Noisy vestibular stimulation improves vestibulospinal function in patients with bilateral vestibulopathy.

OBJECTIVES: To examine the mechanism underlying previously reported ameliorating effects of noisy galvanic vestibular stimulation (GVS) on balance performance in patients with bilateral vestibulopathy (BVP) and determine those patients (incomplete versus complete vestibular loss) that might benefit from this intervention. METHODS: Vestibulospinal reflex thresholds were determined in 12 patients with BVP [2 with complete loss (cBVP) and 10 with residual function (rBVP)]. Patients were stimulated with 1 Hz sinusoidal GVS of increasing amplitudes (0-1.9 mA). Coherence between GVS input and stimulation-induced body motion was determined and psychometric function fits were subsequently used to determine individual vestibulospinal reflex thresholds. The procedure was repeated with an additional application of imperceptible white noise GVS (nGVS). RESULTS: All patients with rBVP but none with cBVP exhibited stimulation-induced vestibulospinal reflex responses with a mean threshold level of 1.26 ± 0.08 mA. Additional nGVS resulted in improved processing of weak subthreshold vestibular stimuli (p = 0.015) and thereby effectively decreased the vestibulospinal threshold in 90% of patients with rBVP (mean reduction 17.3 ± 3.9%; p < 0.001). CONCLUSION: The present findings allow to identify the mechanism by which nGVS appears to stabilize stance and gait performance in patients with BVP. Accordingly, nGVS effectively lowers the vestibular threshold to elicit balance-related reflexes that are required to adequately regulate postural equilibrium. This intervention is only effective in the presence of a residual vestibular functionality, which, however, applies for the majority of patients with BVP. Low-intensity noise stimulation thereby provides a non-invasive treatment option to optimize residual vestibular resources in BVP.

Stochastic resonance in the human vestibular system - Noise-induced facilitation of vestibulospinal reflexes.

BACKGROUND: There is strong evidence that the presence of noise can enhance information processing in sensory systems via stochastic resonance (SR). OBJECTIVES: To examine the presence of SR in human vestibulospinal reflex function. METHODS: Healthy subjects were stimulated with 1 Hz sinusoidal GVS of varying amplitudes (0-1.9 mA). Coherence between GVS input and stimulation-induced motion responses was determined and psychometric function fits were subsequently used to determine individual vestibulospinal reflex thresholds. This procedure was repeated with additional application of imperceptible white noise GVS (nGVS). RESULTS: nGVS significantly facilitated the detectability of weak subthreshold vestibular inputs (p < 0.001) and thereby effectively lowered the vestibulospinal threshold in 90% of participants (p < 0.001, mean reduction: 17.5 ± 14.6%). CONCLUSION: This finding provides evidence for the presence of SR-dynamics in the human vestibular system and gives a functional explanation for previously observed ameliorating effects of low-intensity vestibular noise stimulation on balance control in healthy subjects and patients with vestibular hypofunction.