal direct current stimulation on robotic gait training in patients with chronic supratentorial stroke.

METHODS

Forty patients with chronic supratentorial stroke were randomly assigned into two groups. All patients received ten, 20-minute robotic gait training sessions, five days a week, for two consecutive weeks. Group 1 underwent cathodal transcranial direct current stimulation over the contralesional cerebellar hemisphere + cathodal transcutaneous spinal direct current stimulation in combination with robotic training. Group 2 underwent cathodal transcranial direct current stimulation over the ipsilesional cerebellar hemisphere + cathodal transcutaneous spinal direct current stimulation in combination with robotic training. The primary outcome was the 6-minute walk test performed before, after, and at follow-up at 2 and 4 weeks post-treatment.

RESULTS

No significant difference in the 6-minute walk test between groups was found at the first post-treatment evaluation (P = 0.976), as well as at the 2-week (P = 0.178) and the 4-week (P = 0.069) follow-up evaluations. Both groups showed significant within-group improvements in the 6-minute walk test at all time points.∥Conclusions: Our findings support the hypothesis that cathodal transcranial direct current stimulation over the contralesional or ipsilesional cerebellar hemisphere in combination with cathodal transcutaneous spinal direct current stimulation may lead to similar effects on robotic gait training in chronic supratentorial stroke patients.

Does cerebellar non-invasive brain stimulation affect corticospinal excitability in healthy individuals? A systematic review of literature and meta-analysis

Numerous studies have indicated that non-invasive brain stimulation (NIBS) of the cerebellum could modulate corticospinal excitability (CSE) in young healthy individuals. However, there is no systematic review and meta-analysis that clarifies the effects of cerebellar NIBS on CSE. The aim of this study was to provide a meta-analytic summary of the effects of cerebellar NIBS on CSE. Seven search engines were used to identify any trial evaluating CSE before and after one session of cerebellar NIBS in healthy individuals up to June 2018. Twenty-six studies investigating the corticospinal responses following cerebellar NIBS were included. Meta-analysis was used to pool the findings from included studies. Effects were expressed as mean differences (MD) and the standard deviation (SD). Risk of bias was assessed with the Cochrane tool. Meta-analysis found that paired associative stimulation (PAS) with 2 ms interval, a combination of PAS with 21.5 ms interval and anodal transcranial direct current stimulation, and repetitive transcranial magnetic stimulation with a frequency of < 5 Hz increase CSE (P _PAS2 < 0.00001, P _{PAS21.5 +a-tDCS} = 0.02, P _rTMS = 0.04). However, continuous theta burst stimulation, a combination of PAS with 25 ms interval and anodal transcranial direct current stimulation, and PAS with a 6 ms interval decreased CSE (P _PAS6 < 0.00001, P _cTBS < 0.00001, P _{PAS25 +a-tDCS} = 0.003). The results of this review show that cerebellar NIBS techniques are a promising tool for increasing CSE.

Role of Sensorimotor Cortex in Gestural-Verbal Integration

Action comprehension that is related to language or gestural integration has been shown to engage the motor system in the brain, thus providing preliminary evidence for the gestural-verbal embodiment concept. Based on the involvement of the sensorimotor cortex (M1) in language processing, we aimed to further explore its role in the cognitive embodiment necessary for gestural-verbal integration. As such, we applied anodal (excitatory) and sham transcranial direct current stimulation (tDCS) over the left M1 (with reference electrode over the contralateral supraorbital region) during a gestural-verbal integration task where subjects had to make a decision about the semantic congruency of the gesture (prime) and the word (target). We used a cross-over within-subject design in young subjects. Attentional load and simple reaction time (RT) tasks served as control conditions, applied during stimulation (order of three tasks was counterbalanced). Our results showed that anodal (atDCS) compared to sham tDCS (stDCS) reduced RTs in the gestural-verbal integration task, specifically for incongruent pairs of gestures and verbal expressions, with no effect on control task performance. Our findings provide evidence for the involvement of the sensorimotor system in gestural-verbal integration performance. Further, our results suggest that functional modulation induced by sensorimotor tDCS may be specific to gestural-verbal integration. Future studies should now evaluate the modulatory effect of tDCS on semantic congruency by using tDCS over additional brain regions and include assessments of neural connectivity.

Differential plasticity of extensor and flexor motor cortex representations following visuomotor adaptation

Representations within the primary motor cortex (M1) are capable of rapid functional changes following motor learning, known as use-dependent plasticity. GABAergic inhibition plays a role in use-dependent plasticity. Evidence suggests a different capacity for plasticity of distal and proximal upper limb muscle representations. However, it is unclear whether the motor cortical representations of forearm flexor and extensor muscles also have different capacities for plasticity. The current study used transcranial magnetic stimulation to investigate motor cortex excitability and inhibition of forearm flexor and extensor representations before and after performance of a visuomotor adaptation task that primarily targeted flexors and extensors separately. There was a decrease in extensor and flexor motor-evoked potential (MEP) amplitude after performing the extensor adaptation, but no change in flexor and extensor MEP amplitude after performing the flexor adaptation. There was also a decrease in motor cortical inhibition in the extensor following extensor adaptation, but no change in motor cortical inhibition in the flexor muscle following flexor adaptation or either of the non-prime mover muscles. Findings suggest that the forearm extensor motor cortical representation exhibits plastic change following adaptive motor learning, and broadly support the distinct neural control of forearm flexor and extensor muscles.

The effect of the anodal transcranial direct current stimulation over the cerebellum on the motor cortex excitability

This study was designed to investigate whether the cerebellum has an inhibitory effect on motor cortical excitability. Sixteen healthy adults (age range, 25-50 years, five female) participated in the study. Anodal cerebellar transcranial direct current stimulation (a-cTDCS) was used to modulate cerebellar excitability. A-cTDCS was given for 20 min at 1 mA intensity. The automatic threshold tracking method was used to investigate cortical excitability. Resting motor threshold (RMT), short interval intracortical inhibition (SICI), short interval intracortical facilitation (SICF), intracortical facilitation (ICF), and the input output curve (I-O curve) were motor cortical excitability parameters. a-cTDCS caused a reduction in overall SICI and the reduced SICF for interstimulus intervals (ISIs) to 2.4-4.4 ms. a-cTDCS has no effect on ICF, RMT, and the I-O curve. There were no significant changes in any of these cortical excitability parameters after sham cTDCS. Results of the study indicate that a-cTDCS has a dual (both inhibitory and excitatory) effect on motor cortical excitability, rather than a simple inhibitory effect. The cerebellum modulates both the inhibitory and facilitatory activities of motor cortex (M1) and suggest that cerebello-cerebral motor connectivity is more complex than solely inhibitory or facilitatory connections.

Cerebellar Transcranial Direct Current Stimulation Modulates Corticospinal Excitability During Motor Training

Background: Cerebellar activity can be modulated using cerebellar transcranial direct current stimulation (ctDCS) and, when applied concurrently with task training, has been shown to facilitate cognitive and motor performance. However, how ctDCS facilitates motor performance is not fully understood.

Objective/Hypothesis: To assess the electrophysiological and motor performance effects of ctDCS applied during motor training.

Methods: Fourteen healthy adults (age 28.8 ± 10.5 years) were randomly assigned to complete one session of finger tracking training with either simultaneous bilateral anodal or sham ctDCS. Training was completed in two 15 min epochs with a 5-min break (total 30 min stimulation, 2 mA). Tracking accuracy and corticospinal and intracortical excitability were measured immediately before and after the training period. Motor cortical excitability measures included resting motor threshold (RMT), motor evoked potential (MEP) amplitude, cortical silent period (CSP) and short interval intracortical inhibition (SICI).

Results: There was a significant interaction of Group * Time for MEP amplitude and CSP duration (p < 0.01). Post hoc analysis revealed MEP amplitude was increased in the sham group (p < 0.01), indicating increased corticospinal excitability from baseline while the anodal group displayed a decrease in MEP amplitude (p = 0.023) and prolongation of CSP duration (p < 0.01). SICI and RMT remained unchanged following ctDCS and training. Task accuracy was improved in both groups at post-test with a significant effect of Time (p < 0.01); however, there was no effect of Group (p = 0.45) or interaction of Group * Time (p = 0.83). During training, there was a significant effect of Block (p < 0.01) but no significant effect of Group or interaction effect (p > 0.06).

Conclusions: ctDCS applied during task training is capable of modulating or interfering with practice-related changes in corticospinal excitability without disrupting performance improvement.

Targeting the Human Cerebellum with Transcranial Direct Current Stimulation to Modulate Behavior: a Meta-Analysis

Transcranial direct current stimulation (tDCS) is increasingly used to study motor- and non-motor-related functions of the cerebellum. The aim of the present study was to quantitatively review available studies to estimate the efficacy of cerebellar tDCS in altering motor- and cognitive-related behavioral performance in healthy volunteers. The present meta-analysis included 32 sham-controlled studies. Results from random effects modeling of the cumulative effect size demonstrated that anodal and cathodal tDCS to the cerebellum were effective in changing performance. No evidence for polarity-dependent effects of cerebellar tDCS was found. Current findings establish the feasibility to target motor and non-motor-related cerebellar functions with tDCS, but arguably due to anatomical differences between the cerebellum and cerebral cortex, the polarity of tDCS is not predictive of the direction of the behavioral changes in healthy volunteers.

The cerebellum and dystonia

Dystonia is a heterogeneous disorder characterized by involuntary muscle contractions, twisting movements, and abnormal postures in various body regions. It is widely accepted that the basal ganglia are involved in the pathogenesis of dystonia. A growing body of evidence, however, is challenging the traditional view and suggest that the cerebellum may also play a role in dystonia. Studies on animals indicate that experimental manipulations of the cerebellum lead to dystonic-like movements. Several clinical observations, including those from secondary dystonia cases as well as neurophysiologic and neuroimaging studies in human patients, provide further evidence in humans of a possible relationship between cerebellar abnormalities and dystonia. Claryfing the role of the cerebellum in dystonia is an important step towards providing alternative treatments based on noninvasive brain stimulation techniques.

Cerebellar ataxia

The cerebellum plays an integral role in the control of limb and ocular movements, balance, and walking. Cerebellar disorders may be classified as sporadic or hereditary with clinical presentation varying with the extent and site of cerebellar damage and extracerebellar signs. Deficits in balance and walking reflect the cerebellum's proposed role in coordination, sensory integration, coordinate transformation, motor learning, and adaptation. Cerebellar dysfunction results in increased postural sway, hypermetric postural responses to perturbations and optokinetic stimuli, and postural responses that are poorly coordinated with volitional movement. Gait variability is characteristic and may arise from a combination of balance impairments, interlimb incoordination, and incoordination between postural activity and leg movement. Intrinsic problems with balance lead to a high prevalence of injurious falls. Evidence for pharmacologic management is limited, although aminopyridines reduce attacks in episodic ataxias and may have a role in improving gait ataxia in other conditions. Intensive exercises targeting balance and coordination lead to improvements in balance and walking but require ongoing training to maintain/maximize any effects. Noninvasive brain stimulation of the cerebellum may become a useful adjunct to therapy in the future. Walking aids, orthoses, specialized footwear and seating may be required for more severe cases of cerebellar ataxia.

Transcranial Alternating Current Stimulation (tACS) Mechanisms and Protocols

Perception, cognition and consciousness can be modulated as a function of oscillating neural activity, while ongoing neuronal dynamics are influenced by synaptic activity and membrane potential. Consequently, transcranial alternating current stimulation (tACS) may be used for neurological intervention. The advantageous features of tACS include the biphasic and sinusoidal tACS currents, the ability to entrain large neuronal populations, and subtle control over somatic effects. Through neuromodulation of phasic, neural activity, tACS is a powerful tool to investigate the neural correlates of cognition. The rapid development in this area requires clarity about best practices. Here we briefly introduce tACS and review the most compelling findings in the literature to provide a starting point for using tACS. We suggest that tACS protocols be based on functional brain mechanisms and appropriate control experiments, including active sham and condition blinding.

Impaired Cerebellum to Primary Motor Cortex Associative Plasticity in Parkinson's Disease and Spinocerebellar Ataxia Type 3

Background

Functional perturbation of the cerebellum (CB)–motor cortex (M1) interactions may underlie pathophysiology of movement disorders, such as Parkinson’s disease (PD) and spinocerebellar ataxia type 3 (SCA3). Recently, M1 motor excitability can be bidirectionally modulated in young subjects by corticocortical paired associative stimulation (PAS) on CB and contralateral M1 with transcranial magnetic stimulation (TMS), probably through the cerebello–dentato–thalamo–cortical (CDTC) circuit. In this study, we investigated the CB to M1-associative plasticity in healthy elderly PD and SCA3.

Methods

Ten right-handed PD patients, nine gene-confirmed SCA3 patients, and 10 age-matched healthy controls (HC) were studied. One hundred and twenty pairs of TMS of the left M1 preceded by right lateral CB TMS at an interstimulus interval of 2 (CB → M1 PAS_2ms) and 6 ms (CB → M1 PAS_6ms) were, respectively, applied with at least 1-week interval. M1 excitability was assessed by motor-evoked potential (MEP) amplitude, short-interval intracortical inhibition (SICI), intracortical facilitation (ICF), and cerebellar inhibition (CBI) at the first dorsal interosseous muscle of the right hand before and after the CB → M1 PAS.

Results

The M1 excitability represented by MEP amplitude was significantly facilitated and suppressed in the HC group by CB → M1 PAS_2ms and CB → M1 PAS_6ms, respectively. The bidirectional modulation on MEP amplitude was absent in the PD and SCA3 groups. SICI and the baseline CBI were significantly reduced in the SCA3 group compared to those of the HC group irrespective of the CB → M1 PAS protocols. There was a significant reduction of CBI immediately and 60 min after the CB → M1 PAS protocols in the HC group but not in the patient groups. No significant change of ICF was found.

Conclusion

Corticocortical CB → M1 PAS can induce bidirectional motor cortical plasticity in M1 for healthy aged subjects. The modulation may be independent of the inhibitory neurocircuits, such as SICI and CBI, and the facilitatory mechanism like ICF. Both patients with PD and SCA3 showed impairment of such plasticity, suggesting significant functional perturbation of the CDTC circuit.

Cerebellum: An explanation for dystonia?

Dystonia is a movement disorder that is characterized by involuntary muscle contractions, abnormal movements and postures, as well as by non-motor symptoms, and is due to abnormalities in different brain areas. In this article, we focus on the growing number of experimental studies aimed at explaining the pathophysiological role of the cerebellum in dystonia. Lastly, we highlight gaps in current knowledge and issues that future research studies should focus on as well as some of the potential applications of this research avenue. Clarifying the pathophysiological role of cerebellum in dystonia is an important concern given the increasing availability of invasive and non-invasive stimulation techniques and their potential therapeutic role in this condition.