Vestibular Responses Research Articles

Effects of Vestibular Training on Postural Control of Healthy Adults

Background: Postural stability depends on the integration of multisensory inputs to drive motor outputs. When visual and somatosensory input is available and reliable, this reduces the postural control system’s reliance on the vestibular system. Despite this, vestibular loss can still cause severe postural dysfunction (1,2). Training one or more of the three sensory systems can alter sensory weighting and change postural behavior. Vestibular activation exercises, including horizontal and vertical headshaking, influence vestibular-ocular and -motor responses and have been showed to be effective in vestibular rehabilitation (3–8). 
 Purpose/Hypothesis: To assess sensory reweighting of postural control processing and vestibular-ocular and -motor responses after concurrent vestibular activation with postural training. It was hypothesized that the effect of this training would significantly alter the pattern of sensory weighting by changing the ratio of visual, somatosensory and vestibular dependence needed to maintain postural stability, and significantly decrease vestibular responses.
 Methods: Forty-two young healthy individuals (22 females; 23.0+3.9 years; 1.6+0.1 meters) were randomly assigned into four groups: 1) visual feedback weight shift training (WST) coupled with an active horizontal headshake (HHS), 2) same WST with vertical headshake (VHS), 3) WST with no headshake (NHS) and 4) no training/headshake control (CTL) groups. The headshake groups performed an intensive body WST together with horizontal or vertical rhythmic headshake at 80 to 120 beats/minute. The NHS group performed the WST with no headshake while the controls did not perform any training. Five 15-minute training sessions were performed on consecutive days for one week with the weight shift exercises involving upright limits of stability activities on a flat surface, foam or rocker board (Fig. 1). All groups performed baseline- and post-assessments including sensory organization test (SOT) and force platform ramp perturbations, coupled with electromyographic (EMG) recordings. A video head impulse test was also used to record horizontal vestibulo-ocular reflex (VOR) gain. A between- and within-group repeated measures ANOVA was used to analyze five COP sway variables, the equilibrium and composite scores and sensory ratios of the SOT as well as EMG signals and horizontal VOR gain. Similarly, COP variables, EMG, as well as vestibular reflex data (vertical VOR, vestibulo-collic reflex [VCR] and vestibulo-spinal [VSR] gains) during ramp perturbations were analyzed. Alpha level was set at p<.05.
 Results: The training showed a significant somatosensory downweighting (p=.050) in the headshake groups compared to the other groups. Training also showed significant decreased horizontal VOR gain (p=.040), faster automatic postural response (p=.003) (Figs. 2-4) with improved flexibility (p=.010) in the headshake groups. Muscle activation pattern in medial gastrocnemius (p=.033) was significantly decreased in the headshake.
 Conclusion: The concurrent vestibular activation and weight shift training modifies vestibular-dependent responses after the training intervention as evidenced in somatosensory downweighting, decreased VOR gain, better postural flexibility and faster automatic postural response. Findings suggest this is predominantly due to vestibular adaptation and habituation of VOR, VCR and VSR which induced sensory reweighting.
 Clinical relevance: Findings may be used to guide the development of a vestibular-postural rehabilitation intervention in impaired neurological populations, such as with vestibular disorders or sensory integration problems.

Cerebellar Transcranial Magnetic Stimulation Disrupts Vestibular Reflexes in Standing Balance

Despite its seemingly effortless execution, standing balance is a complex task maintained by the integration of sensory feedback from visual, somatosensory (muscle, skin, joint receptors), and vestibular (inner ear) systems. The vestibular system provides feedback about head acceleration and can be manipulated experimentally by passing an electrical current percutaneously through the mastoid processes. The cerebellum plays an important role in standing balance by integrating sensory feedback to refine activity in postural muscles. A recent study has shown that experimentally depressing cerebellar activity with repetitive transcranial magnetic stimulation can influence postural sway and muscle reflex responses following electrical vestibular stimulation. The aim of this study was to investigate the impact of acute cerebellar stimulation on vestibular muscle reflex responses in different head positions during standing balance. We applied a novel combination of stochastic electrical vestibular stimulation (SVS, ±2.5 mA, 0–20 Hz) and single‐pulse cerebellum transcranial magnetic stimulation (TMS, 60% maximum stimulator output) on fifteen healthy adults (6 males, 9 females; 21–32 years old) in two head positions: head forward and head right. Muscle reflexes were recorded bilaterally with surface electromyography (EMG) from lower leg (soleus and medial gastrocnemius) and neck (sternocleidomastoid) muscles. Time‐based cumulant density and frequency‐based coherence analyses were used to evaluate the relationship between the SVS and evoked EMG responses one second before (Pre) and one second after (Post) cerebellar TMS. 50 TMS pulses were delivered per head position. Preliminary analysis performed on left medial gastrocnemius (LMG) head right data indicated a significant reduction in the short latency (SL, P = 0.0377) and medium latency (ML, P < 0.0001) reflexes following cerebellar TMS (Fig. 1). In addition, a significant drop in the coherence between the SVS and LMG Pre to Post was found using a difference of coherence test. These findings support the cerebellum to be an important center for the integration of vestibular feedback for the control of standing balance.Support or Funding InformationMount Royal University Faculty of Science and TechnologyFigure 1

Vestibulo-Ocular Responses, Visual Field Dependence, and Motion Sickness in Aerobatic Pilots.

BACKGROUND: Aerobatic flight is a challenge for the vestibular system, which is likely to lead to adaptive changes in the vestibular responses of pilots. We investigated whether aerobatic pilots, as individuals who experience intense vestibular stimulation, present modifications of the vestibular-ocular reflex, motion sickness susceptibility and intensity, visual vertical estimation, and visual dependence as compared to normal volunteers.METHODS: To evaluate vestibulo-ocular reflexes, eye movements were recorded with videonystagmography while subjects were rotated on a rotatory chair with the axis of rotation being vertical (canal-ocular reflex) or inclined to 17° (otolith-ocular reflex). Motion sickness was evaluated after the rotatory test using the Graybiel diagnostic criteria. General motion sickness susceptibility and visual field dependence were also evaluated.RESULTS: Averaged data did not show significant difference in canal-ocular reflex and otolith ocular-reflex between groups. However, a significant asymmetry in otolith-driven ocular responses was found in pilots (CW 0.50 ± 1.21° · s-1 vs. CCW 1.59 ± 1.12° · s-1), though visual vertical estimation was not altered in pilots and both groups were found field independent. Pilots were generally less susceptible to motion sickness (MSSQ scores: 2.52 ± 5.59 vs. 13.5 ± 11.36) and less affected by the nauseogenic stimulation (Graybiel diagnostic criteria 3.36 ± 3.81 vs. 8.39 ± 7.01).DISCUSSION: We did not observe the expected habituation in the group of aerobatic pilots. However, there was a significant asymmetry in the otolith-driven ocular responses in pilots, but not in the controls, which may result from the asymmetry in piloting protocols.Kuldavletova O, Tanguy S, Denise P, Quarck G. Vestibulo-ocular responses, visual field dependence, and motion sickness in aerobatic pilots. Aerosp Med Hum Perform. 2020; 91(4):326-331.

Vestibular Responses to Gravity Alterations

Due to the adaptation to environments on Earth, various health-related issues are raised when exposed to different circumstances in space. Of environmental factors in space, gravity alteration has been considered as one of critical environmental changes. The primary inner organ to detect the gravity change is the vestibular system, especially otolith organs, and some limited researches have conducted to understand its mechanical and physiological properties. However, the related consequences were not consistent in despite of well description in systemic effects ranged from the peripheral vestibular system to the central nervous system. Here, we revisited the neuronal and behavioral effects of the gravity alteration on the relevant organs through this review. By representing previous studies for the gravity effects on the peripheral and central vestibular system, this review would provide the concrete understanding of the vestibular responses to the gravity alteration. Also, the physiological responses are expected to provide the useful resources to understand the systemic vestibular responses under the gravity alteration. Keywords: Vestibular system; Gravity; Neuroscience; Otolith 중심 단어: ì „ì •ê¸°ê´€, ì¤‘ë ¥, ì‹ ê²½ê³¼í•™, 이석기관

Neural Mechanisms Underlying High-Frequency Vestibulocollic Reflexes In Humans And Monkeys.

The vestibulocollic reflex is a compensatory response that stabilizes the head in space. During everyday activities, this stabilizing response is evoked by head movements that typically span frequencies from 0 to 30 Hz. Transient head impacts, however, can elicit head movements with frequency content up to 300-400 Hz, raising the question whether vestibular pathways contribute to head stabilization at such high frequencies. Here, we first established that electrical vestibular stimulation modulates human neck motor unit (MU) activity at sinusoidal frequencies up to 300 Hz, but that sensitivity increases with frequency up to a low-pass cutoff of ∼70-80 Hz. To examine the neural substrates underlying the low-pass dynamics of vestibulocollic reflexes, we then recorded vestibular afferent responses to the same electrical stimuli in monkeys. Vestibular afferents also responded to electrical stimuli up to 300 Hz, but in contrast to MUs their sensitivity increased with frequency up to the afferent resting firing rate (∼100-150 Hz) and at higher frequencies afferents tended to phase-lock to the vestibular stimulus. This latter nonlinearity, however, was not transmitted to neck motoneurons, which instead showed minimal phase-locking that decreased at frequencies >75 Hz. Similar to human data, we validated that monkey muscle activity also exhibited low-pass filtered vestibulocollic reflex dynamics. Together, our results show that neck MUs are activated by high-frequency signals encoded by primary vestibular afferents, but undergo low-pass filtering at intermediate stages in the vestibulocollic reflex. These high-frequency contributions to vestibular-evoked neck muscle responses could stabilize the head during unexpected head transients.SIGNIFICANCE STATEMENT Vestibular-evoked neck muscle responses rely on accurate encoding and transmission of head movement information to stabilize the head in space. Unexpected transient events, such as head impacts, are likely to push the limits of these neural pathways since their high-frequency features (0-300 Hz) extend beyond the frequency bandwidth of head movements experienced during everyday activities (0-30 Hz). Here, we demonstrate that vestibular primary afferents encode high-frequency stimuli through frequency-dependent increases in sensitivity and phase-locking. When transmitted to neck motoneurons, these signals undergo low-pass filtering that limits neck motoneuron phase-locking in response to stimuli >75 Hz. This study provides insight into the neural dynamics producing vestibulocollic reflexes, which may respond to high-frequency transient events to stabilize the head.

Differences Between Physical vs. Virtual Evoked Vestibular Responses.

Electrovestibulography (EVestG), a technology purported to measure vestibular activity at the vestibular periphery, was used to compare the vestibular responses to two sensory inputs: (1) back-forward physical tilt (with eyes-open and eyes-closed) and (2) virtual reality replica of the back-forward tilt (eyes-open, physically static). Twenty-seven healthy participants (10 females) were tested. From each of the EVestG recordings, two feature curves: (1) average field potential (FP), and (2) distribution of time intervals between the detected FPs were extracted. For the eyes-closed physical tilt, except for the background segment, the FP response curve was generally wider compared to that evoked during the virtual replica tilt (p < 0.05). Moreover, the eyes-closed physical tilt produced longer time intervals between FP's compared to the virtual stimulus. For this measure, for the background segment, the eyes closed and open physical tilt responses were significantly different (p < 0.05) in both ears (repeated measure experimental design). The results support: (1) both vestibular and visual inputs evoking a measurably different EVestG response, (2) the differences between physical and virtual vestibular responses are dependent on the eyes being either open or closed, and (3) for the stimuli used, the modulation of vestibular afferent activity was measurably smaller for virtual than physical stimulation.

Features of the diagnosis of meniere's disease on the example of a clinical case

Introduction. Meniere’s disease is accompanied by a non-infectious pathology of the inner ear, during which can lead to systemic labyrinth dizziness, nausea, vomiting, hearing loss and tinnitus in one ear. It is a rare disease, affecting up to 200 cases per 100,000 people. Aim. The purpose of this article was to analyze the clinical case of a patient with Meniere's disease. Materials and methods. Patient S., was admitted to the Department of Vascular Pathology of the Brain and Rehabilitation of the State Institution «Institute of Neurology, Psychiatry and Narcology of the NAMS of Ukraine» with complaints of regular attacks of systemic vertigo with a frequency of up to 7 times a week lasting 1–4 hours, sometimes accompanied by vomiting, constant noise and whistling in the left ear. He considers himself ill since 05.11.19, when during training in the gym there was the first case of dizziness and vomiting, which lasted 3 hours. In August 2019, the attacks continued, became more frequent and prolonged, passed on their own, sometimes in a specific position of the head - the head tilted forward with the neck stretched out. Neurological examination revealed that the palpebral fissures and pupils were equal. Eye movements are full, painless. Convergence insufficiency was examined. Corneal reactions are reduced. Horizontal positioning nystagmus was examined. There are no pathological signs, no sensory disturbances. The patient performed coordination tests with uncertainty. The patient showed unsteadiness during Romberg's test. The patient felt pain during the palpation of paravertebral points. The patient experienced exaggerated changes in mood. Results. 1. Brain MRI results showed a thinning and a decrease in the MR signal on T2 from the cochlea of the inner ear to the left. CT scanning of the mastoid processes revealed an increased density of bone structures of the labyrinth nucleus on the left. 2. During the otoneurological examination, the asymmetry of vestibular responses along the labyrinth, slight stimulation of the left labyrinth were established. The patient was diagnosed with peripheral cochleovestibular syndrome on the left, Meniere's syndrome. Conclusions. This clinical case illustrates the importance of collecting a detailed medical history and examination of a patient with complaints of dizziness, hearing loss, tinnitus, and repeated vomiting. To establish a diagnosis and develop a treatment plan for a patient, the coordinated work of a multidisciplinary group is required, which should consist of a neurologist, audiologist, and otolaryngologist.

Development of a conversion model between mechanical and electrical vestibular stimuli.

The vestibular end-organs encode for linear and angular head accelerations in space contributing to our internal representation of self-motion. Activation of the vestibular system with transmastoid electrical current has recently grown in popularity; however, a direct relationship between electrically evoked and mechanically evoked vestibular responses remains elusive in humans. We have developed and tested a mechanical-to-electrical vestibular stimulus conversion model incorporating physiological activation of primary vestibular afferents identified in nonhuman primates. We compared ocular torsional responses between mechanical (chair rotation) and model-derived electrical (binaural-bipolar) stimuli in separate experiments for an angular velocity step change (±10 deg/s over 1 s, ±4-mA peak amplitude; n = 10) and multisine angular velocities (±10 deg/s, 9.7 mA peak to peak, 0.05-1 Hz; n = 5), respectively. Perception of whole body rotation (n = 18) to our step-change stimuli was also evaluated. Ocular torsional slow-phase velocity responses between stimulation types were similar (paired two one-sided tests of equivalence: multiple P < 0.002; one-sample t test: P = 0.178) and correlated (Pearson's coefficient: multiple P < 0.001). Bootstrap analysis of perceived angular velocity likewise showed similarity in perceptual decay dynamics. These data suggest that central processing between stimuli was similar, and our vestibular stimulus conversion model with a conversion factor of ∼0.4 mA per deg/s for an angular velocity step change can generate electrical stimuli that replicates dynamic vestibular activation elicited by mechanical whole body rotations. This proposed vestibular conversion model represents an initial framework for using electrical stimuli to generate mechanically equivalent activation of primary vestibular afferents for use in biomedical applications and immersive reality technologies.NEW & NOTEWORTHY With the growing popularity of electrical vestibular stimulation in biomedical and immersive reality applications, a direct conversion model between electrical and mechanical vestibular stimuli is needed. We developed a model to generate electrical stimuli mimicking the physiological activation of vestibular afferents evoked by mechanical rotations. Ocular and perceptual responses evoked by mechanical and model-derived electrical stimuli were similar, thus providing a critical first step toward generation of electrically induced vestibular responses that have a realistic mechanical equivalent.

Eye Movement Disorders and the Cerebellum.

The cerebellum works as a network hub for optimizing eye movements through its mutual connections with the brainstem and beyond. Here, we review three key areas in the cerebellum that are related to the control of eye movements: (1) the flocculus/paraflocculus (tonsil) complex, primarily for high-frequency, transient vestibular responses, and also for smooth pursuit maintenance and steady gaze holding; (2) the nodulus/ventral uvula, primarily for low-frequency, sustained vestibular responses; and (3) the dorsal vermis/posterior fastigial nucleus, primarily for the accuracy of saccades. Although there is no absolute compartmentalization of function within the three major ocular motor areas in the cerebellum, the structural-functional approach provides a framework for assessing ocular motor performance in patients with disease that involves the cerebellum or the brainstem.

Correlation between Ocular and Vestibular Abnormalities and Convergence Insufficiency in Post-Concussion Syndrome

ABSTRACT The vestibular and oculomotor/visual systems are commonly affected in post-concussion syndrome (PCS). Convergence insufficiency (CI) is the most common ocular abnormality after concussion. Electrovestibulography (EVestG) is a relatively new non-invasive method that measures the peripheral vestibular responses; it has shown abnormal vestibular responses in a PCS. Here, we report the results of investigating the correlation between the vestibular and oculomotor systems in PCS population using EVestG and CI measures. Forty-eight PCS patients were tested using EVestG, out of which 20 also completed the Rivermead post-concussion questionnaire (RPQ). An EVestG feature (Field Potential (FP)-area) was extracted from the stationary part of the EVestG signals. A neuro-ophthalmologist (author BM) measured participants’ CI at near vision using cross-cover examination and a prism-bar. Results indicate: (1) vestibular abnormality (i.e. FP-area) and CI values are significantly correlated in PCS (R = 0.68, p < .01), and (2) there are significant correlations between severity of concussion (i.e. RPQ3) and CI (R = 0.70, p < .01) and between RPQ3 and FP-area (R = −0.56, p < .02). To the best of our knowledge, this is the first study that objectively demonstrates a significant positive correlation between the CI and vestibular systems’ abnormality. These findings are scientifically important as they help localise the pathology of PCS, and are clinically valuable as they help physicians in their decision-making about PCS diagnosis and rehabilitation strategies.

Lower Functional Connectivity in Vestibular-Limbic Networks in Individuals With Subclinical Agoraphobia.

Background: Agoraphobia was described in 1871 as a condition of fear-related alterations in spatial orientation and locomotor control triggered by places or situations that might cause a patient to panic and feel trapped. In contemporary nosology, however, this original concept of agoraphobia was split into two diagnostic entities, i.e., the modern anxiety disorder of agoraphobia, consisting solely of phobic/avoidant symptoms in public spaces, and the recently defined vestibular disorder of persistent postural perceptual dizziness (PPPD), characterized by dizziness, and unsteadiness exacerbated by visual motion stimuli. Previous neuroimaging studies found altered brain activity and connectivity in visual-vestibular networks of patients with PPPD vs. healthy controls. Neuroticism and introversion, which pre-dispose to both agoraphobia and PPPD, influenced brain responses to vestibular and visual motion stimuli in patients with PPPD. Similar neuroimaging studies have not been undertaken in patients with agoraphobia in its current definition. Given their shared history and pre-disposing factors, we sought to test the hypotheses that individuals with agoraphobic symptoms have alterations in visual-vestibular networks similar to those of patients with PPPD, and that these alterations are influenced by neuroticism and introversion.Methods: Drawing from the Human Connectome Project (HCP) database, we matched 52 participants with sub-clinical agoraphobia and 52 control subjects without agoraphobic symptoms on 19 demographic and psychological/psychiatric variables. We then employed a graph-theoretical framework to compare resting-state functional magnetic resonance images between groups and evaluated the interactive effects of neuroticism and introversion on the brain signatures of agoraphobia.Results: Individuals with subclinical agoraphobia had lower global clustering, efficiency and transitivity relative to controls. They also had lower connectivity metrics in two brain networks, one positioned to process incoming visual space-motion information, assess threat, and initiate/inhibit behavioral responses (visuospatial-emotional network) and one positioned to control and monitor locomotion (vestibular-navigational network). Introversion interacted with agoraphobic symptoms to lower the connectivity of the visuospatial-emotional network. This contrasted with previous findings describing neuroticism-associated higher connectivity in a narrower visual-spatial-frontal network in patients with PPPD.Conclusion: Functional connectivity was lower in two brain networks in subclinical agoraphobia as compared to healthy controls. These networks integrate visual vestibular and emotional response to guide movement in space.

Perioperative vestibular assessment and testing

Vestibular disorders can be difficult to accurately diagnose and manage. A careful history and focused physical examination are typically adequate to establish a diagnosis and initiate medical treatment. Vestibular testing is an important component of the work-up, but it is particularly essential for patients being considered for surgical intervention for a vestibular disorder, where the testing can be used to more definitively confirm a suspected diagnosis and to determine baseline vestibular organ function. In this article, we will first briefly review key components of the history and physical examination of patients with vestibular complaints. We will then discuss the most commonly used vestibular tests and their role in the preoperative assessment of patients undergoing vestibular surgery, including nystagmography, caloric testing, rotary chair testing, video head impulse testing, and vestibular evoked myogenic potential testing.

Ascending vestibular pathways to parietal areas MIP and LIPv and efference copy inputs from the medial reticular formation: Functional frameworks for body representations updating and online movement guidance.

The posterior parietal cortex (PPC) serves as a sensorimotor interface by integrating multisensory signals with motor related information for generating and updating body representations and movement plans. Using retrograde transneuronal transfer of rabies virus combined with a conventional tracer, we identified direct and polysynaptic pathways to two PPC areas, the rostral medial intraparietal area (MIP) and the ventral part of the lateral intraparietal area (LIPv) in macaque monkeys. We found that rostral MIP and LIPv receive ascending vestibular pathways, and putative efference copy inputs disynaptically from the medullary medial reticular formation (MRF) where reticulospinal pathways to neck and arm motoneurons originate. LIPv receives minor disynaptic vestibular inputs, and substantial projections from the head movement-related rostral MRF, consistent with head gain modulation of LIPv activity and a role in planning gaze shifts. Rostral MIP is the target of prominent disynaptic pathways from reaching- and head movement-related MRF domains, and major ascending vestibular pathways trisynaptically from both labyrinths, explaining prominent vestibular responses and discrimination between active and passive movements demonstrated in rostral MIP and in the neighboring ventral intraparietal area, which are heavily interconnected. The findings that rostral MIP (belonging to the 'parietal reach region'), receives vestibular inputs as directly as classical vestibular areas, via a parallel channel, and efference copy signals pathways from MRF reticulospinal domains that belong to reach and head movement networks have important implications for the understanding of the role of the PPC in updating body representations and internal models for online guidance of movement.

Neural substrates, dynamics and thresholds of galvanic vestibular stimulation in the behaving primate

Galvanic vestibular stimulation (GVS) uses the external application of electrical current to selectively target the vestibular system in humans. Despite its recent popularity for the assessment/treatment of clinical conditions, exactly how this non-invasive tool activates the vestibular system remains an open question. Here we directly investigate single vestibular afferent responses to GVS applied to the mastoid processes of awake-behaving monkeys. Transmastoid GVS produces robust and parallel activation of both canal and otolith afferents. Notably, afferent activation increases with intrinsic neuronal variability resulting in constant GVS-evoked neuronal detection thresholds across all afferents. Additionally, afferent tuning differs for GVS versus natural self-motion stimulation. Using a stochastic model of repetitive activity in afferents, we largely explain the main features of GVS-evoked vestibular afferent dynamics. Taken together, our results reveal the neural substrate underlying transmastoid GVS-evoked perceptual, ocular and postural responses—information that is essential to advance GVS applicability for biomedical uses in humans.

Superior Canal Dehiscence Syndrome: Relating Clinical Findings With Vestibular Neural Responses From a Guinea Pig Model.

In superior canal dehiscence (SCD), fluid displacement of the endolymph activates type I vestibular hair cells in the crista of the affected canal and thus irregular superior canal (SC) neurons in Scarpa's ganglion, which provides the neurophysiological basis for the clinical presentation of SCD. Patients with SCD display sound- and vibration-induced vertigo/nystagmus and increased amplitudes of vestibular evoked myogenic potentials. Extracellular recordings from n = 25 primary vestibular neurons of 16 female guinea pigs were analyzed. We recorded from the same vestibular neuron before, during and after creating the dehiscence and after closing the dehiscence. Neurobiotin labeling was employed in n = 11 neurons. After SCD, previously unresponsive irregular SC neurons displayed a stimulus-locked increase in discharge during application of air-conducted sound (ACS) or bone-conducted vibration (BCV) for a broad range of frequencies (ACS: 200-4000 Hz; BCV: 500-1500 Hz). This typical response was only observed for irregular SC neurons (n = 19), but not regular SC neurons, or irregular/regular horizontal canal neurons (n = 2 each), and was abolished after closing the dehiscence. Eleven irregular SC neurons responsive to ACS and/or BCV were traced back to calyx synapses in the central crista of the affected superior canal by neurobiotin labeling. Stimulus-locked activation of irregular SC neurons by ACS and BCV is the neurophysiological basis for sound- and vibration-induced vertigo/nystagmus and increased VEMP amplitudes in SCD. The results of the present study help to improve vestibular diagnostics in patients with suspected SCD.

On the Effectiveness of airborne infrasound in eliciting vestibular-evoked myogenic responses

The use of airborne infrasound and other stimuli to elicit (cervical) vestibular-evoked myogenic potentials (cVEMPs) was studied to address the common proposition that infrasound may efficiently stimulate the vestibular system, an effect which may underlie the so-called wind-turbine syndrome. cVEMPs were measured for both ears of 15 normal-hearing subjects using three types of airborne sound stimulation: (1) 500-Hz tone bursts (transient); (2) 500-Hz sinusoidally amplitude-modulated tones at a 40-Hz rate (SAM); and (3) low-frequency and infrasound pure tones (LF/IS). The two former stimulation types served as control and allowed a systematic comparison with (3). It was found that SAM stimulation is effective and appears to be comparable to transient stimulation, as was previously observed in a yet small number of studies. Although the vestibular system is reported to be highly sensitive to low-frequency mechanical vibration, airborne LF/IS stimulation at ∼80–90-phon loudness levels did not elicit significant saccular vestibular responses.

Vestibular Eye Movements Are Heavily Impacted by Visual Motion and Are Sensitive to Changes in Visual Intensities.

Eye movement evaluation constitutes the basis of diagnosis in dizzy patients. Through evaluating ocular torsion and vertical skewing during balance provoking stimulation, the aim of this study was to investigate the impact of vision on a typical vestibular eye movement response. Twelve healthy subjects (six young, six old) were exposed to (1) vestibular (VES), (2) visual (VIS), and (3) visual-vestibular (VIS+VES) stimulation. These were carried out as whole-body roll (VES), optokinetic rolling of visual scenes (VIS), and a combination of both (VIS+VES). Visual scenes were presented at two intensity levels. Eye movement velocities were used to evaluate the relative impact of visual and vestibular stimulation. Torsional velocities were lowest for VIS regardless of age. Velocities for the old group did not differ between VES and VIS+VES, whereas those for the young group were higher for VIS+VES. Regardless of age, amplified visual intensity resulted in an increased torsion-skewing ratio, seen as more degrees of torsion per degrees of skewing. The contributions of VIS and VES in proportion to VIS+VES were calculated as 0.18 (0.08) and 0.74 (0.14), respectively. Our findings demonstrate that vertical skewing is physiologically seen in combination with ocular torsion as a response to visual stimulation, with young subjects exhibiting a more dynamic response. The torsion-skewing ratio was sensitive to small changes in visual intensities, which may prove useful when evaluating visual motion sensitivity. The visual contribution to the vestibular eye movement response highlights the clinical importance of visual examinations when evaluating dizzy patients.

Increased brain responsivity to galvanic vestibular stimulation in bilateral vestibular failure.

In this event-related functional magnetic resonance imaging (fMRI) study we investigated how the brain of patients with bilateral vestibular failure (BVF) responds to vestibular stimuli. We used imperceptible noisy galvanic vestibular stimulation (GVS) and perceptible bi-mastoidal GVS intensities and related the corresponding brain activity to the evoked motion perception. In contrast to caloric irrigation, GVS stimulates the vestibular organ at its potentially intact afferent nerve site.Motion perception thresholds and cortical responses were compared between 26 BVF patients to 27 age-matched healthy control participants. To identify the specificity of vestibular cortical responses we used a parametric design with different stimulus intensities (noisy imperceptible, low perceptible, high perceptible) allowing region-specific stimulus response functions. In a 2 × 3 flexible factorial design all GVS-related brain activities were contrasted with a sham condition that did not evoke perceived motion.Patients had a higher motion perception threshold and rated the vestibular stimuli higher than the healthy participants. There was a stimulus intensity related and region-specific increase of activity with steep stimulus response functions in parietal operculum (e.g. OP2), insula, superior temporal gyrus, early visual cortices (V3) and cerebellum while activity in the hippocampus and intraparietal sulcus did not correlate with vestibular stimulus intensity. Using whole brain analysis, group comparisons revealed increased brain activity in early visual cortices (V3) and superior temporal gyrus of patients but there was no significant interaction, i.e. stimulus-response function in these regions were still similar in both groups. Brain activity in these regions during (high)GVS increased with higher dizziness-related handicap scores but was not related to the degree of vestibular impairment or disease duration. nGVS did not evoke cortical responses in any group.Our data indicate that perceptible GVS-related cortical responsivity is not diminished but increased in multisensory (visual-vestibular) cortical regions despite bilateral failure of the peripheral vestibular organ. The increased activity in early visual cortices (V3) and superior temporal gyrus of BVF patients has several potential implications: (i) their cortical reciprocal inhibitory visuo-vestibular interaction is dysfunctional, (ii) it may contribute to the visual dependency of BVF patients, and (iii) it needs to be considered when BVF patients receive peripheral vestibular stimulation devices, e.g. vestibular implants or portable GVS devices. Imperceptible nGVS did not elicit cortical brain responses making it unlikely that the reported balance improvement of BVF by nGVS is mediated by cortical mechanisms.

Balance Sense: Response Motifs that Pervade the Brain

Measuring how the brain encodes and processes an animal's own motion presents major technical challenges. New approaches demonstrate the viability and merit of measuring vestibular responses throughout the entire brain.

Cellular-Resolution Imaging of Vestibular Processing across the Larval Zebrafish Brain

The vestibular system, which reports on motion and gravity, is essential to postural control, balance, and egocentric representations of movement and space. The motion needed to stimulate the vestibular system complicates studying its circuitry, so we previously developed a method for fictive vestibular stimulation in zebrafish, using optical trapping to apply physical forces to the otoliths. Here, we combine this approach with whole-brain calcium imaging at cellular resolution, delivering a comprehensive map of the brain regions and cellular responses involved in basic vestibular processing. We find responses broadly distributed across the brain, with unique profiles of cellular responses and topography in each region. The most widespread and abundant responses involve excitation that is graded to the stimulus strength. Other responses, localized to the telencephalon and habenulae, show excitation that is only weakly correlated to stimulus strength and that is sensitive to weak stimuli. Finally, numerous brain regions contain neurons that are inhibited by vestibular stimuli, and these neurons are often tightly localized spatially within their regions. By exerting separate control over the left and right otoliths, we explore the laterality of brain-wide vestibular processing, distinguishing between neurons with unilateral and bilateral vestibular sensitivity and revealing patterns whereby conflicting signals from the ears mutually cancel. Our results confirm previously identified vestibular responses in specific regions of the larval zebrafish brain while revealing a broader and more extensive network of vestibular responsive neurons than has previously been described. This provides a departure point for more targeted studies of the underlying functional circuits.