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.
Highlights
Balance control requires the integration of somatosensory, visual and vestibular information [1] and many supraspinal structures have been shown to be involved in this sensory integration [2]
EMG responses induced by cathode right/anode left Galvanic vestibular stimulation (GVS) were observed in 11/14 participants
The short-latency response (SLR) consisted of a suppression of SOL EMG (81.73 ± 10.67%; 6 subjects with a suppression, 1 subject with a facilitation) at a mean latency of 68 ± 3 ms and the medium-latency response (MLR) consisted of a facilitation of SOL EMG (136.26 ± 8.94%; 8 subjects with a facilitation; 1 subject with a suppression) at a latency of 101 ± 4 ms
Summary
Balance control requires the integration of somatosensory, visual and vestibular information [1] and many supraspinal structures have been shown to be involved in this sensory integration [2]. Vestibulocortical interactions during standing hypermetric postural responses [4, 5], observed after unilateral or bilateral vestibular loss, leading to balance and gait deficits. Guzman-Guzman-Lopez et al [15] demonstrated that caloric vestibular stimulation could modulate MEPs in the neck muscles. Their results suggested that the vestibular modulation of corticospinal control of the sterno-cleido-mastoid (SCM) likely occurred at cortical levels. Even though these studies strongly support interactions between vestibular and cortical systems in the control of arm and neck muscles during sitting or lying down, there are none during standing, or any other postural tasks. The nature of vestibulo-cortical interaction required for complex balance control remains unknown
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