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

Modulation of dorsal premotor cortex differentially influences visuomotor adaptation in young and older adults

The communication between dorsal premotor cortex (PMd) and primary motor cortex (M1) is important for visuomotor adaptation, but it is unclear how this relationship changes with advancing age. The present study recruited 21 young and 23 older participants for two experimental sessions during which intermittent theta burst stimulation (iTBS) or sham was applied over PMd. We assessed the effects of PMd iTBS on M1 excitability using motor evoked potentials (MEP) recorded from right first dorsal interosseous when single-pulse transcranial magnetic stimulation (TMS) was applied with posterior-anterior (PA) or anterior-posterior (AP) currents; and adaptation by quantifying error recorded during a visuomotor adaptation task (VAT). PMd iTBS potentiated PA (P < 0.0001) and AP (P < 0.0001) MEP amplitude in both young and older adults. PMd iTBS increased error in young adults during adaptation (P = 0.026), but had no effect in older adults (P = 0.388). Although PMd iTBS potentiated M1 excitability in both young and older adults, the intervention attenuated visuomotor adaptation specifically in young adults.

The cerebellum and the Mirror Neuron System: A matter of inhibition? From neurophysiological evidence to neuromodulatory implications. A narrative review

Mirror neurons show activity during both the execution (AE) and observation of actions (AO). The Mirror Neuron System (MNS) could be involved during motor imagery (MI) as well. Extensive research suggests that the cerebellum is interconnected with the MNS and may be critically involved in its activities. We gathered evidence on the cerebellum's role in MNS functions, both theoretically and experimentally. Evidence shows that the cerebellum plays a major role during AO and MI and that its lesions impair MNS functions likely because, by modulating the activity of cortical inhibitory interneurons with mirror properties, the cerebellum may contribute to visuomotor matching, which is fundamental for shaping mirror properties. Indeed, the cerebellum may strengthen sensory-motor patterns that minimise the discrepancy between predicted and actual outcome, both during AE and AO. Furthermore, through its connections with the hippocampus, the cerebellum might be involved in internal simulations of motor programs during MI. Finally, as cerebellar neuromodulation might improve its impact on MNS activity, we explored its potential neurophysiological and neurorehabilitation implications.

The effects of intermittent theta burst stimulation over dorsal premotor cortex on primary motor cortex plasticity in young and older adults

Previous transcranial magnetic stimulation (TMS) research suggests that the dorsal premotor cortex (PMd) influences neuroplasticity within the primary motor cortex (M1) through indirect (I) wave interneuronal circuits. However, it is unclear how the influence of PMd on the plasticity of M1 I-waves changes with advancing age. This study therefore investigated the neuroplastic effects of intermittent theta burst stimulation (iTBS) to M1 early and late I-wave circuits when preceded by iTBS (PMd iTBS-M1 iTBS) or sham stimulation (PMd sham-M1 iTBS) to PMd in 15 young and 16 older adults. M1 excitability was assessed with motor evoked potentials (MEP) recorded from the right first dorsal interosseous using posterior-anterior (PA) and anterior-posterior (AP) current TMS at standard stimulation intensities (PA1mV, AP1mV) and reduced stimulation intensities (PA0.5mV, early I-waves; AP0.5mV, late I-waves). PMd iTBS-M1 iTBS lowered the expected facilitation of PA0.5mV (to M1 iTBS) in young and older adults (P = 0.009), whereas the intervention had no effect on AP0.5mV facilitation in either group (P = 0.305). The modulation of PA0.5mV following PMd iTBS-M1 iTBS may reflect a specific influence of PMd on different I-wave circuits that are involved in M1 plasticity within young and older adults.

The Effect of Cerebellar rTMS on Modulating Motor Dysfunction in Neurological Disorders: a Systematic Review

The effectiveness of cerebellar repetitive transcranial magnetic stimulation (rTMS) on motor dysfunction in patients with neurological disorders has received increasing attention because of its potential for neuromodulation. However, studies on the neuromodulatory effects, parameters, and safety of rTMS implementation in the cerebellum to alleviate motor dysfunction are limited. This systematic review aimed to evaluate the effectiveness and safety of cerebellar rTMS treatment for motor dysfunction caused by neurological disorders and to review popular stimulation parameters. Five electronic databases-Medline, Web of Science, Scopus, Cochrane Library, and Embase-were searched for relevant research published from inception to July 2022. All randomized controlled trials (RCTs) that reported the effects of cerebellar rTMS combined with behavioral rating scales on motor dysfunction were eligible for enrollment. Additionally, reference lists of the enrolled studies were manually checked. Among 1156 articles screened, 21 RCTs with 666 subjects were included. rTMS conducted on the cerebellum showed an improvement in stroke (spasticity, balance, and gait), cervical dystonia, Parkinson's disease (tremor), cerebellar ataxia, and essential tremor but not in multiple sclerosis. The 8-shaped coil with a diameter of 70 mm was determined as the most common therapeutic choice. None of the studies reported severe adverse events except mild side effects in three. Therefore, rTMS appears to be a promising and safe technique for the treatment of motor dysfunction, targeting the cerebellum to induce motor behavioral improvement. Further rigorous RCTs, including more samples and longer follow-up periods, are required to precisely explore the effective stimulation parameters and possible mechanisms.

A Single Application of Cerebellar Transcranial Direct Current Stimulation Fails to Enhance Motor Skill Acquisition in Parkinson's Disease: A Pilot Study

Parkinson's disease (PD) is a progressive neurodegenerative disorder that leads to numerous impairments in motor function that compromise the ability to perform activities of daily living. Practical and effective adjunct therapies are needed to complement current treatment approaches in PD. Transcranial direct current stimulation applied to the cerebellum (c-tDCS) can increase motor skill in young and older adults. Because the cerebellum is involved in PD pathology, c-tDCS application during motor practice could potentially enhance motor skill in PD. The primary purpose was to examine the influence of c-tDCS on motor skill acquisition in a complex, visuomotor isometric precision grip task (PGT) in PD in the OFF-medication state. The secondary purpose was to determine the influence of c-tDCS on transfer of motor skill in PD. The study utilized a double-blind, SHAM-controlled, within-subjects design. A total of 16 participants completed a c-tDCS condition and a SHAM condition in two experimental sessions separated by a 7-day washout period. Each session involved practice of the PGT concurrent with either c-tDCS or SHAM. Additionally, motor transfer tasks were quantified before and after the practice and stimulation period. The force error in the PGT was not significantly different between the c-tDCS and SHAM conditions. Similarly, transfer task performance was not significantly different between the c-tDCS and SHAM conditions. These findings indicate that a single session of c-tDCS does not elicit acute improvements in motor skill acquisition or transfer in hand and arm tasks in PD while participants are off medications.

Modulation of dorsal premotor cortex differentially influences I-wave excitability in primary motor cortex of young and older adults

Previous research using transcranial magnetic stimulation (TMS) has demonstrated weakened connectivity between dorsal premotor cortex (PMd) and motor cortex (M1) with age. While this alteration is probably mediated by changes in the communication between the two regions, the effect of age on the influence of PMd on specific indirect (I) wave circuits within M1 remains unclear. The present study therefore investigated the influence of PMd on early and late I-wave excitability in M1 of young and older adults. Twenty-two young (mean ± SD, 22.9 ± 2.9 years) and 20 older (66.6 ± 4.2 years) adults participated in two experimental sessions involving either intermittent theta burst stimulation (iTBS) or sham stimulation over PMd. Changes within M1 following the intervention were assessed with motor-evoked potentials (MEPs) recorded from the right first dorsal interosseous muscle. We applied posterior-anterior (PA) and anterior-posterior (AP) current single-pulse TMS to assess corticospinal excitability (PA1mV ; AP1mV ; PA0.5mV , early; AP0.5mV , late), and paired-pulse TMS short intracortical facilitation for I-wave excitability (PA SICF, early; AP SICF, late). Although PMd iTBS potentiated PA1mV and AP1mV MEPs in both age groups (both P < 0.05), the time course of this effect was delayed for AP1mV in older adults (P = 0.001). Furthermore, while AP0.5mV , PA SICF and AP SICF were potentiated in both groups (all P < 0.05), potentiation of PA0.5mV was only apparent in young adults (P < 0.0001). While PMd influences early and late I-wave excitability in young adults, direct PMd modulation of the early circuits is specifically reduced in older adults. KEY POINTS: Interneuronal circuits responsible for late I-waves within primary motor cortex (M1) mediate projections from dorsal premotor cortex (PMd), but this communication probably changes with advancing age. We investigated the effects of intermittent theta burst stimulation (iTBS) to PMd on transcranial magnetic stimulation (TMS) measures of M1 excitability in young and older adults. We found that PMd iTBS facilitated M1 excitability assessed with posterior-anterior (PA, early I-waves) and anterior-posterior (AP, late I-waves) current TMS in young adults, with a stronger effect for AP TMS. M1 excitability assessed with AP TMS also increased in older adults following PMd iTBS, but there was no facilitation for PA TMS responses. We conclude that changes in M1 excitability following PMd iTBS are specifically reduced for the early I-waves in older adults, which could be a potential target for interventions that enhance cortical excitability in older adults.

Cerebellum-Cerebrum paired target magnetic stimulation on balance function and brain network of patients with stroke: A functional near-infrared spectroscopy pilot study

Transcranial magnetic stimulation (TMS) modulation over the cerebellum, primary motor cortex, and supplementary motor cortex individually can improve the balance function of patients with stroke. However, whether their combination could have a better balance modulation effect is uncertain. Therefore, we hypothesized that performing TMS over a combination of these targets can regulate the balance function of patients with stroke. We compared the effects of one-session TMS on eye-open and eye-closed balance conditions in patients with stroke, using different target pairs of unilateral cerebellar (CB-single), cerebellar-primary motor cortex (CB-M1), and cerebellar-supplementary motor area (CB-SMA) targets. A total of 31 patients with stroke were enrolled and randomly divided into three groups to receive single sessions of intermittent theta burst stimulation each. Functional near-infrared spectrum data on resting and standing task states (eye-open and eye-closed) and center of pressure parameters (eye-open and eye-closed) were collected before and after the intervention. Compared with the results in the CB-single group, five intergroup differences in the changes in the center of pressure parameters in the CB-M1 group and two significant differences in the CB-SMA group were observed after one session of intermittent theta burst stimulation. In the CB-SMA group, 12 out of the 14 parameters improved significantly in the EC condition after the intervention. Meanwhile, the functional near-infrared spectrum results showed that the CB-SMA group exhibited a significant inhibitory pattern in the resting-state functional connectivity, which was not observed in the other two groups. In conclusion, we believe that paired targeting of the CB-SMA can reshape the brain network and improve the balance function of patients with stroke.

Long-Term Application of Cerebellar Transcranial Direct Current Stimulation Does Not Improve Motor Learning in Parkinson's Disease

Cerebellar transcranial direct current stimulation (c-tDCS) enhances motor skill acquisition and motor learning in young and old adults. Since the cerebellum is involved in the pathophysiology of Parkinson’s disease (PD), c-tDCS may represent an intervention with potential to improve motor learning in PD. The primary purpose was to determine the influence of long-term application of c-tDCS on motor learning in PD. The secondary purpose was to examine the influence of long-term application of c-tDCS on transfer of motor learning in PD. The study was a randomized, double-blind, SHAM-controlled, between-subjects design. Twenty-one participants with PD were allocated to either a tDCS group or a SHAM stimulation group. Participants completed 9 practice sessions over a 2-week period that involved extensive practice of an isometric pinch grip task (PGT) and a rapid arm movement task (AMT). These practice tasks were performed over a 25-min period concurrent with either anodal c-tDCS or SHAM stimulation. A set of transfer tasks that included clinical rating scales, manual dexterity tests, and lower extremity assessments were quantified in Test sessions at Baseline, 1, 14, and 28 days after the end of practice (EOP). There were no significant differences between the c-tDCS and SHAM groups as indicated by performance changes in the practice and transfer tasks from Baseline to the 3 EOP Tests. The findings indicate that long-term application of c-tDCS does not improve motor learning or transfer of motor learning to a greater extent than practice alone in PD.

Transcranial Direct Current Stimulation as a Treatment Tool for Mild Traumatic Brain Injury

Mild traumatic brain injury (mTBI) has been defined as a transient (<24 h) condition of confusion and/or loss of consciousness for less than 30 min after brain injury and can result in short- and long-term motor and cognitive impairments. Recent studies have documented the therapeutic potential of non-invasive neuromodulation techniques for the enhancement of cognitive and motor function in mTBI. Alongside repetitive transcranial magnetic stimulation (rTMS), the main technique used for this purpose is transcranial direct current stimulation (tDCS). The focus of this review was to provide a detailed, comprehensive (i.e., both cognitive and motor impairment) overview of the literature regarding therapeutic tDCS paradigms after mTBI. A publication search of the PubMed, Scopus, CINAHL, and PsycINFO databases was performed to identify records that applied tDCS in mTBI. The publication search yielded 14,422 records from all of the databases, however, only three met the inclusion criteria and were included in the final review. Based on the review, there is limited evidence of tDCS improving cognitive and motor performance. Surprisingly, there were only three studies that used tDCS in mTBI, which highlights an urgent need for more research to provide additional insights into ideal therapeutic brain targets and optimized stimulation parameters.