The importance of cervical muscles in responses of galvanic body sway test

Body sway in normal subjects was analyzed by means of various methods to study a role of cervical muscles in galvanic body sway test. Galvanic stimulation through the retro-auricular electrode induced an initial response and a deviation response in body sway. When the anodal stimulation was given through the right retro-auricle during standing, a deviation response toward the right side was observed. While keeping a posture weighted on one foot, the stimulation induced a similar response. The stimulation during squatting produced also a deviation response toward the right side. When the head was rotated to the right, the stimulation produced backward responses. When rotated to the left, it produced forward responses. Even without galvanic stimulation, similar responses were also induced by some other method, for example, inclining the head to one side. Galvanic stimulation while sitting resulted in slight but apparent head inclination. The results suggested that cervical muscles played an important role in galvanic body sway test. Initial and deviation responses appeared to be secondarily produced by changes in the cervical muscular tension.     

Direction of galvanically-induced vestibulo-postural responses during active and passive neck torsion

The direction of a postural response induced by galvanic vestibular stimulation depends on the head and trunk position. The relative importance of afferent information (proprioception) and efferent motor command/corollary discharge is unknown. We studied the direction of body sway evoked by galvanic vestibular stimulation in 9 healthy subjects during active and passive head positioning at 0 degrees frontal position, 35 degrees to the left, and 75 degrees to the right, using a custom-built collar. At 0 degrees and 75 degrees there were no significant differences in sway direction between active and passive head positioning. The galvanic stimulation invoked sway toward the anode, mainly in the inter-aural direction. The sway direction differed significantly between active and passive positioning at 35 degrees to the side (p < 0.05). When the head was actively kept in this position, the body sway was mainly in an inter-aural direction. The sway shifted to a naso-occipital direction when the head was passively positioned at 35 degrees. Our results indicate that the afferent proprioceptive information has the largest influence on the direction of the galvanically-induced postural response, although some dependence on efferent motor commands and non-linear cervical proprioception cannot be ruled out entirely.     

Direction of Galvanically-induced Vestibulo-postural Responses during Active and Passive Neck Torsion

The direction of a postural response induced by galvanic vestibular stimulation depends on the head and trunk position. The relative importance of afferent information (proprioception) and efferent motor command/corollary discharge is unknown. We studied the direction of body sway evoked by galvanic vestibular stimulation in 9 healthy subjects during active and passive head positioning at 0° frontal position, 35° to the left, and 75° to the right, using a custom-built collar. At 0° and 75° there were no significant differences in sway direction between active and passive head positioning. The galvanic stimulation invoked sway toward the anode, mainly in the inter-aural direction. The sway direction differed significantly between active and passive positioning at 35° to the side (p&lt;0.05). When the head was actively kept in this position, the body sway was mainly in an inter-aural direction. The sway shifted to a naso-occipital direction when the head was passively positioned at 35°. Our results indicate that the afferent proprioceptive information has the largest influence on the direction of the galvanically-induced postural response, although some dependence on efferent motor commands and non-linear cervical proprioception cannot be ruled out entirely.     

Influence of vibration on the equilibrium of patients with unilateral labyrinthine dysfunction]

To determine the role of proprioceptors of different skeletal muscles in posture control in patients with unilateral labyrinthine dysfunction (ULD), we studied the effect of vibration on those muscles by postulography. Subjects were 59 healthy people and 20 ULD patients. We measured the length of the displacement of the center of gravity, maximum sway length, and sway area. Significant differences were observed between healthy and ULD subjects on stimulation of the muscle groups of the upper and lower extremity during vibration. In patients with ULD, vibration to the dorsal neck muscles caused a deviation toward the diseased side. We speculate that the upper dorsal neck muscle plays an important role in maintaining body balance in the frontal plane in patients with ULD. The upper and lower extremity muscles play a significant role in adjusting standing posture.     

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 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.     

An adequate parameter evaluating the galvanic body sway test: comparison with the caloric test in patients with vestibular schwannomas.

It has been reported that the galvanic body sway test does not correlate with the caloric test. We evaluated the galvanic body sway test in patients with vestibular schwannomas using three parameters: the angle of deviation response onset, the maximum value of the deviation response, and the area of deviation. These parameters reflect velocity, position, and locus of the centre of pressure, respectively. Among these parameters, only the angle of deviation response onset showed unilateral weakness of the response correlating with the canal paresis value, which indicates that velocity is responsible for conduction in the vestibular nerve. However, the galvanic body sway test is apt to be preferred to the caloric test. This might be attributed to the decreased sensitivity of this test.     

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.     

Influence of proprioceptive input from leg, thigh, trunk and neck muscles on the equilibrium of standing]

To investigate and compare the roles of proprioceptive input from the leg, thigh, trunk and neck muscles on equilibrium, we performed static posturography in 50 normal subjects in the standing position on a force platform by applying vibratory stimulations to the muscles. The length of the displacement of the center of gravity, maximum sway length and sway area were measured. The amplitude of the body sway was maximum when the stimulation was applied to the dorsal neck. The forward shift of the center of gravity was also marked by stimulation applied to the dorsal neck. The amplitude of the body sway on stimulation of the leg muscles was also marked, although less than that of dorsal neck stimulation. The backward shift during stimulation of the gastrocnemius and the forward shift during stimulation of the anterior tibialis were remarkable. The results indicate that the leg muscles, which directly regulate the movement of the ankle joint, and the dorsal neck muscles, which change the static equilibrium through the central nervous system, are important for maintaining the standing posture.     

Galvanic-Induced Postural Movements As a Test of Vestibular Function in Humans

Galvanic stimulation produces postural sway and eye movements in humans. Since galvanic currents are thought to exert their effect at the trigger zone of the vestibular nerve, an intact vestibular nerve should be necessary to produce a response. We have used galvanic stimulation in humans to test the hypothesis that intact vestibular nerve fibers are required to obtain a postural sway response. Experimental subjects included normal subjects, patients who had undergone resection of an acoustic neuroma, and patients who had undergone vestibular neurectomy and surgical labyrinthectomy. Our results support the hypothesis that an intact vestibular nerve is necessary to produce a response. Moreover, two patients with recurrent vertigo following vestibular neurectomy and labyrinthectomy, who had absent ice-water caloric test responses in the operated ears, were found to have a positive galvanic response. This result suggested that their recurrent vertigo was based on intact residual vestibular nerve fibers. Although previous research has not yielded a routine clinical use for galvanic stimulation, our results suggest that galvanic stimulation of the vestibular system can provide unique and valuable diagnostic information.     

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.     

Galvanically induced body sway in the anterior-posterior plane

Anterior-posterior body sway was evoked with monopolar bi-aural galvanic stimulus of the vestibular nerves in normal subjects and recorded with a force platform, two experiments being conducted. In an experiment of paired design, 9 normal subjects showed an increase in anterior-posterior sway as compared with lateral sway when exposed to the stimulus. In a second experiment another group of 10 normal subjects were exposed to a galvanic stimulus between a neck electrode and two electrodes placed on the arms, but there was no change in the relationship between anterior-posterior and lateral body sway. It is concluded that monopolar galvanic stimulus of the vestibular nerves can induce anterior-posterior body sway, a phenomenon which can be utilised to investigate the vestibulo-spinal contribution in postural control in the anterior-posterior plane.     

Postural responses evoked by sinusoidal galvanic stimulation of the labyrinth. Influence of head position

In 15 healthy subjects we studied body sway reactions to sinusoidal 0.3 Hz binaural bipolar galvanic current up to 2 mA under three conditions. With the head forward and eyes closed, there is only a periodic lateral displacement of the centre of gravity, following the stimulation with a phase lag. In two other conditions, the head turned to the left or to the right without trunk torsion, the direction of sway was modified in such a way that there were mainly anteroposterior movements. It is thought that this experiment shows the modulatory influence of neck afferents on the direction of vestibulospinal motor effects in man.     

Influence of neck vibration on body sway

To investigate the influence of the cervical input to the equilibrium, the effect of neck vibratory stimulation on body sway was analyzed in 49 normal human subjects. Body perturbations during standing posture were recorded by a force platform with or without vibratory stimulus on the upper cervical region, and analyzed by computer. During the neck vibratory stimulation, the center of gravity was shifted to the forward, and the amplitude of the body sway was increased especially along the front-rear axis. These results indicate that the proprioceptive inputs from the cervical receptors largely modifies the body equilibrium in normal subjects.     

Effects of eye tracking stimulation on movement of body's center of gravity and head in elderly subjects]

This study was designed to investigate the effects of eye tracking stimulation on the movement of the body's center of gravity (CG) and the head in aged persons 60 years old and above. Twenty-three healthy aged subjects ranging from 60 to 79 years old and 10 healthy young subjects were asked to pursue a pendulously moving horizontal target when standing with the feet together. The frequencies of target movement were 0, 0.1, 0.25, 0.5, 0.75, 1.0 and 1.5 Hz. The results were as follows: 1) The body sway locus patterns increased significantly up to around 0.5 Hz of target movement. 2) The total lengths of the locus traced by CG movement and the head movement were larger than that of control subjects throughout the test runs. The aged subjects were also affected by eye tracking stimulation, and showed individual variations. 3) The ratio of the anteroposterior component to the lateral component of CG movement was higher in the elderly subjects than in the control subjects, while head movement was characteristically different. 4) The averaged divisional frequencies were calculated in the range between 0.0195 and 1 Hz. In the anteroposterior component, the averaged divisional frequencies of CG movement and head movement were significantly larger while watching the moving target than when watching a stationary target.     

Significance of pressor input from the human feet in lateral postural control. The effect of hypothermia on galvanically induced body-sway

The significance to human postural control of pressor information from the feet was investigated during vestibular disturbance in seven normal subjects who were exposed to bipolar biaural galvanic stimulation of the vestibular nerves before and after their feet were anaesthetized with hypothermia. The increase in body sway in the lateral plane induced by the galvanic stimulus was enhanced when the feet were anaesthetized, and adaptation of postural control to the galvanic stimulus was delayed. It is concluded that pressor information from the feet contributes significantly to postural control in humans and is important in compensating for vestibular disturbance.     

Influence of vibration to the neck, trunk and lower extremity muscles on equilibrium in normal subjects and patients with unilateral labyrinthine dysfunction

To investigate the role of proprioceptors of different skeletal muscles in postural control, in normal subjects and patients with unilateral labyrinthine dysfunction (ULD), the effect of vibration on these muscles was studied by postulography. The subjects comprised 59 normal subjects and 12 patients with ULD due to resection of acoustic tumours. Sagittal body sway was observed during vibration to the triceps surae, tibialis anterior and upper dorsal neck muscles. No significant change in sway was observed in the frontal plane in normal subjects. Significant differences between normal subjects and patients were found on stimulation of the muscle groups of triceps surae and biceps femoris during vibration. In patients with ULD, vibration to the dorsal neck muscles caused a deviation towards the diseased side. It can be speculated that the upper dorsal neck muscle plays an important role in maintaining the body balance in the frontal plane in patients with ULD. On the other hand, the lower extremity muscles, especially the muscles on the dorsal side of the body, play a significant role in adjusting the standing posture in the sagittal plane.     

Influence of Vibration to the Neck,Trunk and Lower Extremity Muscles on Equilibrium in Normal Subjects and Patients with Unilateral Labyrinthine Dysfunction

To investigate the role of proprioceptors of different skeletal muscles in postural control, in normal subjects and patients with unilateral labyrinthine dysfunction (ULD), the effect of vibration on these muscles was studied by postulography. The subjects comprised 59 normal subjects and 12 patients with ULD due to resection of acoustic tumours. Sagittal body sway was observed during vibration to the triceps surae, tibialis anterior and upper dorsal neck muscles. No significant change in sway was observed in the frontal plane in normal subjects. Significant differences between normal subjects and patients were found on stimulation of the muscle groups of triceps surae and biceps femoris during vibration. In patients with ULD, vibration to the dorsal neck muscles caused a deviation towards the diseased side. It can be speculated that the upper dorsal neck muscle plays an important role in maintaining the body balance in the frontal plane in patients with ULD. On the other hand, the lower extremity muscles, especially the muscles on the dorsal side of the body, play a significant role in adjusting the standing posture in the sagittal plane.     

The N3 potential compared to sound and galvanic vestibular evoked myogenic potential in healthy subjects and in multiple sclerosis patients

Both sound (s-) and galvanic (g-) vestibular-evoked myogenic potential (VEMP) enable us to study the saccular pathways. However, the VEMP can be abnormal for non-vestibular factors, such as insufficient activation of the sterno-cleido-mastoid (SCM) muscle or a lesion that involves the accessory nucleus and/or nerve or the SCM muscle. These drawbacks do not affect another technique that evaluates the saccular function: the N3 potential. We recorded both the s- and the g-VEMP and the N3 potential in a group of 31 healthy subjects to establish a reference range. The N3 potential and the s-VEMP were recordable bilaterally from all the subjects, whereas the g-VEMP was undetectable uni- or bilaterally in 7 subjects. The latency and amplitude values of the s-VEMP did not differ from those of the g-VEMP. For all three techniques, the latency and amplitude values from the right and from the left recording and/or stimulation side were the same. We suggest using normative latency and amplitude values based on the mean and ratio of the right- and left-side values. The s-VEMP, the N3 potential and the auditory evoked response (ABR) were compared in 15 subjects suffering from multiple sclerosis. The three techniques detected a similar number of abnormalities, but these abnormalities were not correlated. This suggests that these different techniques should be regarded as complementary in evaluating saccular function.