Stochastic resonance in the human vestibular system e Noise-induced facilitation of vestibulospinal reflexes M. Wuehr a, * , J.C. Boerner b , C. Pradhan a , J. Decker b , K. Jahn a, c , T. Brandt a, d , R. Schniepp a, b a German Center for Vertigo and Balance Disorders, University of Munich, Germany b Department of Neurology, University of Munich, Germany c Sch€ on Klinik Bad Aibling, Germany d Institute for Clinical Neuroscience, University of Munich, Germany article info Article history: Received 10 October 2017 Accepted 21 October 2017 Available online xxx Keywords: Stochastic resonance Vestibular system Galvanic vestibular stimulation Vestibulospinal reflexes abstract 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 (0e1.9 mA). Coherence between GVS input and stimulation-induced motion responses was determined and psy- chometric function fits were subsequently used to determine individual vestibulospinal reflex thresh- olds. 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. © 2017 Elsevier Inc. All rights reserved. Introduction There is growing evidence that the presence of noise can enhance information processing in sensory systems by a mecha- nism known as stochastic resonance (SR) [1]. Despite accumulating evidence for SR-type dynamics in human proprioceptive, auditory, and visual perception [1], as well as in animal vestibular function [2], it is hitherto unknown whether this phenomenon also applies to the human vestibular system. Recent studies demonstrated that white noise galvanic vestibular stimulation (GVS) at imperceptible levels effectively improves standing and walking stability in healthy subjects and patients with bilateral vestibular hypofunction [3e6]. The purpose of this study was to investigate whether these noise-induced improvements in balance control can be attributed to SR-dynamics in human vestibulospinal function. We therefore determined the threshold for vestibulospinal reflexes evoked by GVS and tested whether an additional imperceptible noisy GVS may facilitate subthreshold information transfer and thereby result in a lowered system's reflex threshold. Materials and methods All experimental procedures were approved by the appropriate Ethics Committee. GVS was applied in 26 healthy subjects (age: 28.2 ± 7.0 years, 8 females) via two pairs of Ag-AgCl electrodes attached bilaterally over the left and right mastoid process. Two constant current stimulators (Model DS5, Digitimer, Hertfordshire, UK) delivered two separate electrical stimuli: (1) a 1 Hz sinusoidal GVS (sGVS) of varying amplitudes (0e1.9 mA, steps of 2.0 mA); (2) an imperceptible zero-mean Gaussian white noise GVS (nGVS) within a frequency range of 0e30 Hz and an intensity set to 80% of the cutaneous threshold for GVS [4,5] (Fig. 1A). * Corresponding author. University of Munich, German Center for Vertigo and Balance Disorders, Marchioninistrasse 15, 81377 Munich, Germany. E-mail address: Max.Wuehr@med.uni-muenchen.de (M. Wuehr). Contents lists available at ScienceDirect Brain Stimulation journal homepage: http://www.journals.elsevier.com/brain-stimulation https://doi.org/10.1016/j.brs.2017.10.016 1935-861X/© 2017 Elsevier Inc. All rights reserved. Brain Stimulation xxx (2017) 1e3 Please cite this article in press as: Wuehr M, et al., Stochastic resonance in the human vestibular system e Noise-induced facilitation of vestibulospinal reflexes, Brain Stimulation (2017), https://doi.org/10.1016/j.brs.2017.10.016