Effects of Noisy Galvanic Vestibular Stimulation on Standing Balance – No Evidence for Stochastic Resonance in Young Healthy Adults

Noisy galvanic vestibular stimulation (nGVS) at imperceptible levels has been shown to reduce body sway. This improvement was commonly attributed to the mechanism of stochastic resonance (SR). However, it has never been explicitly tested whether nGVS-induced effects on body sway consistently follow a SR-like bell-shaped performance curve with optimal improvements at intermediate noise intensities. To test this, body sway in 21 young healthy participants was measured during varying nGVS amplitudes while standing with eyes closed in 3 conditions (quiet stance, sway referencing, sinusoidal platform tilts). Presence of SR-compatible response dynamics in each trial was assessed (1) by a goodness-of-t analysis using an established SR-curve model and (2) by ratings from 3 human experts. In accordance to theory, we found reductions of body sway at one nGVS amplitude in most trials (75-95%). However, only few trials exhibited SR-compatible bell-shaped performance curves with increasing noise amplitudes (10-33%). Instead, body sway measures rather uctuated randomly across nGVS amplitudes. This implies that, at least in young healthy adults, nGVS effects on body sway are incompatible with SR. Thus, previously reported reductions of body sway at particular nGVS intensities more likely result from inherent variations of the performance metric or by other yet unknown mechanisms.


Introduction
Stochastic resonance (SR) is a phenomenon in non-linear systems, where the system's response to weak, sub-threshold signals becomes enhanced in the presence of a weak stochastic interference, i.e., a particular non-zero amount of noise. 1,2 Typically, SR effects will critically depend on the noise amplitude insofar as signal transfer becomes improved at intermediate noise intensities whereas too low or too high noise amplitudes will either not affect or disturb information processing within the system. Accordingly, the characteristic signature for the presence of SR is a noise-induced modulation of the system's output that follows a pseudo-bell-shaped performance curve, which peaks at some particular level of noise that optimally facilitates signal transfer within the system. 3,4 SR-like effects have been demonstrated in animal models at the afferent receptor level for a variety of sensory systems, such as the somatosensory, auditory, and visual system. [5][6][7][8][9] Similar effects have been later veri ed in humans in terms of improvements at the perceptual level as well as in related sensorimotor function. 3,10−14 More recently, analogous effects have been observed in the vestibular system (for a review see 15 ). To induce SR in the vestibular system, most studies used galvanic vestibular stimulation (GVS). GVS is a non-invasive technique that allows to electrically modulate the neuronal activity in the peripheral vestibular endorgans. 16 By applying a noisy form of GVS (nGVS) to healthy young individuals, Galvan-Garza and colleagues could demonstrate that vestibular motion perception can get effectively enhanced in the presence of a low-intensity stochastic vestibular stimulation. 17 Accordingly, they observed that in about 75% of participants nGVS-induced modulations of vestibular perceptual thresholds followed a bell-shaped performance curve with increasing noise amplitude. In agreement with the aforementioned theoretical framework of SR the authors observed a peak at an intermediate "optimal" nGVS intensity. These observations were later con rmed and extended in subsequent studies. [18][19][20] To date, vestibular SR effects have been most extensively studied in the context of human balance control. Bene cial effects of nGVS on human postural control have been reported for young and older healthy individuals 21,22 as well as in patients with peripheral vestibular hypofunction 23 or central neurodegenerative diseases. 24 These reports have been complemented by studies that speci cally analyzed the effects of different stimulus characteristics, 25,26 varying conditions of standing [27][28][29] as well as immediate and after effects of stimulation 30-33 on spontaneous sway patterns. In light of these studies, vestibular SR has been proposed to have immediate clinical implications in terms of a potential treatment option for postural imbalance in the context of a peripheral and/or central vestibular dysfunction. 15 One common shortcoming of previous reports on nGVS effects on balance is, however, that they didn't provide su cient evidence that the observed bene cial effects of nGVS on balance control are actually compatible with the presence of vestibular SR. Accordingly, neither of these studies demonstrated that nGVS-induced modulations of body sway consistently follow a bell-shaped performance curve with increasing nGVS amplitude. Rather, some studies examined the effects of only one xed nGVS amplitude versus sham stimulation. 22,26 Others studied the effects of different nGVS amplitudes but only reported performance modulations at the amplitude at which individuals showed optimally improved balance. 23,31 This approach, however, is statistically problematic since a seeming improvement in performance across several repeated assessments would be likely expected simply due to inherent test-retest variations in the performance metric. Thus, both approaches fail to provide convincing evidence that reported improvements are linked to the mechanism of SR.
The aim of this study was therefore to examine whether previously reported nGVS-induced improvements in balance are consistent with and may be explained by the exhibition of SR. We hypothesized that, in analogy to previously reported effects of nGVS on vestibular perception, 17 nGVS-induced reductions of body sway would follow a bell-shaped performance curve with increasing stimulation intensity consistent with the presence of SR. To this end, we systematically analyzed modulations of body sway in dependence of varying intensities of nGVS in young healthy individuals. nGVS-induced changes in body sway were studied in eyes closed stance during three surface conditions: xed, sway referenced, and sinusoidally tilting. We applied different quantitative and qualitative criteria on a single subject level to determine whether nGVS-induced modulations in body sway were consistent with the presence of SR or rather follow other response dynamics.

Materials And Methods
Participants Twenty-one healthy young subjects (age: 24.0 ± 4.2 years; height: 175 ± 10 cm; weight: 69.5 ± 12.4 kg, 11 females) participated in this study. None of the participants reported any auditory, vestibular, cardiovascular, or orthopedic disorders. All participants gave their written informed consent prior to the experiments. The Ethics Committee of the Ludwig-Maximilians-University approved the study protocol, which was conducted in conformity with the Declaration of Helsinki.

Experimental setup
During experiments, subjects stood either on two force plates (AMTI, Watertown, USA) or on a custombuilt device which allowed to control the tilt angle. Body sway was measured using a camera-based motion capture system (Vicon, Cambridge, UK). Re ective markers were attached to the sacrum, between the shoulder blades, and -not used for analyses -at head and shoulders (Fig. 1A). Stimuli were generated using Matlab scripts and a Simulink model (The Mathworks, Natick, USA) running on a realtime Target PC (Speedgoat, Switzerland). Analog (stimuli, force plates) and camera data were recorded on a PC running Nexus software (Vicon, Cambridge, UK) at 2000 Hz (down-sampled to 100 Hz for the analysis) and 100 Hz sampling rate, respectively. Rubber-electrodes (approx. 10 cm² area) with electrode gel were applied to the mastoid process behind each ear and xed with a head band to deliver bipolar, binaural GVS, which was generate by an isolated bipolar current stimulator (Digitimer, Hertfordshire, UK). The nine nGVS amplitudes were tested with eyes closed in three different platform conditions: xed surface, sway referenced surface, and sinusoidal tilting surface (Fig. 1B). Sway referencing is a condition, in which the platform is commanded to follow the sway of the subject, such that orientation of the subjects' legs is not changing with respect to the platform. Sway referencing was achieved by measuring the movement of the hip via a sway-rod attached to a potentiometer, that was guided by a hook attached to the subjects' hip. Trigonometric calculations were used to calculate the command signal for the platform. As a result, proprioceptive information on body orientation with respect to the platform is decoupled from body orientation in space, making the information virtually useless for the balance task. The sine had a frequency of 0.5 Hz and 0.5° amplitude and was started with a raised cosine to avoid platform jerks. Sway reference and sinusoidal tilt conditions were applied in separate trials for anterior-posterior (AP) and medio-lateral (ML) directions, while both directions were simultaneously recorded in quiet stance.

Procedures
After providing written informed consent, subjects' anthropometric measures were taken. Thereafter, we attached the re ective markers, hooks for sway referencing and nGVS electrodes after skin preparation with abrasive gel and disinfection. Subjects were asked to perform a 120-s long calibration routine in AP and ML direction (see below). Trials were presented in randomized order with short breaks of 1-2 minutes in between and longer breaks if requested by the subjects. Subjects were instructed to close the eyes and 'stand upright and comfortable' and were listening to non-rhythmic audio-books via noisecanceling head-phones to avoid auditory orientation cues and distract from the balancing task.

Data analysis
Center of pressure (COP) was calculated for quiet stance trials as output measure. Center of mass was calculated for sway referencing and sine trials as output measure, since the calculation of the COP was technically not feasible during moving platform conditions.
Center of mass (COM) was obtained from hip and shoulder movements using a calibration routine. 34,35 During the calibration routine, subjects performed slow movements in the ankle and hip joints. Recorded COP trajectories were used as a projection of the COM position in these quasi-static trials. Assuming twosegment mechanics consisting of leg and head-arms-trunk segments, a regression between hip and ankle kinematics (x COP ≈ x COM = Off + A • x hip + B • x sho ) provides calibration factors (Off, A, B), which can be used to obtain x COM sway in conditions where x COM ≠ x COP . Finally, COM angle is calculated from x COM and COM height, as obtained from subject anthropometrics. 36 To con rm the well documented effect of GVS evoking a postural response in ML direction, 37 cumulant density functions between quiet stance COP and nGVS stimulus sequences with corresponding 95% con dence bounds were calculated for the rst 28.16 s of each nGVS amplitude and across all subjects (using neurospec toolbox version 2.11 38 ). For COP in quiet stance and COM in sway referenced platform conditions, path-length was calculated for each nGVS condition and subject using p where T is the trial duration and x i are the individual samples. For the sine condition, body sway amplitude in response to the stimulus was obtained from the sway amplitude spectrum at the stimulus frequency calculated using a scaled Fast Fourier Transform. Data for individual subjects were normalized to body sway measures obtained during the 0-µA nGVS condition and mean +/-standard deviation across subjects were calculated.
For each trial, we initially determined whether body sway measures improved for at least one particular nGVS level compared to baseline condition (i.e., 0 µA nGVS). Since there is no established mathematical de nition of what su ce for the exhibition of SR-like dynamics in response to varying levels of nGVS, we tested two alternative SR criteria, both on the results of single individuals as well as on the group outcomes. For the rst criterion, we performed a goodness-of-t analysis using an established SR-curve t on the relationship between nGVS levels and normalized body sway measures as proposed in Galvan-Garca et al. 17 . The SR function ts a bell-shaped curve on the data based on an equation that was initially developed to describe the general phenomenon of SR 4 and is given by: where A 0 , Ω, and l a determine the depth of the bell, B the y-axis offset and dq the x-axis offset. All ve parameters were tted to experimental data using a global optimization approach using the function 'fmincon' and 'GlobalSearch' option from the Matlab 'Global Optimization Toolbox'. Subsequently, we tested for each trial whether the SR curve t can better explain the experimental data compared to a simple linear t. The comparison was performed using F-test statistics to account for the trade-of between improved residuals and additional free parameters in each t with F 3,4 = 9.1172 for p < .05. The second criterion was more subjective and based the evaluation of three experts that rated experimental data for the presence or absence of SR based on visual inspection of individual response dynamics and their corresponding linear and SR curve ts. 17 This criterion required that at least two of the expert judges identi ed the presence of SR-like dynamics in a trial.

Data availability
The datasets generated and analyzed during the current study are available from the corresponding author on reasonable request.

Results
During all examined stance conditions (quiet stance, sway-referencing, sinusoidal tilt), participants were able to maintain stable balance, except of four single trials (two due to circulation problems and two due to repeated loss of balance during sway referencing) that were excluded from further analysis. During quiet stance with eyes closed, subjects showed slight forward body lean and typical body sway amplitudes (exemplary COM body sway of one participant is presented in Fig. 1C). Body sway was 3-4 times larger in the sway referenced condition (not shown) as compared to quiet stance on a xed surface. During the sinusoidal tilt condition, platform movement evoked sway responses that were visible in the time domain for most subjects. However, they were superimposed by considerable random sway.
In a rst step of analysis, we examined whether nGVS above a certain intensity level (range of applied intensities: 0-199 µA signal RMS) would evoke a postural response in analogy to previously reported effects of GVS on ML postural sway. 37 Visual inspection of corresponding nGVS and body sway traces did not exhibit obvious patterns of covariance (Fig. 1C). Cumulant density estimates (Fig. 2), however, revealed signi cant coupling between nGVS and body sway at the two highest applied nGVS intensities (i.e., 142 and 199 µA signal RMS). This effect was only observed for body sway in ML but not in AP direction. Cumulant density estimates further exhibited a characteristic biphasic pattern with short and medium latency responses at around 350 and 700 ms in agreement with previous reports. 39,40 Hence, this analysis con rmed that the range of applied nGVS stimuli encompassed both suprathreshold nGVS intensities that have a direct modulatory effect on body sway as well as subthreshold nGVS intensities that could potentially induce alterations in body sway via vestibular SR.
In a second step of analysis, we examined whether response dynamics of body sway across the range of applied nGVS intensities might be actually compatible with the presence of vestibular-induced SR. SReffects on body sway would be indicated by a pseudo-bell-shaped response curve with an optimal reduction of body sway at some particular intermediate intensity of nGVS. For this purpose, sway parameters expressing a measure of overall body sway, i.e., sway path-length for quiet and swayreferenced stance conditions and sway amplitude in response to the tilt stimulus for sinusoidal tilt condition, were normalized with respect to the baseline condition (i.e., 0 µA nGVS) and plotted across the range of nGVS amplitudes. Figure 3 to 5 depict group-average and representative individual body sway responses to nGVS and corresponding linear and SR curve ts for the three different examined stance conditions (individual plots for all recorded trials are provided as supplemental material).
In accordance with previous reports, we found reductions of body sway at individual nGVS amplitudes in the majority of subjects and platform conditions (95% and 86% for quiet stance in AP and ML direction, respectively; 84% and 85% for sway-referencing in AP and ML direction, respectively; 75% and 86% for sinusoidal tilt condition in AP and ML direction, respectively). In contrast, however, visual inspection of individual response dynamics by three human judges did only identify a minority of trials where body sway responses exhibited a SR-like pseudo-bell-shaped performance curve across applied nGVS intensities (33% and 10% for quiet stance in AP and ML direction, respectively; 33% and 20% for swayreferencing stance in AP and ML direction, respectively; 25% and 24% for sinusoidal tilt condition in AP and ML direction, respectively). This observation was further supported by the nding that none of the SR-curve ts (both on an individual as well as on a group-average level) yielded a signi cantly better explanatory power as compared to the linear ts. In other words, body sway across tested nGVS amplitudes did not show any systematic changes, but rather showed random uctuations at the groupaverage level.

Discussion
Several previous studies reported that human postural sway can be reduced by the application of a particular intensity of nGVS 21-33 -an effect that was concordantly attributed to the exhibition of SR within vestibular signal transfer. However, none of these studies so far provided systematic evidence for the latter assumption. In this study, we therefore systematically examined whether SR-like changes in body sway across nGVS amplitudes can be actually observed in young healthy individuals. The exhibition of SR in a particular system is typically indicated by a noise-induced modulation of the system's output that follows a pseudo-bell-shaped performance curve with increasing noise intensity, which peaks at a particular optimal noise amplitude where the system's performance becomes optimally enhanced. In accordance with previous reports, we found reduced body sway (for all three tested balance conditions and AP as well as ML directions) at particular nGVS intensities in almost all participants.
However, across the entire range of applied nGVS intensities, individual and grouped body sway modulations exhibited more or less random uctuations and, in most cases, did not exhibit consistent performance curves. This observation suggests that, at least in young healthy individuals, postural improvements at particular nGVS amplitudes are likely to result from inherent variations in the performance metric (i.e., static body sway) rather than being caused by vestibular SR.
Vestibular feedback cues play a minor role during quiet stance on a xed surface, where postural adjustments predominantly rely on somatosensory and -to a lesser extent -visual cues. 35,41 We therefore focused on stance conditions that pronounce the role of vestibular feedback by (a) withdrawal of visual cues (eyes closure) and (b) by manipulating the proprioceptive reference to the Earth vertical through sinusoidal surface tilts or sway referencing. Even under these conditions we did not observe convincing evidence that nGVS induces SR-like reductions in body sway. Our observations are certainly limited by the focus on a young and healthy cohort, in which peripheral vestibular processing presumably operates at a near-to-optimal level and might thus leave little to no potential for externally induced improvements. It is thus conceivable, that SR-like improvements in vestibular balance control might only be observable in the elderly or in patients with vestibular hypofunction, where age-related or pathological vestibular hair cell degeneration has been associated to a decline in peripheral vestibular signal processing. 42,43 However, SR-like effects using nGVS have been reported also for young healthy individuals. Galvan-Garca and colleagues observed in about 75% of young participants, that nGVS-induced modulations of the vestibular threshold for passive motion perception were compatible with SR and followed a bell-shaped performance curve with optimal improvement at a particular intermediate nGVS intensity. 17 Analogously, it was demonstrated that nGVS effectively lowers the threshold to induce vestibulospinal re ex responses in about 90% of young participants. 44 Thus, the question arises why SR-like behavior can be found in perception and simple re exes, but not in standing balance.
One reason could be differences between the processing of vestibular cues in ego motion, sensorimotor re exes, and balance regulation. For instance, vestibulo-ocular re ex thresholds apparently differ from thresholds for vestibular motion perception, in particular in the low frequency range, 45,46 and exhibit different response dynamics to vestibular stimulation or visual-vestibular con ict. 47,48 Differences become even more apparent when comparing processing of vestibular cues at the perceptual and re ex level to that during balance regulation. The reasons for this lie both in the intrinsic multisensory nature and the complex feedback control mechanisms, which constitute postural control. Accordingly, vestibular input in postural control becomes considerably ltered, distorted, or even masked by the interaction with other sensory cues (proprioceptive, visual), the feedback dynamics of the postural control system, the body's low-pass characteristics, and intrinsic noise in neural processing and muscular activations. 49 In line with this, current models of postural control, assume that vestibular cues only become involved in balance regulation after multisensory fusion (in particular with proprioceptive cues) at a late processing stage that is close to the behavioral output. 50,51 Thus, comparisons between nGVS effects on different vestibular-related functions and output measures need to be considered carefully.
Another reason could be that psycho-physical estimates of vestibular perceptual thresholds, which were previously shown to be responsive to nGVS, 17 are designed to be particularly robust against intraindividual performance variations and yield excellent test-to-retest reliability. 52,53 In contrast, test-to-retest variations in standing balance are considerable. 54,55 Thus, inherent trial-to-trial variations in the examined performance metric rather than nGVS-induced effects might govern the response dynamics of static body sway across the range of applied nGVS intensities. However, earlier studies reported nGVS-induced reductions in body sway measures in the order of 10-40%. 23,25 The trial-to-trial variability of sway measures from our current recordings had a standard deviation of 2% for quiet stance, 17% for swayreferencing trials, and 20% for sinusoidal platform tilt trials. Thus, at least the test-to-retest reliability during quiet stance would have been high enough to identify the previously reported 10-40% nGVSinduced improvements.
Finally, we observed that signi cant and consistent nGVS-induced body sway responses started to occur in ML direction for noise RMS intensities at and above 142 µA (approximately corresponding to nGVS peak-to-peak amplitudes of 500 to 700 µA). In accordance in nonhuman primates, neuronal detection thresholds of primary vestibular afferents for GVS applied on the bilateral mastoid processes -a setup analogous to ours -were estimated to lie between 400 to 600 µA peak-to-peak. 56 This suggest that vestibular SR in young healthy individuals with intact peripheral vestibular information processing, should be triggered, if any, by nGVS at amplitudes below these estimated peripheral thresholds. This assumption corresponds to the observation made by Galvan-Garca et al. that optimal nGVS-induced reduction in vestibular perceptual thresholds occurred at nGVS peak-to-peak amplitudes at or below 500 µA. 17 In contrast, previous reports on nGVS-induced improvements of postural sway at stimulation amplitudes way above these estimated detection thresholds (e.g. 1000 µA 21,26,33 ) are therefore unlikely to be attributable to vestibular SR. In these instances, reductions of body sway presumably rather result from a postural stiffening/stabilization response in the presence of an external induced vestibular disturbance as opposed to an effective facilitation of vestibular balance regulation.