Habituation to galvanic vestibular stimulation for analysis of susceptibility to carsickness

OBJECTIVE: In a previous study we developed a standardized procedure for a reproducible quantification of galvanic-induced body sway (GBS). In line with other reports, GBS shows short-term (fast) habituation upon stimulus repetition. The aim of this study was to evaluate whether the degree of short-term habituation to galvanic vestibular stimulation (GVS) is correlated with susceptibility to carsickness. MATERIAL AND METHODS: A total of 24 female subjects underwent computer-controlled GVS as part of a prospective experimental study conducted in a tertiary referral center. A binaural 1-cosinusoidal stimulus of 0.5 Hz and 1 mA was repeated five times. Binaural stimulation was chosen to obtain maximum responses, making possible habituation to repetitive GVS obvious. The groups consisted of 12 subjects suffering from carsickness and 12 healthy subjects. RESULTS: Analysis of the repeated measurements test showed that the GBS-gain curve for the carsickness group was always superimposed on that for the healthy subjects. However, the (absolute) first to fifth GBS gains showed no significant differences (p = 0.134 - 0.995). When comparing short-term habituation in subjects suffering from carsickness versus healthy subjects, the results showed no differences in the mean values of the first (30.534% vs 27.024%), final (42.637% vs 38.544%) and average (35.544% vs 33.644%) habituations (p = 0.875, 0.991 and 0.951, respectively). CONCLUSION: We did not observe any significant differences in sensitivity or habituation of the GBS in carsick subjects compared to healthy subjects. This implies that carsick subjects show a similar ability to discard an irrelevant, non-motion sickness-inducing galvanic stimulus as healthy subjects.     

Habituation to galvanic vestibular stimulation for analysis of susceptibility to carsickness

Objective In a previous study we developed a standardized procedure for a reproducible quantification of galvanic-induced body sway (GBS). In line with other reports, GBS shows short-term (fast) habituation upon stimulus repetition. The aim of this study was to evaluate whether the degree of short-term habituation to galvanic vestibular stimulation (GVS) is correlated with susceptibility to carsickness.

Material and Methods A total of 24 female subjects underwent computer-controlled GVS as part of a prospective experimental study conducted in a tertiary referral center. A binaural 1–cosinusoidal stimulus of 0.5 Hz and 1 mA was repeated five times. Binaural stimulation was chosen to obtain maximum responses, making possible habituation to repetitive GVS obvious. The groups consisted of 12 subjects suffering from carsickness and 12 healthy subjects.

Results Analysis of the repeated measurements test showed that the GBS–gain curve for the carsickness group was always superimposed on that for the healthy subjects. However, the (absolute) first to fifth GBS gains showed no significant differences (p=0.134?0.995). When comparing short-term habituation in subjects suffering from carsickness versus healthy subjects, the results showed no differences in the mean values of the first (30.534% vs 27.024%), final (42.637% vs 38.544%) and average (35.544% vs 33.644%) habituations (p=0.875, 0.991 and 0.951, respectively).

Conclusion We did not observe any significant differences in sensitivity or habituation of the GBS in carsick subjects compared to healthy subjects. This implies that carsick subjects show a similar ability to discard an irrelevant, non-motion sickness-inducing galvanic stimulus as healthy subjects.     

Background on methods of stimulation in galvanic-induced body sway in young healthy adults

The aim of this study was to develop a standardized procedure for reproducible quantification of galvanic-induced body sway (GBS). This was a prospective experimental study conducted in a tertiary referral centre. An exploratory study was first conducted to define the galvanic vestibular stimulation (GVS) method that resulted in the best reproducible responses. Ten subjects underwent computer-controlled GVS using five different types of monaural and binaural stimulation with 2-mA currents. Cosinusoidal stimulation gave the most reproducible responses. The frequency and current variability of this stimulus type were then tested in the same 10 subjects. A monaural continuous 1-cosinusoidal current of 0.5 Hz and 2 mA gave the most reproducible responses (< 20% test-retest variation) and the largest GBS amplitude. The other (sinusoidal) stimuli resulted in variabilities exceeding 50%. This stimulus was thus used for further testing in our normative study. In this study we measured GBS amplitude at 0.5 Hz in 60 subjects, with eyes closed and an inter-feet distance of 0 cm, using a force platform. In addition to body sway, responses included slight dizziness, taste sensations and a tingling sensation at the site of stimulation. Habituation to the applied stimulus was seen. Binaural prestimulation, performed in 50/60 test subjects, is necessary to reduce habituation and achieve optimal reproducibility in order to be able to compare the sensitivity of the left and right vestibular systems. The test-retest variability was determined in detail in 12 additional subjects. Prestimulation reduced habituation, but improved the sensitivity of the method; some test-retest variability persisted (< 20%).     

The effects of stochastic monopolar galvanic vestibular stimulation on human postural sway

Galvanic vestibular stimulation (GVS) is a technique in which small currents are delivered transcutaneously to the afferent nerve endings of the vestibular system through electrodes placed over the mastoid bones. The applied current alters the firing rates of the peripheral vestibular afferents, causing a shift in a standing subject's vestibular perception and a corresponding postural sway. Previously, we showed that in subjects who are facing forward, stochastic bipolar binaural GVS leads to coherent stochastic mediolateral postural sway. The goal of this pilot study was to extend that work and to test the hypothesis that in subjects who are facing forward, stochastic monopolar binaural GVS leads to coherent stochastic anteroposterior postural sway. Stochastic monopolar binaural GVS was applied to ten healthy young subjects. Twenty-four trials, each containing a different galvanic input stimulus from among eight different frequency ranges, were conducted on each subject. Postural sway was evaluated through analysis of the center-of-pressure (COP) displacements under each subject's feet. Spectral analysis was performed on the galvanic stimuli and the COP displacement time series to calculate the coherence spectra. Significant coherence was found between the galvanic input signal and the anteroposterior COP displacement in some of the trials (i.e., at least one) in nine of the ten subjects. In general, the coherence values were highest for the mid-range frequencies that were tested, and lowest for the low- and high-range frequencies. However, the coherence values we obtained were lower than those we previously reported for stochastic bipolar binaural GVS and mediolateral sway. These differences may be due to fundamental characteristics of the vestibular system such as lower sensitivity to symmetric changes in afferent firing dynamics, and/or differences between the biomechanics of anteroposterior and mediolateral sway.     

Galvanic-induced body sway in vestibular schwannoma patients: evidence for stimulation of the central vestibular system

OBJECTIVE: To investigate the various possible sites of excitation by galvanic stimulation by comparing the galvanic-induced body sway (GBS) in vestibular schwannoma (VS) patients with that in healthy subjects. MATERIAL AND METHODS: This was a prospective experimental study conducted in a tertiary referral centre. In a previous study we developed a standardized procedure for reproducible quantification of GBS using a monaural continuous 1-cosinusoidal stimulus (0.5 Hz; 2 mA). In this study, 23 VS patients were tested before and 12 VS patients were also tested after surgical intervention (extirpation of tumour with concomitant vestibular neurectomy) and the results were compared with those obtained in 47 healthy subjects. RESULTS: There were no significant differences in mean total GBS gain between VS patients and healthy subjects (p>0.05) before surgical intervention. After vestibular neurectomy a significant difference in the mean total GBS gain was observed, but GBS was still present after surgery. CONCLUSION: These results prove that galvanic vestibular stimulation excites the central vestibular system even when conduction via the vestibular nerve is hampered.     

Patient and normal three-dimensional eye-movement responses to maintained (DC) surface galvanic vestibular stimulation.

HYPOTHESIS: That disease or dysfunction of vestibular end organs in human patients will reduce or eliminate the contribution of the affected end organs to the total eye-movement response to DC surface galvanic vestibular stimulation (GVS). BACKGROUND: It was assumed that DC GVS (at current of 5 mA) stimulates all vestibular end organs, an assumption that is strongly supported by physiological evidence, including the activation of primary vestibular afferent neurons by galvanic stimulation. Previous studies also have described the oculomotor responses to vestibular activation. Stimulation of individual semicircular canals results in eye movements parallel to the plane of the stimulated canal, and stimulation of the utricular macula produces changes in ocular torsional position. It was also assumed that the total three-dimensional eye-movement response to GVS is the sum of the contributions of the oculomotor drive of all the vestibular end organs. If a particular vestibular end organ were to be diseased or dysfunctional, it was reasoned that its contribution to the GVS-induced oculomotor response would be reduced or absent and that patients thus affected would have a systematic difference in their GVS-induced oculomotor response compared with the response of normal healthy individuals. METHODS: Three-dimensional video eye-movement recording was carried out in complete darkness on normal healthy subjects and patients with various types of vestibular dysfunction, as diagnosed by independent vestibular clinical tests. The eye-movement response to long-duration bilateral and unilateral surface GVS was measured. RESULTS: The pattern of horizontal, vertical, and torsional eye velocity and eye position during GVS of patients independently diagnosed with bilateral vestibular dysfunction, unilateral vestibular dysfunction, CHARGE syndrome (semicircular canal hypoplasia), semicircular canal occlusion, or inferior vestibular neuritis differed systematically from the responses of normal healthy subjects in ways that corresponded to the expectations from the conceptual approach of the study. CONCLUSION: The study reports the first data on the differences between the normal response to GVS and those of patients with a number of clinical vestibular conditions including unilateral vestibular loss, canal block, and vestibular neuritis. The GVS-induced eye-movement patterns of patients with vestibular dysfunction are consistent with the reduction or absence of oculomotor contribution from the end organs implicated in their particular disease condition.     

Response to galvanic vestibular stimulation in patients with unilateral vestibular loss

OBJECTIVES: This study sought to characterize various responses to galvanic vestibular stimulation (GVS) by comparing GVS-induced eye movements in healthy subjects and patients with vestibular function loss. The study also aimed to estimate the clinical significance of GVS tests. Finally, an effort was made to localize the primary excitation site of stimulation in the vestibular system. MATERIALS AND METHODS: Three parameters of response to GVS, spontaneous nystagmus, galvanic stimulating nystagmus (GSN), and postgalvanic stimulating nystagmus (PGSN), were evaluated in 20 normal subjects and 14 patients with complete unilateral vestibular function loss resulting from labyrinthectomy or vestibular neurectomy using a three-dimensional video-electronystagmography technique. RESULTS: In normal subjects, GSN was detected in all subjects and was directed toward the negative electrode. PGSN was also detected but was directed toward the opposite electrode. When the negative electrode was attached to the intact side in unilateral vestibular loss subjects, GSN was always directed toward the negative electrode and PGSN was never observed. When the negative electrode was attached to the lesion side, however, GSN was detected in only one case, and PGSN was observed and directed to the intact side in 13 patients. CONCLUSIONS: The response to GVS in vestibular loss patients differed from that in normal subjects, which suggests that GVS could be useful for estimating the extent of vestibular function loss. The fact that the patterns of GVS response differed so significantly suggests that the primary site of excitation is not central but is instead the peripheral vestibular organ.     

Vestibular evoked myogenic potential (VEMP) with galvanic stimulation in normal subjects

IntroductionThe vestibular evoked myogenic potential (VEMP) generated by galvanic vestibular stimulation (GVS) is related to the vestibulo-spinal pathway. The response recorded from soleus muscle is biphasic with onset of short latency (SL) component around 60 ms and medium latency (ML) component around 100 ms. The first component reflects otolith function (sacule and utricle) and the last deals with semicircular canals.AimTo describe VEMP generated by GVS.MethodsIn this cross-sectional clinical study, VEMP was generated by 2 mA/400 ms binaural GVS, frequency of 5–6 ms that was recorded from soleus muscles of 13 healthy adults, mean age 56 years. The subjects remained standing, head turned contralateral to the GVS applied to the mastoid. Thirty GVS were applied to the mastoid in the position cathode right anode left, followed by 30 in inverted position. SL and ML were measured.ResultsSL and ML components were recorded from both legs of all participants and were similar. The average of SL component was 54 ms and of ML was 112 ms.ConclusionThe components SL and ML of the VEMP response in soleus were reproducible and are useful measures of vestibular-spinal function.© 2014 Associação Brasileira de Otorrinolaringologia e Cirurgia Cérvico-Facial. Published by Elsevier Editora Ltda. All rights reserved.     

Galvanic-induced Body Sway in Vestibular Schwannoma Patients: Evidence for Stimulation of the Central Vestibular System

Objective —To investigate the various possible sites of excitation by galvanic stimulation by comparing the galvanic-induced body sway (GBS) in vestibular schwannoma (VS) patients with that in healthy subjects.

Material and Methods —This was a prospective experimental study conducted in a tertiary referral centre. In a previous study we developed a standardized procedure for reproducible quantification of GBS using a monaural continuous 1–cosinusoidal stimulus (0.5 Hz; 2 mA). In this study, 23 VS patients were tested before and 12 VS patients were also tested after surgical intervention (extirpation of tumour with concomitant vestibular neurectomy) and the results were compared with those obtained in 47 healthy subjects.

Results —There were no significant differences in mean total GBS gain between VS patients and healthy subjects (p>0.05) before surgical intervention. After vestibular neurectomy a significant difference in the mean total GBS gain was observed, but GBS was still present after surgery.

Conclusion —These results prove that galvanic vestibular stimulation excites the central vestibular system even when conduction via the vestibular nerve is hampered.     

Habituation and rate effect in the auditory cortical potentials evoked by trains of stimuli

Summary The effects of the stimulus repetition rate over the habituated auditory cortical evoked responses were studied. The stimulation pattern consisted of trains of pure tone bursts with interstimulus interval (ISI) of 1 s, and intertrain interval (ITI) of 5 s, delivered with constant time and intensity parameters during 93 min. The analysis of the responses was based upon across averaging of the trains, each single response being evaluated in the latency and amplitude parameters. Two time-dependent factors affected the responses in a distinct way: the habituation throughout the whole stimulation and the rate effect within the train. The linear regressions of the time/amplitude functions of the responses were calculated in relation to the duration of ISI and ITI. By introducing a correction factor depending on the repetition rate it was possible to evaluate the relationships between habituation and repetition rate. Changes in the repetition rate do not have any effect on the habituation process. The two phenomena are completely distinct, and they probably have neurophysiologic substrates corresponding to different levels in the central nervous system (CNS).     

Test-retest stability and short-term habituation of the N1 and gamma band response

The gamma band response (GBR) is an exogenous, cortically generated, event-related potential that occurs between 20- and 170-msec post-stimulus onset. The auditory GBR is superimposed on the transient evoked middle and long latency cortical auditory evoked potentials and demonstrates a peak spectral frequency between 30 and 40 Hz. The present investigations were conducted to evaluate the test-retest stability and short-term habituation of the GBR. Both the GBR and N1 were recorded from six normal-hearing, neurologically intact subjects (Investigation 1, test-retest stability) and two subjects with intractable epilepsy with implanted subdural electrode grid arrays (Investigation 2, short-term habituation characteristics). For Investigation 1, the test-retest interval was 1 month. For Investigation 2, 300 samples were acquired per stimulus block (a 10-minute interval) and then subaveraged in blocks of 25 to 50 samples each. Results suggest that (1) like N1, the GBR shows high repeatability (qualitative) and test-retest stability (quantitative) and (2) the GBR does not demonstrate evidence of short-term habituation.     

Does vestibular habituation still have a place in the treatment of benign paroxysmal positional vertigo?

Particle repositioning procedures such as the Epley manoeuvre have become popular in the management of benign paroxysmal positional vertigo (BPPV) at the expense of more traditional therapies such as vestibular habituation. We prospectively analysed the response of consecutive patients presenting with BPPV to treatment with vestibular habituation exercises using a symptom score sheet before and after treatment. This same patient group was then followed up, on average three years, eight months after discharge, to determine their long-term response to treatment. The results of the study demonstrated an excellent short-term response rate to treatment but a high level of recurrence after discharge. Most patients who experienced further symptoms following discharge were however self-reliant and were able to return to their habituation exercises without the need for further medical input. The aim in the management of these patients must be to provide long-term self reliance as well as short-term cure and it may be that this has not yet been well demonstrated with the Epley manoeuvre. It must also be remembered that particle repositioning manoeuvres cannot be used in all patients for example those who suffer with cervical or thoracic spine pathology and that some will fail to respond at all. We believe that vestibular habituation retains a useful role in the treatment of BPPV.     

Binaural interactions of electrically and acoustically evoked responses recorded from the inferior colliculus of guinea pigs

Binaural interactions within the inferior colliculus (IC) elicited by electric and acoustic stimuli were investigated in this study. Using a guinea pig model, binaural acoustic stimuli were presented with different time delays, as were combinations of binaural electric and acoustic stimuli. Averaged evoked potentials were measured using electrodes inserted into the central nucleus of the IC to obtain the binaural interaction component (BIC), computed by subtracting the sum of the two monaural responses from the binaural response. The BICs to acoustic-acoustic stimulation and electric-acoustic stimulation were found to be similar. The BIC amplitude increased with stimulus intensity, but the shapes of the delay functions were similar across the levels tested. The gross-potential data are thus consistent with the thesis that the central auditory system processes binaural electric and acoustic stimuli in a similar manner. These results suggest that the binaural auditory system can process combinations of electric and acoustic stimulation presented across ears and that evoked gross potentials may be used to measure such interaction.     

Binaural interactions of electrically and acoustically evoked responses recorded from the inferior colliculus of guinea pigs

Binaural interactions within the inferior colliculus (IC) elicited by electric and acoustic stimuli were investigated in this study. Using a guinea pig model, binaural acoustic stimuli were presented with different time delays, as were combinations of binaural electric and acoustic stimuli. Averaged evoked potentials were measured using electrodes inserted into the central nucleus of the IC to obtain the binaural interaction component (BIC), computed by subtracting the sum of the two monaural responses from the binaural response. The BICs to acoustic-acoustic stimulation and electric-acoustic stimulation were found to be similar. The BIC amplitude increased with stimulus intensity, but the shapes of the delay functions were similar across the levels tested. The gross-potential data are thus consistent with the thesis that the central auditory system processes binaural electric and acoustic stimuli in a similar manner. These results suggest that the binaural auditory system can process combinations of electric and acoustic stimulation presented across ears and that evoked gross potentials may be used to measure such interaction.     

Symmetry measures of vestibular evoked myogenic potentials using objective detection criteria.

Vestibular evoked myogenic potentials (VEMP) has been put forward as a test to evaluate the symmetry of saccular function. In the present study, the symmetry of VEMP was evaluated in 23 healthy subjects using automatic analysis. In response to binaural clicks with a stimulus repetition rate of 4/s all subjects revealed significant VEMP on both sides. The ipsilateral response to monaural clicks was similar to the response to binaural clicks. Although there were large interindividual variations in amplitude, there were only small variations in latency. VEMP measurements were also assessed using different stimulus rates. At the higher stimulus rates there was a decrease in the VEMP amplitude but there were only small changes in the latency. These findings might suggest that amplitude measures are more likely than latency measures to reveal small vestibular lesions.     

Rapid adaptation of the 2f1-f2 DPOAE in humans: binaural and contralateral stimulation effects

The present data were collected in humans to characterize the effects of monaural and binaural stimulation and contralateral noise on the 2f1-f2 distortion-product otoacoustic emission (DPOAE) adaptation response. DPOAE levels (f2/f1=1.21, L1=70 dB SPL, L2=65 dB SPL) were measured in both ears for a range of f2 frequencies (1.2 to 10.0 kHz). The f2 frequency producing the largest amplitude DPOAE was used for further testing employing three different stimulus conditions: the primary tones were presented to only one ear for 4 s; the two tones were presented simultaneously in both ears; and, contralateral broadband noise (60 dB SPL) was presented for 5 s, beginning 4 s after the onset of the monaural primaries in the test ear. Acoustic reflex thresholds were measured to verify that the middle-ear muscles played no systematic role in the measured DPOAE reductions. Estimates of monaural rapid adaptation levels and time constants agreed well with previous human findings. The magnitude of the rapid adaptation under binaural stimulation, as compared with monaural primaries, was 25% greater on average, though adaptation time constants were comparable. With added contralateral noise, the average DPOAE suppression was 1.1 dB (0.3-2.7 dB). The magnitude of the monaural adaptation and the effects of binaural and contralateral stimulation, however, were smaller than those measured previously in experimental animals, though the time constants were in good agreement.     

Paraspinal muscle response to electrical vestibular stimulation

Galvanic (electrical) vestibular stimulation (GVS) has been used to study the role of the vestibular system in postural control by inducing postural sway in standing subjects. The purpose of this study was to determine the timing and pattern of activation in the paraspinal muscles in response to GVS and to compare these responses with those in the muscles of the lower leg. Binaural-bipolar GVS was applied to the skin overlying the mastoid processes of 10 subjects while they stood on a force plate with their eyes closed. The stimulus consisted of a 0.6 mA 5-pulse sequence. Each pulse lasted for 2 s, followed by 4 s of rest. The centre of pressure (COP) vs. time for each trial was calculated from the reaction forces and moments. Surface electromyographic (EMG) signals from the paraspinal and gastrocnemius muscles were recorded bilaterally. The EMG signals were rectified and integrated (iEMG). The iEMG from the muscles on the cathodal side of the body were then subtracted from the iEMG of the anodal side muscles, to yield a differential EMG (dEMG). Both the paraspinal and gastrocnemius muscles became activated in response to the stimulus. The pattern of activation was consistent with the changes observed in the centre of pressure. The primary response in both muscles acted to move the body toward the anode. This primary response began at 74 +/- 20 ms in the paraspinal muscles and at 118 +/- 18 ms in the gastrocnemius. A second component of the response began at 232 +/- 27 ms in the paraspinal muscles and 262 +/- 54 ms in the gastrocnemius muscles. This second phase of the response was opposite in direction to the primary response and was responsible for decelerating the body and maintaining the deviated position of the centre of mass over the base of support. Following the termination of the stimulus, the opposite pattern of muscle activation in both the paraspinal and the gastrocnemius muscles was observed. The results of this study suggest that the paraspinal muscles may play a significant role in the frontal plane response to vestibular stimulation during stance in humans.     

Excitatory/inhibitory interaction in the LSO revealed by point process modeling.

We studied lateral superior olivary (LSO) unit responses to binaural tone-bursts using a general point process approach. We show that inhibition of the ipsilaterally elicited response by contralateral stimulation cannot be modeled simply as a reduction of the ipsilateral input. Statistical analyses reveal that inhibition operates by scaling the intensity of the point process describing the ipsilateral response. In some cases the scaling process has secondary effects: Binaurally elicited discharges produce bimodal interspike interval histograms from units that produce unimodal interval histograms under monaural stimulation. We present a specific point process model that describes the scaling process and successfully replicates the observed responses to monaural and binaural stimulation of the three types of LSO units: slow choppers, fast choppers, and bimodal units. We interpret scaling as a shunting inhibitory process in these LSO neurons. By relating scaling magnitude to interaural level difference, we demonstrate the spatial sensitivity of LSO units.     

Optokinetic training and vestribular habituation.

The present study deals with the interaction of the visual and vestibular system with special reference to habituation. The results indicate that vestibular responses are modified by repeated uni- and bidirectional optokinetic stimulation. This transfer mechanism is more evident in test subjects after wide angle (foveo-peripheral) than after narrow angle (foveal) optokinetic stimulation. The modification of vestibular responses by means of repetitive optokinetic stimuli is characterized by an enhancement on the one side and a decline of the responses on the other, depending entirely on the nature of the optokinetic stimulus (uni- or bidirectional, foveal or foveo-peripheral). There is some evidence to suggest that these phenonomena are habituation transfer mechanisms. They seem to subserve the improvement of the gain of the vestibulo-ocular reflex. They help the vestibular system to improve the assessment of self-rotation within the environment and to impede the development of visual-vestibular conflicts.     

不同刺激模式前庭诱发肌源性电位的反应特性

目的前庭诱发的肌源性电位(vestibular evoked myogenic potential，VEMP)可用于评价球囊功能及其对称性，比较3种刺激模式VEMP的振幅与潜伏期，希望得出适合临床应用的方法。方法测量21名健康成人3种刺激模式(双侧短声、1侧短声和1侧短声对侧白噪声)的VEMP的潜伏期、振幅，计算出3种情况下的振幅及潜伏期的均值、双侧振幅比值、双侧振幅不对称性。结果3种刺激模式刺激侧振幅与潜伏期无明显差异，但1侧短声刺激对侧振幅低于刺激侧，潜伏期比刺激侧长2～3ms。双侧短声刺激p13波和n23波出现率为100％，1侧短声对侧白噪声刺激p13和n23出现率最低，无特别应用的意义。3种刺激方式VEMPs振幅变化较大。结论VEMP是一种稳定的肌源性电位，双侧短声刺激和1侧短声刺激都不失为较好的刺激方法，双侧给声对双侧听力正常者较为适宜。应用双侧VEMP的振幅比值和对称性，判断双侧球囊的功能。