Galvanic Vestibular Stimulation Advancements for Spatial Disorientation Training

INTRODUCTION: Spatial disorientation (SD) remains the leading contributor to Class A mishaps in the U.S. Navy, consistent with historical trends. Despite this, SD training for military aircrew is largely confined to the classroom and experiential training replicating SD illusions is limited and infrequent. Static flight simulators are most commonly used for training but offer no vestibular stimulation to the flight crew, omitting the source of vestibular-mediated SD.BACKGROUND: We first cover vestibular-mediated SD illusions which may be replicated through galvanic vestibular stimulation (GVS) in a static environment. GVS is a safe, reliable, low-cost avenue for providing vestibular sensory stimulation. We review the underlying mechanisms of GVS such as the excitement and inhibition of the afferent neurons innervating the vestibular system, particularly in the binaural bipolar electrode montage.APPLICATIONS: Two approaches for how GVS may be used to enhance SD training are examined. The first is a means for providing unreliable vestibular sensory perceptions to pilots, and the second details how GVS can be leveraged for replicating vestibular-mediated SD illusions.DISCUSSION: We recommend GVS be pursued as an enhancement to existing SD training. The ability to disorient aircrew in the safe training environment of a static flight simulator would allow for aircrew familiarization to SD, serving as an opportunity to practice life-saving checklist items to recover from SD. A repeatable training profile that could be worn by military aircrew in a static flight simulator may afford a low-cost training solution to the number one cause of fatalities in military aviation.Allred AR, Lippert AF, Wood SJ. Galvanic vestibular stimulation advancements for spatial disorientation training. Aerosp Med Hum Perform. 2024; 95(7):390-398.

Frequency dependence of human thresholds: both perceptual and vestibuloocular reflex thresholds

Although perceptual thresholds have been widely studied, vestibuloocular reflex (VOR) thresholds have received less attention, so the relationship between VOR and perceptual thresholds remains unclear. We compared the frequency dependence of human VOR thresholds to human perceptual thresholds for yaw head rotation in both upright ("yaw rotation") and supine ("yaw tilt") positions, using the same human subjects and motion device. VOR thresholds were generally a little smaller than perceptual thresholds. We also found that horizontal VOR thresholds for both yaw rotation about an Earth-vertical axis and yaw tilt (yaw rotation about an Earth-horizontal axis) were relatively constant across four frequencies (0.2, 0.5, 1, and 2 Hz), with little difference between yaw rotation and yaw tilt VOR thresholds. For yaw tilt stimuli, perceptual thresholds were slightly lower at the lowest frequency and nearly constant at all other (higher) frequencies. However, for yaw rotation, perceptual thresholds increased significantly at the lowest frequency (0.2 Hz). We conclude 1) that VOR thresholds were relatively constant across frequency for both yaw rotation and yaw tilt, 2) that the known contributions of velocity storage to the VOR likely yielded these VOR thresholds that were similar for yaw rotation and yaw tilt for all frequencies tested, and 3) that the integration of otolith and horizontal canal signals during yaw tilt when supine contributes to stable perceptual thresholds, especially relative to the low-frequency perceptual thresholds recorded during yaw rotation.NEW & NOTEWORTHY We describe for the first time that human VOR thresholds differ from human forced-choice perceptual thresholds, with the difference especially evident at frequencies below 0.5 Hz. We also report that VOR thresholds are relatively constant across frequency for both yaw rotation and yaw tilt. These findings are consistent with the idea that high-pass filtering in cortical pathways impacts cognitive decision-making.

Characterization of Vestibular Perception in Patients with Persistent Postural-Perceptual Dizziness

OBJECTIVE

To assess vestibular (i.e., passive self-motion) perception in patients diagnosed with persistent postural-perceptual dizziness (PPPD).

STUDY DESIGN

Case-controlled, cross-sectional, observational investigation.

SETTING

Single-center laboratory-based study.

PATIENTS

Thirteen patients with PPPD, 13 age-matched healthy control volunteers. Of those with PPPD, eight had co-occurring vestibular migraine (VM).

INTERVENTIONS

All participants completed a vestibular threshold test battery reflecting perception with predominant inputs from ( a ) the otoliths (1-Hz interaural y -axis translation, 1-Hz superior-inferior z -axis translation), ( b ) the semicircular canals (2-Hz yaw rotation, 2-Hz tilts in the planes of the vertical canal pairs), and ( c ) and canal-otolith integration (0.5-Hz roll tilt).

MAIN OUTCOME MEASURES

Direction-recognition thresholds for each vestibular threshold test condition.

RESULTS

Across all patients with PPPD, higher thresholds for superior-inferior z -translations thresholds in comparison to age-matched healthy control participants were identified ( p < 0.001). Those patients with co-occurring VM and PPPD (PPPD/+VM) displayed significantly higher z -translation thresholds ( p = 0.006), whereas patients with PPPD without VM (PPPD/-VM) displayed significantly higher roll tilt thresholds ( p = 0.029).

CONCLUSIONS

Patients with PPPD did not display a global worsening of passive self-motion perception as quantified by vestibular perceptual thresholds. Instead, patients with PPPD displayed elevated thresholds for only roll tilt and z -translation thresholds, with the relative change in each threshold impacted by the co-occurrence of VM. Because both z -translation and roll tilt motions are reliant on accurate gravity perception, our data suggest that patients with PPPD may exhibit impaired processing of graviceptive cues.

Human vestibular perceptual thresholds - A systematic review of passive motion perception

BACKGROUND

The vestibular system detects head accelerations within 6 degrees of freedom. How well this is accomplished is described by vestibular perceptual thresholds. They are a measure of perceptual performance based on the conscious evaluation of sensory information. This review provides an integrative synthesis of the vestibular perceptual thresholds reported in the literature. The focus lies on the estimation of thresholds in healthy participants, used devices and stimulus profiles. The dependence of these thresholds on the participants clinical status and age is also reviewed. Furthermore, thresholds from primate studies are discussed.

RESULTS

Thresholds have been measured for frequencies ranging from 0.05 to 5 Hz. They decrease with increasing frequency for five of the six main degrees of freedom (inter-aural, head-vertical, naso-occipital, yaw, pitch). No consistent pattern is evident for roll rotations. For a frequency range beyond 5 Hz, a U-shaped relationship is suggested by a qualitative comparison to primate data. Where enough data is available, increasing thresholds with age and higher thresholds in patients compared to healthy controls can be observed. No effects related to gender or handedness are reported.

SIGNIFICANCE

Vestibular thresholds are essential for next generation screening tools in the clinical domain, for the assessment of athletic performance, and workplace safety alike. Knowledge about vestibular perceptual thresholds contributes to basic and applied research in fields such as perception, cognition, learning, and healthy aging. This review provides normative values for vestibular thresholds. Gaps in current knowledge are highlighted and attention is drawn to specific issues for improving the inter-study comparability in the future.

Head-centric computing for vestibular stimulation under head-free conditions

Background: The vestibular end organs (semicircular canals, saccule and utricle) monitor head orientation and motion. Vestibular stimulation by means of controlled translations, rotations or tilts of the head represents a routine manoeuvre to test the vestibular apparatus in a laboratory or clinical setting. In diagnostics, it is used to assess oculomotor postural or perceptual responses, whose abnormalities can reveal subclinical vestibular dysfunctions due to pathology, aging or drugs. Objective: The assessment of the vestibular function requires the alignment of the motion stimuli as close as possible with reference axes of the head, for instance the cardinal axes naso-occipital, interaural, cranio-caudal. This is often achieved by using a head restraint, such as a helmet or strap holding the head tightly in a predefined posture that guarantees the alignment described above. However, such restraints may be quite uncomfortable, especially for elderly or claustrophobic patients. Moreover, it might be desirable to test the vestibular function under the more natural conditions in which the head is free to move, as when subjects are tracking a visual target or they are standing erect on the moving platform. Here, we document algorithms that allow delivering motion stimuli aligned with head-fixed axes under head-free conditions. Methods: We implemented and validated these algorithms using a MOOG-6DOF motion platform in two different conditions. 1) The participant kept the head in a resting, fully unrestrained posture, while inter-aural, naso-occipital or cranio-caudal translations were applied. 2) The participant moved the head continuously while a naso-occipital translation was applied. Head and platform motion were monitored in real-time using Vicon. Results: The results for both conditions showed excellent agreement between the theoretical spatio-temporal profile of the motion stimuli and the corresponding profile of actual motion as measured in real-time. Conclusion: We propose our approach as a safe, non-intrusive method to test the vestibular system under the natural head-free conditions required by the experiential perspective of the patients.

Vestibular perceptual testing from lab to clinic: a review

Not all dizziness presents as vertigo, suggesting other perceptual symptoms for individuals with vestibular disease. These non-specific perceptual complaints of dizziness have led to a recent resurgence in literature examining vestibular perceptual testing with the aim to enhance clinical diagnostics and therapeutics. Recent evidence supports incorporating rehabilitation methods to retrain vestibular perception. This review describes the current field of vestibular perceptual testing from scientific laboratory techniques that may not be clinic friendly to some low-tech options that may be more clinic friendly. Limitations are highlighted suggesting directions for additional research.

Influence of Visual Feedback on Roll Tilt Perceptual Training

HYPOTHESIS

We hypothesized that the addition of visual feedback during roll tilt perceptual training would yield a significant reduction in vestibular perceptual thresholds relative to a control group.

BACKGROUND

We previously showed that roll tilt vestibular thresholds could be improved through a perceptual training protocol that used a simple auditory cue. Variability in training outcomes within the treatment group suggested that an auditory cue alone may be suboptimal for improving self-motion perception.

METHODS

In 10 healthy adults, roll tilt vestibular thresholds, quantifying the smallest motion that can be reliably perceived, were measured before ("pretraining") and after ("posttraining") a training protocol designed to improve roll tilt perception. The protocol included 1,300 trials of 0.5 Hz whole-body roll tilt over 5 days, with participants being given both an auditory cue ("correct' vs. "incorrect') and visual feedback (viewing a stationary visual scene) after indicating their perceived direction of tilt. A control group (N = 10) underwent only the "pretraining" and "posttraining" assessments.

RESULTS

The training group showed an average decrease in roll tilt vestibular thresholds of 1.7% ± 56%, with training outcomes varying widely. Three individuals showed an average increase in roll tilt thresholds of 69.7%, whereas the remaining seven adults showed an average decrease in thresholds of 32.3%.

CONCLUSION

These data show that visual feedback during roll tilt perceptual training leads to heterogenous outcomes, but in a subset of individuals, it may lead to improvements in perceptual precision. Additional work is needed to define the optimal training parameters, including feedback schema, before investigating potential clinical applications.

Increased roll tilt thresholds are associated with subclinical postural instability in asymptomatic adults aged 21 to 84 years

Background

Balance assessments that intentionally alter the reliability of visual and proprioceptive feedback (e.g., standing on foam with eyes closed) have become a standard approach for identifying vestibular mediated balance dysfunction in older adults. However, such assessments cannot discern which specific element of the vestibular system (e.g., semicircular canal, otolith, or combined canal-otolith) underlies the observed age-related changes in balance performance. The present study was designed to determine the associations between specific sources of vestibular noise and quantitative measures of quiet stance postural control measured during standard "vestibular" balance conditions.

Methods

A group of 52 asymptomatic adults (53.21 ± 19.7, 21 to 84 years) without a history of vestibular or neurologic disorders volunteered for this study. We measured a battery of five vestibular perceptual thresholds that assay vestibular noise with predominant contributions from the vertical canals, lateral canals, utricles, saccules, and the centrally integrated canal-otolith signal. In addition, participants completed two standard balance assessments that were each designed to prioritize the use of vestibular cues for quiet stance postural control-eyes closed on foam (Condition 4 of the Modified Romberg Balance Test) and eyes closed, on a sway referenced support surface (Condition 5 of the Sensory Organization Test).

Results

In age adjusted models, we found strong positive associations between roll tilt vestibular thresholds, a measure of noise in the centrally integrated canal-otolith signal, and the root mean square distance (RMSD) of the anteroposterior and mediolateral center of pressure (CoP) captured during eyes closed stance on a sway referenced support surface. The strength of the association between roll tilt thresholds and the RMSD of the CoP was between 3-times and 30-times larger than the association between postural sway and each of the other vestibular thresholds measured.

Conclusion

We posit that noise in the centrally estimated canal-otolith "tilt" signal may be the primary driver of the subclinical postural instability experienced by older adults during the "vestibular" conditions of balance assessments. Additional testing in adults with clinical balance impairment are needed to identify if roll tilt thresholds may also serve as a surrogate metric by which to detect vestibular mediated balance dysfunction and/or fall risk.

Noise and vestibular perception of passive self-motion

Noise defined as random disturbances is ubiquitous in both the external environment and the nervous system. Depending on the context, noise can degrade or improve information processing and performance. In all cases, it contributes to neural systems dynamics. We review some effects of various sources of noise on the neural processing of self-motion signals at different stages of the vestibular pathways and the resulting perceptual responses. Hair cells in the inner ear reduce the impact of noise by means of mechanical and neural filtering. Hair cells synapse on regular and irregular afferents. Variability of discharge (noise) is low in regular afferents and high in irregular units. The high variability of irregular units provides information about the envelope of naturalistic head motion stimuli. A subset of neurons in the vestibular nuclei and thalamus are optimally tuned to noisy motion stimuli that reproduce the statistics of naturalistic head movements. In the thalamus, variability of neural discharge increases with increasing motion amplitude but saturates at high amplitudes, accounting for behavioral violation of Weber's law. In general, the precision of individual vestibular neurons in encoding head motion is worse than the perceptual precision measured behaviorally. However, the global precision predicted by neural population codes matches the high behavioral precision. The latter is estimated by means of psychometric functions for detection or discrimination of whole-body displacements. Vestibular motion thresholds (inverse of precision) reflect the contribution of intrinsic and extrinsic noise to perception. Vestibular motion thresholds tend to deteriorate progressively after the age of 40 years, possibly due to oxidative stress resulting from high discharge rates and metabolic loads of vestibular afferents. In the elderly, vestibular thresholds correlate with postural stability: the higher the threshold, the greater is the postural imbalance and risk of falling. Experimental application of optimal levels of either galvanic noise or whole-body oscillations can ameliorate vestibular function with a mechanism reminiscent of stochastic resonance. Assessment of vestibular thresholds is diagnostic in several types of vestibulopathies, and vestibular stimulation might be useful in vestibular rehabilitation.

Vestibular perceptual thresholds for rotation about the yaw, roll, and pitch axes

This effort seeks to further assess human perception of self-motion by quantifying and comparing earth-vertical rotational vestibular perceptual thresholds about the yaw, roll, and pitch axes. Early seminal works (Benson Aviat Space Environ Med 60:205-213, 1989) quantified thresholds for yaw, roll, and pitch rotations, using single-cycle sinusoids in angular acceleration with a frequency of 0.3 Hz (3.33 s motion duration) and found yaw thresholds to be significantly lower than roll and pitch thresholds (1.58-1.20 deg/s vs. 2.07 deg/s and 2.04 deg/s, respectively). Our current effort uses modern methods and definitions to reassess if rotational thresholds differ between these three axes of rotation in ten human subjects at 0.3 Hz and additionally across a range of frequencies: 0.1 Hz, 0.3 Hz, and 0.5 Hz. In contrast to the established findings of Benson et al., no statistically significant differences were found between the three rotational axes at 0.3 Hz. Further, no statistically significant differences were found at any of these frequencies. Instead, a consistent pattern was found for yaw, pitch, and roll of increasing thresholds with decreasing rotational frequency, consistent with the brain employing high-pass filter mechanisms for decision-making. We also fill a gap in the literature by extending the quantification of pitch rotation thresholds to 0.1 Hz. Finally, we assessed inter-individual trends between these three frequencies and across all three axes of rotation. In thoroughly considering methodological and other differences between the current and previous studies, we conclude yaw rotation thresholds do not differ from those in roll or pitch.

Improving self-motion perception and balance through roll tilt perceptual training

The present study aimed to determine if a vestibular perceptual learning intervention could improve roll tilt self-motion perception and balance performance. Two intervention groups (n = 10 each) performed 1,300 trials of roll tilt at either 0.5 Hz (2 s/motion) or 0.2 Hz (5 s/motion) distributed over 5 days; each intervention group was provided feedback (correct/incorrect) after each trial. Roll tilt perceptual thresholds, measured using 0.2-, 0.5-, and 1-Hz stimuli, as well as quiet stance postural sway, were measured on day 1 and day 6 of the study. The control group (n = 10) who performed no perceptual training, showed stable 0.2-Hz (+1.48%, P > 0.99), 0.5-Hz (-4.0%, P > 0.99), and 1-Hz (-17.48%, P = 0.20) roll tilt thresholds. The 0.2-Hz training group demonstrated significant improvements in both 0.2-Hz (-23.77%, P = 0.003) and 0.5-Hz (-22.2%, P = 0.03) thresholds. The 0.5-Hz training group showed a significant improvement in 0.2-Hz thresholds (-19.13%, P = 0.029), but not 0.5-Hz thresholds (-17.68%, P = 0.052). Neither training group improved significantly at the untrained 1-Hz frequency (P > 0.05). In addition to improvements in perceptual precision, the 0.5-Hz training group showed a decrease in sway when measured during "eyes open, on foam" (dz = 0.57, P = 0.032) and "eyes closed, on foam" (dz = 2.05, P < 0.001) quiet stance balance tasks. These initial data suggest that roll tilt perception can be improved with less than 5 h of training and that vestibular perceptual training may contribute to a reduction in subclinical postural instability.NEW & NOTEWORTHY Roll tilt vestibular perceptual thresholds, an assay of vestibular noise, were recently found to correlate with postural sway. We therefore hypothesized that roll tilt perceptual training would yield improvements in both perceptual precision and balance. Our data show that roll tilt perceptual thresholds and quiet stance postural sway can be significantly improved after less than 5 h of roll tilt perceptual training, supporting the hypothesis that vestibular noise contributes to increased postural sway.