The Human Horizontal Vestibulo-Ocular Reflex in Response to Active and Passive Head Impulses after Unilateral Vestibular Deafferentation

The Role of the Smartphone in the Diagnosis of Vestibular Hypofunction: A Clinical Strategy for Teleconsultation during the COVID-19 Pandemic and Beyond

Introduction  Vestibular disorders (VDs) are highly prevalent in primary care. Although in general they comprise conditions that are not life-threatening, they are associated with significant functional and physical disability. However, the current coronavirus disease 2019 (COVID-19) pandemic has imposed limitations on the standard treatment of benign conditions, including VDs. In this context, other resources may aid in the diagnosis and management of patients with VDs. It is well known that teleconsultation and teletreatment are both safe and effective alternatives to manage a variety of conditions, and we maintain that VDs should be among these. Objective  To develop a preliminary model of clinical guidelines for the evaluation by teleconsultation of patients with suspected diagnosis of vestibular hypofunction during the COVID-19 pandemic and beyond. Methods  A bibliographic review of the diagnostic feasibility in VDs by teleconsultation was carried out in the LILACS, SciELO, MEDLINE, and PubMed databases; books and specialized websites were also consulted. The legal, regulatory, and technical issues involving digital consultations were reviewed. Results  We found 6 field studies published between 1990 and 2020 in which the efficiency of teleconsultations was observed in the contexts of epidemics and environmental disorders and disadvantageous geographical conditions. After reviewing them, we proposed a strategy to examine and address vestibular complaints related to vestibular hypofunction. Conclusion  The creation of a digital vestibular management algorithm for the identification, counseling, initial intervention, monitoring and targeting of people with possible vestibular hypofunction seems to be feasible, and it will provide a reasonable alternative to in-person evaluations during the COVID-19 pandemic and beyond.

Using Unidirectional Rotations to Improve Vestibular System Asymmetry in Patients with Vestibular Dysfunction

The vestibular system provides information about head movement and mediates reflexes that contribute to balance control and gaze stabilization during daily activities. Vestibular sensors are located in the inner ear on both sides of the head and project to the vestibular nuclei in the brainstem. Vestibular dysfunction is often due to an asymmetry between input from the two sides. This results in asymmetrical neural inputs from the two ears, which can produce an illusion of rotation, manifested as vertigo. The vestibular system has an impressive capacity for compensation, which serves to rebalance how asymmetrical information from the sensory end organs on both sides is processed at the central level. To promote compensation, various rehabilitation programs are used in the clinic; however, they primarily use exercises that improve multisensory integration. Recently, visual-vestibular training has also been used to improve the vestibulo-ocular reflex (VOR) in animals with compensated unilateral lesions. Here, a new method is introduced for rebalancing the vestibular activity on both sides in human subjects. This method consists of five unidirectional rotations in the dark (peak velocity of 320°/s) toward the weaker side. The efficacy of this method was shown in a sequential, double-blinded clinical trial in 16 patients with VOR asymmetry (measured by the directional preponderance in response to sinusoidal rotations). In most cases, VOR asymmetry decreased after a single session, reached normal values within the first two sessions in one week, and the effects lasted up to 6 weeks. The rebalancing effect is due to both an increase in VOR response from the weaker side and a decrease in response from the stronger side. The findings suggest that unidirectional rotation can be used as a supervised rehabilitation method to reduce VOR asymmetry in patients with longstanding vestibular dysfunction.

The Interaction of Pre-programmed Eye Movements With the Vestibulo-Ocular Reflex

The Vestibulo-Ocular Reflex (VOR) works to stabilize gaze during unexpected head movements. However, even subjects who lack a VOR (e.g., vestibulopathic patients) can achieve gaze stability during planned head movements by using pre-programmed eye movements (PPEM). The extent to which PPEM are used by healthy intact subjects and how they interact with the VOR is still unclear. We propose a model of gaze stabilization which makes several claims: (1) the VOR provides ocular stability during unexpected (i.e., passive) head movements; (2) PPEM are used by both healthy and vestibulopathic subjects during planned (i.e., active) head movements; and (3) when a passive perturbation interrupts an active head movement in intact animals (i.e., combined passive and active head movement) the VOR works with PPEM to provide compensation. First, we show how our model can reconcile some seemingly conflicting findings in earlier literature. We then test the above-mentioned predictions against data we collected from both healthy and vestibular-lesioned guinea pigs. We found that (1) vestibular-lesioned animals showed a dramatic decrease in compensatory eye movements during passive head movements, (2) both populations showed improved ocular compensation during active vs. passive head movements, and (3) during combined active and passive head movements, eye movements compensated for both the active and passive component of head velocity. These results support our hypothesis that while the VOR provides compensation during passive head movements, PPEM are used by both intact and lesioned subjects during active movements and further, that PPEM work together with the VOR to achieve gaze stability.

New insights into vestibular-saccade interaction based on covert corrective saccades in patients with unilateral vestibular deficits

In response to passive high-acceleration head impulses, patients with low vestibulo-ocular reflex (VOR) gains often produce covert (executed while the head is still moving) corrective saccades in the direction of deficient slow phases. Here we examined 23 patients using passive, and 9 also active, head impulses with acute (< 10 days from onset) unilateral vestibular neuritis and low VOR gains. We found that when corrective saccades are larger than 10°, the slow-phase component of the VOR is inhibited, even though inhibition increases further the time to reacquire the fixation target. We also found that 1) saccades are faster and more accurate if the residual VOR gain is higher, 2) saccades also compensate for the head displacement that occurs during the saccade, and 3) the amplitude-peak velocity relationship of the larger corrective saccades deviates from that of head-fixed saccades of the same size. We propose a mathematical model to account for these findings hypothesizing that covert saccades are driven by a desired gaze position signal based on a prediction of head displacement using vestibular and extravestibular signals, covert saccades are controlled by a gaze feedback loop, and the VOR command is modulated according to predicted saccade amplitude. A central and novel feature of the model is that the brain develops two separate estimates of head rotation, one for generating saccades while the head is moving and the other for generating slow phases. Furthermore, while the model was developed for gaze-stabilizing behavior during passively induced head impulses, it also simulates both active gaze-stabilizing and active gaze-shifting eye movements.NEW & NOTEWORTHY During active or passive head impulses while fixating stationary targets, low vestibulo-ocular gain subjects produce corrective saccades when the head is still moving. The mechanisms driving these covert saccades are poorly understood. We propose a mathematical model showing that the brain develops two separate estimates of head rotation: a lower level one, presumably in the vestibular nuclei, used to generate the slow-phase component of the response, and a higher level one, within a gaze feedback loop, used to drive corrective saccades.

Role of the Patient's History of Vestibular Symptoms in the Clinical Evaluation of the Bedside Head-Impulse Test

Objective

Our aim was to identify the role of the investigators’ knowledge of the patient’s history of vestibular symptoms (PVH) in the clinical evaluation of the bedside head-impulse test (bHIT). We hypothesized that this knowledge will reduce uncertainty and improve bHIT accuracy when compared to quantitative analysis of the vestibulo-ocular reflex by video head-impulse test (vHIT).

Methods

We looked for changes in the clinical assessment of the bHIT in 594 consecutive patients before and after taking PVH. bHIT was performed by 12 clinical neurologists with various clinical experience in neuro-otological diseases (novices to long-standing experts). vHIT was analyzed by four experts being blinded for the patients’ clinical presentation and history of symptoms. The confidence of bHIT and vHIT was rated (0–100%).

Results

One hundred fifty-four (15%) of 1,030 bHIT of all eligible patients (n = 515) were rated pathological. Thirty-five (22.7%) of them were rated bilateral vestibulopathies. Sensitivity of bHIT reached 56.3%, its specificity 92.4%; the positive predictive value (PPV) was 41.5% and the negative predictive value 95.7%. These data did not differ between bHIT before and after PVH. bHIT after PVH (post-bHIT) differed from pre-bHIT in 44.3%, usually with regard to the level of confidence but also in polarity (5%). The accuracy of changes in bHIT depended on the direction of change: a “normal” post-bHIT was correct in 92.3% while only 39.8% of pathological post-bHIT were pathological on vHIT. However, sensitivity of a pathological post-bHIT depended on the clinical experience in taking PVH and bHIT: the PPV was 20.5% in novices as compared to 69.6% in experts.

Conclusion

The study shows that PVH changes the certainty and/or polarity of the clinical evaluation of bHIT. Unlike expected, the increase in confidence in post-bHIT is associated with a consistently high specificity but no increase in sensitivity. Accuracy of changes in post-bHIT depends on the investigators’ clinical experience: it increases only in experts but not novices. Since novices show only a poor PPV and moderate sensitivity of bHIT, pathological bHITs should be controlled by vHIT, even in patients with a positive PVH. By contrast, confirmed normal post-bHIT is usually correct.

Acute unilateral loss of vestibular function

Sudden unilateral loss of vestibular function is the most severe condition that can occur in the vestibular system. The clinical syndrome is caused by the physiologic properties of the vestibulo-ocular reflex (VOR) arc. In the normal situation, the two peripheral vestibular end organs are connected to a functional unit in coplanar pairs of semicircular canals working in a push-pull mode. "Push-pull" mode means that, when one side is excited, the other side is inhibited, and vice versa due to two mechanisms. First, first-order vestibular afferents are bipolar cells. They have a tonic firing rate that is modulated up or down depending on the direction of rotation. Second, via inhibitory neural connections of second-order vestibular neurons between the vestibular nuclei (vestibular commissural system), the excited side inhibits further the contralateral side. The neural signals are encoded as the difference of the change in firing rate of the vestibular neurons modulating the tonic firing rate on both sides in opposite directions (one side up, the contralateral side down). When the head is not moving, the two peripheral vestibular end organs generate a resting firing rate, which is exactly equal on both sides. When the head is rotated, for example, to the right, the right-sided first-order vestibular afferents increase their discharge rate and the left-sided ones decrease their firing rate. This leads to increase in firing rate of also the type I second-order vestibular neurons in the vestibular nuclei, which synapse with inhibitory type II neurons on the contralateral side, further decreasing the firing rate in the second-order vestibular neurons in the contralateral vestibular nucleus. When the direction of head rotation is reversed, the behavior of the type I neurons on the two sides of the head is reversed. The same relation exists between the coplanar vertical canal afferents on the two sides of the head. When there is unilateral damage to the end organ or the vestibular nerve, the resting firing frequency is drastically reduced or even silenced on the lesioned side, thereby creating a tonic imbalance between the normal resting firing on the healthy side and the lesioned side. This tonic imbalance mimics a permanent rotation toward the healthy side (the side with the higher firing rate), resulting, via the VOR, in a slow-phase drift of the eyes toward the side of the lesion, interrupted by rapid quick-phase resetting eye movements toward the healthy side. This leads to the typical vestibular spontaneous horizontal-torsional nystagmus together with rotational vertigo and postural imbalance, with the tendency to fall toward the lesioned side. The tonic imbalance with the hallmark of spontaneous nystagmus usually recovers within days to weeks after the lesion due to the central restoration of tonic activity on the lesioned side. The dynamic changes, however, might be long-lasting when the peripheral sensors do not recover their function. This causes asymmetric VOR responses, with weaker responses when the head is rotated rapidly toward the lesioned side, leading to transient oscillopsia.

Association between vestibulo-ocular reflex suppression, balance, gait, and fall risk in ageing and neurodegenerative disease: protocol of a one-year prospective follow-up study

Background

Falls frequency increases with age and particularly in neurogeriatric cohorts. The interplay between eye movements and locomotion may contribute substantially to the occurrence of falls, but is hardly investigated. This paper provides an overview of current approaches to simultaneously measure eye and body movements, particularly for analyzing the association of vestibulo-ocular reflex (VOR) suppression, postural deficits and falls in neurogeriatric risk cohorts. Moreover, VOR suppression is measured during head-fixed target presentation and during gaze shifting while postural control is challenged. Using these approaches, we aim at identifying quantitative parameters of eye-head-coordination during postural balance and gait, as indicators of fall risk.

Methods/Design

Patients with Progressive Supranuclear Palsy (PSP) or Parkinson’s disease (PD), age- and sex-matched healthy older adults, and a cohort of young healthy adults will be recruited. Baseline assessment will include a detailed clinical assessment, covering medical history, neurological examination, disease specific clinical rating scales, falls-related self-efficacy, activities of daily living, neuro-psychological screening, assessment of mobility function and a questionnaire for retrospective falls. Moreover, participants will simultaneously perform eye and head movements (fixating a head-fixed target vs. shifting gaze to light emitting diodes in order to quantify vestibulo-ocular reflex suppression ability) under different conditions (sitting, standing, or walking). An eye/head tracker synchronized with a 3-D motion analysis system will be used to quantify parameters related to eye-head-coordination, postural balance, and gait. Established outcome parameters related to VOR suppression ability (e.g., gain, saccadic reaction time, frequency of saccades) and motor related fall risk (e.g., step-time variability, postural sway) will be calculated. Falls will be assessed prospectively over 12 months via protocols and monthly telephone interviews.

Discussion

This study protocol describes an experimental setup allowing the analysis of simultaneously assessed eye, head and body movements. Results will improve our understanding of the influence of the interplay between eye, head and body movements on falls in geriatric high-risk cohorts.

Electronic supplementary material

The online version of this article (doi:10.1186/s12883-015-0447-5) contains supplementary material, which is available to authorized users.

Compensatory saccades benefit from prediction during head impulse testing in early recovery from vestibular deafferentation

The head impulse test (HIT) can identify a deficient vestibulo-ocular reflex (VOR) by the compensatory saccade (CS) generated once the head stops moving. The inward HIT is considered safer than the outward HIT, yet might have an oculomotor advantage given that the subject would presumably know the direction of head rotation. Here, we compare CS latencies following inward (presumed predictable) and outward (more unpredictable) HITs after acute unilateral vestibular nerve deafferentation. Seven patients received inward and outward HITs delivered at six consecutive postoperative days (POD) and again at POD 30. All head impulses were recorded by portable video-oculography. CS included those occurring during (covert) or after (overt) head rotation. Inward HITs included mean CS latencies (183.48 ms ± 4.47 SE) that were consistently shorter than those generated during outward HITs in the first 6 POD (p = 0.0033). Inward HITs induced more covert saccades compared to outward HITs, acutely. However, by POD 30 there were no longer any differences in latencies or proportions of CS and direction of head rotation. Patients with acute unilateral vestibular loss likely use predictive cues of head direction to elicit early CS to keep the image centered on the fovea. In acute vestibular hypofunction, inwardly applied HITs may risk a preponderance of covert saccades, yet this difference largely disappears within 30 days. Advantages of inwardly applied HITs are discussed and must be balanced against the risk of a false-negative HIT interpretation.

The video head impulse test

CONCLUSION

Additional research is needed to validate the importance of the video head impulse tests (vHIT), but it provides an important contribution to the evaluation of anterior and posterior semicircular canal disorders.

OBJECTIVES

To share observations of the vHIT test in clinical neurotology and to discuss the significance of the study findings.

METHODS

This study comprised 200 patients with a clinical history of vestibular disturbances who were submitted to a vHIT including all six semicircular canals.

RESULTS

Abnormal responses of the anterior and posterior canals were found in several patients, either alone or combined with altered responses in the lateral canals. A unilateral hypoactive response of a posterior canal was found in a patient with a small vestibular schwannoma.

Nystagmus and saccadic intrusions

We review current concepts of nystagmus and saccadic oscillations, applying a pathophysiological approach. We begin by discussing how nystagmus may arise when the mechanisms that normally hold gaze steady are impaired. We then describe the clinical and laboratory evaluation of patients with ocular oscillations. Next, we systematically review the features of nystagmus arising from peripheral and central vestibular disorders, nystagmus due to an abnormal gaze-holding mechanism (neural integrator), and nystagmus occurring when vision is compromised. We then discuss forms of nystagmus for which the pathogenesis is not well understood, including acquired pendular nystagmus and congenital forms of nystagmus. We then summarize the spectrum of saccadic disorders that disrupt steady gaze, from intrusions to flutter and opsoclonus. Finally, we review current treatment options for nystagmus and saccadic oscillations, including drugs, surgery, and optical methods. Examples of each type of nystagmus are provided in the form of figures.

Peaks and troughs of three-dimensional vestibulo-ocular reflex in humans

The three-dimensional vestibulo-ocular reflex (3D VOR) ideally generates compensatory ocular rotations not only with a magnitude equal and opposite to the head rotation but also about an axis that is collinear with the head rotation axis. Vestibulo-ocular responses only partially fulfill this ideal behavior. Because animal studies have shown that vestibular stimulation about particular axes may lead to suboptimal compensatory responses, we investigated in healthy subjects the peaks and troughs in 3D VOR stabilization in terms of gain and alignment of the 3D vestibulo-ocular response. Six healthy upright sitting subjects underwent whole body small amplitude sinusoidal and constant acceleration transients delivered by a six-degree-of-freedom motion platform. Subjects were oscillated about the vertical axis and about axes in the horizontal plane varying between roll and pitch at increments of 22.5° in azimuth. Transients were delivered in yaw, roll, and pitch and in the vertical canal planes. Eye movements were recorded in with 3D search coils. Eye coil signals were converted to rotation vectors, from which we calculated gain and misalignment. During horizontal axis stimulation, systematic deviations were found. In the light, misalignment of the 3D VOR had a maximum misalignment at about 45°. These deviations in misalignment can be explained by vector summation of the eye rotation components with a low gain for torsion and high gain for vertical. In the dark and in response to transients, gain of all components had lower values. Misalignment in darkness and for transients had different peaks and troughs than in the light: its minimum was during pitch axis stimulation and its maximum during roll axis stimulation. We show that the relatively large misalignment for roll in darkness is due to a horizontal eye movement component that is only present in darkness. In combination with the relatively low torsion gain, this horizontal component has a relative large effect on the alignment of the eye rotation axis with respect to the head rotation axis.

Vestibular system may provide equivalent motor actions regardless of the number of body segments involved in the task

The vestibulospinal system likely plays an essential role in motor equivalence--the ability to reach the desired motor goal despite intentional or imposed changes in the number of body segments involved in the task. To test this hypothesis, we compared the ability of healthy subjects and patients with unilateral vestibular lesions (surgical acoustic neuroma resection 0.6 to 6.7 yr before the study) to maintain either the same hand position or the same trajectory of within arm reach movements while flexing the trunk, in the absence of vision. In randomly selected trials, the trunk motion was prevented by an electromagnetic device. Healthy subjects were able to preserve the hand position or trajectory by modifying the elbow and shoulder joint rotations in a condition-dependent way, at a minimal latency of about 60 ms after the trunk movement onset. In contrast, six of seven patients showed deficits in the compensatory angular modifications at least in one of two tasks so that 30-100% of the trunk displacement was not compensated and thus influenced the hand position or trajectory. Results suggest that vestibular influences evoked by the head motion during trunk flexion play a major role in maintaining the consistency of arm motor actions in external space despite changes in the number of body segments involved. Our findings also suggest that despite long-term plasticity in the vestibular system and related neural structures, unilateral vestibular lesion may reduce the capacity of the nervous system to achieve motor equivalence.

The effect of binocular eye position and head rotation plane on the human torsional vestibuloocular reflex

We examined how the gain of the torsional vestibulo-ocular reflex (VOR) (defined as the instantaneous eye velocity divided by inverted head velocity) in normal humans is affected by eye position, target distance, and the plane of head rotation. In six normal subjects we measured three-dimensional (3D) eye and head rotation axes using scleral search coils, and 6D head position using a magnetic angular and linear position measurement device, during low-amplitude (approximately 20 degrees ), high-velocity (approximately 200 degrees/s), high-acceleration (approximately 4000 degrees /s2) rapid head rotations or 'impulses.' Head impulses were imposed manually and delivered in five planes: yaw (horizontal canal plane), pitch, roll, left anterior-right posterior canal plane (LARP), and right anterior-left posterior canal plane (RALP). Subjects were instructed to fix on one of six targets at eye level. Targets were either straight-ahead, 20 degrees left or 20 degrees right from midline, at distance 15 or 124 cm from the subject. Two subjects also looked at more eccentric targets, 30 degrees left or 30 degrees right from midline. We found that the vertical and horizontal VOR gains increased with the proximity of the target to the subject. Previous studies suggest that the torsional VOR gain should decrease with target proximity. We found, however, that the torsional VOR gain did not change for all planes of head rotation and for both target distances. We also found a dynamic misalignment of the vertical positions of the eyes during the torsional VOR, which was greatest during near viewing with symmetric convergence. This dynamic vertical skew during the torsional VOR arises, in part, because when the eyes are converged, the optical axes are not parallel to the naso-occipital axes around which the eyes are rotating. In five of six subjects, the average skew ranged 0.9 degrees -2.9 degrees and was reduced to <0.4 degrees by a 'torsional' quick-phase (around the naso-occipital axis) occurring <110 ms after the onset of the impulse. We propose that the torsional quick-phase mechanism during the torsional VOR could serve at least three functions: (1) resetting the retinal meridians closer to their usual orientation in the head, (2) correcting for the 'skew' deviation created by misalignment between the axes around which the eyes are rotating and the line of sight, and (3) taking the eyes back toward Listing's plane.

Angular and Linear Vestibulo-Ocular Responses in Humans

A new technique is introduced to measure linear and angular vestibulo-ocular responses in three dimensions. Using a three-dimensional motion platform, human subjects underwent whole-body rotations and translations. Eye movements were measured with an infrared video recording device and/or with scleral search coils. Subjects were tested with sinusoidal stimulation and impulses under light and dark conditions. The results show that for sinusoidal stimulation, torsion compensatory eye movements (roll stimulation) have a low gain compared to horizontal (yaw) and vertical (pitch) compensatory eye movements. With impulses, we reliably assessed gain and delay for rotations in yaw, pitch, and roll. Under this stimulus condition the gain for roll (torsion eye movements) was also low compared to yaw and pitch (horizontal and vertical eye movements). For translations, the gain of eye-movement responses varied between 0.7 and 1 in the light. In the dark, responses were lower and more variable.

Gaze Position Corrective Eye Movements in Normal Subjects and in Patients with Vestibular Deficits

Eye movements in response to high-acceleration head rotations (thrusts) in the horizontal plane from patients with unilateral (UVD) or bilateral vestibular loss (BVD) were recorded. The rapid, gaze-position corrections (GPCs) that appeared when vestibulo-ocular reflex (VOR) slow phases were undercompensatory were characterized. For comparison, eye movements from normal subjects who were asked to generate saccades in the direction opposite head rotation (in the same direction as slow phases) were recorded. This normal-subject model produced responses with spatial and temporal characteristics similar to those from GPCs in patients as follows: When head rotations were generated actively, compared with passively, gaze-position errors and corresponding GPCs were smaller and occurred earlier. During passively generated head thrusts, GPCs still occurred when head rotations were made in total darkness, though their accuracy decreased as the requirement for maintaining gaze on a specific location in space was relaxed. Time of onset of GPCs was not rigidly tied to head kinematics (peak velocity or peak acceleration). Speeds of GPCs, however, were lower than speeds of similar-sized, head-fixed saccades. Finally, during passive and active head thrusts in patients, sustained, high-frequency (20 to 30 Hz) oscillations that appeared as tiny saccades were occasionally observed, one immediately following the other, resembling a compensatory slow-phase response. Taken together, the results suggest that one strategy for overcoming a VOR deficit is to enlist the saccadic system to produce an oculomotor response that is required to compensate for head rotation. This response may come in the form of high-velocity GPCs or smaller-amplitude oscillations.

Intrinsic membrane properties of vertebrate vestibular neurons: function, development and plasticity

Central vestibular neurons play an important role in the processing of body motion-related multisensory signals and their transformation into motor commands for gaze and posture control. Over recent years, medial vestibular nucleus (MVN) neurons and to a lesser extent other vestibular neurons have been extensively studied in vivo and in vitro, in a range of species. These studies have begun to reveal how their intrinsic electrophysiological properties may relate to their response patterns, discharge dynamics and computational capabilities. In vitro studies indicate that MVN neurons are of two major subtypes (A and B), which differ in their spike shape and after-hyperpolarizations. This reflects differences in particular K(+) conductances present in the two subtypes, which also affect their response dynamics with type A cells having relatively low-frequency dynamics (resembling "tonic" MVN cells in vivo) and type B cells having relatively high-frequency dynamics (resembling "kinetic" cells in vivo). The presence of more than one functional subtype of vestibular neuron seems to be a ubiquitous feature since vestibular neurons in the chick and frog also subdivide into populations with different, analogous electrophysiological properties. The ratio of type A to type B neurons appears to be plastic, and may be determined by the signal processing requirements of the vestibular system, which are species-variant. The membrane properties and discharge pattern of type A and type B MVN neurons develop largely post-natally, through the expression of the underlying ion channel conductances. The membrane properties of MVN neurons show rapid and long-lasting plastic changes after deafferentation (unilateral labyrinthectomy), which may serve to maintain their level of activity and excitability after the loss of afferent inputs.

Do predictive mechanisms improve the angular vestibulo-ocular reflex in vestibular neuritis?

Recovery from vestibular neuritis (VN) is often incomplete which leads to persistent vestibular imbalance during rapid head movements. Patients with unilateral vestibular lesions have a larger gain of the horizontal vestibulo-ocular reflex during active compared to passive head movements. To test whether this gain increase is related to predictive mechanisms we studied 15 patients with VN and 14 control subjects during predictable and unpredictable passive horizontal head impulses in the light and darkness. The vestibulo-ocular reflex showed a significantly shorter latency and higher gain in the light for predictable head impulses towards the ipsilesional side. However, this effect is small and might contribute but cannot exclusively account for the gain increase during active head movements.