Assessment of vestibulocortical interactions during standing in healthy subjects

Abstract

The vestibular system is essential to produce adequate postural responses enabling voluntary movement. However, how the vestibular system influences corticospinal output during postural tasks is still unknown. Here, we examined the modulation exerted by the vestibular system on corticospinal output during standing. Healthy subjects (n = 25) maintained quiet standing, head facing forward with eyes closed. Galvanic vestibular stimulation (GVS) was applied bipolarly and binaurally at different delays prior to transcranial magnetic stimulation (TMS) which triggered motor evoked potentials (MEPs). With the cathode right/anode left configuration, MEPs in right Soleus (SOL) muscle were significantly suppressed when GVS was applied at ISI = 40 and 130ms before TMS. With the anode right/cathode left configuration, no significant changes were observed. Changes in the MEP amplitude were then compared to changes in the ongoing EMG when GVS was applied alone. Only the decrease in MEP amplitude at ISI = 40ms occurred without change in the ongoing EMG, suggesting that modulation occurred at a premotoneuronal level. We further investigated whether vestibular modulation could occur at the motor cortex level by assessing changes in the direct corticospinal pathways using the short-latency facilitation of the SOL Hoffmann reflex (H-reflex) by TMS. None of the observed modulation occurred at the level of motor cortex. Finally, using the long-latency facilitation of the SOL H-reflex, we were able to confirm that the suppression of MEP at ISI = 40ms occurred at a premotoneuronal level. The data indicate that vestibular signals modulate corticospinal output to SOL at both premotoneuronal and motoneuronal levels during standing.

Cortical Effects of Noisy Galvanic Vestibular Stimulation Using Functional Near-Infrared Spectroscopy

“…Stochastic resonance refers to the improved ability to detect stimuli, or better output signal quality, in a nonlinear system with the addition of noise [ 19 , 20 ]. The effects of nGVS are likely the result of the fact that the vestibular afferents terminate in the vestibular nuclei, which can influence motor behavior through their connections with oculomotor circuitry and via the vestibulospinal tract [ 21 , 22 , 23 ]. Since the vestibular nuclei have dense connections with the thalamus, nGVS has the potential to affect a variety of brain regions and thus behavior [ 24 ].…”

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

“…Additional maneuvers have been added to evaluate the effect of GVS on the H-reflex, for example, it was found that posture influences H-reflex amplitude facilitated by GVS ( Tanaka et al, 2021 ). The head position must also be considered while assessing H-reflex amplitude ( Okada et al, 2018 ; Nepveu et al, 2020 ). In 10 healthy subjects, Kennedy and Inglis (2002) found that H-reflex modulation by GVS showed a larger response with anodal stimulation and subjects turning to the right, and cathodal stimulation with subjects turning to the left, than any other polarity or head position combination ( Kennedy and Inglis, 2002 ).…”

“…Another study used GVS and TMS as vestibular and corticospinal input, respectively, to determine its influence at premotor (H-reflex) and motoneuron (electromyography) levels by analyzing amplitude and latency. The results suggested that modulation with GVS occurs at premotor neuron and motor neuron levels but is not influenced by cortical activity ( Nepveu et al, 2020 ). Moreover, cerebellar repetitive TMS followed by noisy GVS facilitated the H-reflex ratio excitability recorded 100 ms after a vestibulospinal function test performed with squared GVS in healthy adults.…”