Cerebellar transcranial direct current stimulation modulates the effect of cerebellar transcranial magnetic stimulation on the excitability of spinal reflex

Effects of Anodal tDCS Applied Over the Cerebellum Combined with Physical Therapy on Center of Gravity Sway in a Patient with Cerebellar Ataxia: A Single-Case Study

Damage to the cerebellum results in dysfunctional standing postural control. Patients with cerebellar ataxia have a larger sway in the center of gravity (COG) while standing. Transcranial direct current stimulation (tDCS) has been applied in the rehabilitation of patients with central nervous system disorders; however, its effect on COG sway in patients with cerebellar ataxia remains unknown. We aimed to confirm the effects of anodal cerebellar tDCS (ctDCS) combined with physical therapy on COG sway in a patient with cerebellar ataxia using a retrospective ABA single-case study design. This study involved a patient with left cerebellar hemorrhage. Walking and postural balance rehabilitation were conducted in phase A. Anodal ctDCS was combined with the walking and postural balance rehabilitation in phase B. We measured COG sway in the open- and closed-eyes standing conditions daily throughout all the phases. In the open-eyes standing condition, there was no significant change in COG sway in phase B. Conversely, in the closed-eyes standing condition, the circumferential area, total sway path length, and anteroposterior sway path length decreased in phase B. No change was observed in the mediolateral sway path length. The combination of anodal ctDCS and physical therapy may decrease COG sway in patients with cerebellar ataxia in the closed-eyes standing condition, and its effect may be greater in the anteroposterior direction.

Reduced Cerebellar Brain Inhibition and Vibrotactile Perception in Response to Mechanical Hand Stimulation at Flutter Frequency

The cerebellum is traditionally considered a movement control structure because of its established afferent and efferent anatomical and functional connections with the motor cortex. In the last decade, studies also proposed its involvement in perception, particularly somatosensory acquisition and prediction of the sensory consequences of movement. However, compared to its role in motor control, the cerebellum's specific role or modulatory influence on other brain areas involved in sensory perception, specifically the primary sensorimotor cortex, is less clear. In the present study, we explored whether peripherally applied vibrotactile stimuli at flutter frequency affect functional cerebello-cortical connections. In 17 healthy volunteers, changes in cerebellar brain inhibition (CBI) and vibration perception threshold (VPT) were measured before and after a 20-min right hand mechanical stimulation at 25 Hz. 5 Hz mechanical stimulation of the right foot served as an active control condition. Performance in a Grooved Pegboard test (GPT) was also measured to assess stimulation's impact on motor performance. Hand stimulation caused a reduction in CBI (13.16%) and increased VPT but had no specific effect on GPT performance, while foot stimulation had no significant effect on all measures. The result added evidence to the functional connections between the cerebellum and primary motor cortex, as shown by CBI reduction. Meanwhile, the parallel increase in VPT indirectly suggests that the cerebellum influences the processing of vibrotactile stimulus through motor-sensory interactions.

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.

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.

Impact of Repetitive Transcranial Magnetic Stimulation to the Cerebellum on Performance of a Ballistic Targeting Movement

This study aimed to investigate the effects of repetitive transcranial magnetic stimulation (rTMS) of the cerebellum on changes in motor performance during a series of repetitive ballistic-targeting tasks. Twenty-two healthy young adults (n = 12 in the active-rTMS group and n = 10 in the sham rTMS group) participated in this study. The participants sat on a chair in front of a monitor and fixed their right forearms to a manipulandum. They manipulated the handle with the flexion/extension of the wrist to move the bar on the monitor. Immediately after a beep sound was played, the participant moved the bar as quickly as possible to the target line. After the first 10 repetitions of the ballistic-targeting task, active or sham rTMS (1 Hz, 900 pulses) was applied to the right cerebellum. Subsequently, five sets of 100 repetitions of this task were conducted. Participants in the sham rTMS group showed improved reaction time, movement time, maximum velocity of movement, and targeting error after repetition. However, improvements were inhibited in the active-rTMS group. Low-frequency cerebellar rTMS may disrupt motor learning during repetitive ballistic-targeting tasks. This supports the hypothesis that the cerebellum contributes to motor learning and motor-error correction in ballistic-targeting movements.

Cerebellar Intermittent Theta-Burst Stimulation Reduces Upper Limb Spasticity After Subacute Stroke: A Randomized Controlled Trial

Objective: This study aims to explore the efficacy of cerebellar intermittent theta-burst stimulation (iTBS) on upper limb spasticity in subacute stroke patients.

Methods: A total of 32 patients with upper limb spasticity were enrolled and randomly assigned to treatment with cerebellar iTBS or sham stimulation before conventional physical therapy daily for 2 weeks. The primary outcomes included the modified Ashworth scale (MAS), the modified Tardieu scale (MTS), and the shear wave velocity (SWV). The secondary outcomes were the H-maximum wave/M-maximum wave amplitude ratio (H_max/M_max ratio), motor-evoked potential (MEP) latency and amplitude, central motor conduction time (CMCT), and the Barthel Index (BI). All outcomes were evaluated at baseline and after 10 sessions of intervention.

Results: After the intervention, both groups showed significant improvements in the MAS, MTS, SWV, and BI. In addition, patients treated with cerebellar iTBS had a significant increase in MEP amplitude, and patients treated with sham stimulation had a significant decrease in H_max/M_max ratio. Compared with the sham stimulation group, the MAS, MTS, and SWV decreased more in the cerebellar iTBS group.

Conclusion: Cerebellar iTBS is a promising adjuvant tool to reinforce the therapeutic effect of conventional physical therapy in upper limb spasticity management after subacute stroke (Chinese Clinical Trial Registry: ChiCTR1900026516).

Posture influences on vestibulospinal tract excitability

The human vestibulospinal tract has important roles in postural control, but it has been unknown whether vestibulospinal tract excitability is influenced by the body's postures. We investigated whether postures influence the vestibulospinal tract excitability by a neurophysiological method, i.e., applying galvanic vestibular stimulation (GVS) 100 ms before tibial nerve stimulation evoking the soleus H-reflex. GVS is a percutaneous stimulation, and it has not been clarified how the cutaneous input from GVS influences the facilitation effect of cathodal GVS on the soleus H-reflex amplitude. In Experiment 1, we evaluated the effects of GVS on the soleus H-reflex amplitude of subjects in the prone, supine, and sitting positions in random order to clarify the differences in the GVS effects among these postures. In Experiment 2, to determine whether the effects of GVS in the supine and sitting positions are due solely to cutaneous input from GVS, we provided GVS and cutaneous stimulations as conditioning stimuli and compared the effects in both postures. Interaction effects between postures and stimulus conditions were observed in both experiments. The facilitation rate of the maximum H-reflex amplitude by GVS in the sitting position was significantly higher than those in the prone and supine positions (Experiment 1). The facilitation rate of GVS was significantly larger than the cutaneous stimulation only in the sitting position (Experiment 2). These results indicate that vestibulospinal tract excitability may be higher in the sitting position than in either lying position (prone and supine), due mainly to the increased need for postural control.

Single-Session Cerebellar Transcranial Direct Current Stimulation Affects Postural Control Learning and Cerebellar Brain Inhibition in Healthy Individuals

Cerebellar transcranial direct current stimulation (ctDCS) modulates cerebellar activity and postural control. However, the effects of ctDCS on postural control learning and the mechanisms associated with these effects remain unclear. To examine the effects of single-session ctDCS on postural control learning and cerebellar brain inhibition (CBI) of the primary motor cortex in healthy individuals. In this triple-blind, sham-controlled study, 36 participants were allocated randomly to one of three groups: (1) anodal ctDCS group, (2) cathodal ctDCS group, and (3) sham ctDCS group. ctDCS (2 mA) was applied to the cerebellar brain for 20 min prior to six blocks of standing postural control training (each block consisted of five trials of a 30-s tracking task). CBI and corticospinal excitability of the tibialis anterior muscle were assessed at baseline, immediately after, 1 day after, and 7 days after training. Skill acquisition following training was significantly reduced in both the anodal and cathodal ctDCS groups compared with the sham ctDCS group. Changes in performance measured 1 day after and 7 days after training did not differ among the groups. In the anodal ctDCS group, CBI significantly increased after training, whereas corticospinal excitability decreased. Anodal ctDCS-induced CBI changes were correlated with the learning formation of postural control (r = 0.55, P = 0.04). Single-session anodal and cathodal ctDCS could suppress the skill acquisition of postural control in healthy individuals. The CBI changes induced by anodal ctDCS may affect the learning process of postural control.

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.

Cerebellum-mediated trainability of eye and head movements for dynamic gazing

Objective

To investigate whether gaze stabilization exercises (GSEs) improve eye and head movements and whether low-frequency cerebellar repetitive transcranial magnetic stimulation (rTMS) inhibits GSE trainability.

Methods

25 healthy adults (real rTMS, n = 12; sham rTMS, n = 13) were recruited. Real or sham rTMS was performed for 15 min (1 Hz, 900 stimulations). The center of the butterfly coil was set 1 cm below the inion in the real rTMS. Following stimulation, 10 trials of 1 min of a GSE were conducted at 1 min intervals. In the GSE, the subjects were instructed to stand upright and horizontally rotate their heads according to a beeping sound corresponding to 2 Hz and with a gaze point ahead of them. Electrooculograms were used to estimate the horizontal gaze direction of the right eye, and gyroscopic measurements were performed to estimate the horizontal head angular velocity during the GSE trials. The percentage change from the first trial of motion range of the eye and head was calculated for each measurement. The percent change of the eye/head range ratio was calculated to assess the synchronous changes of the eye and head movements as the exercise increased.

Results

Bayesian two-way analysis of variance showed that cerebellar rTMS affected the eye motion range and eye/head range ratio. A post hoc comparison (Bayesian t-test) showed evidence that the eye motion range and eye/head range ratio were reduced in the fifth, sixth, and seventh trials compared with the first trial sham stimulation condition.

Conclusions

GSEs can modulate eye movements with respect to head movements, and the cerebellum may be associated with eye–head coordination trainability for dynamic gazing during head movements.