Reliability and laterality of the soleus H-reflex following galvanic vestibular stimulation in healthy individuals

Changes in vestibular-related responses to combined noisy galvanic vestibular stimulation and cerebellar transcranial direct current stimulation

Vestibular nuclei and cerebellar function comprise vestibular neural networks that control vestibular-related responses. However, the vestibular-related responses to simultaneous stimulation of these regions are unclear. This study aimed to examine whether the combination of noisy galvanic vestibular stimulation (nGVS) and cerebellar transcranial direct current stimulation (ctDCS) using a complex transcranial electrical stimulation device alters vestibular-dominant standing stability and vestibulo-ocular reflex (VOR) function. The center of foot pressure (COP) sway and VOR of participants (28 healthy, young adults) were assessed under four conditions of transcranial electrical stimulation using nGVS and ctDCS. The COP was calculated with the participant standing on a soft-foam surface with eyes closed using a force plate to evaluate body sway. VOR measurements were collected via passive head movements and fixation on a target projected onto the front wall using a video head impulse test (vHIT). VOR gain was calculated in six directions using a semicircular canal structure based on the ratio of eye movement to head movement. The nGVS + ctDCS and nGVS + sham ctDCS conditions decreased COP sway compared to the sham nGVS + ctDCS and sham nGVS + sham ctDCS conditions. No significant differences were observed in the main effect of stimulation or the interaction of stimulation and direction on the vHIT parameters. The results of this study suggest that postural stability may be independently affected by nGVS. Our findings contribute to the basic neurological foundation for the clinical application of neurorehabilitation using transcranial electrical stimulation of the vestibular system.

Effects of postural-control training with different sensory reweightings in a patient with body lateropulsion: a single-subject design study

INTRODUCTION

Body lateropulsion (BL) is an active lateral tilt of the body during standing or walking that is thought to be affected by a lesion of the vestibulospinal tract (VST) and the subjective visual vertical (SVV) tilt. Interventions for BL have not been established.

OBJECTIVE

We examined the effects of postural-control training with different sensory reweighting on standing postural control in a patient with BL.

METHODS

The patient had BL to the left when standing or walking due to a left-side medullary and cerebellar infarct. This study was a single-subject A-B design with follow-up: Phase A was postural-control training with visual feedback; phase B provided reweighting plantar somatosensory information. Postural control, VST excitability, and SVV were measured.

RESULTS

At baseline and phase A, the patient could not stand with eyes-closed on a rubber mat, but became able to stand in phase B. The mediolateral center of pressure (COP) position did not change significantly, but the COP velocity decreased significantly during phase B and the follow-up on the firm surface. VST excitability was lower on the BL versus the non-BL side, and the SVV deviated to the right throughout the study.

CONCLUSION

Postural-control training with reweighting somatosensory information might improve postural control in a patient with BL.

Post-activation depression of the Hoffman reflex is not altered by galvanic vestibular stimulation in healthy subjects

The comprehension of the neural elements interacting in the spinal cord affected by vestibular input will contribute to the understanding of movement execution in normal and pathological conditions. In this context, Hoffman's reflex (H-reflex) has been used to evaluate transient excitability changes on the spinal cord descending pathways. The post-activation depression (P-AD) of the H-reflex consists of evoking consecutive responses (>1 Hz) provoking an amplitude depression, which has been shown to diminish in pathological conditions (i.e., spasticity, diabetic neuropathy). Galvanic Vestibular Stimulation (GVS) is a non-invasive method that activates the vestibular afferents and has been used to study the excitability of the H-reflex applied as a conditioning pulse. To our knowledge, there are no reports evaluating the P-AD during and after GVS. Our primary aim was to determine if GVS alters the P-AD evoked by stimulating the tibial nerve at 0.1, 1, 5, and 10 Hz, recording in the gastrocnemius and soleus muscles. Direct current stimulation of 2.0 ± 0.6 mA with the cathode ipsilateral (Ipsi) or contralateral (Contra) to the H-reflex electrode montage was applied bilaterally over the mastoid process in 19 healthy subjects. The P-AD's immediate post-GVS response (P Ipsi, P Contra) was also analyzed. Secondarily, we analyzed the excitability of the H-reflex during GVS. Responses evoked at 0.1 Hz with GVS, post-GVS, and a Control (no GVS) condition were used for comparisons. Our results show that P-AD persisted in all subjects despite increased excitability induced by GVS: statistical significance was found when comparing P-AD at 1, 5, and 10 Hz with the corresponding condition (Control, Ipsi, P Ipsi, Contra, P Contra) at 0.1 Hz (p < 0.001). Additionally, the increase in excitability produced by GVS was quantified for the first H-reflex of each P-AD stimulation frequency. The percentage change for all GVS conditions surpassed the Control by at least 20%, being statistically significant for Contra compared to Control (p < 0.01). In summary, although GVS increases the excitability of the vestibulospinal pathway at a premotor level, the neural inhibitory mechanism present in P-AD remains unaltered in healthy subjects.

Relationships between changes in lateral vestibulospinal tract excitability and postural control by dynamic balance intervention in healthy individuals: A preliminary study

Introduction

We conducted dynamic balance or static intervention on healthy young adults to examine the changes in lateral vestibulospinal tract (LVST) excitability and postural control that ensued following dynamic balance intervention and to investigate the correlation between these changes.

Methods

Twenty-eight healthy young adults were randomly assigned to either the dynamic balance group or the control group. They performed either a dynamic balance or static intervention for 10 trials of 30 s each and were assessed for head jerks during the intervention to confirm adaptation to the intervention. The dynamic balance intervention consisted of maintaining balance on a horizontally unstable surface, whereas the control intervention involved standing in the same foot position as the dynamic balance intervention on a stable surface while completing a maze task. LVST excitability and postural stability were assessed before and after the interventions. LVST excitability was assessed as the change rate in the soleus H-reflex amplitude with galvanic vestibular stimulation (GVSH). The velocity and area of the center of pressure (COP) were examined in the eyes closed/foam rubber condition.

Results

No significant main and interaction effects (task, time) were observed for GVSH and COP variables. In the dynamic balance intervention, head jerk significantly decreased, and GVSH-change and changes in head jerk and COP area were significantly negatively correlated.

Discussion

The LVST excitability change for the dynamic balance intervention varied among the participants, although increased LVST excitability may have been related to increased postural stability.

Vestibulo-Ocular Reflex Is Modulated by Noisy Galvanic Vestibular Stimulation

We investigated whether noisy galvanic vestibular stimulation (nGVS) modulates the vestibulo-ocular reflex (VOR) and whether this effect is correlated with the effect of nGVS on body sway. Thirty healthy young adults participated. The video head impulse test (vHIT) was used to estimate the ratio of eye motion velocity/head motion velocity to VOR-gain. The gain 60 ms after the start of head motion (VOR-gain-60 ms) and regression slope (RS) (i.e., gain in eye and head motion; VOR-gain-RS) were calculated. The total path length of the foot center of pressure (COP-TL) during upright standing was calculated to estimate body sway. Noisy Galvanic Vestibular Stimulation at 0.2, 0.6, 1.2 mA, or sham stimulation (direct current: 0 mA) was delivered to the bilateral mastoid process in random order during vHIT and COP measurements. Application of nGVS at 0.2 mA significantly reduced VOR-gain-RS, while application of nGVS at 0.6 mA significantly increased COP-TL. Vestibulo-ocular reflex-gain-60 ms differed significantly between 0.2 and 1.2 mA. There was no significant correlation between COP-TL and VOR-related parameters. These findings suggest that nGVS at 0.2 mA inhibits the VOR, while nGVS at 0.6 mA increases body sway during upright standing, although there may be no relationship between the respective effects in healthy individuals.