The effects of galvanic stimulation on the human vestibulo-ocular reflex

Reduction of cybersickness during and immediately following noisy galvanic vestibular stimulation

The mechanism underlying cybersickness during virtual reality (VR) exposure is still poorly understood, although research has highlighted a causal role for visual-vestibular sensory conflict. Recently established methods for reducing cybersickness include galvanic vestibular stimulation (GVS) to mimic absent vestibular cues in VR, or vibration of the vestibular organs to add noise to the sensory modality. Here, we examined if applying noise to the vestibular system using noisy-current GVS affects sickness severity in VR. Participants were exposed to one of the two VR games that were classified as either moderately or intensely nauseogenic. The VR content lasted for 50 min and was broken down into three blocks: 30 min of gameplay during exposure to either noisy GVS (± 1750 μA) or sham stimulation (0 μA), and 10 min of gameplay before and after this block. We characterized the effects of noisy GVS in terms of post-minus-pre-exposure cybersickness scores. In the intense VR condition, we found a main effect of noisy vestibular stimulation on a verbal cybersickness scale, but not for questionnaire measures of cybersickness. Participants reported lower cybersickness scores during and directly after exposure to GVS. However, this difference was quickly extinguished (~ 3-6 min) after further VR exposure, indicating that sensory adaptation did not persist after stimulation was terminated. In contrast, there were no differences between the sham and GVS group for the moderate VR content. The results show the potential for reducing cybersickness with non-invasive sensory stimulation. We address possible mechanisms for the observed effects, including noise-induced sensory re-weighting.

Ocular torsion responses to sinusoidal electrical vestibular stimulation

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We measured ocular torsion responses to sinusoidal Electrical Vestibular Stimulation.

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Responses were observed at all frequencies from 0.05 to 20 Hz.

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Gain and phase analysis suggest the stimulus is interpreted by the CNS as velocity.

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Our non-invasive method assesses torsional VOR at frequencies impossible with natural stimuli.

Background

Eye movements evoked by electrical vestibular stimulation (EVS) offer potential for diagnosing vestibular dysfunction. However, ocular recording techniques are often too invasive or impractical for routine clinical use. Furthermore, the kinematic nature of the EVS signal is not fully understood in terms of movement sensations.

New method

We apply sinusoidal EVS stimuli varying from 0.05 to 20 Hz, and record the eye in darkness using an infrared camera. Eye movement was measured offline using commercially available software to track iris striations. Response gain and phase were calculated separately for eye position, velocity and acceleration across all frequencies, to determine how the brain interprets the EVS signal.

Results

Ocular torsion responses were observed at the same frequency as the stimulus, for all frequencies, while lateral/vertical responses were minimal or absent. Response gain and phase resembled previously reported responses to natural rotation, but only when analysing eye velocity, not position or acceleration.

Comparison with existing method(s)

Our method offers a simple, affordable, reliable and non-invasive method for tracking the ocular response to EVS. It is more convenient than scleral coil recordings, or marking the sclera to aid video tracking. It also allows us to assess the torsional VOR at frequencies not possible with natural stimuli.

Conclusions

Ocular torsion responses to EVS can be readily assessed using sinusoidal stimuli combined with an infrared camera. Gain and phase analysis suggests that the central nervous system interprets the stimulus as head roll velocity. Future work will assess the diagnostic potential for patients with vestibular disorders.

Core Body Temperature Effects on the Mouse Vestibulo-ocular Reflex

Core body temperature has been shown to affect vestibular end-organ and nerve afferents so that their resting discharge rate and sensitivity increase with temperature. Our aim was to determine whether these changes observed in extracellular nerve recordings of anaesthetized C57BL/6 mice corresponded to changes in the behavioural vestibulo-ocular reflex (VOR) of alert mice. The VOR drives eye rotations to keep images stable on the retina during head movements. We measured the VOR gain (eye velocity/head velocity) and phase (delay between vestibular stimulus and response) during whole-body sinusoidal rotations ranging 0.5-12 Hz with peak velocity 50 or 100 °/s in nine adult C57BL/6 mice. We also measured the VOR during whole-body transient rotations with acceleration 3000 or 6000 °/s² reaching a plateau of 150 or 300 °/s. These measures were obtained while the mouse's core body temperature was held at either 32 or 37 °C for at least 35 min before recording. The temperature presentation order and timing were pseudo-randomized. We found that a temperature increase from 32 to 37 °C caused a significant increase in sinusoidal VOR gain of 17 % (P < 0.001). Temperature had no other effects on the behavioural VOR. Our data suggest that temperature effects on regularly firing afferents best correspond to the changes that we observed in the VOR gain.

Transmastoid galvanic stimulation does not affect the vergence-mediated gain increase of the human angular vestibulo-ocular reflex

Vergence is one of several viewing contexts that require an increase in the angular vestibular-ocular reflex (aVOR) response. A previous monkey study found that the vergence-mediated gain (eye/head velocity) increase of the aVOR was attenuated by 64 % when anodic currents, which preferentially lower the activity of irregularly firing vestibular afferents, were delivered to both labyrinths. We sought to determine whether there was similar evidence implicating a role for irregular afferents in the vergence-mediated gain increase of the human aVOR. Our study is based upon analysis of the aVOR evoked by head rotations, delivered passively while subjects viewed a near (15 cm) or far (124 cm) target and applying galvanic vestibular stimulation (GVS) via surface electrodes. We tested 12 subjects during 2-3 sessions each. Vestibular stimuli consisted of passive whole-body rotations (sinusoids from 0.05-3 Hz and 12-25°/s, and transients with peak ~15°, 50°/s, 500°/s(2)) and head-on-body impulses (peak ~30°, 150°/s, 3,000°/s(2)). GVS was on for 10 s every 20 s. All polarity combinations were tested, with emphasis on uni- and bi-lateral anodic inhibition. The average stimulus current was 5.9 ± 1.6 mA (range: 3-9.5 mA), vergence angle (during near viewing) was 22.6 ± 2.8° and slow-phase eye velocity caused by left anodic current stimulation with head stationary was -3.4 ± 1.1°/s, -0.2 ± 0.6°/s and 2.5 ± 1.4°/s (torsion, vertical, horizontal). No statistically significant GVS effects were observed, suggesting that surface electrode GVS has no effect on the vergence-mediated gain increase of the aVOR at the current levels (~6 mA) tolerated by most humans. We conclude that clinically practical transmastoid GVS does not effectively silence irregular afferents and hypothesize that currents >10 mA are needed to reproduce the monkey results.

Probing residual vestibular function with galvanic stimulation in vestibular loss patients

INTRODUCTION

Galvanic vestibular stimulation (GVS) was used to probe the residual vestibular function in unilateral vestibular loss.

METHODS

39 control subjects, 60 patients areflexic to caloric tests, 21 patients with pre-operative acoustic neurinoma, and 26 patients with surgical deafferentation were tested with 4mA trapezoidal GVS. Horizontal slow phase velocity and vertical slow phase velocity components of the slow phase of induced nystagmus were recorded with videonystagmography.

RESULTS

All patients had lower horizontal slow phase velocity compared with intact subject. In caloric areflexic patients, the intact ear responded similarly to controls. The lesioned ear was less excitable but could be inhibited as in controls. In patients with acoustic neurinoma, the intact ear was normally excitable but less sensitive to inhibition. The lesioned ear, although not excitable, could be inhibited. In patients with surgical deafferentation, the intact ear could be barely excited and not inhibited. The lesioned ear could not be excited but slightly inhibited. These results fit well with previous data in animal models after unilateral vestibular loss.

CONCLUSION

Unilateral vestibular patients exhibit a progressive deterioration of the horizontal canal function, corresponding to 4 different patterns of GVS response. This study confirms that GVS is useful to probe evolution of the residual vestibular function in patients.

Vestibulospinal responses in motor incomplete spinal cord injury

OBJECTIVE

Postural instability limits ambulatory capacity in patients with spinal cord injury (SCI). Galvanic vestibular stimulation (GVS) was used to investigate the integrity of vestibulospinal pathways and related changes in postural responses in SCI.

METHODS

Binaural bipolar galvanic stimuli of 400 ms duration and 3 mA intensity were applied in 8 motor incomplete SCI and 8 control subjects who stood facing towards the left. EMG responses were recorded from the right soleus muscle and the trajectory of the centre of pressure (CoP) was measured with a force plate.

RESULTS

There was no difference in excitability and amplitude of the responses between the groups. However, the latency and duration of the medium latency EMG response and all CoP responses were significantly longer in the SCI group. Additionally, postural stability was reduced in the SCI group, as shown by a greater tendency to fall due to GVS.

CONCLUSIONS

Despite early EMG responses proving the basic connectivity of the direct vestibulospinal pathways, the delayed GVS responses suggest a vestibulospinal deficit in the SCI subjects.

SIGNIFICANCE

GVS can be applied in incomplete SCI to supplement the neurological examination by revealing changes in vestibulospinal responses and impairment of postural stability.

Feedforward versus feedback modulation of human vestibular-evoked balance responses by visual self-motion information

Visual information modulates the balance response evoked by a pure vestibular perturbation (galvanic vestibular stimulation, GVS). Here we investigate two competing hypotheses underlying this visual-vestibular interaction. One hypothesis assumes vision acts in a feedforward manner by altering the weight of the vestibular channel of balance control. The other assumes vision acts in a feedback manner through shifts in the retinal image produced by the primary response. In the first experiment we demonstrate a phenomenon that is predicted by both hypotheses: the GVS-evoked balance response becomes progressively smaller as the amount of visual self-motion information is increased. In the second experiment we independently vary the pre-stimulus and post-stimulus visual environments. The rationale is that feedback effects would depend only upon the post-stimulus visual environment. Although the post-stimulus visual environment did affect later parts of the response (after approximately 400 ms), the pre-stimulus visual environment had a strong influence on the size of the early part of the response. We conclude that both feedforward and feedback mechanisms act in concert to modulate the GVS-evoked response. We suggest this dual interaction that we observe between visual and vestibular channels is likely to apply to all sensory channels that contribute to balance control.

EMG responses evoked by the termination of galvanic (DC) vestibular stimulation: 'off-responses'

OBJECTIVE

Vestibular responses in soleus electromyography (EMG) evoked by the sudden onset of galvanic (DC) stimulation ('on-responses') have been described in detail previously. The aim of the present study was to describe responses in soleus triggered by the termination of galvanic stimulation ('off-responses').

METHODS

In 10 healthy human subjects, we studied responses to transmastoid (bilateral) stimuli of 200 ms and 2 s average duration and 3 or 4 mA intensity. We obtained both on- and off-responses using the same raw data. EMG activity was recorded onto tape while current pulses of systematically varying duration were delivered. Averaged on-responses were obtained by triggering from the beginning of the current pulses. Averaged off-responses were obtained by triggering from the termination of the current pulses.

RESULTS

Short-latency (SL) and medium latency (ML) off-responses were both obtained in all but one study. The SL and the ML components of the off-responses were present and had similar latencies and amplitudes, but opposite excitability, to the on-responses obtained with the same stimuli.

CONCLUSIONS

Off-responses to galvanic vestibular stimulation can be recorded from soleus EMG. Our findings imply that vestibular SL and ML reflex responses in the legs are dependent on the change in the rate of vestibular nerve discharge, not its absolute level. Both on- and off-responses have properties appropriate to a role in maintaining body stability.

Short-latency eye movements evoked by near-threshold galvanic vestibular stimulation

To investigate whether the primary planes of eye and body responses to galvanic vestibular stimulation (GVS) are congruent, we have measured the binocular, three-dimensional eye movements (scleral coil technique) to bilateral bipolar GVS in six normal human subjects. Stimulation intensities were kept deliberately low in order to characterize the response to near-threshold intensities of stimulation (0.1-0.9 mA) that had been used previously to characterise body postural responses. Stimuli were applied for 4 s, but only the early responses that occurred within the initial 300 ms of turning the current on or off were measured. At intensities of 0.1-0.7 mA the 'on' response consisted almost exclusively of a torsional slow phase eye movement in which the top of the eyes rotated towards the anode. The latency of the torsional response was ca. 46 ms. A weak polarity-dependent disconjugate response was also observed in which the intorting eye elevated and the extorting eye depressed ('skew eye deviation'). When the current was turned off similar responses occurred in the reverse direction. Removal of the visual fixation light-emitting diode (LED) had no consistent effect on the short-latency ocular responses. The direction of the ocular response was similar to that of the postural response and is compatible with GVS inducing an apparent dynamic roll-tilt of the head towards the cathode. However, weak horizontal eye movements, which became more prominent as the stimulus intensity was increased to 0.9 mA, were also observed. This suggests that an additional weak rotational component about the yaw axis, or a component of lateral translation in the frontal plane, is contained in the GVS-evoked signal. The overall pattern of eye movement suggests that semicircular canal afferents contribute to these low-intensity GVS responses.

Galvanic and acoustic vestibular stimulation activate different populations of vestibular afferents

OBJECTIVE

To deduce whether similar or distinct populations of vestibular afferents are activated by acoustic and galvanic vestibular stimulation by comparing the effectiveness of 'matched' stimuli in eliciting vestibulospinal reflexes.

METHODS

Twelve subjects (5 men, 7 women) underwent individual 'matching' of 2 ms tone burst and galvanic stimuli, using vestibulocollic reflexes so that corrected reflex amplitudes to tone burst and galvanic stimuli were within 10% of each other. These same intensities were then administered using 20 ms durations to determine whether they were equally effective in evoking vestibulospinal responses.

RESULTS

Corrected reflex amplitudes for vestibulocollic responses to tone burst and galvanic stimulation were not significantly different for the right (P=0.45) or left (P=0.68) sides. All subjects had vestibulospinal responses to galvanic stimulation (average intensity 4.0 mA for both sides). The short latency (SL) and medium latency (ML) components of the vestibulospinal reflexes were larger after galvanic compared to tone burst stimulation in 11 of 12 subjects (P<0.01).

CONCLUSIONS

Despite evoking equal-sized vestibulocollic reflexes, there was a clear dissociation between the magnitude of tone burst and galvanic-induced vestibulospinal reflexes. Galvanic stimulation evoked SL and ML reflexes in all subjects. Tone burst stimuli evoked only small SL reflexes and, in most cases, no ML reflexes. Acoustically-evoked vestibulocollic reflexes are likely to be due to saccular excitation. The limited effectiveness of longer tone burst stimuli to evoke ML vestibulospinal reflexes suggests that saccular afferents have, at most, only a minor role in the production of these reflexes. We conclude that galvanic stimulation is more effective in eliciting vestibulospinal reflexes than tone burst stimulation, and that the two methods activate different populations of vestibular afferents.

Galvanic vestibular stimulation evokes sensations of body rotation

Psychophysical experiments identified effects of galvanic vestibular stimulation (GVS) on the perception of whole-body angular rotation. Subjects lay supine on a platform that could rotate about a vertical axis through the vestibular axis so that linear movements were excluded. Movements were applied sufficiently above perception threshold to enable a reliable report of direction and movement size. In some trials, binaural GVS was applied concurrently at 1-2 mA. When GVS that was incongruent with the movement was applied, subjects reported lesser spin, on average cancelling the movement perception. When the GVS and movement were congruent, subjects reported greater spin. We conclude that GVS produces a vestibular signal of rotation, probably though an effect on semicircular canals.