Introduction
Balance control is a complex, multisensory process that depends on the integration of visual, proprioceptive, and vestibular inputs to generate appropriate reflexive responses and adaptive motor behaviours that maintain postural stability. Balance impairments are common in older adults and various clinical populations, leading to increased risk for falls. Emerging research has highlighted the potential clinical utility of noisy subthreshold vestibular stimulation (termed, ‘noisy galvanic vestibular stimulation’) applied to the mastoids to reduce imbalance. However, our understanding of the mechanisms underpinning these effects remains poor. This study tested the hypothesis that noisy galvanic vestibular stimulation enhances balance by modulating spinal reflexes.
Methods
19 healthy young adults took part in our study. Participants received noisy galvanic vestibular stimulation (± 0.35 mA) during standing whilst H-reflexes were measured. H-reflex was elicited through stimulation in the popliteal fossa and the EMG was measured across the right soleus. Standing H-reflex amplitude was measured during standing balance (eyes closed), whilst receiving either active or sham vestibular stimulation. The protocol was conducted on both a foam and a solid surface to compare across different levels of vestibular dependence: standing on foam with eyes closed results in the CNS relying predominately on vestibular input to regular balance. Average normalised H-reflex amplitudes were calculated for each condition. All participants provided informed consent and the study was approved by the local ethics committee.
Results
In total we analysed 1520 H-reflexes (n=80 per participant). There was no significant change in normalised H-reflex amplitude between sham and active stimulation on the solid surface (t(18)=-0.481, p=0.636). However, on the foam surface we found a significant increase in H-reflex amplitude during active stimulation (t(18)=2.149, p=0.046).
Discussion
Our results indicate that noisy galvanic vestibular stimulation modulates spinal reflexes during challenging, vestibular-dominant balance conditions (standing on a foam surface). Our findings support spinal reflex modulation as a possible mechanism for the improvements in balance seen with noisy galvanic vestibular stimulation. The contrasting results on the foam and solid surfaces are a likely result of sensory reweighting, whereby the body up-regulates vestibular contributions to balance and down regulates the visual and somatosensory inputs which are inherently less reliable under these conditions (eyes closed and foam). This increased reliance on vestibular inputs likely potentiates the effect of vestibular stimulation. Our results provide new evidence for the possible mechanism/s behind noisy galvanic vestibular stimulation’s effect on balance, which may help further enhance future clinical use.