Horizontal vestibuloocular reflex evoked by high-acceleration rotations in the squirrel monkey. I. Normal responses

Corrective saccades in acute vestibular neuritis: studying the role of prediction with automated passively induced head impulses

When the demands for visual stabilization during head rotations overwhelm the ability of the vestibuloocular reflex (VOR) to produce compensatory eye movements, the brain produces corrective saccades that bring gaze toward the fixation target, even without visual cues (covert saccades). What triggers covert saccades and what might be the role of prediction in their generation are unknown. We studied 14 subjects with acute vestibular neuritis. To minimize variability of the stimulus, head impulses were imposed with a motorized torque generator with the subject on a bite bar. Predictable and unpredictable (timing, amplitude, direction) stimuli were compared. Distributions of covert corrective saccade latencies were analyzed with a "LATER" (linear approach to threshold with ergodic rate) approach. On the affected side, VOR gain was higher (0.47 ± 0.28 vs. 0.39 ± 0.22, P ≪ 0.001) with predictable than unpredictable head impulses, and gaze error at the end of the head movement was less (5.4 ± 3.3° vs. 6.9 ± 3.3°, P ≪ 0.001). Analyzing trials with covert saccades, gaze error at saccade end was significantly less with predictable than unpredictable head impulses (4.2 ± 2.8° vs. 5.5 ± 3.2°, P ≪ 0.001). Furthermore, covert corrective saccades occurred earlier with predictable than unpredictable head impulses (140 ± 37 vs. 153 ± 37 ms, P ≪ 0.001). Using a LATER analysis with reciprobit plots, we were able to divide covert corrective saccades into two classes, early and late, with a break point in the range of 88-98 ms. We hypothesized two rise-to-threshold decision mechanisms for triggering early and late covert corrective saccades, with the first being most engaged when stimuli are predictable.NEW & NOTEWORTHY We successfully used a LATER (linear approach to threshold with ergodic rate) analysis of the latencies of corrective saccades in patients with acute vestibular neuritis. We found two types of covert saccades: early (<90 ms) and late (>90 ms) covert saccades. Predictability led to an increase in VOR gain and a decrease in saccade latency.

Vestibulo-ocular reflex gain improvements at peak head acceleration and velocity following onset of unilateral vestibular neuritis: Insights into neural compensation mechanisms

BACKGROUND AND AIMS

An acute unilateral peripheral vestibular deficit (aUPVD) due to vestibular neuritis causes deficient yaw axis vestibular ocular reflex (VOR) gains. Using video head impulse tests (vHITs), we examined phasic and tonic velocity gains of the VOR over time to determine if these differed at onset and during subsequent improvement.

METHODS

The VOR responses of 61 patients were examined within 5 days of aUPVD onset, and 3 and 7 weeks later using vHIT with mean peak yaw angular velocities of 177°/s (sd 45°/s) and mean peak accelerations of 3660°/s2 (sd 1300°/s2). The phasic velocity or acceleration gain (aG) was computed as the ratio of eye to head velocity around peak head acceleration, and the tonic velocity gain (vG) was calculated as the same ratio around peak head velocity.

RESULTS

aG increased ipsi-deficit from 0.45 at onset to 0.67 at 3 weeks and 7 weeks later, and vG increased ipsi-deficit from 0.29 to 0.51 and 0.53, respectively, yielding a significant time effect (p < 0.001). Deficit side aG was significantly greater (p < 0.001) than vG at all time points. Deficit side gain improvements in aG and vG were similar. Contra-deficit aG increased from 0.86 to 0.95 and 0.94 at 3 weeks and 7 weeks, and vG contra-deficit increased from 0.84, to 0.89 and 0.87, respectively, also yielding a significant time effect (p = 0.004). Contra-deficit aG and vG were normal at 3 weeks. Mean canal paresis values improved from 91% to 67% over the 7 weeks.

CONCLUSIONS

Acceleration and velocity VOR gains on the deficit side are reduced by aUPVD and improve most in the first 3 weeks after aUPVD onset. Deficit side aG is consistently higher than deficit side vG following an aUPVD, suggesting that acceleration rather than velocity sensitive compensatory neural mechanisms are predominant during the compensation process for aUPVD.

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.

A Once-Daily High Dose of Intraperitoneal Ascorbate Improves Vestibulo-ocular Reflex Compensation After Unilateral Labyrinthectomy in the Mouse

Ascorbate potentiates the response of nicotinic-acetylcholine-receptors containing α9 and α10 subunits found predominantly in the efferent systems of the inner ear, such as the efferent vestibular system (EVS). Prior mouse studies have shown that an attenuated EVS results in reduced vestibulo-ocular reflex (VOR) gain (=eye_velocity/head_velocity) plasticity in intact (VOR adaptation) and surgically-lesioned (VOR compensation) mice. We sought to determine whether ascorbate-treatment could improve VOR recovery after vestibular organ injury, possibly through potentiation of the EVS pathway. We tested 10 cba129 mice, 5 received ascorbate-treatment and 5 did not, but otherwise experienced the same conditions. Ascorbate-treatment comprised a once-daily intraperitoneal injection of L-form reduced ascorbate (4 g/kg) in 0.2 ml saline starting 1 week before, and ending 4 weeks after, unilateral labyrinthectomy surgery. These were deliberately high doses to determine the ascorbate effects on recovery. Baseline, acute, and chronic sinusoidal VOR gains (frequency and velocity ranges: 0.2-10 Hz, 20-100 deg/s) were measured 3-5 days before, 3-5 days after, and 28-31 days after labyrinthectomy. Mice treated with ascorbate had acute ipsilesional VOR gains 12 % higher compared to control mice (+45.2 ± 14.9 % from baseline versus +33.7 ± 15.4 %, P < 0.001). Similarly, chronic ipsilesional and contralesional VOR gains were respectively 16 % (+74.3 ± 16.3 % from baseline versus +58.1 ± 15.8 %, P < 0.001) and 13 % (+78.6 ± 16.0 % versus +65.6 ± 10.9 %, P < 0.001) higher compared to control mice. These data suggest ascorbate-treatment had a prophylactic effect reducing acute loss, and helped recovery during acute to chronic stages of compensation. One possible mechanism is that an ascorbate-enhanced EVS drives an increase in the number and sensitivity of irregular-discharging primary vestibular afferents, important for VOR plasticity.

Vestibular Precision at the Level of Perception, Eye Movements, Posture, and Neurons

Precision and accuracy are two fundamental properties of any system, including the nervous system. Reduced precision (i.e., imprecision) results from the presence of neural noise at each level of sensory, motor, and perceptual processing. This review has three objectives: (1) to show the importance of studying vestibular precision, and specifically that studying accuracy without studying precision ignores fundamental aspects of the vestibular system; (2) to synthesize key hypotheses about precision in vestibular perception, the vestibulo-ocular reflex, posture, and neurons; and (3) to show that groups of studies that are thoughts to be distinct (e.g., perceptual thresholds, subjective visual vertical variability, neuronal variability) are actually "two sides of the same coin" - because the methods used allow results to be related to the standard deviation of a Gaussian distribution describing the underlying neural noise. Vestibular precision varies with age, stimulus amplitude, stimulus frequency, body orientation, motion direction, pathology, medication, and electrical/mechanical vestibular stimulation, but does not vary with sex. The brain optimizes precision during integration of vestibular cues with visual, auditory, and/or somatosensory cues. Since a common concern with precision metrics is time required for testing, we describe approaches to optimize data collection and provide evidence that fatigue and session effects are minimal. Finally, we summarize how precision is an individual trait that is correlated with clinical outcomes in patients as well as with performance in functional tasks like balance. These findings highlight the importance of studying vestibular precision and accuracy, and that knowledge gaps remain.

The instantaneous training demand drives vestibulo-ocular reflex adaptation

The vestibulo-ocular reflex (VOR) maintains stable vision during rapid head rotations by rotating the eyes in the opposite direction to the head. The latency between onset of the head rotation and onset of the eye rotation is 5-8 ms in healthy humans. However, VOR latency can be 3-4 times larger in patients treated with intra-tympanic gentamicin. A prior study showed that latency can be trained with head rotations at 0.2 Hz. We sought to determine how the VOR is affected when a delay between vestibular and visual stimuli is added during adaptation training with high-frequency head rotations (impulses), where the VOR is the main vision-stabilizing system. Using a variant of the incremental VOR adaptation technique, the delay between head rotation onset and movement onset of a visual target was gradually increased. With this training, the instantaneous VOR gain demand (= target/head velocity) varied from less than unity to greater than unity during each head impulse, albeit in a consistent and repeatable way. We measured the active and passive VOR gain and latency before and after VOR adaptation training in healthy normal subjects. There was no significant change in VOR latency across subjects; however, there was a significant decrease in VOR gain of - 6.0 ± 4.5%. These data suggest that during high-frequency head rotations delay/latency is interpreted as a changing instantaneous VOR gain demand. Although the gain demand in this study had a complex trajectory, adaptation was evident with the VOR seeming to use an average of the instantaneous gain demand.

Vestibulo-ocular reflex characteristics during unidirectional translational whole-body vibration without head restriction

The vestibulo-ocular reflex (VOR) plays a crucial role in ocular stability. However, VOR characteristics under realistic whole-body vibration conditions, particularly without head restriction, remain unclear. The aim of this study was to characterise the VOR over a wide range of whole-body vibration frequencies (0.7-10 Hz), such as occur when driving a car. Eye and head movements were measured in response to unidirectional translational whole-body vibration that resembled actual vehicle vibrations. The VOR was then modelled by regressing eye velocity data on multiple head movement components. Results showed that the VOR was explained by angular velocity, linear acceleration, and linear jerk components of the head movements. Because the VOR in response to head linear-jerk components disrupted ocular stability in the current experimental setup, our results suggest that degraded vision in whole-body vibratory environments might be partially attributable to jerky head movements. Practitioner summary: The vestibulo-ocular reflex (VOR) during unidirectional translational whole-body vibration without head restriction was modelled using multiple head movement components, with the aim of characterising the VOR. Results showed that the VOR was explained by angular velocity, linear acceleration, and linear jerk components of head movements.

Residual vestibular function after vestibular schwannoma surgery

OBJECTIVES

This study aimed to assess vestibular function in 39 patients who underwent neurectomy for vestibular schwannoma.

METHOD

Semicircular canal reactivity was measured by video head-impulse test using high-frequency passive head acceleration. Response gain was calculated as a ratio between the areas under the eye-velocity curve and the head-velocity curve.

STATISTICAL ANALYSIS

Student t-test was used for to compare quantitative variables. ANOVA was used to test inter-group differences in categoric variables.

RESULTS

In all cases, surgery-side gain on head impulse test was low, with increased gain asymmetry. A subgroup of 7 patients (18%) showed relatively high gain in vestibulo-ocular reflex on the surgery side. Caloric reaction was absent in all cases. These findings indicate that residual vestibular function can be conserved following vestibular schwannoma extirpation.

CONCLUSION

Cases with moderate vestibulo-ocular reflex gain were a subgroup with partial conservation of vestibular nerve fibers. Whether this is a predictor of better functional prognosis remains to be elucidated. Higher gain correlated with less extensive surgery and sparing of the inferior vestibular nerve. Low gain correlated with complete vestibular neurectomy. This information may guide rehabilitation strategy following surgery.

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.

Are Covert Saccade Functionally Relevant in Vestibular Hypofunction?

The vestibulo-ocular reflex maintains gaze stabilization during angular or linear head accelerations, allowing adequate dynamic visual acuity. In case of bilateral vestibular hypofunction, patients use saccades to compensate for the reduced vestibulo-ocular reflex function, with covert saccades occurring even during the head displacement. In this study, we questioned whether covert saccades help maintain dynamic visual acuity, and evaluated which characteristic of these saccades are the most relevant to improve visual function. We prospectively included 18 patients with chronic bilateral vestibular hypofunction. Subjects underwent evaluation of dynamic visual acuity in the horizontal plane as well as video recording of their head and eye positions during horizontal head impulse tests in both directions (36 ears tested). Frequency, latency, consistency of covert saccade initiation, and gain of covert saccades as well as residual vestibulo-ocular reflex gain were calculated. We found no correlation between residual vestibulo-ocular reflex gain and dynamic visual acuity. Dynamic visual acuity performance was however positively correlated with the frequency and gain of covert saccades and negatively correlated with covert saccade latency. There was no correlation between consistency of covert saccade initiation and dynamic visual acuity. Even though gaze stabilization in space during covert saccades might be of very short duration, these refixation saccades seem to improve vision in patients with bilateral vestibular hypofunction during angular head impulses. These findings emphasize the need for specific rehabilitation technics that favor the triggering of covert saccades. The physiological origin of covert saccades is discussed.

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.

Angular vestibuloocular reflex responses in Otop1 mice. II. Otolith sensor input improves compensation after unilateral labyrinthectomy

The role of the otoliths in mammals in the normal angular vestibuloocular reflex (VOR) was characterized in an accompanying study based on the Otopetrin1 (Otop1) mouse, which lacks functioning otoliths because of failure to develop otoconia but seems to have otherwise normal peripheral anatomy and neural circuitry. That study showed that otoliths do not contribute to the normal horizontal (rotation about Earth-vertical axis parallel to dorso-ventral axis) and vertical (rotation about Earth-vertical axis parallel to interaural axis) angular VOR but do affect gravity context-specific VOR adaptation. By using these animals, we sought to determine whether the otoliths play a role in the angular VOR after unilateral labyrinthectomy when the total canal signal is reduced. In five Otop1 mice and five control littermates we measured horizontal and vertical left-ear-down and right-ear-down sinusoidal VOR (0.2–10 Hz, 20–100°/s) during the early (3–5 days) and plateau (28–32 days) phases of compensation after unilateral labyrinthectomy and compared these measurements with baseline preoperative responses from the accompanying study. From similar baselines, acute gain loss was ~25% less in control mice, and chronic gain recovery was ~40% more in control mice. The acute data suggest that the otoliths contribute to the angular VOR when there is a loss of canal function. The chronic data suggest that a unilateral otolith signal can significantly improve angular VOR compensation. These data have implications for vestibular rehabilitation of patients with both canal and otolith loss and the development of vestibular implants, which currently only mimic the canals on one side.

NEW & NOTEWORTHY This is the first study examining the role of the otoliths (defined here as the utricle and saccule) on the acute and chronic angular vestibuloocular reflex (VOR) after unilateral labyrinthectomy in an animal model in which the otoliths are reliably inactivated and the semicircular canals preserved. This study shows that the otolith signal is used to augment the acute angular VOR and help boost VOR compensation after peripheral injury.

Nonhuman primate vestibuloocular reflex responses to prosthetic vestibular stimulation are robust to pulse timing errors caused by temporal discretization

Electrical stimulation of vestibular afferent neurons to partially restore semicircular canal sensation of head rotation and the stabilizing reflexes that sensation supports has potential to effectively treat individuals disabled by bilateral vestibular hypofunction. Ideally, a vestibular implant system using this approach would be integrated with a cochlear implant, which would provide clinicians with a means to simultaneously treat loss of both vestibular and auditory sensation. Despite obvious similarities, merging these technologies poses several challenges, including stimulus pulse timing errors that arise when a system must implement a pulse frequency modulation-encoding scheme (as is used in vestibular implants to mimic normal vestibular nerve encoding of head movement) within fixed-rate continuous interleaved sampling (CIS) strategies used in cochlear implants. Pulse timing errors caused by temporal discretization inherent to CIS create stair step discontinuities of the vestibular implant's smooth mapping of head velocity to stimulus pulse frequency. In this study, we assayed electrically evoked vestibuloocular reflex responses in two rhesus macaques using both a smooth pulse frequency modulation map and a discretized map corrupted by temporal errors typical of those arising in a combined cochlear-vestibular implant. Responses were measured using three-dimensional scleral coil oculography for prosthetic electrical stimuli representing sinusoidal head velocity waveforms that varied over 50-400°/s and 0.1-5 Hz. Pulse timing errors produced negligible effects on responses across all canals in both animals, indicating that temporal discretization inherent to implementing a pulse frequency modulation-coding scheme within a cochlear implant's CIS fixed pulse timing framework need not sacrifice performance of the combined system's vestibular implant portion. NEW & NOTEWORTHY Merging a vestibular implant system with existing cochlear implant technology can provide clinicians with a means to restore both vestibular and auditory sensation. Pulse timing errors inherent to integration of pulse frequency modulation vestibular stimulation with fixed-rate, continuous interleaved sampling cochlear implant stimulation would discretize the smooth head velocity encoding of a combined device. In this study, we show these pulse timing errors produce negligible effects on electrically evoked vestibulo-ocular reflex responses in two rhesus macaques.

Eye Position Error Influence over "Open-Loop" Smooth Pursuit Initiation

The oculomotor system integrates a variety of visual signals into appropriate motor plans, but such integration can have widely varying time scales. For example, smooth pursuit eye movements to follow a moving target are slower and longer lasting than saccadic eye movements and it has been suggested that initiating a smooth pursuit eye movement involves an obligatory "open-loop" interval in which new visual motion signals presumably cannot influence the ensuing motor plan for up to 100 ms after movement initiation. However, this view is contrary to the idea that the oculomotor periphery has privileged access to short-latency visual signals. Here, we show that smooth pursuit initiation is sensitive to visual inputs, even in open-loop intervals. We instructed male rhesus macaque monkeys to initiate saccade-free smooth pursuit eye movements and injected a transient, instantaneous eye position error signal at different times relative to movement initiation. We found robust short-latency modulations in eye velocity and acceleration, starting only ∼50 ms after transient signal occurrence and even during open-loop pursuit initiation. Critically, the spatial direction of the injected position error signal had predictable effects on smooth pursuit initiation, with forward errors increasing eye acceleration and backward errors reducing it. Catch-up saccade frequencies and amplitudes were also similarly altered ∼50 ms after transient signals, much like the well known effects on microsaccades during fixation. Our results demonstrate that smooth pursuit initiation is highly sensitive to visual signals and that catch-up saccade generation is reset after a visual transient.SIGNIFICANCE STATEMENT Smooth pursuit eye movements allow us to track moving objects. The first ∼100 ms of smooth pursuit initiation are characterized by smooth eye acceleration and are overwhelmingly described as being "open-loop"; that is, unmodifiable by new visual motion signals. We found that all phases of smooth pursuit, including the so-called open-loop intervals, are reliably modifiable by visual signals. We injected transient flashes resulting in very brief, spatially specific position error signals to smooth pursuit and observed very short-latency changes in smooth eye movements to minimize such errors. Our results highlight the flexibility of the oculomotor system in reacting to environmental events and suggest a functional role for the pervasiveness of visual sensitivity in oculomotor control brain regions.

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.

Bilateral Vestibulopathy: Vestibular Function, Dynamic Visual Acuity and Functional Impact

Introduction: Patients suffering from bilateral vestibular hypofunction (BVH) often experience ataxia as well as visual instability. Even though progress has been made in vestibular testing, insights regarding vestibular deficit in BVH remain incomplete since no method allows evaluation of frequency ranges of vestibular sensors in a continuous way. The aim of our study was to give a detailed description of the level of vestibular deficit in different ranges of vestibular stimulation and an exhaustive evaluation of the functional impact including dynamic visual acuity (DVA) in a cohort of BVH patients in different etiologies.

Methods: We prospectively included 20 patients with chronic BVH. All patients underwent clinical evaluation and functional assessment including evaluation of their symptoms related to BVH, quality of life questionnaire and DVA in the horizontal and vertical plane. Patients underwent vestibulo-ocular reflex (VOR) testing using rotatory chair, caloric stimulation and video head impulse (vHIT) in the plane of the 6 canals, and cervical and ocular Vestibular evoked myogenic potentials.

Results: Mean rotatory VOR gain was 0.07 (SD = 0.07). Mean rotatory VOR gain during vHIT for the lateral, anterior and posterior canals was respectively < 0.28, < 0.34, and < 0.20. Mean loss of DVA in the 4 directions was >0.30 LogMAR. In our population fall frequency was significantly higher in patients with lower UniPedal Stance Test (UPST), higher Dizziness Handicap Inventory and Ataxia Numeric Scale (ANS) scores, as well as greater loss of upwards DVA. Patients with ototoxic BVH had a significantly higher residual VOR gain during vHIT in the anterior canal plane and lower DHI than other patients. In the general population anterior canal function was significantly higher than lateral or posterior canal function.

Conclusions: This study gives extensive descriptive results of residual vestibular function, DVA and quality of life in a population of patients suffering from severe BVH. UPST and ANS are good indicators for fall risk in case of BVH. Gentamicin induced BVH seems to have a lesser impact on quality of life than other etiologies.Anterior semi-circular canal function seems less deteriorated than lateral and posterior function.

Sound abnormally stimulates the vestibular system in canal dehiscence syndrome by generating pathological fluid-mechanical waves

Individuals suffering from Tullio phenomena experience dizziness, vertigo, and reflexive eye movements (nystagmus) when exposed to seemingly benign acoustic stimuli. The most common cause is a defect in the bone enclosing the vestibular semicircular canals of the inner ear. Surgical repair often corrects the problem, but the precise mechanisms underlying Tullio phenomenon are not known. In the present work we quantified the phenomenon in an animal model of the condition by recording fluid motion in the semicircular canals and neural activity evoked by auditory-frequency stimulation. Results demonstrate short-latency phase-locked afferent neural responses, slowly developing sustained changes in neural discharge rate, and nonlinear fluid pumping in the affected semicircular canal. Experimental data compare favorably to predictions of a nonlinear computational model. Results identify the biophysical origin of Tullio phenomenon in pathological sound-evoked fluid-mechanical waves in the inner ear. Sound energy entering the inner ear at the oval window excites fluid motion at the location of the defect, giving rise to traveling waves that subsequently excite mechano-electrical transduction in the vestibular sensory organs by vibration and nonlinear fluid pumping.

Reduced Purposeful Head Movements During Community Ambulation Following Unilateral Vestibular Loss

Background. Individuals with unilateral vestibular hypofunction (UVH) alter their movement and reduce mobility to try to stabilize their gaze and avoid symptoms of dizziness and vertigo. Objective. To determine if individuals with UVH 6 weeks after surgery demonstrate altered head and trunk kinematics during community ambulation. Methods. A total of 15 vestibular schwannoma patients with documented postoperative unilateral vestibular loss and 9 healthy controls with symmetrical vestibulo-ocular reflexes participated in this cross-sectional study. Head kinematics (head turn frequency, amplitude, and velocity) and head-trunk coordination during community ambulation were obtained from inertial measurement units for all head movements and within specific amplitudes of head movement. Results. Individuals with UVH made smaller (mean 26° [SD = 3°] vs 32° [SD = 6°]), fewer (mean 133 [SD = 59] vs 221 [SD = 64]), and slower (mean 75°/s [SD = 8°/s] vs 103°/s [SD = 23°/s]) head turns than healthy individuals ( P < .05) but did not demonstrate significantly increased head-trunk coupling (mean 38% [SD = 18%] vs 31% [SD = 11%], P = .22). When small (≤45°) and large (>45°) head turns were considered separately, individuals with UVH demonstrated increased head-trunk coupling compared with healthy individuals for large head turns (mean 54% [SD = 23%] vs 33% [SD = 10%], P = .005). Conclusions. This study demonstrated that although walking at an adequate speed, individuals with UVH made fewer, smaller, and slower head movements symmetrically in both directions compared with healthy individuals and did not decouple their head movement relative to their trunk when required to make larger purposeful head turns during community ambulation.

Cold Thermal Irrigation Decreases the Ipsilateral Gain of the Vestibulo-Ocular Reflex

OBJECTIVES

During head rotations, neuronal firing rates increase in ipsilateral and decrease in contralateral vestibular afferents. At low accelerations, this "push-pull mechanism" is linear. At high accelerations, however, the change of firing rates is nonlinear in that the ipsilateral increase of firing rate is larger than the contralateral decrease. This mechanism of stronger ipsilateral excitation than contralateral inhibition during high-acceleration head rotation, known as Ewald's second law, is implemented within the nonlinear pathways. The authors asked whether caloric stimulation could provide an acceleration signal high enough to influence the contribution of the nonlinear pathway to the rotational vestibulo-ocular reflex gain (rVOR gain) during head impulses.

DESIGN

Caloric warm (44°C) and cold (24, 27, and 30°C) water irrigations of the left ear were performed in 7 healthy human subjects with the lateral semicircular canals oriented approximately earth-vertical (head inclined 30° from supine) and earth-horizontal (head inclined 30° from upright).

RESULTS

With the lateral semicircular canal oriented earth-vertical, the strongest cold caloric stimulus (24°C) significantly decreased the rVOR gain during ipsilateral head impulses, while all other irrigations, irrespective of head position, had no significant effect on rVOR gains during head impulses to either side.

CONCLUSIONS

Strong caloric irrigation, which can only be achieved with cold water, reduces the rVOR gain during ipsilateral head impulses and thus demonstrates Ewald's second law in healthy subjects. This unilateral gain reduction suggests that cold-water caloric irritation shifts the set point of the nonlinear relation between head acceleration and the vestibular firing rate toward a less acceleration-sensitive zone.

Noisy vestibular stimulation increases gait speed in normals and in bilateral vestibulopathy

BACKGROUND

Galvanic vestibular stimulation delivered as zero-mean current noise (noisy GVS) has been shown to improve static and dynamic postural stability probably by enhancing vestibular information.

OBJECTIVE

/Hypothesis: To examine the effect of an imperceptible level of noisy GVS on dynamic locomotion in normal subjects as well as in patients with bilateral vestibulopathy.

METHODS

Walking performance of 19 healthy subjects and 12 patients with bilateral vestibulopathy at their preferred speed was examined during application of noisy GVS with an amplitude ranging from 0 to 1000 μA. The gait velocity, stride length and stride time were analyzed.

RESULTS

Noisy GVS had significant effects on gait velocity, stride length and stride time in healthy subjects as well as in patients with bilateral vestibulopathy (p < 0.05). The optimal amplitude of noisy GVS improved gait velocity by 10.9 ± 1.2%, stride length by 5.7 ± 1.2% and stride time by 4.6 ± 7% (p < 0.0001) compared to the control session in healthy subjects. The optimal stimulus improved gait velocity by 12.8 ± 1.3%, stride length by 8.3 ± 1.1% and stride time by 3.7 ± 7% (p < 0.0001) in patients with bilateral vestibulopathy. The improved values of these parameters of locomotion by noisy GVS in the patients were not significantly different from those in healthy subjects in the control condition (p > 0.4).

CONCLUSION

Noisy GVS is effective in improving gait performance in healthy subjects as well as in patients with bilateral vestibulopathy.

The modulation of hippocampal theta rhythm by the vestibular system

The vestibular system is a sensory system that has evolved over millions of years to detect acceleration of the head, both rotational and translational, in three dimensions. One of its most important functions is to stabilize gaze during unexpected head movement; however, it is also important in the control of posture and autonomic reflexes. Theta rhythm is a 3- to 12-Hz oscillating EEG signal that is intimately linked to self-motion and is also known to be important in learning and memory. Many studies over the last two decades have shown that selective activation of the vestibular system, using either natural rotational or translational stimulation, or electrical stimulation of the peripheral vestibular system, can induce and modulate theta activity. Furthermore, inactivation of the vestibular system has been shown to significantly reduce theta in freely moving animals, which may be linked to its impairment of place cell function as well as spatial learning and memory. The pathways through which vestibular information modulate theta rhythm remain debatable. However, vestibular responses have been found in the pedunculopontine tegmental nucleus (PPTg) and activation of the vestibular system causes an increase in acetylcholine release into the hippocampus, probably from the medial septum. Therefore, a pathway from the vestibular nucleus complex and/or cerebellum to the PPTg, supramammillary nucleus, posterior hypothalamic nucleus, and septum to the hippocampus is likely. The modulation of theta by the vestibular system may have implications for vestibular effects on cognitive function and the contribution of vestibular impairment to the risk of dementia.

Feasibility and Validity of Discriminating Yaw Plane Head-on-Trunk Motion Using Inertial Wearable Sensors

A consequence of vestibular loss is increased coupling of head-on-trunk motion, particularly in the yaw plane, which adversely affects community mobility in these patients. Inertial sensors may provide a means of better understanding normal decoupling behaviors in community environments, but demonstration of their validity and responsiveness is needed. This paper examined the validity and measurement sensitivity of inertial sensors in quantifying yaw plane head-trunk decoupling during unrestricted and restricted cervical motion conditions in healthy adults. Peak head turn amplitude and velocity, head-trunk coupling, and trunk turn lag were simultaneously measured using wearable inertial sensors and a motion capture system. Agreement between motion capture and the inertial sensors was excellent (intraclass correlation coefficients(2,1) >.75) for all measured outcomes during a static head turn task and for peak head turn velocity and trunk turn lag during a walking task. Cervical collar use significantly reduced head turn amplitude and velocity, and increased coupling of head-on-trunk motion (p<.02). Measurement of head and trunk coordination during gait activities using inertial sensors is valid and feasible. Amplitude and velocity outcomes were most reliable and responsive to experimental alterations in head motion. Using inertial sensors to quantify abnormal kinematics following vestibular loss may provide insights into recovery of head-trunk coordination in these individuals.

Core Body Temperature Effects on the Mouse Vestibulo-ocular Reflex

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

Rhesus Cochlear and Vestibular Functions Are Preserved After Inner Ear Injection of Saline Volume Sufficient for Gene Therapy Delivery

Sensorineural losses of hearing and vestibular sensation due to hair cell dysfunction are among the most common disabilities. Recent preclinical research demonstrates that treatment of the inner ear with a variety of compounds, including gene therapy agents, may elicit regeneration and/or repair of hair cells in animals exposed to ototoxic medications or other insults to the inner ear. Delivery of gene therapy may also offer a means for treatment of hereditary hearing loss. However, injection of a fluid volume sufficient to deliver an adequate dose of a pharmacologic agent could, in theory, cause inner ear trauma that compromises functional outcome. The primary goal of the present study was to assess that risk in rhesus monkeys, which closely approximates humans with regard to middle and inner ear anatomy. Secondary goals were to identify the best delivery route into the primate ear from among two common surgical approaches (i.e., via an oval window stapedotomy and via the round window) and to determine the relative volumes of rhesus, rodent, and human labyrinths for extrapolation of results to other species. We measured hearing and vestibular functions before and 2, 4, and 8 weeks after unilateral injection of phosphate-buffered saline vehicle (PBSV) into the perilymphatic space of normal rhesus monkeys at volumes sufficient to deliver an atoh1 gene therapy vector. To isolate effects of injection, PBSV without vector was used. Assays included behavioral observation, auditory brainstem responses, distortion product otoacoustic emissions, and scleral coil measurement of vestibulo-ocular reflexes during whole-body rotation in darkness. Three groups (N = 3 each) were studied. Group A received a 10 μL transmastoid/trans-stapes injection via a laser stapedotomy. Group B received a 10 μL transmastoid/trans-round window injection. Group C received a 30 μL transmastoid/trans-round window injection. We also measured inner ear fluid space volume via 3D reconstruction of computed tomography (CT) images of adult C57BL6 mouse, rat, rhesus macaque, and human temporal bones (N = 3 each). Injection was well tolerated by all animals, with eight of nine exhibiting no signs of disequilibrium and one animal exhibiting transient disequilibrium that resolved spontaneously by 24 h after surgery. Physiologic results at the final, 8-week post-injection measurement showed that injection was well tolerated. Compared to its pretreatment values, no treated ear's ABR threshold had worsened by more than 5 dB at any stimulus frequency; distortion product otoacoustic emissions remained detectable above the noise floor for every treated ear (mean, SD and maximum deviation from baseline: -1.3, 9.0, and -18 dB, respectively); and no animal exhibited a reduction of more than 3 % in vestibulo-ocular reflex gain during high-acceleration, whole-body, passive yaw rotations in darkness toward the treated side. All control ears and all operated ears with definite histologic evidence of injection through the intended site showed similar findings, with intact hair cells in all five inner ear sensory epithelia and intact auditory/vestibular neurons. The relative volumes of mouse, rat, rhesus, and human inner ears as measured by CT were (mean ± SD) 2.5 ± 0.1, 5.5 ± 0.4, 59.4 ± 4.7 and 191.1 ± 4.7 μL. These results indicate that injection of PBSV at volumes sufficient for gene therapy delivery can be accomplished without destruction of inner ear structures required for hearing and vestibular sensation.

Low-intensity ultrasound activates vestibular otolith organs through acoustic radiation force

The present study examined the efficacy of 5 MHz low-intensity focused ultrasound (LiFU) as a stimulus to remotely activate inner ear vestibular otolith organs. The otolith organs are the primary sensory apparati responsible for detecting orientation of the head relative to gravity and linear acceleration in three-dimensional space. These organs also respond to loud sounds and vibration of the temporal bone. The oyster toadfish, Opsanus tau, was used to facilitate unobstructed acoustic access to the otolith organs in vivo. Single-unit responses to amplitude-modulated LiFU were recorded in afferent neurons identified as innervating the utricle or the saccule. Neural responses were equivalent to direct mechanical stimulation, and arose from the nonlinear acoustic radiation force acting on the otolithic mass. The magnitude of the acoustic radiation force acting on the otolith was measured ex vivo. Results demonstrate that LiFU stimuli can be tuned to mimic directional forces occurring naturally during physiological movements of the head, loud air conducted sound, or bone conducted vibration.

The mammalian efferent vestibular system plays a crucial role in vestibulo-ocular reflex compensation after unilateral labyrinthectomy

The α9-nicotinic acetylcholine receptor (α9-nAChR) subunit is expressed in the vestibular and auditory periphery, and its loss of function could compromise peripheral input from the predominantly cholinergic efferent vestibular system (EVS). A recent study has shown that α9-nAChRs play an important role in short-term vestibulo-ocular reflex (VOR) adaptation. We hypothesize that α9-nAChRs could also be important for other forms of vestibular plasticity, such as that needed for VOR recovery after vestibular organ injury. We measured the efficacy of VOR compensation in α9 knockout mice. These mice have deletion of most of the gene (chrna9) encoding the nAChR and thereby lack α9-nAChRs. We measured the VOR gain (eye velocity/head velocity) in 20 α9 knockout mice and 16 cba129 controls. We measured the sinusoidal (0.2-10 Hz, 20-100°/s) and transient (1,500-6,000°/s2) VOR in complete darkness before (baseline) unilateral labyrinthectomy (UL) and then 1, 5, and 28 days after UL. On day 1 after UL, cba129 mice retained ~50% of their initial function for contralesional rotations, whereas α9 knockout mice only retained ~20%. After 28 days, α9 knockout mice had ~50% lower gain for both ipsilesional and contralesional rotations compared with cba129 mice. Cba129 mice regained ~75% of their baseline function for ipsilesional and ~90% for contralesional rotations. In contrast, α9 knockout mice only regained ~30% and ~50% function, respectively, leaving the VOR severely impaired for rotations in both directions. Our results show that loss of α9-nAChRs severely affects VOR compensation, suggesting that complimentary central and peripheral EVS-mediated adaptive mechanisms might be affected by this loss.NEW & NOTEWORTHY Loss of the α9-nicotinic acetylcholine receptor (α9-nAChR) subunit utilized by the efferent vestibular system (EVS) has been shown to significantly affect vestibulo-ocular reflex (VOR) adaptation. In our present study we have shown that loss of α9-nAChRs also affects VOR compensation, suggesting that the mammalian EVS plays an important role in vestibular plasticity, in general, and that VOR compensation is a more distributed process than previously thought, relying on both central and peripheral changes.

Effect of Noisy Galvanic Vestibular Stimulation on Ocular Vestibular-Evoked Myogenic Potentials to Bone-Conducted Vibration

OBJECTIVE

Galvanic vestibular stimulation (GVS) delivered as zero-mean current noise (noisy GVS) has been shown to improve static and dynamic postural stability probably by enhancing vestibular information. The purpose of this study was to examine the effect of an imperceptible level noisy GVS on ocular vestibular-evoked myogenic potentials (oVEMPs) in response to bone-conducted vibration (BCV).

MATERIALS AND METHODS

oVEMPs to BCV were measured during the application of white noise GVS with an amplitude ranging from 0 to 300 µA [in root mean square (RMS)] in 20 healthy subjects. Artifacts in the oVEMPs caused by GVS were reduced by inverting the waveforms of noisy GVS in the later half of the stimulus from the one in the early half. We examined the amplitudes of N1 and N1-P1 and their latencies.

RESULTS

Noisy GVS significantly increased the N1 and N1-P1 amplitudes (p < 0.05) whereas it had no significant effects on N1 or P1 latencies (p > 0.05). Noisy GVS had facilitatory effects in 79% of ears. The amplitude of the optimal stimulus was 127 ± 14 µA, and it increased the N1 and N1-P1 amplitude by 75.9 ± 15% and 47.7 ± 9.1%, respectively, as compared with 0 µA session (p < 0.05).

CONCLUSION

Noisy GVS can increase the amplitude of oVEMPs to BCV in healthy subjects probably via stochastic resonance. The results of the present study suggest that noisy GVS may improve static and dynamic postural stability by enhancing the function of the vestibular afferents.

Noisy galvanic vestibular stimulation induces a sustained improvement in body balance in elderly adults

Vestibular dysfunction causes postural instability, which is prevalent in the elderly. We previously showed that an imperceptible level of noisy galvanic vestibular stimulation (nGVS) can improve postural stability in patients with bilateral vestibulopathy during the stimulus, presumably by enhancing vestibular information processing. In this study, we investigated the after-effects of an imperceptible long-duration nGVS on body balance in elderly adults. Thirty elderly participants underwent two nGVS sessions in a randomised order. In Session 1, participants received nGVS for 30 min twice with a 4-h interval. In Session 2, participants received nGVS for 3 h. Two-legged stance tasks were performed with eyes closed while participants stood on a foam rubber surface, with and without nGVS, and parameters related to postural stability were measured using posturography. In both sessions, the postural stability was markedly improved for more than 2 h after the cessation of the stimulus and tended to decrease thereafter. The second stimulation in Session 1 caused a moderate additional improvement in body balance and promoted the sustainability of the improvement. These results suggest that nGVS can lead to a postural stability improvement in elderly adults that lasts for several hours after the cessation of the stimulus, probably via vestibular neuroplasticity.

Responses of non-eye-movement central vestibular neurons to sinusoidal yaw rotation in compensated macaques after unilateral semicircular canal plugging

After vestibular labyrinth injury, behavioral measures of vestibular performance recover to variable degrees (vestibular compensation). Central neuronal responses after unilateral labyrinthectomy (UL), which eliminates both afferent resting activity and sensitivity to movement, have been well-studied. However, unilateral semicircular canal plugging (UCP), which attenuates angular-velocity detection while leaving afferent resting activity intact, has not been extensively studied. The current study reports response properties of yaw-sensitive non-eye-movement rhesus macaque vestibular neurons after compensation from UCP. The responses at a series of frequencies (0.1-2 Hz) and peak velocities (15-210°/s) were compared between neurons recorded before and at least 6 wk after UCP. The gain (sp/s/°/s) of central type I neurons (responding to ipsilateral yaw rotation) on the side of UCP was reduced relative to normal controls at 0.5 Hz, ±60°/s [0.48 ± 0.30 (SD) normal, 0.32 ± 0.15 ipsilesion; 0.44 ± 0.2 contralesion]. Type II neurons (responding to contralateral yaw rotation) after UCP have reduced gain (0.40 ± 0.27 normal, 0.35 ± 0.25 ipsilesion; 0.25 ± 0.18 contralesion). The difference between responses after UCP and after UL is primarily the distribution of type I and type II neurons in the vestibular nuclei (type I neurons comprise 66% in vestibular nuclei normally; 51% ipsilesion UCP; 59% contralesion UCP; 38% ipsilesion UL; 65% contralesion UL) and the magnitude of the responses of type II neurons ipsilateral to the lesion. These differences suggest that the need to compensate for unilateral loss of resting vestibular nerve activity after UL necessitates a different strategy for recovery of dynamic vestibular responses compared to after UCP.

Vestibular Performance During High-Acceleration Stimuli Correlates with Clinical Decline in SCA6

In spinocerebellar ataxia type 6 (SCA6), the vestibular dysfunction and its correlation with other clinical parameters require further exploration. We determined vestibular responses over a broad range of stimulus acceleration in 11 patients with SCA6 (six men, age range=33-72 years, mean age±SD=59±12 years) using bithermal caloric irrigations, rotary chair, and head impulse tests. Correlations were also pursued among disability scores, as measured using the International Cooperative Ataxia Rating Scale, disease duration, age at onset, cytosine-adenine-guanine (CAG) repeat length, and the gain of the vestibulo-ocular reflex (VOR). In response to relatively low-acceleration, low-frequency rotational and bithermal caloric stimuli, the VOR gains were normal or increased regardless of the severity of disease. On the other hand, with relatively high-acceleration, high-frequency head impulses, there was a relative increase in gain in the mildly affected patients and a decrease in gain in the more severely affected patients and gains were negatively correlated with the severity of disease (Spearman correlation, R=-0.927, p<0.001). Selective decrease of the vestibular responses during high-acceleration, high-frequency stimuli may be ascribed to degeneration of either the flocculus or vestibular nuclei. The performance of the VOR during high-acceleration, high-frequency head impulses may be a quantitative indicator of clinical decline in SCA6.

The role of GABAB receptors in the vestibular oculomotor system in mice

Systemic administration of a gamma-amino butyric acid type B (GABAB) receptor agonist, baclofen, affects various physiological and psychological processes. To date, the effects on oculomotor system have been well characterized in primates, however those in mice have not been explored. In this study, we investigated the effects of baclofen focusing on vestibular-related eye movements. Two rotational paradigms, i.e. sinusoidal rotation and counter rotation were employed to stimulate semicircular canals and otolith organs in the inner ear. Experimental conditions (dosage, routes and onset of recording) were determined based on the prior studies exploring the behavioral effects of baclofen in mice. With an increase in dosage, both canal and otolith induced ocular responses were gradually affected. There was a clear distinction in the drug sensitivity showing that eye movements derived from direct vestibulo-ocular reflex pathways were relatively unaltered, while the responses through higher-order neural networks in the vestibular system were substantially decreased. These findings were consistent with those observed in primates suggesting a well-conserved role of GABAB receptors in the oculomotor system across frontal-eyed and lateral-eyed animals. We showed here a previously unrecognized effect of baclofen on the vestibular oculomotor function in mice. When interpreting general animal performance under the drug, the potential contribution of altered balance system should be taken into consideration.

Physiology of central pathways

The relative simplicity of the neural circuits that mediate vestibular reflexes is well suited for linking systems and cellular levels of analyses. Notably, a distinctive feature of the vestibular system is that neurons at the first central stage of sensory processing in the vestibular nuclei are premotor neurons; the same neurons that receive vestibular-nerve input also send direct projections to motor pathways. For example, the simplicity of the three-neuron pathway that mediates the vestibulo-ocular reflex leads to the generation of compensatory eye movements within ~5ms of a head movement. Similarly, relatively direct pathways between the labyrinth and spinal cord control vestibulospinal reflexes. A second distinctive feature of the vestibular system is that the first stage of central processing is strongly multimodal. This is because the vestibular nuclei receive inputs from a wide range of cortical, cerebellar, and other brainstem structures in addition to direct inputs from the vestibular nerve. Recent studies in alert animals have established how extravestibular signals shape these "simple" reflexes to meet the needs of current behavioral goal. Moreover, multimodal interactions at higher levels, such as the vestibular cerebellum, thalamus, and cortex, play a vital role in ensuring accurate self-motion and spatial orientation perception.

The mammalian efferent vestibular system plays a crucial role in the high-frequency response and short-term adaptation of the vestibuloocular reflex

Although anatomically well described, the functional role of the mammalian efferent vestibular system (EVS) remains unclear. Unlike in fish and reptiles, the mammalian EVS does not seem to play a role in modulation of primary afferent activity in anticipation of active head movements. However, it could play a role in modulating long-term mechanisms requiring plasticity such as vestibular adaptation. We measured the efficacy of vestibuloocular reflex (VOR) adaptation in α9-knockout mice. These mice carry a missense mutation of the gene encoding the α9 nicotinic acetylcholine receptor (nAChR) subunit. The α9 nAChR subunit is expressed in the vestibular and auditory periphery, and its loss of function could compromise peripheral input from the predominantly cholinergic EVS. We measured the VOR gain (eye velocity/head velocity) in 26 α9-knockout mice and 27 cba129 control mice. Mice were randomly assigned to one of three groups: gain-increase adaptation (1.5×), gain-decrease adaptation (0.5×), or no adaptation (baseline, 1×). After adaptation training (horizontal rotations at 0.5 Hz with peak velocity 20°/s), we measured the sinusoidal (0.2-10 Hz, 20-100°/s) and transient (1,500-6,000°/s(2)) VOR in complete darkness. α9-Knockout mice had significantly lower baseline gains compared with control mice. This difference increased with stimulus frequency (∼ 5% <1 Hz to ∼ 25% >1 Hz). Moreover, vestibular adaptation (difference in VOR gain of gain-increase and gain-decrease adaptation groups as % of gain increase) was significantly reduced in α9-knockout mice (17%) compared with control mice (53%), a reduction of ∼ 70%. Our results show that the loss of α9 nAChRs moderately affects the VOR but severely affects VOR adaptation, suggesting that the EVS plays a crucial role in vestibular plasticity.

Velocity-selective adaptation of the horizontal and cross-axis vestibulo-ocular reflex in the mouse

One commonly observed phenomenon of vestibulo-ocular reflex (VOR) adaptation is a frequency-selective change in gain (eye velocity/head velocity) and phase (relative timing between the vestibular stimulus and response) based on the frequency content of the adaptation training stimulus. The neural mechanism behind this type of adaptation is not clear. Our aim was to determine whether there were other parameter-selective effects on VOR adaptation, specifically velocity-selective and acceleration-selective changes in the horizontal VOR gain and phase. We also wanted to determine whether parameter selectivity was also in place for cross-axis adaptation training (a visual-vestibular training stimulus that elicits a vestibular-evoked torsional eye movement during horizontal head rotations). We measured VOR gain and phase in 17 C57BL/6 mice during baseline (no adaptation training) and after gain-increase, gain-decrease and cross-axis adaptation training using a sinusoidal visual-vestibular (mismatch) stimulus with whole-body rotations (vestibular stimulus) with peak velocity 20 and 50°/s both with a fixed frequency of 0.5 Hz. Our results show pronounced velocity selectivity of VOR adaptation. The difference in horizontal VOR gain after gain-increase versus gain-decrease adaptation was maximal when the sinusoidal testing stimulus matched the adaptation training stimulus peak velocity. We also observed similar velocity selectivity after cross-axis adaptation training. Our data suggest that frequency selectivity could be a manifestation of both velocity and acceleration selectivity because when one of these is absent, e.g. acceleration selectivity in the mouse, frequency selectivity is also reduced.

Sleep deprivation has no effect on dynamic visual acuity in military service members who are healthy

BACKGROUND

The risk of traumatic brain injury (TBI) and comorbid posttraumatic dizziness is elevated in military operational environments. Sleep deprivation is known to affect a service member's performance while deployed, although little is known about its effects on vestibular function. Recent findings suggest that moderate acceleration step rotational stimuli may elicit a heightened angular vestibulo-ocular reflex (aVOR) response relative to low-frequency sinusoidal stimuli after 26 hours of sleep deprivation. There is concern that a sleep deprivation-mediated elevation in aVOR function could confound detection of comorbid vestibular pathology in service members with TBI. The term "dynamic visual acuity" (DVA) refers to an individual's ability to see clearly during head movement and is a behavioral measure of aVOR function. The Dynamic Visual Acuity Test (DVAT) assesses gaze instability by measuring the difference between head-stationary and head-moving visual acuity.

OBJECTIVE

The purpose of this study was to investigate the effects of 26 hours of sleep deprivation on DVA as a surrogate for aVOR function.

DESIGN

This observational study utilized a repeated-measures design.

METHODS

Twenty soldiers with no history of vestibular insult or head trauma were assessed by means of the DVAT at angular head velocities of 120 to 180°/s. Active and passive yaw and pitch impulses were obtained before and after sleep deprivation.

RESULTS

Yaw DVA remained unchanged as the result of sleep deprivation. Active pitch DVA diminished by -0.005 LogMAR (down) and -0.055 LogMAR (up); passive pitch DVA was degraded by -0.06 LogMAR (down) and -0.045 LogMAR (up).

LIMITATIONS

Sample homogeneity largely confounded accurate assessment of test-retest reliability in this study, resulting in intraclass correlation coefficients lower than those previously reported.

CONCLUSIONS

Dynamic visual acuity testing in soldiers who are healthy revealed no change in gaze stability after rapid yaw impulses and subclinical changes in pitch DVA after sleep deprivation. Findings suggest that DVA is not affected by short-term sleep deprivation under clinical conditions.

The horizontal angular vestibulo-ocular reflex: a nonlinear mechanism for context-dependent responses

Studies of the vestibulo-ocular reflex (VOR) have revealed that this type of involuntary eye movement is influenced by viewing distance. This paper presents a bilateral model for the horizontal angular VOR in the dark based on realistic physiological mechanisms. It is shown that by assigning proper nonlinear neural computations at the premotor level, the model is capable of replicating target-distance-dependent VOR responses that are in agreement with geometrical requirements. Central premotor responses in the model are also shown to be consistent with experimental observations. Moreover, the model performance after simulated unilateral canal plugging also reproduces experimental observations, an emerging property. Such local nonlinear computations could similarly generate context-dependent behaviors in other more complex motor systems.

Postural responses to electrical stimulation of the vestibular end organs in human subjects

A multichannel vestibular prosthesis that delivers electrical stimulation to the perilymph of individual semicircular canals is a potential new treatment modality for patients with vestibular deficiencies. Most research in this field has evaluated the efficacy of this approach by its ability to reproduce eye movements in response to head rotations. Our group has developed such a device and implanted it in four human subjects with intractable unilateral Meniere's disease. This allows us to evaluate individual semicircular canal contribution to the control of balance and posture in human subjects. In this report, we demonstrate that electrical stimulation trains delivered to the perilymph of individual semicircular canals elicit postural responses specific to the particular canal stimulated, with some current spread to adjacent end organs. Modulation of stimulation current modulates the amplitude of the postural response. However, eye movements elicited by the same electrical stimuli were not consistent with postural responses in magnitude or direction in all subjects. Taken together, these findings support the feasibility of a vestibular prosthesis for the control of balance and illustrate new challenges for the development of this technology.

Multichannel vestibular prosthesis employing modulation of pulse rate and current with alignment precompensation elicits improved VOR performance in monkeys

An implantable prosthesis that stimulates vestibular nerve branches to restore the sensation of head rotation and the three-dimensional (3D) vestibular ocular reflex (VOR) could benefit individuals disabled by bilateral loss of vestibular sensation. Our group has developed a vestibular prosthesis that partly restores normal function in animals by delivering biphasic current pulses via electrodes implanted in semicircular canals. Despite otherwise promising results, this approach has been limited by insufficient velocity of VOR response to head movements that should inhibit the implanted labyrinth and by misalignment between direction of head motion and prosthetically elicited VOR. We report that significantly larger VOR eye velocities in the inhibitory direction can be elicited by adapting a monkey to elevated baseline stimulation rate and current prior to stimulus modulation and then concurrently modulating ("co-modulating") both rate and current below baseline levels to encode inhibitory angular head velocity. Co-modulation of pulse rate and current amplitude above baseline can also elicit larger VOR eye responses in the excitatory direction than do either pulse rate modulation or current modulation alone. Combining these stimulation strategies with a precompensatory 3D coordinate transformation improves alignment and magnitude of evoked VOR eye responses. By demonstrating that a combination of co-modulation and precompensatory transformation strategies achieves a robust VOR response in all directions with significantly improved alignment in an animal model that closely resembles humans with vestibular loss, these findings provide a solid preclinical foundation for application of vestibular stimulation in humans.

The vestibular system implements a linear-nonlinear transformation in order to encode self-motion

Early vestibular processing in macaque monkeys is inherently nonlinear and is optimized to detect specific features of self-motion.

Although it is well established that the neural code representing the world changes at each stage of a sensory pathway, the transformations that mediate these changes are not well understood. Here we show that self-motion (i.e. vestibular) sensory information encoded by VIIIth nerve afferents is integrated nonlinearly by post-synaptic central vestibular neurons. This response nonlinearity was characterized by a strong (∼50%) attenuation in neuronal sensitivity to low frequency stimuli when presented concurrently with high frequency stimuli. Using computational methods, we further demonstrate that a static boosting nonlinearity in the input-output relationship of central vestibular neurons accounts for this unexpected result. Specifically, when low and high frequency stimuli are presented concurrently, this boosting nonlinearity causes an intensity-dependent bias in the output firing rate, thereby attenuating neuronal sensitivities. We suggest that nonlinear integration of afferent input extends the coding range of central vestibular neurons and enables them to better extract the high frequency features of self-motion when embedded with low frequency motion during natural movements. These findings challenge the traditional notion that the vestibular system uses a linear rate code to transmit information and have important consequences for understanding how the representation of sensory information changes across sensory pathways.

Understanding how the coding of sensory information changes at different stages of sensory processing remains a fundamental challenge in systems neuroscience. Here we address this question by studying early sensory processing in vestibular pathways of monkeys, a system for which sensory stimuli are relatively easy to describe. Peripheral vestibular afferents detect and encode head motion in space to ensure posture and gaze is accurate and stable during everyday life. In this study, we show that central vestibular neurons nonlinearly integrate their afferent inputs, which helps explain the mechanisms that generate enhanced feature detection in sensory pathways. In addition, our results overturn conventional wisdom that early vestibular processing is linear, revealing a striking boosting nonlinearity that is a hallmark of the first central stage of vestibular processing. Studies from other sensory systems have shown that higher-order neurons can more efficiently detect specific features of sensory input, and that nonlinear transformations can increase this efficiency. We suggest that nonlinear integration of afferent input by central vestibular neurons extends their coding range and facilitates the detection of natural vestibular stimuli.

Does orbital proprioception contribute to gaze stability during translation?

Translational motion induces retinal image slip which varies with object distance. The brain must know binocular eye position in real time in order to scale eye movements so as to minimize retinal slip. Two potential sources of eye position information are orbital proprioception and an internal representation of eye position derived from central ocular motor signals. To examine the role of orbital proprioceptive information, the position of the left eye was perturbed by microstimulation of the left abducens nerve during translational motion to the right or left along the interaural axis in two rhesus macaques. Microstimulation rotated the eye laterally, activating eye muscle proprioceptors, while keeping central motor commands undisturbed. We found that microstimulation-induced eye position changes did not affect the translational VOR in the abductive (lateral rectus) direction, but it did influence the responses in the adductive (medial rectus) direction. Our findings demonstrate that proprioceptive inputs appear to be involved in the TVOR responses at least during ipsilateral head movements and proprioceptive influences on the TVOR may involve vergence-related signals to the oculomotor nucleus. However, internal representation of eye position, derived from central ocular motor signals, likely plays the dominant role in providing eye position information for scaling eye movements during translational motion, particularly in the abducent direction.

Cellular and network contributions to vestibular signal processing: impact of ion conductances, synaptic inhibition, and noise

Head motion-related sensory signals are transformed by second-order vestibular neurons (2°VNs) into appropriate commands for retinal image stabilization during body motion. In frogs, these 2°VNs form two distinct subpopulations that have either tonic or highly phasic intrinsic properties, essentially compatible with low-pass and bandpass filter characteristics, respectively. In the present study, physiological data on cellular properties of 2°VNs of the grass frog (Rana temporaria) have been used to construct conductance-based spiking cellular models that were fine-tuned by fitting to recorded spike-frequency data. The results of this approach suggest that low-threshold, voltage-dependent potassium channels in phasic and spike-dependent potassium channels in tonic 2°VNs are important contributors to the differential, yet complementary response characteristics of the two vestibular subtypes. Extension of the cellular model with conductance-based synapses allowed simulation of afferent excitation and evaluation of the emerging properties of local feedforward inhibitory circuits. This approach revealed the relative contributions of intrinsic and synaptic factors on afferent signal processing in phasic 2°VNs. Additional extension of the single-cell model to a population model allowed testing under more natural conditions including asynchronous afferent labyrinthine input and synaptic noise. This latter approach indicated that the feedforward inhibition from the local inhibitory network acts as a high-pass filter, which reinforces the impact of the intrinsic membrane properties of phasic 2°VNs on peak response amplitude and timing. Thus, the combination of cellular and network properties enables phasic 2°VNs to work as a noise-resistant detector, suitable for central processing of short-duration vestibular signals.

Restoration of 3D vestibular sensation in rhesus monkeys using a multichannel vestibular prosthesis

Profound bilateral loss of vestibular hair cell function can cause chronically disabling loss of balance and inability to maintain stable vision during head and body movements. We have previously shown that chinchillas rendered bilaterally vestibular-deficient via intratympanic administration of the ototoxic antibiotic gentamicin regain a more nearly normal 3-dimensional vestibulo-ocular reflex (3D VOR) when head motion information sensed by a head-mounted multichannel vestibular prosthesis (MVP) is encoded via rate-modulated pulsatile stimulation of vestibular nerve branches. Despite significant improvement versus the unaided condition, animals still exhibited some 3D VOR misalignment (i.e., the 3D axis of eye movement responses did not precisely align with the axis of head rotation), presumably due to current spread between a given ampullary nerve's stimulating electrode(s) and afferent fibers in non-targeted branches of the vestibular nerve. Assuming that effects of current spread depend on relative orientation and separation between nerve branches, anatomic differences between chinchilla and human labyrinths may limit the extent to which results in chinchillas accurately predict MVP performance in humans. In this report, we describe the MVP-evoked 3D VOR measured in alert rhesus monkeys, which have labyrinths that are larger than chinchillas and temporal bone anatomy more similar to humans. Electrodes were implanted in five monkeys treated with intratympanic gentamicin to bilaterally ablate vestibular hair cell mechanosensitivity. Eye movements mediated by the 3D VOR were recorded during passive sinusoidal (0.2-5 Hz, peak 50°/s) and acceleration-step (1000°/s(2) to 150°/s) whole-body rotations in darkness about each semicircular canal axis. During constant 100 pulse/s stimulation (i.e., MVP powered ON but set to stimulate each ampullary nerve at a constant mean baseline rate not modulated by head motion), 3D VOR responses to head rotation exhibited profoundly low gain [(mean eye velocity amplitude)/(mean head velocity amplitude) < 0.1] and large misalignment between ideal and actual eye movements. In contrast, motion-modulated sinusoidal MVP stimuli elicited a 3D VOR with gain 0.4-0.7 and axis misalignment of 21-38°, and responses to high-acceleration transient head rotations exhibited gain and asymmetry closer to those of unilaterally gentamicin-treated animals (i.e., with one intact labyrinth) than to bilaterally gentamicin-treated animals without MVP stimulation. In comparison to responses observed under similar conditions in chinchillas, acute responses to MVP stimulation in rhesus macaque monkeys were slightly better aligned to the desired rotation axis. Responses during combined rotation and prosthetic stimulation were greater than when either stimulus was presented alone, suggesting that the central nervous system uses MVP input in the context of multisensory integration. Considering the similarity in temporal bone anatomy and VOR performance between rhesus monkeys and humans, these observations suggest that an MVP will likely restore a useful level of vestibular sensation and gaze stabilization in humans.

Unidirectional rotations produce asymmetric changes in horizontal VOR gain before and after unilateral labyrinthectomy in macaques

Unilateral vestibular lesions cause marked asymmetry in the horizontal vestibulo-ocular reflex (VOR) during rapid head rotations, with VOR gain being lower for head rotations toward the lesion than for rotations in the opposite direction. Reducing this gain asymmetry by enhancing ipsilesional responses would be an important step toward improving gaze stability following vestibular lesions. To that end, there were two goals in this study. First, we wanted to determine whether we could selectively increase VOR gain in only one rotational direction in normal monkeys by exposing them to a training session comprised of a 3-h series of rotations in only one direction (1,000°/s² acceleration to a plateau of 150°/s for 1 s) while they wore 1.7 × magnifying spectacles. Second, in monkeys with unilateral vestibular lesions, we designed a paradigm intended to reduce the gain asymmetry by rotating the monkeys toward the side of the lesion in the same way as above but without spectacles. There were three main findings (1) unidirectional rotations with magnifying spectacles result in gain asymmetry in normal monkeys, (2) gain asymmetry is reduced when animals are rotated towards the side of the labyrinthectomy via the ipsilesional rotation paradigm, and (3) repeated training causes lasting reduction in VOR gain asymmetry.

Interactions between intrinsic membrane and emerging network properties determine signal processing in central vestibular neurons

Head/body motion-related sensory signals are transformed in second-order vestibular neurons (2°VN) into commands for appropriate motor reactions that stabilize gaze and posture during locomotion. In all vertebrates, these neurons form functional subgroups with different membrane properties and response dynamics, compatible with the necessity to process a wide range of motion-related sensory signals. In frog, 2°VN subdivide into two well-defined populations with distinctly different intrinsic membrane properties, discharge dynamics and synaptic response characteristics. Tonic 2°VN form low-pass filters with membrane properties that cause synaptic amplification, whereas phasic 2°VN form band-pass filters that cause shunting of repetitive inputs. The different, yet complementary, filter properties render tonic neurons suitable for integration and phasic neurons for differentiation and event detection. Specific insertion of phasic 2°VN into local vestibular networks of inhibitory interneurons reinforces the functional consequences of the intrinsic membrane properties of this particular cell type with respect to the processing of afferent sensory signals. Thus, the combination of matching intrinsic cellular and emerging network properties generates sets of neuronal elements that form adjustable, frequency-tuned filter components for separate transformation of the various dynamic aspects of head motion-related signals. The overall frequency tuning of central vestibular neurons differs between vertebrates along with variations in species-specific locomotor dynamics, thereby illustrating an ecophysiological plasticity of the involved neuronal elements. Moreover, separation into multiple, dynamically different subtypes at any neuronal level along the vestibulo-motor reflex pathways suggests an organization of head motion-related sensory-motor transformation in parallel, frequency-tuned channels.