Pilot study of a new rehabilitation tool: improved unilateral short-term adaptation of the human angular vestibulo-ocular reflex

Short-term learning of the vestibulo-ocular reflex induced by a custom interactive computer game

Retinal image slip during head rotation drives motor learning in the rotational vestibulo-ocular reflex (VOR) and forms the basis of gaze-stability exercises that treat vestibular dysfunction. Clinical exercises, however, are unengaging, cannot easily be titrated to the level of impairment, and provide neither direct feedback nor tracking of the patient's adherence, performance, and progress. To address this, we have developed a custom application for VOR training based on an interactive computer game. In this study, we tested the ability of this game to induce VOR learning in individuals with normal vestibular function, and we compared the efficacy of single-step and incremental learning protocols. Eighteen participants played the game twice on different days. All participants tolerated the game and were able to complete both sessions. The game scenario incorporated a series of brief head rotations, similar to active head impulses, that were paired with a dynamic acuity task and with a visual-vestibular mismatch (VVM) intended to increase VOR gain (single-step: 300 successful trials at ×1.5 viewing; incremental: 100 trials each of ×1.13, ×1.33, and ×1.5 viewing). Overall, VOR gain increased by 15 ± 4.7% (mean ± 95% CI, P < 0.001). Gains increased similarly for active and passive head rotations, and, contrary to our hypothesis, there was little effect of the learning strategy. This study shows that an interactive computer game provides robust VOR training and has the potential to deliver effective, engaging, and trackable gaze-stability exercises to patients with a range of vestibular dysfunctions.NEW & NOTEWORTHY This study demonstrates the feasibility and efficacy of a customized computer game to induce motor learning in the high-frequency rotational vestibulo-ocular reflex. It provides a physiological basis for the deployment of this technology to clinical vestibular rehabilitation.

Vestibulo-Ocular Reflex Short-Term Adaptation Is Halved After Compensation for Unilateral Labyrinthectomy

Several prior studies, including those from this laboratory, have suggested that vestibulo-ocular reflex (VOR) adaptation and compensation are two neurologically related mechanisms. We therefore hypothesised that adaptation would be affected by compensation, depending on the amount of overlap between these two mechanisms. To better understand this overlap, we examined the effect of gain-increase (gain = eye velocity/head velocity) adaptation training on the VOR in compensated mice since both adaptation and compensation mechanisms are presumably driving the gain to increase. We tested 11 cba129 controls and 6 α9-knockout mice, which have a compromised efferent vestibular system (EVS) known to affect both adaptation and compensation mechanisms. Baseline VOR gains across frequencies (0.2 to 10 Hz) and velocities (20 to 100°/s) were measured on day 28 after unilateral labyrinthectomy (UL) and post-adaptation gains were measured after gain-increase training on day 31 post-UL. Our findings showed that after chronic compensation gain-increase adaptation, as a percentage of baseline, in both strains of mice (~14%), was about half compared to their previously reported healthy, non-operated counterparts (~32%). Surprisingly, there was no difference in gain-increase adaptation between control and α9-knockout mice. These data support the notion that adaptation and compensation are separate but overlapping processes. They also suggest that half of the original adaptation capacity remained in chronically compensated mice, regardless of EVS compromise associated with α9-knockout mice, and strongly suggest VOR adaptation training is a viable treatment strategy for vestibular rehabilitation therapy and, importantly, augments the compensatory process.

Reporting of exercise dose and dosage and outcome measures for gaze stabilisation in the literature: a scoping review

OBJECTIVES

The concept of this review is to examine and quantify the reporting of parameters of dose (duration, speed, head excursion) and dosage (daily and weekly frequency, duration) for gaze stabilisation exercises and to report on outcome measures used to assess change in gaze stabilisation following intervention. This review includes any population completing gaze stabilisation exercises.

DESIGN

Scoping review.

METHODS

We searched key terms in the following databases: PubMed, CINAHL, Scopus and Cochrane. Two researchers reviewed titles, abstracts and full-text articles for inclusion. Data retrieved included: patient diagnosis, specific interventions provided, dose and dosage of gaze stabilisation interventions and outcome measures.

RESULTS

From the initial 1609 results, 138 studies were included. Data extraction revealed that only 13 studies (9.4%) reported all parameters of dose and dosage. Most studies used other interventions in addition to gaze stabilisation exercises. Half of the studies did not use a clinical or instrumented outcome measure of gaze stability, using only patient-reported outcome measures. Clinical tests of gaze stability were used in 21.1% of studies, and instrumented measures of gaze stability were used in 14.7% of studies.

CONCLUSIONS

Full reporting of the dose and dosage of gaze stabilisation interventions is infrequent, impairing the ability to translate current evidence into clinical care. Most studies did not use a clinical or instrumented measure of gaze stabilisation as outcome measures, questioning the validity of intervention effects. Improved reporting and use of outcome measures are necessary to establish optimal intervention parameters for those with gaze stability impairments.

Vestibular Rehabilitation for Peripheral Vestibular Hypofunction: An Updated Clinical Practice Guideline From the Academy of Neurologic Physical Therapy of the American Physical Therapy Association

Background:

Uncompensated vestibular hypofunction can result in symptoms of dizziness, imbalance, and/or oscillopsia, gaze and gait instability, and impaired navigation and spatial orientation; thus, may negatively impact an individual's quality of life, ability to perform activities of daily living, drive, and work. It is estimated that one-third of adults in the United States have vestibular dysfunction and the incidence increases with age. There is strong evidence supporting vestibular physical therapy for reducing symptoms, improving gaze and postural stability, and improving function in individuals with vestibular hypofunction. The purpose of this revised clinical practice guideline is to improve quality of care and outcomes for individuals with acute, subacute, and chronic unilateral and bilateral vestibular hypofunction by providing evidence-based recommendations regarding appropriate exercises.

Methods:

These guidelines are a revision of the 2016 guidelines and involved a systematic review of the literature published since 2015 through June 2020 across 6 databases. Article types included meta-analyses, systematic reviews, randomized controlled trials, cohort studies, case-control series, and case series for human subjects, published in English. Sixty-seven articles were identified as relevant to this clinical practice guideline and critically appraised for level of evidence.

Results:

Based on strong evidence, clinicians should offer vestibular rehabilitation to adults with unilateral and bilateral vestibular hypofunction who present with impairments, activity limitations, and participation restrictions related to the vestibular deficit. Based on strong evidence and a preponderance of harm over benefit, clinicians should not include voluntary saccadic or smooth-pursuit eye movements in isolation (ie, without head movement) to promote gaze stability. Based on moderate to strong evidence, clinicians may offer specific exercise techniques to target identified activity limitations and participation restrictions, including virtual reality or augmented sensory feedback. Based on strong evidence and in consideration of patient preference, clinicians should offer supervised vestibular rehabilitation. Based on moderate to weak evidence, clinicians may prescribe weekly clinic visits plus a home exercise program of gaze stabilization exercises consisting of a minimum of: (1) 3 times per day for a total of at least 12 minutes daily for individuals with acute/subacute unilateral vestibular hypofunction; (2) 3 to 5 times per day for a total of at least 20 minutes daily for 4 to 6 weeks for individuals with chronic unilateral vestibular hypofunction; (3) 3 to 5 times per day for a total of 20 to 40 minutes daily for approximately 5 to 7 weeks for individuals with bilateral vestibular hypofunction. Based on moderate evidence, clinicians may prescribe static and dynamic balance exercises for a minimum of 20 minutes daily for at least 4 to 6 weeks for individuals with chronic unilateral vestibular hypofunction and, based on expert opinion, for a minimum of 6 to 9 weeks for individuals with bilateral vestibular hypofunction. Based on moderate evidence, clinicians may use achievement of primary goals, resolution of symptoms, normalized balance and vestibular function, or plateau in progress as reasons for stopping therapy. Based on moderate to strong evidence, clinicians may evaluate factors, including time from onset of symptoms, comorbidities, cognitive function, and use of medication that could modify rehabilitation outcomes.

Discussion:

Recent evidence supports the original recommendations from the 2016 guidelines. There is strong evidence that vestibular physical therapy provides a clear and substantial benefit to individuals with unilateral and bilateral vestibular hypofunction.

Limitations:

The focus of the guideline was on peripheral vestibular hypofunction; thus, the recommendations of the guideline may not apply to individuals with central vestibular disorders. One criterion for study inclusion was that vestibular hypofunction was determined based on objective vestibular function tests. This guideline may not apply to individuals who report symptoms of dizziness, imbalance, and/or oscillopsia without a diagnosis of vestibular hypofunction.

Disclaimer:

These recommendations are intended as a guide to optimize rehabilitation outcomes for individuals undergoing vestibular physical therapy. The contents of this guideline were developed with support from the American Physical Therapy Association and the Academy of Neurologic Physical Therapy using a rigorous review process. The authors declared no conflict of interest and maintained editorial independence.

Video Abstract available for more insights from the authors (see the Video, Supplemental Digital Content 1, available at: http://links.lww.com/JNPT/A369).

Comparison of Incremental Vestibulo-ocular Reflex Adaptation Training Versus x1 Training in Patients With Chronic Peripheral Vestibular Hypofunction: A Two-Year Randomized Controlled Trial

BACKGROUND AND PURPOSE

A crossover, double-blinded randomized controlled trial to investigate once-daily incremental vestibulo-ocular reflex (VOR) adaptation (IVA) training over 2 years in people with stable and chronic peripheral vestibular hypofunction.

METHODS

Twenty-one patients with peripheral vestibular hypofunction were randomly assigned to intervention-then-control (n = 12) or control-then-intervention (n = 9) groups. The task consisted of either x1 (control) or IVA training, once daily every day for 15 minutes over 6-months, followed by a 6-month washout, then repeated for arm 2 of the crossover. Primary outcome: vestibulo-ocular reflex gain. Secondary outcomes: compensatory saccades, dynamic visual acuity, static balance, gait, and subjective symptoms. Multiple imputation was used for missing data. Between-group differences were analyzed using a linear mixed model with repeated measures.

RESULTS

On average patients trained once daily 4 days per week. IVA training resulted in significantly larger VOR gain increase (active: 20.6% ± 12.08%, P = 0.006; passive: 30.6% ± 25.45%, P = 0.016) compared with x1 training (active: -2.4% ± 12.88%, P = 0.99; passive: -0.6% ± 15.31%, P = 0.68) (P < 0.001). The increased IVA gain did not significantly reduce with approximately 27% persisting over the washout period. x1 training resulted in greater reduction of compensatory saccade latency (P = 0.04) and increase in amplitude (P = 0.02) compared with IVA training. There was no difference between groups in gait and balance measures; however, only the IVA group had improved total Dizziness Handicap Inventory (P = 0.006).

DISCUSSION AND CONCLUSIONS

Our results suggest IVA improves VOR gain and reduces perception of disability more than conventional x1 training. We suggest at least 4 weeks of once-daily 4 days-per-week IVA training should be part of a comprehensive vestibular rehabilitation program.Video Abstract available for more insights from the authors (see the Video, Supplemental Digital Content 1, available at: http://links.lww.com/JNPT/A356).

Short-Latency Covert Saccades - The Explanation for Good Dynamic Visual Performance After Unilateral Vestibular Loss?

Background: Functional head impulse test (fHIT) tests the ability of the vestibulo-ocular reflex (VOR) to allow visual perception during head movements. Our previous study showed that active head movements to the side with a vestibular lesion generated a dynamic visual performance that were as good as during movements to the intact side. Objective: To examine the differences in eye position during the head impulse test when performed with active and passive head movements, in order to better understand the role of the different saccade properties in improving visual performance. Method: We recruited 8 subjects with complete unilateral vestibular loss (4 men and 4 women, mean age 47 years) and tested them with video Head Impulse Test (vHIT) and Functional Head Impulse Test (fHIT) during passive and active movements while looking at a target. We assessed the mean absolute position error of the eye during different time frames of the head movement, the peak latency and the peak velocity of the first saccade, as well as the visual performance during the head movement. Results: Active head impulses to the lesioned side generated dynamic visual performances that were as good as when testing the intact side. Active head impulses resulted in smaller position errors during the visual perception task (p = 0.006) compared to passive head-impulses and the position error during the visual perception time frame correlated with shorter latencies of the first saccade (p < 0.001). Conclusion: Actively generated head impulses toward the side with a complete vestibular loss resulted in a position error within or close to the margin necessary to obtain visual perception for a brief period of time in patients with chronic unilateral vestibular loss. This seems to be attributed to the appearance of short-latency covert saccades, which position the eyes in a more favorable position during head movements.

Once-Daily Incremental Vestibular-Ocular Reflex Adaptation Training in Patients With Chronic Peripheral Vestibular Hypofunction: A 1-Week Randomized Controlled Study

BACKGROUND AND PURPOSE

This was a double-blinded randomized controlled study to investigate the effects of once-daily incremental vestibulo-ocular reflex (VOR) training over 1 week in people with chronic peripheral vestibular hypofunction.

METHODS

A total of 24 patients with peripheral vestibular hypofunction were randomly assigned to intervention (n = 13) or control (n = 11) groups. Training consisted of either x1 (control) or incremental VOR adaptation exercises, delivered once daily for 15 minutes over 4 days in 1 week. Primary outcome: VOR gain with video-oculography. Secondary outcomes: Compensatory saccades measured using scleral search coils, dynamic visual acuity, static balance, gait, and subjective symptoms. Between-group differences were analyzed with a linear mixed-model with repeated measures.

RESULTS

There was a difference in the VOR gain increase between groups (P < 0.05). The incremental training group gain increased during active (13.4% ± 16.3%) and passive (12.1% ± 19.9%) head impulse testing (P < 0.02), whereas it did not for the control group (P = 0.59). The control group had reduced compensatory saccade latency (P < 0.02). Both groups had similarly improved dynamic visual acuity scores (P < 0.05). Both groups had improved dynamic gait index scores (P < 0.002); however, only the incremental group had improved scores for the 2 walks involving head oscillations at approximately 2 Hz (horizontal: P < 0.05; vertical: P < 0.02), increased gait speed (P < 0.02), and step length (P < 0.01) during normal gait, and improved total Dizziness Handicap Inventory (P < 0.05).

CONCLUSIONS

Our results suggest incremental VOR adaptation significantly improves gain, gait with head rotation, balance during gait, and symptoms in patients with chronic peripheral vestibular hypofunction more so than conventional x1 gaze-stabilizing exercises.Video Abstract available for more insights from the authors (see the Video, Supplemental Digital Content 1, available at: http://links.lww.com/JNPT/A336).

Two conditions to fully recover dynamic canal function in unilateral peripheral vestibular hypofunction patients

BACKGROUND

The crucial role of early vestibular rehabilitation (VR) to recover a dynamic semicircular canal function was recently highlighted in patients with unilateral vestibular hypofunction (UVH). However, wide inter-individual differences were observed, suggesting that parameters other than early rehabilitation are involved.

OBJECTIVE

The aim of the study was to determine to what extent the degree of vestibular loss assessed by the angular vestibulo-ocular reflex (aVOR) gain could be an additional parameter interfering with rehabilitation in the recovery process. And to examine whether different VR protocols have the same effectiveness with regard to the aVOR recovery.

METHODS

The aVOR gain and the percentage of compensatory saccades were recorded in 81 UVH patients with the passive head impulse test before and after early VR (first two weeks after vertigo onset: N = 43) or late VR (third to sixth week after onset: N = 38) performed twice a week for four weeks. VR was performed either with the unidirectional rotation paradigm or gaze stability exercises. Supplementary outcomes were the dizziness handicap inventory (DHI) score, and the static and dynamic subjective visual vertical.

RESULTS

The cluster analysis differentiated two distinct populations of UVH patients with pre-rehab aVOR gain values on the hypofunction side below 0.20 (N = 42) or above 0.20 (N = 39). The mean gain values were respectively 0.07±0.05 and 0.34±0.12 for the lateral canal (p < 0.0001), 0.09±0.06 and 0.44±0.19 for the anterior canal (p < 0.0001). Patients with aVOR gains above 0.20 and early rehab fully recovered dynamic horizontal canal function (0.84±0.14) and showed very few compensatory saccades (18.7% ±20.1%) while those with gains below 0.20 and late rehab did not improve their aVOR gain value (0.16±0.09) and showed compensatory saccades only (82.9% ±23.7%). Similar results were found for the anterior canal function. Recovery of the dynamic function of the lateral canal was found with both VR protocols while it was observed with the gaze stability exercises only for the anterior canal. All the patients reduced their DHI score, normalized their static SVV, and exhibited uncompensated dynamic SVV.

CONCLUSIONS

Early rehab is a necessary but not sufficient condition to fully recover dynamic canal function. The degree of vestibular loss plays a crucial role too, and to be effective rehabilitation protocols must be carried out in the plane of the semicircular canals.

Absence of a vergence-mediated vestibulo-ocular reflex gain increase does not preclude adaptation

BACKGROUND

The gain (eye-velocity/head-velocity) of the angular vestibuloocular reflex (aVOR) during head impulses can be increased while viewing near-targets and when exposed to unilateral, incremental retinal image velocity error signals. It is not clear however, whether the tonic or phasic vestibular pathways mediate these gain increases.

OBJECTIVE

Determine whether a shared pathway is responsible for gain enhancement between vergence and adaptation of aVOR gain in patients with unilateral vestibular hypofunction (UVH).

MATERIAL AND METHODS

20 patients with UVH were examined for change in aVOR gain during a vergence task and after 15-minutes of ipsilesional incremental VOR adaptation (uIVA) using StableEyes (a device that controls a laser target as a function of head velocity) during horizontal passive head impulses. A 5 % aVOR gain increase was defined as the threshold for significant change.

RESULTS

11/20 patients had >5% vergence-mediated gain increase during ipsi-lesional impulses. For uIVA, 10/20 patients had >5% ipsi-lesional gain increase. There was no correlation between the vergence-mediated gain increase and gain increase after uIVA training.

CONCLUSION

Vergence-enhanced and uIVA training gain increases are mediated by separate mechanisms and/or vestibular pathways (tonic/phasic). The ability to increase the aVOR gain during vergence is not prognostic for successful adaptation training.

Repeated video head impulse testing in patients is a stable measure of the passive vestibulo-ocular reflex

Objectives

The video head impulse test (vHIT) is used as a measure of compensation yet it's stability in patients with vestibular pathology is unknown.

Methods

144 patients (n = 72 female, mean 54.46 ± 15.8 years) were grouped into one of three primary diagnoses (Peripheral, Central, or Mixed). Subjects were further categorized based on sex (male versus female), ear (left versus right; ipsilesional versus contralesional), age (six groups ranging from 19 to 84 years), and duration between visits (five groups, mean 191.46 ± SE 29.42 days, median 55.5 days). The gain of the VOR during passive head rotation was measured for each semicircular canal (horizontal, anterior, posterior).

Results

There was no difference in the VOR gain within any semicircular canal between the two visits (horizontal: p = 0.179; anterior: p = 0.628; posterior: p = 0.613). However, the VOR gain from the horizontal canals was higher than the vertical canals for each visit (p < 0.001). Patients diagnosed with peripheral vestibular pathology had significantly lower (p ≤ 0.001) horizontal semicircular canal gains at each visit. There was no difference in VOR gain between sex (p = 0.215) or age groupings (p = 0.331). Test-retest reliability of vHIT in patient subjects is good (ICC = 0.801) and the VOR gain values across two separate visits were significant and positively correlated (r = 0.67) regardless of sex, ear, age, or duration between visits.

Conclusion

The vHIT is a stable measure of VOR gain over two different times across a variety of vestibular patients with no influence of age or sex.

Retinal Image Slip Must Pass the Threshold for Human Vestibulo-Ocular Reflex Adaptation

We sought to determine whether repeated vestibulo-ocular reflex (VOR) adaptation training to increase the VOR gain (eye/head velocity) had a lasting effect in normal subjects and whether there was a retinal image slip tolerance threshold for VOR adaptation. We used the unilateral incremental VOR adaptation technique and horizontal active (self-generated, predictable) head impulses as the vestibular stimulus. Both active and passive (imposed, unpredictable) head impulse VOR gains were measured before and after unilateral incremental VOR adaptation training. The adapting side was pseudo-randomized for left or right. We tested ten normal subjects over one block (10 sessions over 12 days) of VOR adaptation training and testing, immediately followed by a second block (5 sessions over 19 days) of testing only without training. Our findings show robust short-term VOR adaptation of ~ 10 % immediately after each 15-min training session, but that the daily pre-adaptation gain was most different on days 1 and 2, and for subsequent training days before saturating to ~ 5 % greater than the pre-adaptation gain on day 1. This increase was partially retained for 19 days after regular training stopped. The data suggest that stable vision in normal subjects is maintained when there is < 5 % deviation in VOR gain from the original baseline, which corresponds to < 9°/s retinal image slip. Below this threshold, there is poor adaptive drive to return the gain to its original baseline value.

Human vestibulo-ocular reflex adaptation is frequency selective

The vestibulo-ocular reflex (VOR) is the only system that maintains stable vision during rapid head rotations. The VOR gain (eye/head velocity) can be trained to increase using a vestibular-visual mismatch stimulus. We sought to determine whether low-frequency (sinusoidal) head rotation during training leads to changes in the VOR during high-frequency head rotation testing, where the VOR is more physiologically relevant. We tested eight normal subjects over three sessions. For training protocol 1, subjects performed active sinusoidal head rotations at 1.3 Hz while tracking a laser target, whose velocity incrementally increased relative to head velocity so that the VOR gain required to stabilize the target went from 1.1 to 2 over 15 min. Protocol 2 was the same as protocol 1, except that head rotations were at 0.5 Hz. For protocol 3, head rotation frequency incrementally increased from 0.5 to 2 Hz over 15 min, while the VOR gain required to stabilize the target was kept at 2. We measured the active and passive, sinusoidal (1.3Hz) and head impulse VOR gains before and after each protocol. Sinusoidal and head impulse VOR gains increased in protocols 1 and 3; however, although the sinusoidal VOR gain increase was ~20%, the related head impulse gain increase was only ~10%. Protocol 2 resulted in no-gain adaptation. These data show human VOR adaptation is frequency selective, suggesting that if one seeks to increase the higher-frequency VOR response, i.e., where it is physiologically most relevant, then higher-frequency head movements are required during training, e.g., head impulses.NEW & NOTEWORTHY This study shows that human vestibulo-ocular reflex adaptation is frequency selective at frequencies >0.3 Hz. The VOR in response to mid- (1.3 Hz) and high-frequency (impulse) head rotations were measured before and after mid-frequency sinusoidal VOR adaptation training, revealing that the mid-frequency gain change was higher than high-frequency gain change. Thus, if one seeks to increase the higher-frequency VOR response, where it is physiologically most relevant, then higher-frequency head movements are required during training.

New advances regarding adaptation of the vestibulo-ocular reflex

This is a review summarizing the development of vestibulo-ocular reflex (VOR) adaptation behavior with relevance to rehabilitation over the last 10 years and examines VOR adaptation using head-on-body rotations, specifically the influence of training target contrast, position and velocity error signal, active vs. passive head rotations, and sinusoidal vs. head impulse rotations. This review discusses optimization of the single VOR adaptation training session, consolidation between repeated training sessions, and dynamic incremental VOR adaptation. Also considered are the effects of aging and the roles of the efferent vestibular system, cerebellum, and otoliths on angular VOR adaptation. Finally, this review examines VOR adaptation findings in studies using whole body rotations.

VOR adaptation training and retention in a patient with profound bilateral vestibular hypofunction

A novel training method known as incremental VOR adaptation (IVA) can improve the vestibulo-ocular reflex (VOR) gain for both active and passive head rotation by coupling active head rotations with a laser-projected target that moves in the opposite direction of the head at a fraction of the head velocity. A 51-year-old male with bilateral vestibular hypofunction participated in a research protocol using a portable IVA device for 645 days. Passive VOR gains improved 179% to 600%; standing posture and gait also improved. Motor learning within the vestibular system using the IVA method is possible after severe vestibular pathology. Laryngoscope, 129:2568-2573, 2019.

Rebalancing the Vestibular System by Unidirectional Rotations in Patients With Chronic Vestibular Dysfunction

Introduction: Vestibular dysfunction is a common disorder that results in debilitating symptoms. Even after full compensation, the vestibulo-ocular reflex (VOR) could be further improved by using rehabilitation exercises and visual-vestibular adaptation. We hypothesized that in patients with asymmetric vestibular function, the system could be rebalanced by unidirectional rotations toward the weaker side (i.e., a pure vestibular stimulation).

Methods: Sixteen subjects (5 female and 11 male, 43.2 ± 17.0 years old) with chronic vestibular dysfunction that was non-responsive to other types of medical treatment were recruited for the study (ClinicalTrials.gov Identifier: NCT01080430). Subjects had VOR asymmetry quantified by an abnormal directional preponderance (DP) with rotation test and no previous history of central vestibular problems or fluctuating peripheral vestibular disorders. They participated either in the short-term study (one session) or the long-term study (7 visits over 5 weeks). Rehabilitation consisted of five trapezoid unidirectional rotations (peak velocity of 320°/s) toward the weaker side. Care was taken to slowly stop the rotation in order to avoid stimulation in the opposite direction during deceleration. To study the short-term effect, VOR responses were measured before and 10, 40, and 70 min after a single unidirectional rotational rehabilitation session. For long-term effects, the VOR gain was measured before and 70min after rehabilitation in each session.

Results: We observed a significant decrease in VOR asymmetry even 10 min after one rehabilitation session (short-term study). With consecutive rehabilitation sessions in the long-term study, DP further decreased to reach normal values during the first 2 sessions and only one subjects required further rehabilitation after week 4. This change in DP was due to an increase in responses during rotations toward the weaker side and a decrease in VOR responses during rotations in the other direction.

Conclusion: Our results show that unidirectional rotation can reduce the VOR imbalance and asymmetry in patients with previously compensated vestibular dysfunction and could be used as an effective supervised method for vestibular rehabilitation even in patients with longstanding vestibular dysfunction.

Simultaneous and opposing horizontal VOR adaptation in humans suggests functionally independent neural circuits

The healthy vestibulo-ocular reflex (VOR) ensures that images remain on the fovea of the retina during head rotation to maintain stable vision. VOR behavior can be measured as a summation of linear and nonlinear properties although it is unknown whether asymmetric VOR adaptation can be performed synchronously in humans. The purpose of the present study is twofold. First, examine whether the right and left VOR gains can be synchronously adapted in opposing directions. Second, to investigate whether the adaptation context transfers between both sides. Three separate VOR adaptation sessions were randomized such that the VOR was adapted Up-bilaterally, Down-bilaterally, or Mixed (one side up, opposite side down). Ten healthy subjects completed the study. Subjects were tested while seated upright, 1 meter in front of a wall in complete dark. Each subject made active (self-generated) head impulse rotations for 15 min while viewing a gradually increasing amount of retinal slip. VOR training demand changed by 10% every 90 s. The VOR changed significantly for all training conditions. No significant differences in the magnitude of VOR gain changes between training conditions were found. The human VOR can be simultaneously driven in opposite directions. The similar magnitude of VOR gain changes across training conditions suggests functionally independent VOR circuits for each side of head rotation that mediate simultaneous and opposing VOR adaptations.

NEW & NOTEWORTHY Our results indicate that humans have the adaptive capacity for concurrent and opposing directions of vestibulo-ocular reflex (VOR) motor learning. Context specificity of VOR adaptation is dependent on the error signal being unilateral or bilateral, which we illustrate via a lack of VOR gain transfer using unique adaptive demands.

Human Vestibulo-Ocular Reflex Adaptation Training: Time Beats Quantity

The vestibulo-ocular reflex (VOR) is the main gaze stabilising system during rapid head movements. The VOR is highly plastic and its gain (eye/head velocity) can be increased via training that induces an incrementally increasing retinal image slip error signal to drive VOR adaptation. Using the unilateral incremental VOR adaptation technique and horizontal active head impulses as the vestibular stimulus, we sought to determine the factors important for VOR adaptation including: the total training time, ratio and number of head impulses to each side (adapting and non-adapting sides; the adapting side was pseudo-randomised left or right) and exposure time to the visual target during each head impulse. We tested 11 normal subjects, each over 5 separate sessions and training protocols. The basic training protocol (protocol one) consisted of unilateral incremental VOR adaptation training lasting 15 min with the ratio of head impulses to each side 1:1. Each protocol varied from the basic. For protocol two, the ratio of impulses were in favour of the adapting side by 2:1. For protocol three, all head impulses were towards the adapting side and the training only lasted 7.5 min. For protocol four, all impulses were towards the adapting side and lasted 15 min. For protocol five, all head impulses were to the adapting side and the exposure time to the visual target during each impulse was doubled. We measured the active and passive VOR gains before and after the training. Albeit with small sample size, our data suggest that the total training time and the visual target exposure time for each head impulse affected adaptation, whereas the total number and repetition rate of head impulses did not. These data have implications for vestibular rehabilitation, suggesting that quality and duration of VOR adaptation exercises are more important than rapid repetition of exercises.

Human Vestibulo-Ocular Reflex Adaptation: Consolidation Time Between Repeated Training Blocks Improves Retention

We sought to determine if separating vestibulo-ocular reflex (VOR) adaptation training into training blocks with a consolidation (rest) period in between repetitions would result in improved VOR adaptation and retention. Consolidation of motor learning refers to the brain benefitting from a rest period after prior exposure to motor training. The role of consolidation on VOR adaptation is unknown, though clinicians often recommend rest periods as a part of vestibular rehabilitation. The VOR is the main gaze stabilising system during rapid head movements. The VOR is highly plastic and its gain (eye/head velocity) can be increased via training that induces an incrementally increasing retinal image slip error signal to drive VOR adaptation. The unilateral incremental adaptation technique typically consists of one 15-min training block leading to an increase in VOR gain of ~ 10 % towards the training side. We tested nine normal subjects, each over six separate sessions/days. Three training protocols/sessions were 5 min each (1 × 5-min training) and three training protocols/sessions were 55 min each. Each 55-min protocol comprised 5-min training, 20-min rest, 5-min training, 20-min rest, 5-min training (3 × 5-min training). Active and passive VOR gains were measured before and after training. For training with consolidation breaks, VOR gain retention was measured over 1 h. The VOR gain increase after 1 × 5-min training was 3.1 ± 2.1 % (P < 0.01). One might expect that repeating this training three times would result in × 3 total increase of 9.3 %; however, the gain increase after 3 × 5-min training was only 7.1 ± 2.8 % (P < 0.001), suggesting that consolidation did not improve VOR adaptation for our protocols. However, retention was improved by the addition of consolidation breaks, i.e. gains did not decrease over 1 h (P = 0.43). These data suggest that for optimal retention VOR adaptation exercises should be performed over shorter repeated blocks.

Acute Bilateral Superior Branch Vestibular Neuropathy

The rapid onset of a bilateral vestibular hypofunction (BVH) is often attributed to vestibular ototoxicity. However, without any prior exposure to ototoxins, the idiopathic form of BVH is most common. Although sequential bilateral vestibular neuritis (VN) is described as a cause of BVH, clinical evidence for simultaneous and acute onset bilateral VN is unknown. We describe a patient with an acute onset of severe gait ataxia and oscillopsia with features compatible with acute BVH putatively due to a bilateral VN, which we serially evaluated with clinical and laboratory vestibular function testing over the course of 1 year. Initially, bilateral superior and horizontal semicircular canals and bilateral utricles were impaired, consistent with damage to both superior branches of each vestibular nerve. Hearing was spared. Only modest results were obtained following 6 months of vestibular rehabilitation. At a 1-year follow-up, only the utricular function of one side recovered. This case is the first evidence supporting an acute presentation of bilateral VN as a cause for BVH, which would not have been observed without critical assessment of each of the 10 vestibular end organs.

Optimal Human Passive Vestibulo-Ocular Reflex Adaptation Does Not Rely on Passive Training

The vestibulo-ocular reflex (VOR) is the main vision-stabilising system during rapid head movements in humans. A visual-vestibular mismatch stimulus can be used to train or adapt the VOR response because it induces a retinal image slip error signal that drives VOR motor learning. The training context has been shown to affect VOR adaptation. We sought to determine whether active (self-generated) versus passive (externally imposed) head rotation vestibular training would differentially affect adaptation and short-term retention of the active and passive VOR responses. Ten subjects were tested, each over six separate 1.5-h sessions. We compared active versus passive head impulse (transient, rapid head rotations with peak velocity ~ 150 °/s) VOR adaptation training lasting 15 min with the VOR gain challenged to increment, starting at unity, by 0.1 every 90 s towards one side only (this adapting side was randomised to be either left or right). The VOR response was tested/measured in darkness at 10-min intervals, 20-min intervals, and two single 60-min interval sessions for 1 h post-training. The training was active or passive for the 10- and 20-min interval sessions, but only active for the two single 60-min interval sessions. The mean VOR response increase due to training was ~ 10 % towards the adapting side versus ~2 % towards the non-adapting side. There was no difference in VOR adaptation and retention between active and passive VOR training. The only factor to affect retention was exposure to a de-adaptation stimulus. These data suggest that active VOR adaptation training can be used to optimally adapt the passive VOR and that adaptation is completely retained over 1 h as long as there is no visual feedback signal driving de-adaptation.

The Effect of Visual Contrast on Human Vestibulo-Ocular Reflex Adaptation

The vestibulo-ocular reflex (VOR) is the main retinal image stabilising mechanism during rapid head movement. When the VOR does not stabilise the world or target image on the retina, retinal image slip occurs generating an error signal that drives the VOR response to increase or decrease until image slip is minimised, i.e. VOR adaptation occurs. Visual target contrast affects the human smooth pursuit and optokinetic reflex responses. We sought to determine if contrast also affected VOR adaptation. We tested 12 normal subjects, each over 16 separate sessions. For sessions 1-14, the ambient light level (lx) during adaptation training was as follows: dark, 0.1, 0.2, 0.3, 0.5, 0.7, 1, 2, 8, 16, 32, 64, 128 and 255 lx (light level for a typical room). For sessions 15-16, the laser target power (related to brightness) was halved with ambient light at 0 and 0.1 lx. The adaptation training lasted 15 min and consisted of left/right active head impulses. The VOR gain was challenged to increment, starting at unity, by 0.1 every 90 s for rotations to the designated adapting side and fixed at unity towards the non-adapting side. We measured active and passive VOR gains before and after adaptation training. We found that for both the active and passive VOR, there was a significant increase in gain only towards the adapting side due to training at contrast level 1.5 k and above (2 lx and below). At contrast level 261 and below (16 lx and above), adaptation training resulted in no difference between adapting and non-adapting side gains. Our modelling suggests that a contrast threshold of ~ 1000, which is 60 times higher than that provided by typical room lighting, must be surpassed for robust active and passive VOR adaptation. Our findings suggest contrast is an important factor for adaptation, which has implication for rehabilitation programs.

Stability of the aVOR to Repeat Head Impulse Testing

OBJECTIVE

The angular vestibulo-ocular reflex (aVOR) is known to be influenced by factors such as arousal and cognition during traditional vestibular function testing. However, the inherent variability of the aVOR to head impulse testing has not been explicitly examined. The purpose of this study was to determine the variability of the aVOR to active and passive head impulses using the gold standard scleral search coil method to record head and eye rotation.

STUDY DESIGN

Descriptive.

SETTING

Tertiary referral center.

PATIENTS

Twenty six healthy control subjects agreed to active and passive horizontal head impulse testing on at least two separate sessions from two unique institutions. An additional 27 individuals with cochlear implantation (CI) underwent passive horizontal and vertical semicircular canal plane head impulse testing. Test sessions were separated from 3 to 210 days in the normal subjects and from 49 to 537 days in the subjects with CI.

MAIN OUTCOME MEASURE(S)

Reliability of the angular VOR gain (eye velocity/head velocity) over time.

RESULTS

In the healthy control subjects, there was no difference in aVOR gain between right and left ears, between session one and session two, or between active (self-generated, 0.99 ± 0.08) or passive (imposed, 1.0 ± 0.08) head impulses. In the patients, we also found the aVOR gain very stable over time. However, the aVOR gains of the patients were different across the semicircular canal planes tested (p < 0.001) with the four vertical semicircular canals having lower aVOR gains than the two horizontal canals.

CONCLUSIONS

Our data suggest the aVOR gain is quite stable when tested across unique days in healthy controls and patients with auditory-only inner ear pathology.

Vestibular Compensation in Unilateral Patients Often Causes Both Gain and Time Constant Asymmetries in the VOR

The vestibulo-ocular reflex (VOR) is essential in our daily life to stabilize retinal images during head movements. Balanced vestibular functionality secures optimal reflex performance which otherwise can be distorted by peripheral vestibular lesions. Luckily, vestibular compensation in different neuronal sites restores VOR function to some extent over time. Studying vestibular compensation gives insight into the possible mechanisms for plasticity in the brain. In this work, novel experimental analysis tools are employed to reevaluate the VOR characteristics following unilateral vestibular lesions and compensation. Our results suggest that following vestibular lesions, asymmetric performance of the VOR is not only limited to its gain. Vestibular compensation also causes asymmetric dynamics, i.e., different time constants for the VOR during leftward or rightward passive head rotation. Potential mechanisms for these experimental observations are provided using simulation studies.