The human semicircular canal model of galvanic vestibular stimulation

A vector summation model of the action of galvanic stimuli on the semicircular canals has been shown to explain empirical balance and perceptual responses to binaural-bipolar stimuli. However, published data suggest binaural-monopolar stimuli evoke responses that are in the reverse direction of the model prediction. Here, we confirm this by measuring balance responses to binaural-monopolar stimulation as movements of the upper trunk. One explanation for the discrepancy is that the galvanic stimulus might evoke an oppositely directed balance response from the otolith organs that sums with and overrides the semicircular canal response. We tested this hypothesis by measuring sway responses across the full range of head pitch. The results showed some modulation of sway with pitch such that the maximal response occurred with the head in the primary position. However, the effect fell a long way short of that required to reverse the canal sway response. This indicates that the model is incomplete. Here, we examine alterations to the model that could explain both the bipolar and monopolar-evoked behavioural responses. An explanation was sought by remodelling the canal response with more recent data on the orientation of the individual canals. This improved matters but did not reverse the model prediction. However, the model response could be reversed by either rotating the entire labyrinth in the skull or by altering the gains of the individual canals. The most parsimonious solution was to use the more recent canal orientation data coupled with a small increase in posterior canal gain.     

Direct perturbation of neural integrator by bilateral galvanic vestibular stimulation

Caloric vestibular stimulation (CVS) and galvanic vestibular stimulation (GVS) act primarily on the peripheral vestibular system. Although the electrical current applied during GVS is thought to flow through peripheral vestibular organs, some current may spread into areas within the central nervous system, particularly when the bilateral galvanic vestibular stimulation (bGVS) method is used. According to Alexander’s law, the magnitude of nystagmus increases with eccentric gaze movement, due to the function of the neural integrator (NI); thus, if the information for vestibular stimulation corresponds to Alexander’s law, the peripheral vestibular organ is stimulated. Therefore, it would appear that if CVS results comply with Alexander’s law, and bGVS results do not, the sites stimulated by bGVS are not perfectly located in the peripheral vestibular area. In our experiments on normal human subjects, the magnitude of nystagmus under CVS increased with rising gaze eccentricity in the direction that the magnitude of the nystagmus increases, and this change was found to follow Alexander’s law. However, in the case of nystagmus under bGVS, results did not follow Alexander’s law. In addition, study of the influences of bGVS at different current intensities on nystagmus magnitude showed that bGVS at 5 mA distorted nystagmus magnitude more than at 3 mA, which suggests bGVS acts not only on the peripheral vestibular nerves, but also on some areas of the central nervous system, particularly the NI. According to our experiments, bGVS directly affects neural integrator function.     

Habituation to galvanic vestibular stimulation for analysis of postural control abilities in gymnasts

The possible correlation between postural control abilities in gymnasts and the sensitivity for and the degree of short-term habituation to galvanic vestibular stimulation (GVS) was studied. Seven balance trained young girls (Dutch National Junior Gymnasts Championship) versus seven non-trained girls and twenty-five women underwent computer-controlled GVS using a monaural continuous 1-cosinusoidal stimulus of 0.5 Hz and 2 mA, repeated three times on each side [Balter, Stokroos, Boumans, Kingma, Acta Otolaryngol. (in press); Balter, Stokroos, Eterman, Paredis, Orbons, Kingma, Acta Otolaryngol. (in press)]. Results showed that mean total galvanic-induced body sway (GBS) gain was significantly lower in the trained and untrained girls compared to the adult women (P < 0.05). Mean habituation to GVS (learning abilities), however, showed no significant differences between the three groups. We suggest that the superior balance control in professional gymnasts is primarily achieved through motor training and not by learning abilities or a higher sensitivity of the vestibular system [Neurosci. Lett. 225 (1998) 155].     

Position and velocity responses to galvanic vestibular stimulation in human subjects during standing

Galvanic vestibular stimulation (GVS) in animals modulates the firing of otolith and semicircular canal afferents alike. Here, we look for postural responses evoked by GVS from the otolith organs and semicircular canals. To minimise the modifying effects of somatosensory input on the response, low-intensity (0.3–0.5 mA) GVS was applied for 8 s while subjects stood on foam rubber with the feet together and strapped to the floor. The response had three phases: (i) a rapid movement during the first second, (ii) a slower movement that persisted throughout the stimulus, and (iii) a rapid partial return movement after GVS stopped. The three movement velocities were significantly different. The GVS response therefore appears to be the sum of a step response that returns to the starting point when the stimulus stops, and a constant-velocity ramp response for the duration of the stimulus without a return movement. Subjects' responses differed in size and profile, some with the step or ramp responses almost exclusively but most with a combination of both. The 'step-plus-ramp' model was tested by comparing the three velocities. If the responses add, the initial velocity should not be different from the sum of the velocities during the ramp-only period and the step-only period at offset. ANOVA and pairwise comparisons confirmed this. It is concluded that postural responses to GVS arise through stimulation of both otolith and canal afferents.     

A direct comparison of local dynamic stability during unperturbed standing and walking

Standing and walking are very different tasks. It might be reasonable, therefore, to assume that the mechanisms used to maintain the stability of standing and walking should be quite different. However, many studies have shown that postural stability measures can generally predict risk of falls, even though most falls occur during locomotor tasks and not during postural tasks. This suggests that there is at least some commonality among the mechanisms governing the control of both standing and walking. The present study was conducted to determine whether the postural stability either is or is not directly related to locomotor stability. Twenty healthy adults, age 18–73 years, walked on a motorized treadmill at their preferred walking speed for three trials of 5 min. They also stood on a force plate for three trials of 5 min. Both tasks were performed without imposing any additional external perturbations. The motion of each subject’s trunk segment was recorded and described using a multi-dimensional state space defined in the same manner for both tasks. Local dynamic stability was quantified from the mean divergence over time of locally perturbed trajectories in state space, which was parameterized as a double exponential process. Divergence parameters were compared to determine the relationship between local dynamic stability during standing and walking. Standing and walking exhibited local dynamic stability properties that were significantly different (P<0.001) and not correlated (P>0.1). Divergence parameters were also compared to traditional center of pressure (COP) measures obtained from standing trials. COP measures were significantly correlated to local divergence parameters for standing, but not to those for walking. This study provides direct evidence that the mechanisms governing standing and walking stability are significantly different.     

Influence of galvanic vestibular stimulation on egocentric and object-based mental transformations

The vestibular system analyses angular and linear accelerations of the head that are important information for perceiving the location of one’s own body in space. Vestibular stimulation and in particular galvanic vestibular stimulation (GVS) that allow a systematic modification of vestibular signals has so far mainly been used to investigate vestibular influence on sensori-motor integration in eye movements and postural control. Comparatively, only a few behavioural and imaging studies have investigated how cognition of space and body may depend on vestibular processing. This study was designed to differentiate the influence of left versus right anodal GVS compared to sham stimulation on object-based versus egocentric mental transformations. While GVS was applied, subjects made left-right judgments about pictures of a plant or a human body presented at different orientations in the roll plane. All subjects reported illusory sensations of body self-motion and/or visual field motion during GVS. Response times in the mental transformation task were increased during right but not left anodal GVS for the more difficult stimuli and the larger angles of rotation. Post-hoc analyses suggested that the interfering effect of right anodal GVS was only present in subjects who reported having imagined turning themselves to solve the mental transformation task (egocentric transformation) as compared to those subjects having imagined turning the picture in space (object-based mental transformation). We suggest that this effect relies on shared functional and cortical mechanisms in the posterior parietal cortex associated with both right anodal GVS and mental imagery. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.

The effects of stochastic galvanic vestibular stimulation on human postural sway

 Galvanic vestibular stimulation serves to modulate the continuous firing level of the peripheral vestibular afferents. It has been shown that the application of sinusoidally varying, bipolar galvanic currents to the vestibular system can lead to sinusoidally varying postural sway. Our objective was to test the hypothesis that stochastic galvanic vestibular stimulation can lead to coherent stochastic postural sway. Bipolar binaural stochastic galvanic vestibular stimulation was applied to nine healthy young subjects. Three different stochastic vestibular stimulation signals, each with a different frequency content (0–1 Hz, 1–2 Hz, and 0–2 Hz), were used. The stimulation level (range 0.4–1.5 mA, peak to peak) was determined on an individual basis. Twenty 60-s trials were conducted on each subject – 15 stimulation trials (5 trials with each stimulation signal) and 5 control (no stimulation) trials. During the trials, subjects stood in a relaxed, upright position with their head facing forward. Postural sway was evaluated by using a force platform to measure the displacements of the center of pressure (COP) under each subject’s feet. Cross-spectral measures were used to quantify the relationship between the applied stimulus and the resulting COP time series. We found significant coherency between the stochastic vestibular stimulation signal and the resulting mediolateral COP time series in the majority of trials in 8 of the 9 subjects tested. The coherency results for each stimulation signal were reproducible from trial to trial, and the highest degree of coherency was found for the 1- to 2-Hz stochastic vestibular stimulation signal. In general, for the nine subjects tested, we did not find consistent significant coherency between the stochastic vestibular stimulation signals and the anteroposterior COP time series. This work demonstrates that, in subjects who are facing forward, bipolar binaural stochastic galvanic stimulation of the vestibular system leads to coherent stochastic mediolateral postural sway, but it does not lead to coherent stochastic anteroposterior postural sway. Our finding that the coherency was highest for the 1- to 2-Hz stochastic vestibular stimulation signal may be due to the intrinsic dynamics of the quasi-static postural control system. In particular, it may result from the effects of the vestibular stimulus simply being superimposed upon the quiet-standing COP displacements. By utilizing stochastic stimulation signals, we ensured that the subjects could not predict a change in the vestibular stimulus. Thus, our findings indicate that subjects can act as ”responders” to galvanic vestibular stimulation. Received: 13 March 1998 / Accepted: 8 October 1998     

Influence of galvanic vestibular stimulation on postural recovery during sudden falls

To assess vestibular influences on recovery of balance during sudden falls, we measured the postural responses of five healthy subjects to a hold and release perturbation coupled with galvanic vestibular stimulation (GVS). Two electrode pairs were located with the anterior electrode of each pair over the mastoid process and the posterior electrode over the trapezius muscle on the same side. Bilateral unipolar GVS was generated 60 ms after a holding force against the sternum was released by individually driving left and right electrode pairs as cathode or anode at 1 mA for 12 s or 2 mA for 6 s. We computed the frequency and damping parameters of a multi-link inverted pendulum model of the body which best fit the transient postural oscillations after release for each subject. These parameters did not differ significantly across conditions indicating the GVS did not modify the preset overall strategy of postural recovery. The intensity and polarity of GVS significantly biased both the postural lean during the oscillatory period and the resting postural stance achieved during stimulation, deviating them forward for cathodal stimulation and backward for anodal. The residuals of the multi-link fit, the frequency spectra of the actual body sway ripples about the modeled sway, were different across conditions. Because GVS affected postural bias but not dynamics, it is likely that it provided erroneous velocity signals leading to vestibulospinal compensations in segmental stiffness and damping mechanisms. Our findings are consistent with theoretical analyses of the influence of GVS on the semicircular canals and otolith organs of the inner ear.     

Influence of vestibular and visual stimulation on split-belt walking

We investigated the influence of vestibular (caloric ear irrigation) and visual (optokinetic) stimulation on slow and fast split-belt walking. The velocity of one belt was fixed (1.5 or 5.0–6.0 km/h) and subjects (N = 8 for vestibular and N = 6 for visual experiments) were asked to adjust the velocity of the other belt to a level at which they perceived the velocity of both the belts as equal. Throughout all experiments, subjects bimanually held on to the space-fixed handles along the treadmill, which provided haptic information on body orientation. While the optokinetic stimulus (displayed on face-mounted virtual reality goggles) had no effect on belt velocity adjustments compared to control trials, cold-water ear irrigation during slow (but not fast) walking effectively influenced belt velocity adjustments in seven of eight subjects. Only two of these subjects decreased the velocity of the ipsilateral belt, consistent with the ipsilateral turning toward the irrigated ear in the Fukuda stepping test. The other five subjects, however, increased the velocity of the ipsilateral belt. A straight-ahead sense mechanism can explain both decreased and increased velocity adjustments. Subjects decrease or increase ipsilateral belt velocity depending on whether the vestibular stimulus is interpreted as an indicator of the straight-ahead direction (decreased velocity) or as an error signal relative to the straight-ahead direction provided by the haptic input from the space-fixed handles along the treadmill (increased velocity). The missing effect during fast walking corroborates the findings by others that the influence of vestibular tone asymmetry on locomotion decreases at higher gait velocities.     

Effects of galvanic vestibular stimulation on human posture and perception while standing

This study examines three hypotheses that have been proposed to explain the effects of galvanic vestibular stimulation (GVS) in standing human subjects. The first assumes realignment to an altered representation of vertical. GVS-evoked body tilt produced under conditions of different stability was compared with perceptions of the vertical which subjects indicated by two means, a visual line and a manipulandum. GVS produced body tilt that increased in unstable conditions but there were no differences in the perceived vertical in any condition. The second hypothesis is that the altered vestibular signal is interpreted as a tilt of the support surface. The postural response evoked by tilting the support surface was compared with the GVS response under conditions of varying stability. These responses were different, particularly for the lower body where movements were oppositely directed. Standing on foam augmented GVS responses whereas standing with feet apart augmented platform-tilt responses. The third hypothesis is that GVS produces an illusion of movement, and this causes a reaction in the opposite direction. Perception of movement during GVS was determined in standing and immobilised subjects. Although immobilised subjects experienced illusions of movement in the direction opposite the sway response, this only happened after long periods of stimulation and never for standing where subjects accurately reported the true direction of sway. Thus, the results do not support any of these proposals. Instead, they and other observations support a simpler interpretation that the GVS signal is consistent with head movement and evokes an automated response to stabilise the head in space.     

Comparison of human ocular torsion patterns during natural and galvanic vestibular stimulation

Galvanic vestibular stimulation (GVS) is reported to induce interindividually variable tonic ocular torsion (OT) and superimposed torsional nystagmus. It has been proposed that the tonic component results from the activation of otolith afferents. We tested our hypothesis that both the tonic and the phasic OT are mainly due to semicircular canal (SCC) stimulation by examining whether the OT patterns elicited by GVS can be reproduced by pure SCC stimulations. Using videooculography we measured the OT of six healthy subjects while two different stimuli with a duration of 20 s were applied: 1) transmastoidal GVS steps of 2 mA with the head in a pitched nose-down position and 2) angular head rotations around a combined roll-yaw axis parallel to the gravity vector with the head in the same position. The stimulation profile was individually scaled to match the nystagmus properties from GVS and consisted of a sustained velocity step of 4-12 degrees /s on which a velocity ramp of 0.67-2 degrees /s(2) was superimposed. Since blinks were reported to induce transient torsional eye movements, the subjects were also asked to blink once 10 s after stimulus onset. Analysis of torsional eye movements under both conditions revealed no significant differences. Thus we conclude that both the tonic and the phasic OT responses to GVS can be reproduced by pure rotational stimulations and that the OT-related effects of GVS on SCC afferents are similar to natural stimulations at small amplitudes.     

Modulation of muscle sympathetic bursts by sinusoidal galvanic vestibular stimulation in human subjects

There is controversy as to whether the vestibulosympathetic reflexes demonstrated in experimental animals actually exist in human subjects. While head-down neck flexion and off-vertical axis rotation can increase muscle sympathetic nerve activity (MSNA) in awake subjects, we recently showed that bipolar galvanic vestibular stimulation (GVS) does not. However, it is possible that our stimuli (2 mA, 1 s)—although capable of causing strong postural and occulomotor responses—were too brief. To address this issue we activated vestibular afferents using continuous sinusoidal (0.5–0.8 Hz, 60–100 cycles, ±2 mA) bipolar binaural GVS in 11 seated subjects. Sinusoidal GVS evoked robust vestibular illusions of “rocking in a boat” or “swinging from side to side.” Cross-correlation analysis revealed a cyclic modulation of MSNA ranging from 31 to 86% across subjects (mean ± SE 58 ± 5%), with total MSNA increasing by 156 ± 19% (P = 0.001). Furthermore, we documented de novo synthesis of sympathetic bursts that were coupled to the sinusoidal input, such that two bursts—rather than the obligatory single burst—could be generated within a cardiac interval. This demonstrates that the human vestibular apparatus exerts a potent facilitatory influence on MSNA that potentially operates independently of the baroreceptor system.     

Effects of deuterium oxide and galvanic vestibular stimulation on visual cortical cell function

/he spontaneous and evoked unit activities of complex visual cortical cells were recorded from Brodmann's area 18 in immobilized, unanesthetized cats before, during, and after stimulation of the vestibular system. The vestibular system was stimulated by intravenous injection of deuterium oxide (D2O)--a noted nystagmogenic agent (14)--or by direct galvanic stimulation of the labyrinth. Measures of the receptive-field areas, poststimulus time histograms, directional preferences, and the optimal speed of the light bar stimulating the cell were obtained before and after the application of D2O. Directional preferences were determined in a novel manner, using a method derived from a hierarchical clustering technique (19). Data were collected and analyzed from a) visual cortical cells in cats with intact labyrinths, b) visual cortical cells in cats following bilateral labrinthectomies, and c) nonvisual cortical cells in cats with intact labyrinths. In cats with intact labyrinths, D2O changed the optimal length of the light bar that was able to stimulate the cortical cell as well as the path on which it evoked the response of the cell. Both values, which constitute the receptive field of the cell, changed approximately proportionately. This effect usually lasts for less than 4.5 h. The other cellular characteristics were also altered by the D2O. Galvanic stimulation of the labyrinth resembles, in its effects, the injection of D2O. In labyrinth-intact cats, the time course of area 18 spontaneous activity dramatically increased 30 min or more after D2O was administered. It peaked 2-3 h later and still had not returned to preinjection levels even 7 h after the D2O administration. In bilaterally labyrinthectomized cats, the spontaneous activity of the visual cells (and the other cellular characteristics studied) did not change following D2O administration. In nonvisual cells from labyrinth-intact cats, the spontaneous activity demonstrated a slight but significant decrease over time after D2O injection. (The other measures, however, did not change.) In pilot studies (about 2 wk prior to the electrophysiological experiments), the cats were injected with D2O. Within 8-10 min afterward, signs of positional nystagmus commenced; and within 30 min, problems in maintaining balance were noted. This continued for 7-8 h before disappearing. In the labyrinthectomized animals, such effects were not observed. These results, therefore, add support to other evidence that suggests that D2O works directly through the vestibular apparatus to produce the effects it does (and not through interference with certain cellular processes).(ABSTRACT TRUNCATED AT 400 WORDS)     

Treadmill validation of an over-ground walking test to predict peak oxygen consumption

Summary The purpose of this study was to determine whether a test developed to predict maximal oxygen consumption (VO_2max) during over-ground walking, was similarly valid as a predictor of peak oxygen consumption (VO₂) when administered during a 1-mile (1.61 km) treadmill walk. Treadmill walk time, mean heart rate over the last 2 full min of the walk test, age, and body mass were entered into both generalized (GEN Eq.) and gender-specific (GSP Eq.) prediction equations. Overall results indicated a highly significant linear relationship between observed peakVO₂ and GEN Eq. predicted values (r=0.91), a total error (TE) of 5.26 ml · kg^–1 · min^–1 and no significant difference between observed and predicted peakVO₂ mean values. The peakVO₂ for women (n = 75) was predicted accurately by GSP Eq. (r = 0.85; TE = 4.5 ml · kg^–1 · min^–1), but was slightly overpredicted by GEN Eq. (overall mean difference = 1.4 ml · kg^–1 · min^–1;r=0.86; TE = 4.56 ml · kg^–1 · min^–1). No significant differences between observed peakVO₂ and either GEN Eq. (r=0.85; TE=4.3 ml · kg^–1 · min^–1) or GSP Eq. (r=0.85; TE = 4.8 ml · kg^–1 · min^–1)predicted values were noted for men (n=48) with peakVO₂ values less than or equal to 55 ml · kg^–1 · min^–1. However, both equations significantly underpredicted peakVO₂ for the remaining high peakVO₂ men (n = 22). In conclusion, the over-ground walking test, when administered on a treadmill, is a valid method of predicting peakVO₂ but underpredicts peakVO₂ of subjects with observed high peakVO₂ values. Present address: Human Performance Laboratory State University, Muncie, IN 47306, USA     

Low-frequency galvanic vestibular stimulation evokes two peaks of modulation in skin sympathetic nerve activity

We have previously shown that sinusoidal galvanic vestibular stimulation (sGVS), delivered bilaterally at 0.2–2.0 Hz, evokes a potent entrainment of sympathetic outflow to muscle and skin. Most recently, we showed that stimulation at 0.08–0.18 Hz generates two bursts of modulation of muscle sympathetic nerve activity (MSNA), more pronounced at 0.08 Hz, which we interpreted as reflecting bilateral projections from the vestibular nuclei to the medullary nuclei responsible for the generation of MSNA. Here, we test the hypothesis that these very low frequencies of sGVS modulate skin sympathetic nerve activity (SSNA) in a similar fashion. SSNA was recorded via tungsten microelectrodes inserted into the left common peroneal nerve in 11 awake-seated subjects. Bipolar binaural sGVS (±2 mA, 100 cycles) was applied to the mastoid processes at 0.08, 0.13 and 0.18 Hz. As with MSNA, cross-correlation analysis revealed two bursts of modulation of SSNA for each cycle of stimulation but, unlike MSNA, this modulation was equally pronounced at all frequencies. These results further support our conclusion that bilateral sGVS causes cyclical modulation of the left and right vestibular nerves and a resultant modulation of sympathetic outflow that reflects the summed activity of bilateral projections from the vestibular nuclei onto, in this case, the primary output nuclei responsible for SSNA—the medullary raphé. Furthermore, these findings emphasise the role of the vestibular system in the control of skin sympathetic outflow, and the cutaneous expression of motion sickness: pallor and sweat release. Indeed, vestibular modulation of SSNA was higher in those subjects reporting nausea than in those who did not report nausea during this low-frequency sGVS.     

The integration of neural information by a passive kinetic stimulus and galvanic vestibular stimulation in the lateral vestibular nucleus

Despite an easy control and the direct effects on vestibular neurons, the clinical applications of galvanic vestibular stimulation (GVS) have been restricted because of its unclear activities as input. On the other hand, some critical conclusions have been made in the peripheral and the central processing of neural information by kinetic stimuli with different motion frequencies. Nevertheless, it is still elusive how the neural responses to simultaneous GVS and kinetic stimulus are modified during transmission and integration at the central vestibular area. To understand how the neural information was transmitted and integrated, we examined the neuronal responses to GVS, kinetic stimulus, and their combined stimulus in the vestibular nucleus. The neuronal response to each stimulus was recorded, and its responding features (amplitude and baseline) were extracted by applying the curve fitting based on a sinusoidal function. Twenty-five (96.2%) comparisons of the amplitudes showed that the amplitudes decreased during the combined stimulus (p < 0.001). However, the relations in the amplitudes (slope = 0.712) and the baselines (slope = 0.747) were linear. The neuronal effects by the different stimuli were separately estimated; the changes of the amplitudes were mainly caused by the kinetic stimulus and those of the baselines were largely influenced by GVS. Therefore, the slopes in the comparisons implied the neural sensitivity to the applied stimuli. Using the slopes, we found that the reduced amounts of the neural information were transmitted. Overall, the comparisons of the responding features demonstrated the linearity and the subadditivity in the neural transmission.     

Effects of galvanic vestibular stimulation on postural limb reflexes and neurons of spinal postural network

Quadrupeds maintain the dorsal side up body orientation due to the activity of the postural control system driven by limb mechanoreceptors. Binaural galvanic vestibular stimulation (GVS) causes a lateral body sway toward the anode. Previously, we have shown that this new position is actively stabilized, suggesting that GVS changes a set point in the reflex mechanisms controlling body posture. The aim of the present study was to reveal the underlying neuronal mechanisms. Experiments were performed on decerebrate rabbits. The vertebral column was rigidly fixed, whereas hindlimbs were positioned on a platform. Periodic lateral tilts of the platform caused postural limb reflexes (PLRs): activation of extensors in the loaded and flexing limb and a decrease in extensor activity in the opposite (unloaded and extending) limb. Putative spinal interneurons were recorded in segments L4-L5 during PLRs, with and without GVS. We have found that GVS enhanced PLRs on the cathode side and reduced them on the anode side. This asymmetry in PLRs can account for changes in the stabilized body orientation observed in normal rabbits subjected to continuous GVS. Responses to platform tilts (frequency modulation) were observed in 106 spinal neurons, suggesting that they can contribute to PLR generation. Two neuron groups were active in opposite phases of the tilt cycle of the ipsi-limb: F-neurons in the flexion phase, and E-neurons in the extension phase. Neurons were driven mainly by afferent input from the ipsi-limb. If one supposes that F- and E-neurons contribute, respectively, to excitation and inhibition of extensor motoneurons, one can expect that the pattern of response to GVS in F-neurons will be similar to that in extensor muscles, whereas E-neurons will have an opposite pattern. We have found that ~40% of all modulated neurons meet this condition, suggesting that they contribute to the generation of PLRs and to the GVS-caused changes in PLRs.     

Between-subject variability and within-subject reliability of the human eye-movement response to bilateral galvanic (DC) vestibular stimulation

Recent studies have shown that responses to surface galvanic vestibular stimulation (GVS) show substantial interindividual variation. Between-subject variability may be due to individual differences between subjects, or to the poor reliability of the test, or to differences in test details, or to host factors. The aim of the present study was to compare variability between and within subjects in binocular 3-D eye-movement responses to long-duration, maintained, large-amplitude, bilateral, bipolar, surface GVS. Subjects were seated and restrained, and in one condition fixated a small, centrally located visual target; in the other condition, testing was carried out in complete darkness. Surface GVS of 5 mA, with a rectangular waveform was delivered bilaterally for 5 min while eye movements were measured using computerised video-oculography (VTM). In the first experiment, ten subjects participated in both conditions in one session, and in the second experiment, two subjects participated in both conditions for a total of five repeated sessions. The stimulation was well tolerated by all subjects and produced a change in torsional position with the upper pole of both eyes rolling towards the anode and away from the cathode in all subjects in both conditions. Although little vertical nystagmus was evident in either condition, most subjects showed relatively strong horizontal nystagmus (slow phases towards the anode) in darkness. This study confirms previous observations that the torsional response to GVS is highly variable between subjects, whilst also showing for the first time that eye-movement responses to GVS show good within-subject repeatability. This study also demonstrates considerable between-subject variability in the relative ratios of response components (torsional and horizontal nystagmus, torsional position), whereas the relatively small within-subject variability can be characterised more by changes in the overall amplitude of the eye-movement response. Subjects show idiosyncratic oculomotor response patterns to GVS, varying slightly in absolute magnitude between sessions. Thus, GVS may be a more reliable stimulus than may have been anticipated from the literature.