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.     

Directional Plasticity Rapidly Improves 3D Vestibulo-Ocular Reflex Alignment in Monkeys Using a Multichannel Vestibular Prosthesis

Bilateral loss of vestibular sensation can be disabling. We have shown that a multichannel vestibular prosthesis (MVP) can partly restore vestibular sensation as evidenced by improvements in the 3-dimensional angular vestibulo-ocular reflex (3D VOR). However, a key challenge is to minimize misalignment between the axes of eye and head rotation, which is apparently caused by current spread beyond each electrode’s targeted nerve branch. We recently reported that rodents wearing a MVP markedly improve 3D VOR alignment during the first week after MVP activation, probably through the same central nervous system adaptive mechanisms that mediate cross-axis adaptation over time in normal individuals wearing prisms that cause visual scene movement about an axis different than the axis of head rotation. We hypothesized that rhesus monkeys would exhibit similar improvements with continuous prosthetic stimulation over time. We created bilateral vestibular deficiency in four rhesus monkeys via intratympanic injection of gentamicin. A MVP was mounted to the cranium, and eye movements in response to whole-body passive rotation in darkness were measured repeatedly over 1 week of continuous head motion-modulated prosthetic electrical stimulation. 3D VOR responses to whole-body rotations about each semicircular canal axis were measured on days 1, 3, and 7 of chronic stimulation. Horizontal VOR gain during 1 Hz, 50 °/s peak whole-body rotations before the prosthesis was turned on was <0.1, which is profoundly below normal (0.94 ± 0.12). On stimulation day 1, VOR gain was 0.4–0.8, but the axis of observed eye movements aligned poorly with head rotation (misalignment range ∼30–40 °). Substantial improvement of axis misalignment was observed after 7 days of continuous motion-modulated prosthetic stimulation under normal diurnal lighting. Similar improvements were noted for all animals, all three axes of rotation tested, for all sinusoidal frequencies tested (0.05–5 Hz), and for high-acceleration transient rotations. VOR asymmetry changes did not reach statistical significance, although they did trend toward slight improvement over time. Prior studies had already shown that directional plasticity reduces misalignment when a subject with normal labyrinths views abnormal visual scene movement. Our results show that the converse is also true: individuals receiving misoriented vestibular sensation under normal viewing conditions rapidly adapt to restore a well-aligned 3D VOR. Considering the similarity of VOR physiology across primate species, similar effects are likely to occur in humans using a MVP to treat bilateral vestibular deficiency.     

Human Vestibulo-Ocular Reflex Adaptation Reduces when Training Demand Variability Increases

One component of vestibular rehabilitation in patients with vestibulo-ocular reflex (VOR) hypofunction is gaze-stabilizing exercises that seek to increase (adapt) the VOR response. These prescribed home-based exercises are performed by the patient and thus their use/training is inherently variable. We sought to determine whether this variability affected VOR adaptation in ten healthy controls (× 2 training only) and ten patients with unilateral vestibular hypofunction (× 1 and × 2 training). During × 1 training, patients actively (self-generated, predictable) move their head sinusoidally while viewing a stationary fixation target; for × 2 training, they moved their outstretched hand anti-phase with their head rotation while attempting to view a handheld target. We defined the latter as manual × 2 training because the subject manually controls the target. In this study, head rotation frequency during training incrementally increased 0.5–2 Hz over 20 min. Active and passive (imposed, unpredictable) sinusoidal (1.3-Hz rotations) and head impulse VOR gains were measured before and after training. We show that for controls, manual × 2 training resulted in significant sinusoidal and impulse VOR adaptation of ~ 6 % and ~ 3 %, respectively, though this was ~two-thirds lower than increases after computer-controlled × 2 training (non-variable) reported in a prior study. In contrast, for patients, there was an increase in impulse but not sinusoidal VOR response after a single session of manual × 2 training. Patients had more than double the variability in VOR demand during manual × 2 training compared to controls, which could explain why adaptation was not significant in patients. Our data suggest that the clinical × 1 gaze-stabilizing exercise is a weak stimulus for VOR adaptation.     

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

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

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

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

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

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

Characterization and Inflammatory Response of Perivascular-Resident Macrophage-Like Melanocytes in the Vestibular System

A large number of perivascular cells expressing both macrophage and melanocyte characteristics (named perivascular-resident macrophage-like melanocytes, PVM/Ms), previously found in the intra-strial fluid–blood barrier, are also found in the blood–labyrinth barrier area of the vestibular system in normal adult cochlea, including in the three ampullae of the semicircular canals (posterior, superior, and horizontal), utricle, and saccule. The cells were identified as PVM/Ms, positive for the macrophage and melanocyte marker proteins F4/80 and GSTα4. Similar to PVM/Ms present in the stria vascularis, the PVM/Ms in the vestibular system are closely associated with microvessels and structurally intertwined with endothelial cells and pericytes, with a density in normal (unstimulated) utricle of 225 ± 43/mm²; saccule 191 ± 25/mm²; horizontal ampullae 212 ± 36/mm²; anterior ampullae 238 ± 36/mm²; and posterior ampullae 223 ± 64/mm². Injection of bacterial lipopolysaccharide into the middle ear through the tympanic membrane causes the PVM/Ms to activate and arrange in an irregular pattern along capillary walls in all regions within a 48-h period. The inflammatory response significantly increases vascular permeability and leakage. The results underscore the morphological complexity of the blood barrier in the vestibular system, with its surrounding basal lamina, pericytes, as well as second line of defense in PVM/Ms. PVM/Ms may be important to maintain blood barrier integrity and initiating local inflammatory response in the vestibular system.     

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

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

Enhanced Vestibulo-ocular Reflex to Electrical Vestibular Stimulation in Meniere’s Disease

Meniere’s disease is characterized by sporadic episodes of vertigo, nystagmus, fluctuating sensorineural hearing loss, tinnitus and aural pressure. Since Meniere’s disease can affect different regions of the vestibular labyrinth, we investigated if electrical vestibular stimulation (EVS) which excites the entire vestibular labyrinth may be useful to reveal patchy endorgan pathology. We recorded three-dimensional electrically evoked vestibulo-ocular reflex (eVOR) to transient EVS using bilateral, bipolar 100-ms current steps at intensities of 0.9, 2.5, 5.0, 7.5 and 10.0 mA with dual-search coils in 12 unilateral Meniere’s patients. Their results were compared to 17 normal subjects. Normal eVOR had tonic and phasic spatiotemporal properties best described by the torsional component, which was four times larger than horizontal and vertical components. At EVS onset and offset of 8.9 ms latency, there were phasic eVOR initiation (M = 1,267 °/s²) and cessation (M = −1,675 °/s²) acceleration pulses, whereas during the constant portion of the EVS, there was a maintained tonic eVOR (M = 9.1 °/s) at 10 mA. However in Meniere’s disease, whilst latency of EVS onset and offset was normal at 9.0 ms, phasic eVOR initiation (M = 1,720 °/s²) and cessation (M = −2,523 °/s²) were enlarged at 10 mA. The initiation profile was a bimodal response, whilst the cessation profile frequently did not return to baseline. The tonic eVOR (M = 20.5 °/s) exhibited a ramped enhancement of about twice normal at 10 mA. Tonic eVOR enhancement was present for EVS >0.9 mA and disproportionately enhanced the torsional, vertical and horizontal components. These eVOR abnormalities may be a diagnostic indicator of Meniere’s disease and may explain the vertigo attacks in the presence of declining mechanically evoked vestibular responses.     

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.     

VOR gain and phase in active head rotation tests of normal subjects and patients with peripheral labyrinthine lesions

The gain and phase of the vestibulo-ocular reflex (VOR) were studied by active head rotation tests in normal subjects and in patients with unilateral or bilateral lesions of vestibular function. The examination was performed under two conditions: alert-in-dark and with spatially-fixed target. The results were evaluated using a simplified model of vestibular response. Under alert-in-dark condition, the VOR grain and phase deficits were observed on rotation to the affected side in patients with unilateral lesions and bilaterally in patients with bilateral lesions. Under spatially-fixed-target condition, these patients showed a decrease in gain at higher frequencies but no phase lag was observed. The principally new advantage was that not only VOR gain but also VOR phase could be quantified using this active head rotation test. Therefore, for diagnosing VOR dysfunction, this active head rotation test is more useful than the active head rotation tests previously reported.     

Motorized head impulse rotator in patients with vestibular schwannoma

Conclusion. Motorized head impulse rotator is an effective technique to assess peripheral vestibular function. Approximately a quarter of patients with vestibular schwannoma (VS) had preserved preoperative responses. Vestibular disability could not be predicted based on vestibulo-ocular reflex (VOR) performance during motion stimuli, or in the caloric test. Objectives. To explore motorized head impulse rotator for evaluation of angular horizontal VOR in patients with VS, and to compare these responses to those of the caloric test and the symptoms. Patients and methods. We prospectively recorded head and eye position during unpredictable motorized head impulses in 38 patients with VS. We calculated gain and asymmetry of VOR (mean±95% CI), and the results were compared to those of the caloric test and a questionnaire regarding dizziness, hearing and quality of life. Results. The VOR during motorized impulses was abnormal in 71% of patients. Asymmetry in gain correlated significantly (p<0.001) with unilateral weakness in the caloric test. Preoperative gain was significantly lowered to 0.83±0.08 on the ipsilateral side compared to 0.98±0.06 on the contralateral side. Postoperative gain on the operated side of 0.53±0.05 was significantly different from preoperative gain (p<0.001). Findings in vestibular tests did not correlate with subjective sensation of dizziness.     

Recovery of the High-Acceleration Vestibulo-ocular Reflex After Vestibular Neuritis

Vestibular neuritis (VN) usually leads to a sudden gain asymmetry of the high-acceleration horizontal vestibulo-ocular reflex (VOR). We asked whether this asymmetry decreases over time indicating peripheral recovery and/or central compensation. The horizontal VOR during rapid rotational head impulses to both sides was recorded with search coils in 37 patients at different time periods (1-240 weeks) after the onset of VN. In ten patients, sequential measurements were performed. Gains of the VOR during head impulses toward the ipsilesional side significantly increased after the initial drop (average gains: < 1 week: 0.35; 1-4 weeks: 0.33; 4-40 weeks: 0.55; 40-240 weeks: 0.50). Gains on the contralesional side, however, were only slightly reduced and showed no significant change. We conclude that, in contrast to patients after hemilabyrinthectomy or unilateral vestibular neurectomy, the ocular response to ipsilesional rotations in patients after VN improves over time. This finding suggests that ipsilesional recovery is peripheral or, if central, depends on spared peripheral function. The physiology of linear and nonlinear VOR pathways predicts a considerable gain reduction for contralesional head impulses if central compensation mechanisms are not engaged. Thus, the relatively preserved gain on the contralesional side can be explained only by central "upregulation". Apparently, for high accelerations of the head, effective central compensation after VN does not aim to balance the gains of the VOR but tries to boost the contralesional gain close to normal.     

Vestibulo-ocular responses during static head roll and three-dimensional head impulses after vestibular neuritis

This study aimed to investigate whether unilateral vestibular neuritis (VN) causes the same deficits of ocular counter-roll during static head roll (OCR(S)) and dynamic vestibulo-ocular reflex gains during head impulses (VOR(HI)) as unilateral vestibular deafferentation (VD). Ten patients with acute and 14 patients with chronic vestibular paralysis after VN were examined. The testing battery included fundus photography of both eyes with the head upright (binocular cyclorotation) and dual search coil recordings in a three-field magnetic frame. With one dual search coil on the right eye and the other on the forehead, the following stimuli were given: i) Halmagyi-Curthoys head impulses about the vertical, horizontal and torsional axes. ii) Static roll positions of the head up to 20 degrees right- and left-ear-down by movement of the neck. The comparison group consisted of 19 healthy subjects. Compared with the VD-patients, as reported in the literature, acute VN-patients showed the same pattern of OCR(S) gain reduction and binocular cyclorotation (CRb). The main feature that distinguished chronic VN-patients from chronic VD-patients was the normalization of the torsional VOR(HI) gain to the affected side, whereas the VOR(HI) gains in the horizontal and vertical directions did not show recovery (as in the patients with chronic VD). Chronic VN-patients differed from acute VN-patients by: i) symmetrical OCR(S) gains, ii) a less pronounced CRb toward the affected side, and iii) a normal torsional VOR(HI) gain toward the affected side. Since the ipsilesional torsional VOR(HI) gain did not recover in VD-patients, the normalization of this gain in our VN-patients can only be explained by a (partial) recovery of otolith function on the side of the lesion after the neuritis.     

Comparison of head thrust test with head autorotation test reveals that the vestibulo-ocular reflex is enhanced during voluntary head movements

OBJECTIVES: To compare 2 clinical tests of vestibular function, the head autorotation test (HART) and the head thrust test (HTT), and to determine why they give disparate results in patients with known unilateral vestibular deficiency (UVD) due to labyrinthectomy. METHODS: We used scleral coils to measure the horizontal (yaw) vestibulo-ocular reflex (VOR) in 5 healthy human subjects and in 11 patients who underwent labyrinthectomy. We used 2 paradigms. Using HART, subjects visually fixated a target during self-generated, swept-frequency, sinusoidal, horizontal head rotations. Using HTT, patients fixated the target during horizontal head thrusts delivered randomly in direction and time. RESULTS: In subjects without UVD, eye movements were almost perfectly compensatory for both paradigms. In subjects with UVD, VOR gain for ipsilesional head thrusts was low for both paradigms, but significantly (P<.001) higher (less abnormal) for HART (0.60 +/- 0.13) than for HTT (0.14 +/- 0.13). Contralesional gain was reduced for both, to 0.64 +/- 0.20 for HART and to 0.57 +/- 0.17 for HTT. Because ipsilesional and contralesional gains were not statistically different for HART (P =.69), comparison of VOR gains for half-cycle responses to the HART stimulus could not reliably identify the side of the known lesion. In contrast, HTT consistently identified the side of the lesion for all subjects with UVD. To investigate whether preprogramming contributes to the boost in VOR as measured by HART, we compared the gain and response delay of eye movements during actively self-generated and passively received head thrusts. For subjects without UVD, response delays were shorter for active (6 +/- 1 milliseconds) than for passive (12 +/- 1 milliseconds) HTT. For ipsilesional rotations of subjects with UVD, active HTT yielded a significantly higher gain (0.44 +/- 0.20) (P<.001) and a shorter delay (15 +/- 6 milliseconds) (P<.001) than did passive HTT (0.14 +/- 0.13 and 37 +/- 15 milliseconds, respectively). Contralesional test results revealed a similar performance boost for active head movements. Data are given as mean +/- SD. CONCLUSION: When comparison of half-cycle gains is used to identify the lesion side, self-generated predictable head movement paradigms, such as HART and active HTT, are less accurate than passive HTT in the characterization of UVD, in part because preprogramming can augment the VOR during voluntary head movements.     

A new vestibulo-ocular reflex recording system designed for routine vestibular clinical use.

A new vestibulo-ocular reflex (VOR) recording system was developed, which consists of an infrared eye camera, a small velocity sensor and a frequency modulator. Using this system, the head velocity signal was frequency modulated and simultaneously recorded as a sound signal on the audio track of a Hi8 video recorder with eye images. This device enabled recording of the VOR response in routine vestibular clinical practice. The reliability and effectiveness of this system were estimated by recording and analysing the VOR response against manually controlled rotation in normal subjects (n = 22) and in patients with unilateral severe vestibular hypofunction (n = 11). VOR gain on clockwise rotation viewed from the top was defined as R gain, and counterclockwise rotation as L gain. Directional preponderance (DP%) was also calculated. VOR gain towards the diseased side was significantly lower than that towards the intact side, and also significantly lower than that of normal subjects. DP% of unilateral vestibular hypofunction cases was significantly larger than that of normal subjects. These findings indicate that this VOR recording system reliably detects severe unilateral vestibular hypofunction.     

Invariance of vestibulo-ocular reflex gain to head impulses in pitch at different initial eye-in-orbit elevations: Implications for Alexander's law

Abstract Conclusions: These findings are in line with previous data on the horizontal vestibulo-ocular reflex (VOR) from this laboratory and suggest that eye position signals do not modulate natural vestibular responses. Hence, the Alexander's law (AL) phenomenon cannot be interpreted simply as a consequence of vestibular or oculomotor nuclei activity modulation with desired gaze. Background: AL states that the intensity of the spontaneous nystagmus of a patient with a unilateral vestibular lesion grows with increasing gaze in the direction of the fast phase. Some of the mechanisms proposed to account for the gaze effects assume a direct modification of the normal VOR by eye position signals. We tested the validity of these assumptions and investigated the effects of gaze direction on the normal vertical human VOR in the behaviorally relevant high frequency range. Methods: Head and eye movements were recorded with the search coil method during passive head impulses in pitch, while subjects were asked to hold gaze at various elevation angles in 8° steps within ± 16° from the straight ahead reference position. Results: Upward and downward head rotations produced VOR gains of similar magnitude. Furthermore, the gain remained unaffected by eye-in-orbit position for both upward and downward head impulses.     

Dynamic Displacement of Normal and Detached Semicircular Canal Cupula

The dynamic displacement of the semicircular canal cupula and modulation of afferent nerve discharge were measured simultaneously in response to physiological stimuli in vivo. The adaptation time constant(s) of normal cupulae in response to step stimuli averaged 36 s, corresponding to a mechanical lower corner frequency for sinusoidal stimuli of 0.0044 Hz. For stimuli equivalent to 40–200 deg/s of angular head velocity, the displacement gain of the central region of the cupula averaged 53 nm per deg/s. Afferents adapted more rapidly than the cupula, demonstrating the presence of a relaxation process that contributes significantly to the neural representation of angular head motions by the discharge patterns of canal afferent neurons. We also investigated changes in time constants of the cupula and afferents following detachment of the cupula at its apex—mechanical detachment that occurs in response to excessive transcupular endolymph pressure. Detached cupulae exhibited sharply reduced adaptation time constants (300 ms–3 s, n = 3) and can be explained by endolymph flowing rapidly over the apex of the cupula. Partially detached cupulae reattached and normal afferent discharge patterns were recovered 5–7 h following detachment. This regeneration process may have relevance to the recovery of semicircular canal function following head trauma.     

Head impulses after unilateral vestibular deafferentation validate Ewald's second law.

To determine the relative contributions of ampullofugal (AF) and ampullopetal (AP) stimulation of the horizontal semicircular canal (HSCC) to the horizontal vestibulo-ocular reflex (HVOR), 12 patients were studied 1 year after total unilateral vestibular deafferentation (UVD). Compensatory eye movement responses to impulses of horizontal head rotation were studied using magnetic search coils. The head impulses were rapid (up to 3000 deg/sec/sec) passive, unpredictable, step displacements of horizontal angular head position with respect to the trunk. The results from these 12 patients were compared with results from 30 normal subjects. An HVOR deficit was found to each side. The HVOR in response to head impulses toward the deafferented side, a response generated exclusively by ampullofugal stimulation of the single functioning HSCC, was severely deficient with an average gain of 0.25; the HVOR in response to head impulses toward the intact side, a response generated exclusively by ampullopetal stimulation of the single functioning HSCC, was mildly but significantly deficient compared with normal subjects. These results show that rapid, unpredictable head movements, unlike slow, predictable head movements, do demonstrate the AP-AF HVOR asymmetry, which could be expected from consideration of the behavior of single vestibular afferent neurons, an asymmetry that is expressed by Ewald's 2nd Law.     

Does High‐Frequency Pseudo‐random Rotational Chair Testing Increase the Diagnostic Yield of the ENG Caloric Test in Detecting Bilateral Vestibular Loss in the Dizzy Patient?

OBJECTIVES: To assess the incremental diagnostic yield of testing vestibulo-ocular (VOR) gain with high-frequency pseudo-random rotational chair (PsRRC) over testing with bithermal electronystagmography caloric tests in the dizzy patient, particularly in detecting bilateral vestibular loss. PATIENTS AND METHODS: One hundred ninety-eight patients presenting with dizziness underwent PsRRC and caloric testing. The VOR gain on PsRRC was measured at 0.32 to 5.0 Hz, with gain categorized as normal or decreased. PsRRC results were compared with caloric responses, also categorized as normal, or into graded categories of unilateral or bilateral vestibular loss. RESULTS: Reduced PsRRC gain was found in 29 (15%) patients, and reduced caloric tests responses in 70 (35%), with 25 (13%) having bilateral loss. Of patients with reduced chair gain, 25 of 29 (86%) demonstrated bilateral caloric loss. PsRRC gain was normal in most patients with unilateral caloric weakness, but was decreased in all patients with bilateral caloric weakness. The probability of a patient with completely normal caloric responses having an abnormal rotation chair in this study group was under 1% (1 of 128). CONCLUSIONS: PsRRC testing does not offer much additional diagnostic benefit when caloric responses are normal. It is useful in specific conditions, such as unilateral caloric loss for which the patient is not compensating, borderline caloric loss when traditional water caloric tests cannot be used, or for monitoring progressive bilateral vestibular loss.