Galvanic Vestibular Stimulation Modifies Vection Paths in Healthy Subjects

Caloric vestibular stimulation induces vestibular circular vection even with a conflicting visual display presented in a virtual reality headset

This study explored visual-vestibular sensory integration when the vestibular system receives self-motion information using caloric irrigation. The objectives of this study were to (1) determine if measurable vestibular circular vection can be induced in healthy participants using caloric vestibular stimulation and (2) determine if a conflicting visual display could impact vestibular vection. In Experiment 1 (E1), participants had their eyes closed. Air caloric vestibular stimulation cooled the endolymph fluid of the horizontal semi-circular canal inducing vestibular circular vection. Participants reported vestibular circular vection with a potentiometer knob that measured circular vection direction, speed, and duration. In Experiment 2 (E2), participants viewed a stationary display in a virtual reality headset that did not signal self-motion while receiving caloric vestibular stimulation. This produced a visual-vestibular conflict. Participants indicated clockwise vection in the left ear and counter-clockwise vection in right ear in a significant proportion of trials in E1 and E2. Vection was significantly slower and shorter in E2 compared to E1. E2 results demonstrated that during visual-vestibular conflict, visual and vestibular cues are used to determine self-motion rather than one system overriding the other. These results are consistent with optimal cue integration hypothesis.

Quantifying virtual self-motion sensations induced by galvanic vestibular stimulation

BACKGROUND

The vestibular system provides a comprehensive estimate of self-motion in 3D space. Widely used to artificially stimulate the vestibular system, binaural-bipolar square-wave Galvanic Vestibular Stimulation (GVS) elicits a virtual sensation of roll rotation. Postural responses to GVS have been clearly delineated, however quantifying the perceived virtual rotation vector has not been fully realised.

OBJECTIVE

We aimed to quantify the perceived virtual roll rotation vector elicited by GVS using a psychophysical approach on a 3D turntable.

METHODS

Participants were placed supine on the 3D turntable and rotated around the naso-occipital axis while supine and received square-wave binaural-bipolar GVS or sham stimulation. GVS amplitudes and intensities were systematically manipulated. The turntable motion profile consisted of a velocity step of 20°/s2 until the trial velocity between 0-20°/s was reached, followed by a 1°/s ramp until the end of the trial. In a psychophysical adaptive staircase procedure, we systematically varied the roll velocity to identify the exact velocity that cancelled the perceived roll sensation induced by GVS.

RESULTS

Participants perceived a virtual roll rotation towards the cathode of approximately 2°/s velocity for 1 mA GVS and 6°/s velocity for 2.5 mA GVS. The observed values were stable across repetitions.

CONCLUSIONS

Our results quantify for the first time the perceived virtual roll rotations induced by binaural-bipolar square-wave GVS. Importantly, estimates were based on perceptual judgements, in the absence of motor or postural responses and in a head orientation where the GVS-induced roll sensation did not interact with the perceived direction of gravity. This is an important step towards applications of GVS in different settings, including sensory substitution or Virtual Reality.

Antero-Posterior vs. Lateral Vestibular Input Processing in Human Visual Cortex

Visuo-vestibular integration is crucial for locomotion, yet the cortical mechanisms involved remain poorly understood. We combined binaural monopolar galvanic vestibular stimulation (GVS) and functional magnetic resonance imaging (fMRI) to characterize the cortical networks activated during antero-posterior and lateral stimulations in humans. We focused on functional areas that selectively respond to egomotion-consistent optic flow patterns: the human middle temporal complex (hMT+), V6, the ventral intraparietal (VIP) area, the cingulate sulcus visual (CSv) area and the posterior insular cortex (PIC). Areas hMT+, CSv, and PIC were equivalently responsive during lateral and antero-posterior GVS while areas VIP and V6 were highly activated during antero-posterior GVS, but remained silent during lateral GVS. Using psychophysiological interaction (PPI) analyses, we confirmed that a cortical network including areas V6 and VIP is engaged during antero-posterior GVS. Our results suggest that V6 and VIP play a specific role in processing multisensory signals specific to locomotion during navigation.

Inducing circular vection with tactile stimulation encircling the waist

In general, moving sensory stimuli (visual and auditory) can induce illusory sensations of self-motion (i.e. vection) in the direction opposite of the sensory stimulation. The aim of the current study was to examine whether tactile stimulation encircling the waist could induce circular vection (around the body's yaw axis) and to examine whether this type of stimulation would influence participants' walking trajectory and balance. We assessed the strength and direction of perceived self-motion while vision was blocked and while either receiving tactile stimulation encircling the waist clockwise or counterclockwise or no tactile stimulation. Additionally, we assessed participants' walking trajectory and balance while receiving these different stimulations. Tactile stimulation encircling the waist was found to lead to self-reported circular vection in a subset of participants. In this subset of participants, circular vection was on average experienced in the same direction as the tactile stimulation. Additionally, perceived rotatory self-motion in participants that reported circular vection correlated with balance (i.e., sway velocity and the standard error of the mean in the medio-lateral dimension). The fact that, in this subset of participants, subjective reports of vection correlated with objective outcome measures indicates that tactile stimulation encircling the waist might indeed be able to induced circular vection.

Cortical Correlates of the Simulated Viewpoint Oscillation Advantage for Vection

Behavioural studies have consistently found stronger vection responses for oscillating, compared to smooth/constant, patterns of radial flow (the simulated viewpoint oscillation advantage for vection). Traditional accounts predict that simulated viewpoint oscillation should impair vection by increasing visual–vestibular conflicts in stationary observers (as this visual oscillation simulates self-accelerations that should strongly stimulate the vestibular apparatus). However, support for increased vestibular activity during accelerating vection has been mixed in the brain imaging literature. This fMRI study examined BOLD activity in visual (cingulate sulcus visual area — CSv; medial temporal complex — MT+; V6; precuneus motion area — PcM) and vestibular regions (parieto-insular vestibular cortex — PIVC/posterior insular cortex — PIC; ventral intraparietal region — VIP) when stationary observers were exposed to vection-inducing optic flow (i.e., globally coherent oscillating and smooth self-motion displays) as well as two suitable control displays. In line with earlier studies in which no vection occurred, CSv and PIVC/PIC both showed significantly increased BOLD activity during oscillating global motion compared to the other motion conditions (although this effect was found for fewer subjects in PIVC/PIC). The increase in BOLD activity in PIVC/PIC during prolonged exposure to the oscillating (compared to smooth) patterns of global optical flow appears consistent with vestibular facilitation.

Vection Latency Is Reduced by Bone-Conducted Vibration and Noisy Galvanic Vestibular Stimulation

Studies of the illusory sense of self-motion elicited by a moving visual surround (‘vection’) have revealed key insights about how sensory information is integrated. Vection usually occurs after a delay of several seconds following visual motion onset, whereas self-motion in the natural environment is perceived immediately. It has been suggested that this latency relates to the sensory mismatch between visual and vestibular signals at motion onset. Here, we tested three techniques with the potential to reduce sensory mismatch in order to shorten vection onset latency: noisy galvanic vestibular stimulation (GVS) and bone conducted vibration (BCV) at the mastoid processes, and body vibration applied to the lower back. In Experiment 1, we examined vection latency for wide field visual rotations about the roll axis and applied a burst of stimulation at the start of visual motion. Both GVS and BCV reduced vection latency by two seconds compared to the control condition, whereas body vibration had no effect on latency. In Experiment 2, the visual stimulus rotated about the pitch, roll, or yaw axis and we found a similar facilitation of vection by both BCV and GVS in each case. In a control experiment, we confirmed that air-conducted sound administered through headphones was not sufficient to reduce vection onset latency. Together the results suggest that noisy vestibular stimulation facilitates vection, likely due to an upweighting of visual information caused by a reduction in vestibular sensory reliability.

Future challenges for vection research: definitions, functional significance, measures, and neural bases

This paper discusses four major challenges facing modern vection research. Challenge 1 (Defining Vection) outlines the different ways that vection has been defined in the literature and discusses their theoretical and experimental ramifications. The term vection is most often used to refer to visual illusions of self-motion induced in stationary observers (by moving, or simulating the motion of, the surrounding environment). However, vection is increasingly being used to also refer to non-visual illusions of self-motion, visually mediated self-motion perceptions, and even general subjective experiences (i.e., “feelings”) of self-motion. The common thread in all of these definitions is the conscious subjective experience of self-motion. Thus, Challenge 2 (Significance of Vection) tackles the crucial issue of whether such conscious experiences actually serve functional roles during self-motion (e.g., in terms of controlling or guiding the self-motion). After more than 100 years of vection research there has been surprisingly little investigation into its functional significance. Challenge 3 (Vection Measures) discusses the difficulties with existing subjective self-report measures of vection (particularly in the context of contemporary research), and proposes several more objective measures of vection based on recent empirical findings. Finally, Challenge 4 (Neural Basis) reviews the recent neuroimaging literature examining the neural basis of vection and discusses the hurdles still facing these investigations.

Impact of the motion and visual complexity of the background on players' performance in video game-like displays

The visual interfaces of virtual environments such as video games often show scenes where objects are superimposed on a moving background. Three experiments were designed to better understand the impact of the complexity and/or overall motion of two types of visual backgrounds often used in video games on the detection and use of superimposed, stationary items. The impact of background complexity and motion was assessed during two typical video game tasks: a relatively complex visual search task and a classic, less demanding shooting task. Background motion impaired participants' performance only when they performed the shooting game task, and only when the simplest of the two backgrounds was used. In contrast, and independently of background motion, performance on both tasks was impaired when the complexity of the background increased. Eye movement recordings demonstrated that most of the findings reflected the impact of low-level features of the two backgrounds on gaze control.

Augmenting sensory-motor conflict promotes adaptation of postural behaviors in a virtual environment

We present results from a series of studies that investigated how multimodal mismatches in a virtual environment modified postural response organization. Adaptation of motor commands to functional circumstances is driven directly by error signals. Thus, motor relearning should increase when performing in environments containing sensory mismatch. We hypothesized that kinematics of the response would be linked to specific characteristics of the sensory array. Sensory weighting was varied by: 1) rotating the visual field about the talo-crural joint or the interaural axis, 2) adding stochastic vibrations at the sole of the foot, and 3) combining galvanic vestibular stimulation with rotations of the visual field. Results indicated that postural responses are shaped by the location of a sensory disturbance and also by the processing demands of the environmental array. Sensory-motor demands need to be structured when developing therapeutic interventions for patients with balance disorders.

Effects of active and passive viewpoint jitter on vection in depth

Recent studies have shown that the vection in depth experienced by stationary observers viewing constant velocity radial flow can be enhanced by adding simulated viewpoint jitter/oscillation. This study examined the effect of manipulating visual-vestibular conflict on the perceived strength and speed of vection in depth. Four conditions were examined: (i) radial flow without viewpoint jitter viewed by stationary observers (consistent visual-vestibular inputs); (ii) radial flow with viewpoint jitter synchronized to lateral head oscillation (consistent inputs); (iii) radial flow with viewpoint jitter viewed by stationary observers (inconsistent inputs); (iv) radial flow without viewpoint jitter viewed during head oscillation (inconsistent inputs). We found that the strength and perceived speed of vection in depth was always greater when simulated viewpoint jitter was introduced. No further vection enhancement was found when this jitter was generated by active head oscillation-even though passive jitter conditions should have generated significant sensory conflicts, whereas active jitter conditions would not. Active head oscillation without display jitter also had little effect, producing similar vection strength/speed ratings to stationary observation of non-jittering optic flow. Horizontal eye tracking suggested that retinal stimulation was similar between comparable active and passive viewing conditions. This stabilization of the retinal image across active and passive conditions appeared to be due to cooperative engagement of the translational vestibuloocular reflex and the visually driven ocular following response. Rather than providing evidence for synergistic integration of self-motion perception, these findings obtained with low-frequency sensory stimuli suggest that self-motion perception is dominated by visual processing centres.