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<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.     

Head and body sway in response to vertical visual stimulation

CONCLUSIONS: Postural responses differed according to the stimulus direction, i.e. vertical visual stimulation induced head rather than trunk displacements. Accordingly, it could be that center of foot pressure (COP) responses tended to underestimate the postural sway during visual stimulation. OBJECTIVES: To investigate head and body sway in response to vertical visual surround motion, and to examine the correlation between the displacements of head and body segments derived from video-motion analysis and COP measurements. MATERIAL AND METHODS: Postural sway was assessed in 10 young female subjects by video-motion analysis of four different head and body segments, and by use of force-plate posturography. Head and body sway in the pitch plane was induced by rotating a random pattern of dots about the subject's inter-aural axis at a constant acceleration of 1 degree/s(2) or a constant velocity of 60 degrees/s in darkness. RESULTS: Generally, head displacement was greater than that of other body parts during vertical optokinetic stimulation (OKS). In most subjects, maximum head displacements were induced in the same direction as the visual motion. Downward OKS induced a forward head and body sway. The COP trajectory correlated well with the displacements of each head and body segment during downward OKS. In contrast, postural responses to upward OKS were complicated in terms of their time course. The correlation coefficient between each head and body segment and the COP varied among individuals for upward OKS.     

Effect of head extension on equilibrium in normal subjects

A dynamic posturography system was used to test the effect of 55 degrees head extension on postural sway in 20 normal subjects. There was a highly significant increase in sway with head extension under two conditions; in both, the support surface moves proportionally to body sway angle (sway-referenced feedback). The largest increase in sway occurred when the eyes were closed and the support surface was sway-referenced. This latter condition removes vision, reduces the effectiveness of ankle proprioception, and forces the subject to depend mostly on vestibular information for equilibrium. We suggest that head extension increases sway because the utricular otoliths are put into a disadvantageous position. This may be another example of the role of utricular input in the control of balance.     

Sound-evoked postural responses in normal subjects

A pattern of sound-induced paroxysms of the eye and head and other spinal motor neuron synkinesis (Tullio's phenomenon) in human subjects always implies either a pathological contiguity of the tympano-ossicular chain and membranous labyrinth or a dehiscence of the bone overlying the superior semicircular canal. However, it has become clear in the last decade that sound-evoked vestibular stimulation is not only a sign of disease but also a physiological phenomenon, The examination of such physiologically sound-induced vestibular (saccular) responses contributes today to the clinical testing of the vestibular organ, mainly in the form of vestibular-evoked myogenic potentials. In this study it was observed that, in a group of 20 normal subjects, a 500 Hz tonal stimulus of high intensity (105 dB HL = 118.5 dB SPL), applied monoaurally, elicited postural responses. Each subject was studied under 4 different conditions: (i) head facing forwards, eyes open; (ii) head facing forwards, eyes closed; (iii) head rotated approximately 90 degrees to the right, eyes closed: and (iv) head rotated approximately 90 degrees to the left, eyes closed. Body sway, measured using a force platform, was recorded in all subjects, with eyes either open or closed. Postural responses, which were also elicited with a 250 Hz tonal stimulus, were not observed with a tone of 2000 Hz, with legs slightly flexed or with binaural stimulation. The postural sway (head facing forwards, eyes open or closed) was in a lateral direction towards the stimulated ear: with the stimulus applied to the right ear the subject had postural sway towards the right, with the stimulus applied to the left ear towards the left. When the head was rotated approximately 90 degrees sideways and the stimulus was given facing forwards (i.e. head rotated contralaterally to stimulated ear) the postural sway was in a forward direction; when the head was rotated approximately 90 degrees sideways and the stimulus was given facing backwards (i.e. head rotated ipsilaterally to stimulated ear) the postural sway was in a backward direction. The mean values (mm) of body sway obtained with the head facing forwards and the eyes closed were higher than those with the eyes open (21.7 and 22.8 vs 15.7 and 14.7 for the right and left ears, respectively); higher mean values were obtained with the head turned to the side contralateral to the ear stimulated and the eyes closed (29.3 and 24.8 for the right and left ears, respectively). Under this condition the body sway was mainly in a forward direction. The sound-evoked vestibulopostural reflex seems to be a useful test for exploring the saccular function and, as a click-evoked vestibulocollic reflex, can be considered a physiological Tullio phenomenon.     

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.     

Head and body sway in response to vertical visual stimulation

Conclusions. Postural responses differed according to the stimulus direction, i.e. vertical visual stimulation induced head rather than trunk displacements. Accordingly, it could be that center of foot pressure (COP) responses tended to underestimate the postural sway during visual stimulation. Objectives. To investigate head and body sway in response to vertical visual surround motion, and to examine the correlation between the displacements of head and body segments derived from video-motion analysis and COP measurements. Material and methods. Postural sway was assessed in 10 young female subjects by video-motion analysis of four different head and body segments, and by use of force-plate posturography. Head and body sway in the pitch plane was induced by rotating a random pattern of dots about the subject's inter-aural axis at a constant acceleration of 1°/s² or a constant velocity of 60°/s in darkness. Results. Generally, head displacement was greater than that of other body parts during vertical optokinetic stimulation (OKS). In most subjects, maximum head displacements were induced in the same direction as the visual motion. Downward OKS induced a forward head and body sway. The COP trajectory correlated well with the displacements of each head and body segment during downward OKS. In contrast, postural responses to upward OKS were complicated in terms of their time course. The correlation coefficient between each head and body segment and the COP varied among individuals for upward OKS.     

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 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.     

Short-term Adaptation in the Peripheral Auditory System is Related to the AMPA Receptor

A pattern of sound-induced paroxysms of the eye and head and other spinal motor neuron synkinesis (Tullio's phenomenon) in human subjects always implies either a pathological contiguity of the tympano-ossicular chain and membranous labyrinth or a dehiscence of the bone overlying the superior semicircular canal. However, it has become clear in the last decade that sound-evoked vestibular stimulation is not only a sign of disease but also a physiological phenomenon. The examination of such physiologically sound-induced vestibular (saccular) responses contributes today to the clinical testing of the vestibular organ, mainly in the form of vestibular-evoked myogenic potentials. In this study it was observed that, in a group of 20 normal subjects, a 500 Hz tonal stimulus of high intensity (105 dB HL=118.5 dB SPL), applied monoaurally, elicited postural responses. Each subject was studied under 4 different conditions: (i) head facing forwards, eyes open; (ii) head facing forwards, eyes closed; (iii) head rotated &;#44 90° to the right, eyes closed; and (iv) head rotated 90° to the left, eyes closed. Body sway, measured using a force platform, was recorded in all subjects, with eyes either open or closed. Postural responses, which were also elicited with a 250 Hz tonal stimulus, were not observed with a tone of 2000 Hz, with legs slightly flexed or with binaural stimulation. The postural sway (head facing forwards, eyes open or closed) was in a lateral direction towards the stimulated ear: with the stimulus applied to the right ear the subject had postural sway towards the right, with the stimulus applied to the left ear towards the left. When the head was rotated &;#44 90° sideways and the stimulus was given facing forwards (i.e. head rotated contralaterally to stimulated ear) the postural sway was in a forward direction; when the head was rotated &;#44 90° sideways and the stimulus was given facing backwards (i.e. head rotated ipsilaterally to stimulated ear) the postural sway was in a backward direction. The mean values (mm) of body sway obtained with the head facing forwards and the eyes closed were higher than those with the eyes open (21.7 and 22.8 vs 15.7 and 14.7 for the right and left ears, respectively); higher mean values were obtained with the head turned to the side contralateral to the ear stimulated and the eyes closed (29.3 and 24.8 for the right and left ears, respectively). Under this condition the body sway was mainly in a forward direction. The sound-evoked vestibulopostural reflex seems to be a useful test for exploring the saccular function and, as a click-evoked vestibulocollic reflex, can be considered a physiological Tullio phenomenon.     

Influence of Neck Proprioception on Vibration-induced Postural Sway

Objective --Several reports have shown that the direction of the postural responses induced by vestibular stimulation is affected by the positions of the neck and torso. The aim of this study was to investigate whether the postural responses to vibratory proprioceptive stimulation of the calf muscles are affected by the position of the head and thus by proprioceptive and vestibular information from the neck and head. Material and Methods --Ten normal subjects were exposed to vibratory proprioceptive stimulation of the calf muscles when the head was maintained in five different positions: in a neutral position facing forwards, with the head turned to the right or left sides or with the head tilted backwards or forwards. Body movements were evaluated by analyzing the anteroposterior and lateral torques induced towards the supporting surface. Results --The analysis showed that only the anteroposterior body sway was significantly affected by the position of the head. The anteroposterior postural responses were primarily increased during the tests with the head tilted backwards or forwards, whereas the postural responses were unaffected by head torsion towards the sides. The lateral responses were primarily affected by vision and not by the position of the head. Conclusion --The findings suggest that the responses evoked by vibratory proprioceptive stimulation of the calf muscles may be affected by different mechanisms, either by purely proprioceptive information or by an interaction between proprioceptive and vestibular information. Moreover, the increasing difference between the test conditions over time suggests that fatigue of the neck muscles may be one of the factors affecting the responses induced by the perturbations.     

Influence of neck proprioception on vibration-induced postural sway

OBJECTIVE: Several reports have shown that the direction of the postural responses induced by vestibular stimulation is affected by the positions of the neck and torso. The aim of this study was to investigate whether the postural responses to vibratory proprioceptive stimulation of the calf muscles are affected by the position of the head and thus by proprioceptive and vestibular information from the neck and head. MATERIAL AND METHODS: Ten normal subjects were exposed to vibratory proprioceptive stimulation of the calf muscles when the head was maintained in five different positions: in a neutral position facing forwards, with the head turned to the right or left sides or with the head tilted backwards or forwards. Body movements were evaluated by analyzing the anteroposterior and lateral torques induced towards the supporting surface. RESULTS: The analysis showed that only the anteroposterior body sway was significantly affected by the position of the head. The anteroposterior postural responses were primarily increased during the tests with the head tilted backwards or forwards, whereas the postural responses were unaffected by head torsion towards the sides. The lateral responses were primarily affected by vision and not by the position of the head. CONCLUSIONS: The findings suggest that the responses evoked by vibratory proprioceptive stimulation of the calf muscles may be affected by different mechanisms, either by purely proprioceptive information or by an interaction between proprioceptive and vestibular information. Moreover, the increasing difference between the test conditions over time suggests that fatigue of the neck muscles may be one of the factors affecting the responses induced by the perturbations.     

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.     

Effect of head orientation on human postural stability following unilateral vestibular ablation.

Effective interpretation of vestibular inputs to postural control requires that orientation of head on body is known. Postural stability might deteriorate when vestibular information and neck information are not properly coupled, as might occur with vestibular pathology. Postural sway was assessed in unilateral vestibulopathic patients before and acutely, 1, 4, and 18+ months after unilateral vestibular ablation (UVA) as well as in normal subjects. Postural equilibrium with eyes closed was quantified as scaled pk-pk sway during 20 s trials in which the support surface was modulated proportionally with sway. Subjects were tested with the head upright and facing forward, turned 45 degrees right, and 45 degrees left. Equilibrium was uninfluenced by head orientation in normal subjects. In contrast, patients after UVA showed both a general reduction in stability and a right/left head orientation-dependent asymmetry. These abnormalities adaptively recovered with time. It is concluded that vestibular inputs to postural control are interpreted within a sensory-motor context of head-on-body orientation.     

Short and long-term postural learning to withstand galvanic vestibular perturbations

We investigated changes of postural responses to repeated bipolar galvanic vestibular stimulation on 5 consecutive days and once again after 3 months. Subjects consisted of 21 healthy volunteers. Except for the first day did the induced torque variance in response to galvanic vestibular stimulation not decrease within each test session, but there was a major reduction from day to day (p< 0.001) reflecting a continued processing of the postural experience gained during the stimulation. The decreased end level magnitude of postural responses after 5 days was retained after 3 months. The galvanic stimulation failed to induce larger torque variance compared to quiet stance toward the end of the 5 days as well as after 3 months, indicating a down-regulation of a repeated erroneous vestibular stimulation by the postural control system - i.e. sensory reweighting. This argues that a major adaptation effect to galvanic vestibular perturbation takes place after the exposure to the stimulation - similar to the concept of the consolidation process involved in motor learning. This should be considered when repeatedly assessing vestibular function both clinically and in studies. It implies that sensory training involved in rehabilitation from vestibular diseases/deficiencies should be executed with spaced intervals in order to procure more efficient learning processes and in the end, a better function.     

The deviation of the body's sway center with galvanic stimulation

We investigated galvanic body sway of ten healthy adults and eight patients by using the averaging program for measuring T1 and T2, which were latencies at the onset and at the cutoff of galvanic stimulation, respectively, and the plotting program, which adopted 1.73 sec as T1 and 0.39 sec as T2, for calculating the coordinates of the body's sway center. We estimated galvanic body sway with the difference between the coordinate of the body's sway center during stimulation and that during no stimulation. The deviation in lateral direction on the healthy subjects ranged from 0.7 or 0.9 to 2.1 cm at right- or left-side stimulation. One of the patients with peripheral disorders revealed differences between right- and left-side stimulation. The other patients did not reveal any differences. In antero-posterior direction six of the healthy subjects deviated backward at right-side stimulation and nine subjects deviated backward at left-side stimulation. The patients with peripheral disorders did not reveal any differences. The other patients revealed some differences. Waves computed by the averaging program do not always show the quantity of galvanic body sway. The deviation of the body's sway center shows the quantity of body sway consistently. The body's sway center is considered to be a suitable parameter in the galvanic body sway test for statistical analysis.     

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.     

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.     

Eye movements during active head turning with different vestibular and cervical input

Eye movements were measured in 15 volunteers during vestibulo-ocular reflex (VOR), cervico-ocular reflex with the head fixed from the ceiling (passive COR), during voluntary stabilization of the head in space while the trunk was moved sinusoidally (active COR) and active head movements with and without additional vestibular or cervical stimuli. The subjects were sitting with eyes covered on a rotating chair swinging sinusoidally at 40 degrees peak to peak amplitude at 0.05, 0.1 and 0.2 Hz. The saccadic activity during passive COR is below the VOR and increases slightly during active COR. During voluntary head movements it shows a marked increase and is further activated if cervical or vestibular stimuli are added. The amplitudes of eye shifts of passive and active COR are not different. During active head movements and more with additional cervical or vestibular input, they increase significantly. The phase of the maximum eye shifts to head position is anticompensatory during passive COR and compensatory during VOR. The phase lead of about 45 degrees during active head movements is less during active COR but is larger with additional cervical and vestibular stimuli reaching 90 degrees.     

Postural stability after vestibular schwannoma surgery

Factors related to postural stability and the course of recovery after vestibular schwannoma surgery were analyzed in a retrospective study of 177 patients and in a prospective study of 44 patients. Before surgery, 7 of the 44 patients in the prospective group complained of postural instability. The body sway velocity was measured. In the retrospective study, the sway velocity was abnormal under nonvisual control in 63% of the patients and under visual control in 34%. The sway velocity correlated with tumor size, smooth pursuit deficit, and postoperative work history. We compared patients with abnormal postural control to those with normal postural control, and found that the former relied to a greater extent on visual information and the latter on proprioceptive information. In the prospective study, the sway velocity increased after the operation, and 1 year after surgery it had not returned to preoperative values. In the logistic regression analysis, a translabyrinthine approach, rehabilitation, the patient's age, and preserved function of the facial nerve predicted a good outcome of postural stability. The retrosigmoid approach and depression were the most significant risk factors for postoperative gait difficulties. Vestibular rehabilitation increased the reliance on proprioception for maintenance of postural control and improved postural stability.