Eye movements evoked by selective saccular nerve stimulation in cats

Binocular 3-D otolith-ocular reflexes: responses of chinchillas to natural and prosthetic stimulation after ototoxic injury and vestibular implantation

The otolith end organs inform the brain about gravitational and linear accelerations, driving the otolith-ocular reflex (OOR) to stabilize the eyes during translational motion (e.g., moving forward without rotating) and head tilt with respect to gravity. We previously characterized OOR responses of normal chinchillas to whole body tilt and translation and to prosthetic electrical stimulation targeting the utricle and saccule via electrodes implanted in otherwise normal ears. Here we extend that work to examine OOR responses to tilt and translation stimuli after unilateral intratympanic gentamicin injection and to natural/mechanical and prosthetic/electrical stimulation delivered separately or in combination to animals with bilateral vestibular hypofunction after right ear intratympanic gentamicin injection followed by surgical disruption of the left labyrinth at the time of electrode implantation. Unilateral intratympanic gentamicin injection decreased natural OOR response magnitude to about half of normal, without markedly changing OOR response direction or symmetry. Subsequent surgical disruption of the contralateral labyrinth at the time of electrode implantation surgery further decreased OOR magnitude during natural stimulation, consistent with bimodal-bilateral otolith end organ hypofunction (ototoxic on the right ear, surgical on the left ear). Delivery of pulse frequency- or pulse amplitude-modulated prosthetic/electrical stimulation targeting the left utricle and saccule in phase with whole body tilt and translation motion stimuli yielded responses closer to normal than the deficient OOR responses of those same animals in response to head tilt and translation alone.NEW & NOTEWORTHY Previous studies to expand the scope of prosthetic stimulation of the otolith end organs showed that selective stimulation of the utricle and saccule is possible. This article further defines those possibilities by characterizing a diseased animal model and subsequently studying its responses to electrical stimulation alone and in combination with mechanical motion. We show that we can partially restore responses to tilt and translation in animals with unilateral gentamicin ototoxic injury and contralateral surgical disruption.

Discordant horizontal-torsional nystagmus: a sign of posterior semicircular canal dysfunction

In central as well as peripheral vestibular lesions, right-beating horizontal nystagmus is almost always associated with clockwise (top poles of the eyes beating to the right ear) torsional nystagmus when observed and vice versa (concordant nystagmus). This study aimed to determine the etiologies and mechanisms of horizontal and torsional nystagmus beating in the opposite directions (discordant nystagmus). We reviewed the medical records of 16 consecutive patients with discordant horizontal-torsional nystagmus who had been evaluated at the dizziness clinics of Seoul National University Bundang Hospital (n = 11, from March 2003 to March 2021) and Korea University Medical Center (n = 5, from March 2019 to March 2021). The underlying etiologies included inferior vestibular neuritis (n = 7), Meniere's disease (n = 4), internuclear ophthalmoplegia (n = 3), medullary hemorrhage (n = 1), and normal pressure hydrocephalus (n = 1). The torsional nystagmus decreased during the gaze in the same direction (for instance, during rightward gaze in clockwise nystagmus) and increased during the gaze in the opposite direction. Head-impulse tests (HITs) were positive for the ipsilesional posterior canal (PC) in all 11 patients with unilateral peripheral vestibulopathy and two of the three patients with unilateral central vestibulopathy. Discordant horizontal-torsional nystagmus may be observed in peripheral as well as central lesions. Given the findings of HITs and modulation of spontaneous nystagmus during lateral gazes, discordant horizontal-torsional nystagmus may be ascribed to selective damage of the excitatory or inhibitory pathway from the PC that innervates the ipsilateral superior oblique and contralateral inferior rectus muscles.

Skull Vibration Induced Nystagmus Test: Correlations with Semicircular Canal and Otolith Asymmetries

Background: To establish in patients with peripheral vestibular disorders relations between skull vibration-induced nystagmus (SVIN) different components (horizontal, vertical, torsional) and the results of different structurally related vestibular tests. Methods: SVIN test, canal vestibular test (CVT: caloric test + video head impulse test: VHIT), otolithic vestibular test (OVT: ocular vestibular evoked myogenic potential oVEMP + cervical vestibular evoked myogenic potential cVEMP) performed on the same day in 52 patients with peripheral vestibular diseases (age < 65 years), and 11 control patients were analyzed. Mixed effects logistic regression analysis was performed to assert whether the presence of nystagmus in SVIN (3D analysis) have an association with the presence of peripheral vestibular dysfunction measured by vestibular explorations (CVT or OVT). Results: We obtained different groups: Group-Co (control group), Group-VNT (dizzy patients with no vestibular tests alterations), Group-O (OVT alterations only), Group-C (CVT alterations only), Group-M (mixed alterations). SVIN-SPV horizontal component was significantly higher in Group-M than in the other groups (p = 0.005) and correlated with alterations of lateral-VHIT (p < 0.001), caloric test (p = 0.002) and oVEMP (p = 0.006). SVIN-SPV vertical component was correlated with the anterior-VHIT and oVEMP alterations (p = 0.007; p = 0.017, respectively). SVIN-SPV torsional component was correlated with the anterior-VHIT positivity (p = 0.017). SVIN was the only positive test for 10% of patients (83% of Group-VNT). Conclusion: SVIN-SPV analysis in dizzy patients shows significant correlation to both CVT and OVT. SVIN horizontal component is mainly relevant to both vestibular tests exploring lateral canal and utricle responses. SVIN-SPV is significantly higher in patients with combined canal and otolith lesions. In some patients with dizziness, SVIN may be the only positive test.

Therapy-Resistant Atypical Downbeat Nystagmus with Vertigo Confined to Specific Head-Hanging Positions: Mapping to the Gravity Vector on a Multi-Axis Turntable

Downbeat nystagmus (DBN) observed in head-hanging positions, may be of central or peripheral origin. Central DBN in head-hanging positions is mostly due to a disorder of the vestibulo-cerebellum, whereas peripheral DBN is usually attributed to canalolithiasis of an anterior semicircular canal. Here, we describe an atypical case of a patient who, after head trauma, experienced severe and stereotypic vertigo attacks after being placed in various head-hanging positions. Vertigo lasted 10–15 s and was always associated with a robust DBN. The provocation of transient vertigo and DBN, which both showed no decrease upon repetition of maneuvers, depended on the yaw orientation relative to the trunk and the angle of backward pitch. On a motorized, multi-axis turntable, we identified the two-dimensional Helmholtz coordinates of head positions at which vertigo and DBN occurred (y-axis: horizontal, space-fixed; z-axis: vertical, and head-fixed; x-axis: torsional, head-fixed, and unchanged). This two-dimensional area of DBN-associated head positions did not change when whole-body rotations took different paths (e.g., by forwarding pitch) or were executed with different velocities. Moreover, the intensity of DBN was also independent of whole-body rotation paths and velocities. So far, therapeutic approaches with repeated liberation maneuvers and cranial vibrations were not successful. We speculate that vertigo and DBN in this patient are due to macular damage, possibly an unstable otolithic membrane that, in specific orientations relative to gravity, slips into a position causing paroxysmal stimulation or inhibition of macular hair cells.

Ictal downbeat nystagmus in Ménière disease: A cross-sectional study

OBJECTIVE

To determine the mechanism of ictal downbeat nystagmus in Ménière disease (MD), we compared the head impulse gain of the vestibulo-ocular reflex (VOR) for each semicircular canal between patients with (n = 7) and without (n = 70) downbeat nystagmus during attacks of MD.

METHODS

We retrospectively analyzed the results of video-oculography, video head-impulse tests, and cervical vestibular-evoked myogenic potentials (VEMPs) in 77 patients with definite MD who were evaluated during an attack.

RESULTS

Pure or predominant downbeat nystagmus was observed in 7 patients (9%) with unilateral MD during the attacks. All 7 patients showed spontaneous downbeat nystagmus without visual fixation with a slow phase velocity ranging from 1.5 to 11.2°/s (median 5.4, interquartile range 3.7-8.5). All showed a transient decrease of the head impulse VOR gains for the posterior canals (PCs) in both ears (n = 4) or in the affected ear (n = 3). Cervical VEMPs were decreased in the affected (n = 2) or both ears (n = 2) when evaluated during the attacks. Downbeat nystagmus disappeared along with normalization of the VOR gains for PCs after the attacks in all patients. During the attacks, the head impulse VOR gains for the PC on the affected side were lower in the patients with ictal downbeat nystagmus than in those without (Mann-Whitney U test, p < 0.001), while the gains for other semicircular canals did not differ between the groups.

CONCLUSION

Downbeat nystagmus may be observed during attacks of MD due to an asymmetry in the vertical VOR or saccular dysfunction. MD should be considered in recurrent audiovestibulopathy and ictal downbeat nystagmus.

The role of vertical semicircular canal function in the vertical component of skull vibration-induced nystagmus

Background: Generally, vertical component of the skull vibratory nystagmus (VCN) is ignored in the clinical practise. Thus, the relative contribution of the vestibular organs in the presence of VCN remains unknown.Objectives: To determine the association between vertical semicircular canal (vSCC) function and the presence of VCN.Material and methods: Comparisons were made between Video Head Impulse Test and SVINT (100 Hz) results at the time of the acute peripheral vestibular lesion (PVL) and at the post-acute phase in patients diagnosed PVL. Later on, a paired analysis was performed restricting the assessments to patients with vestibular explorations in both the acute and post-acute phases.Results: In an univariable analysis, larger mean total gain differences (TGD) between vSCC VOR gains, significantly related with the appearance of VCN in nystagmography in the acute phase (p = .001), unlike the post-acute phase (p = .46). After a multivariate analysis, mean TGD was the only predictive factor of the VCN (p = .013). In the paired analysis, we found an increase in the post-acute phase mean TGD, approaching zero value.Conclusions and significance: Global relation between all vertical canals has at least a contributory role in the presence of the vertical component of nystagmus in SVINT.

Evidence-based diagnostic use of VEMPs : From neurophysiological principles to clinical application

BACKGROUND

Vestibular evoked myogenic potentials (VEMPs) are increasingly being used for testing otolith organ function.

OBJECTIVE

This article provides an overview of the anatomical, biomechanical and neurophysiological principles underlying the evidence-based clinical application of ocular and cervical VEMPs (oVEMPs and cVEMPs).

MATERIAL AND METHODS

Systematic literature search in PubMed until April 2019.

RESULTS

Sound and vibration at a frequency of 500 Hz represent selective vestibular stimuli for the otolith organs. The predominant specificity of oVEMPs for contralateral utricular function and of cVEMPs for ipsilateral saccular function is defined by the different central projections of utricular and saccular afferents. VEMPs are particularly useful in the diagnosis of superior canal dehiscence and otolith organ specific vestibular dysfunction and as an alternative diagnostic approach in situations when video oculography is not possible or useful.

CONCLUSION

The use of VEMPs is a simple, safe, reliable and selective test of dynamic function of otolith organs.

Binocular 3D otolith-ocular reflexes: responses of chinchillas to prosthetic electrical stimulation targeting the utricle and saccule

From animal experiments by Cohen and Suzuki et al. in the 1960s to the first-in-human clinical trials now in progress, prosthetic electrical stimulation targeting semicircular canal branches of the vestibular nerve has proven effective at driving directionally appropriate vestibulo-ocular reflex eye movements, postural responses, and perception. That work was considerably facilitated by the fact that all hair cells and primary afferent neurons in each canal have the same directional sensitivity to head rotation, the three canals' ampullary nerves are geometrically distinct from one another, and electrically evoked three-dimensional (3D) canal-ocular reflex responses approximate a simple vector sum of linearly independent components representing relative excitation of each of the three canals. In contrast, selective prosthetic stimulation of the utricle and saccule has been difficult to achieve, because hair cells and afferents with many different directional sensitivities are densely packed in those endorgans and the relationship between 3D otolith-ocular reflex responses and the natural and/or prosthetic stimuli that elicit them is more complex. As a result, controversy exists regarding whether selective, controllable stimulation of electrically evoked otolith-ocular reflexes (eeOOR) is possible. Using micromachined, planar arrays of electrodes implanted in the labyrinth, we quantified 3D, binocular eeOOR responses to prosthetic electrical stimulation targeting the utricle, saccule, and semicircular canals of alert chinchillas. Stimuli delivered via near-bipolar electrode pairs near the maculae elicited sustained ocular countertilt responses that grew reliably with pulse rate and pulse amplitude, varied in direction according to which stimulating electrode was employed, and exhibited temporal dynamics consistent with responses expected for isolated macular stimulation.NEW & NOTEWORTHY As the second in a pair of papers on Binocular 3D Otolith-Ocular Reflexes, this paper describes new planar electrode arrays and vestibular prosthesis architecture designed to target the three semicircular canals and the utricle and saccule. With this technological advancement, electrically evoked otolith-ocular reflexes due to stimulation via utricle- and saccule-targeted electrodes were recorded in chinchillas. Results demonstrate advances toward achieving selective stimulation of the utricle and saccule.

[Evidence-based diagnostic use of VEMPs : From neurophysiological principles to clinical application. German version]

BACKGROUND

Vestibular evoked myogenic potentials (VEMPs) are increasingly being used for testing otolith organ function.

OBJECTIVE

This article provides an overview of the anatomical, biomechanical and neurophysiological principles of an evidence-based clinical application of ocular and cervical VEMPs (oVEMPs and cVEMPs).

MATERIAL AND METHODS

Systematic literature search in PubMed until April 2019.

RESULTS

Sound and vibration at a frequency of 500 Hz represent selective vestibular stimuli for the otolith organs. The predominant specificity of oVEMPs for contralateral utricular function and of cVEMPs for ipsilateral saccular function is defined by the different neuronal projections of the utricle and the saccule. VEMPs are particularly useful in the diagnosis of superior canal dehiscence and otolith organ-specific vestibular dysfunction and as an alternative diagnostic approach in situations when video oculography is not possible or useful.

CONCLUSION

The use of VEMPs is a simple, safe, reliable and selective test of dynamic function of otolith organs.

Concepts and Physiological Aspects of the Otolith Organ in Relation to Electrical Stimulation

BACKGROUND

This paper discusses some of the concepts and major physiological issues in developing a means of electrically stimulating the otolithic system, with the final goal being the electrical stimulation of the otoliths in human patients. It contrasts the challenges of electrical stimulation of the otolith organs as compared to stimulation of the semicircular canals. Electrical stimulation may consist of trains of short-duration pulses (e.g., 0.1 ms duration at 400 Hz) by selective electrodes on otolith maculae or otolithic afferents, or unselective maintained DC stimulation by large surface electrodes on the mastoids - surface galvanic stimulation.

SUMMARY

Recent anatomical and physiological results are summarized in order to introduce some of the unique issues in electrical stimulation of the otoliths. The first challenge is that each otolithic macula contains receptors with opposite polarization (opposing preferred directions of stimulation), unlike the uniform polarization of receptors in each semicircular canal crista. The puzzle is that in response to the one linear acceleration in the one macula, some otolithic afferents have an increased activation whereas others have decreased activation. Key Messages: At the vestibular nucleus this opposite receptor hair cell polarization and consequent opposite afferent input allow enhanced response to the one linear acceleration, via a "push-pull" neural mechanism in a manner analogous to the enhancement of semicircular canal responses to angular acceleration. Within each otolithic macula there is not just one uniform otolithic neural input to the brain - there are very distinctly different channels of otolithic neural inputs transferring the neural data to the brainstem. As a simplification these channels are characterized as the sustained and transient systems. Afferents in each system have different responses to stimulus onset and maintained stimulation and likely different projections, and most importantly different thresholds for activation by electrical stimulation and different adaptation rates to maintained stimulation. The implications of these differences are considered.

Electrical Vestibular Stimulation in Humans: A Narrative Review

BACKGROUND

In patients with bilateral vestibulopathy, the regular treatment options, such as medication, surgery, and/or vestibular rehabilitation, do not always suffice. Therefore, the focus in this field of vestibular research shifted to electrical vestibular stimulation (EVS) and the development of a system capable of artificially restoring the vestibular function. Key Message: Currently, three approaches are being investigated: vestibular co-stimulation with a cochlear implant (CI), EVS with a vestibular implant (VI), and galvanic vestibular stimulation (GVS). All three applications show promising results but due to conceptual differences and the experimental state, a consensus on which application is the most ideal for which type of patient is still missing.

SUMMARY

Vestibular co-stimulation with a CI is based on "spread of excitation," which is a phenomenon that occurs when the currents from the CI spread to the surrounding structures and stimulate them. It has been shown that CI activation can indeed result in stimulation of the vestibular structures. Therefore, the question was raised whether vestibular co-stimulation can be functionally used in patients with bilateral vestibulopathy. A more direct vestibular stimulation method can be accomplished by implantation and activation of a VI. The concept of the VI is based on the technology and principles of the CI. Different VI prototypes are currently being evaluated regarding feasibility and functionality. So far, all of them were capable of activating different types of vestibular reflexes. A third stimulation method is GVS, which requires the use of surface electrodes instead of an implanted electrode array. However, as the currents are sent through the skull from one mastoid to the other, GVS is rather unspecific. It should be mentioned though, that the reported spread of excitation in both CI and VI use also seems to induce a more unspecific stimulation. Although all three applications of EVS were shown to be effective, it has yet to be defined which option is more desirable based on applicability and efficiency. It is possible and even likely that there is a place for all three approaches, given the diversity of the patient population who serves to gain from such technologies.

Virtual Rhesus Labyrinth Model Predicts Responses to Electrical Stimulation Delivered by a Vestibular Prosthesis

To better understand the spread of prosthetic current in the inner ear and to facilitate design of electrode arrays and stimulation protocols for a vestibular implant system intended to restore sensation after loss of vestibular hair cell function, we created a model of the primate labyrinth. Because the geometry of the implanted ear is complex, accurately modeling effects of prosthetic stimuli on vestibular afferent activity required a detailed representation of labyrinthine anatomy. Model geometry was therefore generated from three-dimensional (3D) reconstructions of a normal rhesus temporal bone imaged using micro-MRI and micro-CT. For systematically varied combinations of active and return electrode location, the extracellular potential field during a biphasic current pulse was computed using finite element methods. Potential field values served as inputs to stochastic, nonlinear dynamic models for each of 2415 vestibular afferent axons, each with unique origin on the neuroepithelium and spiking dynamics based on a modified Smith and Goldberg model. We tested the model by comparing predicted and actual 3D vestibulo-ocular reflex (VOR) responses for eye rotation elicited by prosthetic stimuli. The model was individualized for each implanted animal by placing model electrodes in the standard labyrinth geometry based on CT localization of actual implanted electrodes. Eye rotation 3D axes were predicted from relative proportions of model axons excited within each of the three ampullary nerves, and predictions were compared to archival eye movement response data measured in three alert rhesus monkeys using 3D scleral coil oculography. Multiple empirically observed features emerged as properties of the model, including effects of changing active and return electrode position. The model predicts improved prosthesis performance when the reference electrode is in the labyrinth's common crus (CC) rather than outside the temporal bone, especially if the reference electrode is inserted nearly to the junction of the CC with the vestibule. Extension of the model to human anatomy should facilitate optimal design of electrode arrays for clinical application.

Vestibular-evoked myogenic potentials

The vestibular-evoked myogenic potential (VEMP) is a short-latency potential evoked through activation of vestibular receptors using sound or vibration. It is generated by modulated electromyographic signals either from the sternocleidomastoid muscle for the cervical VEMP (cVEMP) or the inferior oblique muscle for the ocular VEMP (oVEMP). These reflexes appear to originate from the otolith organs and thus complement existing methods of vestibular assessment, which are mainly based upon canal function. This review considers the basis, methodology, and current applications of the cVEMP and oVEMP in the assessment and diagnosis of vestibular disorders, both peripheral and central.

Binocular misalignments elicited by altered gravity provide evidence for nonlinear central compensation

Increased ocular positioning misalignments upon exposure to altered gravity levels (g-levels) have been strongly correlated with space motion sickness (SMS) severity, possibly due to underlying otolith asymmetries uncompensated in novel gravitational environments. We investigated vertical and torsional ocular positioning misalignments elicited by the 0 and 1.8 g g-levels of parabolic flight and used these data to develop a computational model to describe how such misalignments might arise. Ocular misalignments were inferred through two perceptual nulling tasks: Vertical Alignment Nulling (VAN) and Torsional Alignment Nulling (TAN). All test subjects exhibited significant differences in ocular misalignments in the novel g-levels, which we postulate to be the result of healthy individuals with 1 g-tuned central compensatory mechanisms unadapted to the parabolic flight environment. Furthermore, the magnitude and direction of ocular misalignments in hypo-g and hyper-g, in comparison to 1 g, were nonlinear and nonmonotonic. Previous linear models of central compensation do not predict this. Here we show that a single model of the form a + bg (ε), where a, b, and ε are the model parameters and g is the current g-level, accounts for both the vertical and torsional ocular misalignment data observed inflight. Furthering our understanding of oculomotor control is critical for the development of interventions that promote adaptation in spaceflight (e.g., countermeasures for novel g-level exposure) and terrestrial (e.g., rehabilitation protocols for vestibular pathology) environments.

Pulsed infrared radiation excites cultured neonatal spiral and vestibular ganglion neurons by modulating mitochondrial calcium cycling

Cochlear implants are currently the most effective solution for profound sensorineural hearing loss, and vestibular prostheses are under development to treat bilateral vestibulopathies. Electrical current spread in these neuroprostheses limits channel independence and, in some cases, may impair their performance. In comparison, optical stimuli that are spatially confined may result in a significant functional improvement. Pulsed infrared radiation (IR) has previously been shown to elicit responses in neurons. This study analyzes the response of neonatal rat spiral and vestibular ganglion neurons in vitro to IR (wavelength = 1,863 nm) using Ca(2+) imaging. Both types of neurons responded consistently with robust intracellular Ca(2+) ([Ca(2+)]i) transients that matched the low-frequency IR pulses applied (4 ms, 0.25-1 pps). Radiant exposures of ∼637 mJ/cm(2) resulted in continual neuronal activation. Temperature or [Ca(2+)] variations in the media did not alter the IR-evoked transients, ruling out extracellular Ca(2+) involvement or primary mediation by thermal effects on the plasma membrane. While blockage of Na(+), K(+), and Ca(2+) plasma membrane channels did not alter the IR-evoked response, blocking of mitochondrial Ca(2+) cycling with CGP-37157 or ruthenium red reversibly inhibited the IR-evoked [Ca(2+)]i transients. Additionally, the magnitude of the IR-evoked transients was dependent on ryanodine and cyclopiazonic acid-dependent Ca(2+) release. These results suggest that IR modulation of intracellular calcium cycling contributes to stimulation of spiral and vestibular ganglion neurons. As a whole, the results suggest selective excitation of neurons in the IR beam path and the potential of IR stimulation in future auditory and vestibular prostheses.

New perspectives on vestibular evoked myogenic potentials

PURPOSE OF REVIEW

Although the vestibular evoked myogenic potential (VEMP) measured from the cervical muscles (cVEMP, cervical VEMP) is well described and has documented clinical utility, its analogue recorded from the extraocular muscles (oVEMP, ocular VEMP) has been described only recently and is currently emerging as an additional test of otolith function. This review will, therefore, summarize recent developments in VEMP research with a focus on the oVEMP.

RECENT FINDINGS

Recent studies suggest that the oVEMP is produced by otolith afferents in the superior vestibular nerve division, whereas the cVEMP evoked by sound is thought to be an inferior vestibular nerve reflex. Correspondingly, the oVEMP correlates better with caloric and subjective visual vertical tests than sound-cVEMPs. cVEMPs are more complicated than often thought, as shown by the presence of crossed responses and conflicting results of recent vibration studies. Altered inner ear mechanics produced by the vestibular diseases superior semicircular canal dehiscence and Ménière's disease lead to changes in the preferred frequency of the oVEMP and cVEMP.

SUMMARY

The oVEMP provides complementary diagnostic information to the cVEMP and is likely to be a useful addition to the diagnostic test battery in neuro-otology.

Sound-induced vertigo after cochlear implantation

AIM

Postoperative vertigo is a well-known complication after cochlear implantation. The aim of the study was to investigate whether the electrical stimulation of the auditory structures via cochlear implant electrodes can affect the vestibular system and induce vertigo.

MATERIALS AND METHODS

In the first group, 114 patients were surveyed retrospectively via questionnaires to evaluate the occurrence and frequency of sound-induced vertigo after cochlear implantation. In the second group of 26 patients, the effects of electrical stimulation on the vestibular system were studied prospectively.

RESULTS

In the first group of patients without any preoperative sound-induced vertigo (n = 104), 20 patients (18%) reported sound-induced vertigo, which occurred after cochlear implantation. In the second group, an acoustic stimulus delivered via the speech processor of the cochlear implant elicited a vestibular evoked myogenic potential response in 4 of the 26 patients as a sign of vestibular costimulation (of the macula sacculi as part of the otolith organs). Horizontal and vertical nystagmus was triggered, whereas utricular function and postural stability remained unchanged. No correlation was found between C/M levels and the vestibular evoked myogenic potentials and nystagmus responses.

CONCLUSION

Sound-induced vertigo can occur in cochlear implantees. This seems to be primarily caused by electrical costimulation of the sacculus as part of the otolith organs.

Differences between otolith- and semicircular canal-activated neural circuitry in the vestibular system

In the last two decades, we have focused on establishing a reliable technique for focal stimulation of vestibular receptors to evaluate neural connectivity. Here, we summarize the vestibular-related neuronal circuits for the vestibulo-ocular reflex, vestibulocollic reflex, and vestibulospinal reflex arcs. The focal stimulating technique also uncovered some hidden neural mechanisms. In the otolith system, we identified two hidden neural mechanisms that enhance otolith receptor sensitivity. The first is commissural inhibition, which boosts sensitivity by incorporating inputs from bilateral otolith receptors, the existence of which was in contradiction to the classical understanding of the otolith system but was observed in the utricular system. The second mechanism, cross-striolar inhibition, intensifies the sensitivity of inputs from both sides of receptive cells across the striola in a single otolith sensor. This was an entirely novel finding and is typically observed in the saccular system. We discuss the possible functional meaning of commissural and cross-striolar inhibition. Finally, our focal stimulating technique was applied to elucidate the different constructions of axonal projections from each vestibular receptor to the spinal cord. We also discuss the possible function of the unique neural connectivity observed in each vestibular receptor system.

Virtual labyrinth model of vestibular afferent excitation via implanted electrodes: validation and application to design of a multichannel vestibular prosthesis

To facilitate design of a multichannel vestibular prosthesis that can restore sensation to individuals with bilateral loss of vestibular hair cell function, we created a virtual labyrinth model. Model geometry was generated through 3-dimensional (3D) reconstruction of microMRI and microCT scans of normal chinchillas (Chinchilla lanigera) acquired with 30-48 μm and 12 μm voxels, respectively. Virtual electrodes were positioned based on anatomic landmarks, and the extracellular potential field during a current pulse was computed using finite element methods. Potential fields then served as inputs to stochastic, nonlinear dynamic models for each of 2,415 vestibular afferent axons with spiking dynamics based on a modified Smith and Goldberg model incorporating parameters that varied with fiber location in the neuroepithelium. Action potential propagation was implemented by a well validated model of myelinated fibers. We tested the model by comparing predicted and actual 3D angular vestibulo-ocular reflex (aVOR) axes of eye rotation elicited by prosthetic stimuli. Actual responses were measured using 3D video-oculography. The model was individualized for each animal by placing virtual electrodes based on microCT localization of real electrodes. 3D eye rotation axes were predicted from the relative proportion of model axons excited within each of the three ampullary nerves. Multiple features observed empirically were observed as emergent properties of the model, including effects of active and return electrode position, stimulus amplitude and pulse waveform shape on target fiber recruitment and stimulation selectivity. The modeling procedure is partially automated and can be readily adapted to other species, including humans.

Cross-axis adaptation improves 3D vestibulo-ocular reflex alignment during chronic stimulation via a head-mounted multichannel vestibular prosthesis

By sensing three-dimensional (3D) head rotation and electrically stimulating the three ampullary branches of a vestibular nerve to encode head angular velocity, a multichannel vestibular prosthesis (MVP) can restore vestibular sensation to individuals disabled by loss of vestibular hair cell function. However, current spread to afferent fibers innervating non-targeted canals and otolith end organs can distort the vestibular nerve activation pattern, causing misalignment between the perceived and actual axis of head rotation. We hypothesized that over time, central neural mechanisms can adapt to correct this misalignment. To test this, we rendered five chinchillas vestibular deficient via bilateral gentamicin treatment and unilaterally implanted them with a head-mounted MVP. Comparison of 3D angular vestibulo-ocular reflex (aVOR) responses during 2 Hz, 50°/s peak horizontal sinusoidal head rotations in darkness on the first, third, and seventh days of continual MVP use revealed that eye responses about the intended axis remained stable (at about 70% of the normal gain) while misalignment improved significantly by the end of 1 week of prosthetic stimulation. A comparable time course of improvement was also observed for head rotations about the other two semicircular canal axes and at every stimulus frequency examined (0.2-5 Hz). In addition, the extent of disconjugacy between the two eyes progressively improved during the same time window. These results indicate that the central nervous system rapidly adapts to multichannel prosthetic vestibular stimulation to markedly improve 3D aVOR alignment within the first week after activation. Similar adaptive improvements are likely to occur in other species, including humans.

A critical review of the neurophysiological evidence underlying clinical vestibular testing using sound, vibration and galvanic stimuli

In addition to activating cochlear receptors, air conducted sound (ACS) and bone conducted vibration (BCV) activate vestibular otolithic receptors, as shown by neurophysiological evidence from animal studies--evidence which is the foundation for using ACS and BCV for clinical vestibular testing by means of vestibular-evoked myogenic potentials (VEMPs). Recent research is elaborating the specificity of ACS and BCV on vestibular receptors. The evidence that saccular afferents can be activated by ACS has been mistakenly interpreted as showing that ACS only activates saccular afferents. That is not correct - ACS activates both saccular and utricular afferents, just as BCV activates both saccular and utricular afferents, although the patterns of activation for ACS and BCV do not appear to be identical. The otolithic input to sternocleidomastoid muscle appears to originate predominantly from the saccular macula. The otolithic input to the inferior oblique appears to originate predominantly from the utricular macula. Galvanic stimulation by surface electrodes on the mastoids very generally activates afferents from all vestibular sense organs. This review summarizes the physiological results, the potential artifacts and errors of logic in this area, reconciles apparent disagreements in this field. The neurophysiological results on BCV have led to a new clinical test of utricular function - the n10 of the oVEMP. The cVEMP tests saccular function while the oVEMP tests utricular function.

The effect of gaze direction on the ocular vestibular evoked myogenic potential produced by air-conducted sound

OBJECTIVE

To assess the effects of vertical and horizontal gaze, head rotation, body position, and vision on the ocular vestibular evoked myogenic potential (OVEMP) produced by air-conducted (AC) sound.

METHODS

Ten normal subjects were stimulated by 500 Hz 2 ms AC tone bursts at 136-142 dB peak SPL. OVEMPs were recorded from electrodes placed beneath the eyes. Angles of vertical gaze ranged from maximal downward to upward gaze in increments of 5-10 degrees . Horizontal gaze was measured during elevation and ranged from 20 degrees adduction to 20 degrees abduction.

RESULTS

Increasing vertical gaze increased OVEMP amplitude, especially for the contralateral eye (neutral vs maximal upward gaze; contra: 1.0 vs 2.6 microV; ipsi: 0.8 vs 0.9 microV; P<0.001). OVEMPs from the contralateral eye peaked significantly earlier in the upward gaze positions (contra: 9.2 ms; ipsi: 10.4 ms; P<0.001), but peaked later during downward gaze (contra: 14.2 ms; ipsi: 11.4 ms; P=0.014). There were small effects of horizontal gaze and supine body position, but no effects of head rotation or vision.

CONCLUSIONS

OVEMP amplitudes are strongly modulated by gaze position. Truncal position also affects OVEMP amplitude.

SIGNIFICANCE

This study quantifies the effect of gaze on the OVEMP and demonstrates the importance of controlling for gaze in clinical and experimental studies.

The human sound-evoked vestibulo-ocular reflex and its electromyographic correlate

OBJECTIVE

Sound and vibration evoke a short-latency eye movement or "sound-evoked vestibulo-ocular reflex" (VOR) and an infraorbital surface potential: the "ocular vestibular-evoked myogenic potential" (OVEMP). We examined their relationship by measuring the modulation of both responses by gaze and stimulus parameters.

METHODS

In seven subjects with superior semicircular-canal dehiscence (SCD) and six controls, the sound-evoked VOR was measured in 3D using scleral search coils. OVEMPs were recorded simultaneously, using surface electromyography.

RESULTS

Eye movement onset (11.6+/-0.8ms) coincided with the OVEMP peak (12.1+/-0.35ms). OVEMP and VOR magnitudes were 5-15 times larger in SCD compared with controls. OVEMP amplitudes were maximal on upgaze and abolished on downgaze; VOR magnitudes were unaffected. When stimulus type was changed from sound to vibration, OVEMP and VOR changed concordantly: increasing in controls and decreasing in SCD. OVEMP and VOR tuned to identical stimulus frequencies. OVEMP and VOR magnitudes on upgaze were significantly correlated (R=0.83-0.97).

CONCLUSION

Selective decrease of the OVEMP upon downgaze is consistent with relaxation or retraction of the inferior oblique muscles. The temporal relationship of OVEMP and VOR and their identical modulation by external factors confirms a common origin.

SIGNIFICANCE

Sound-evoked OVEMP and VOR represent the electrical and mechanical correlates of the same vestibulo-ocular response.

Does acute dysfunction of the saccular afferents affect the subjective visual horizontal in patients with vestibular neurolabyrinthitis?

CONCLUSIONS

The results of a series of studies including the present study suggest that acute dysfunction of the utricular afferents accompanied by acute dysfunction of the saccular afferents might require more time for the compensation of the otolith-ocular system than acute utricular dysfunction that was not accompanied by acute saccular dysfunction. Perhaps the inputs from the saccule also have some contribution to the subjective visual horizontal (SVH).

OBJECTIVE

To clarify if acute dysfunction of the saccular afferents affects the SVH.

PATIENTS AND METHODS

Twenty-six patients with vestibular neurolabyrinthitis (20 men and 6 women, 23-67 years of age) were enrolled in this study. They had undergone measurement of SVH at the early stage (within 1 month after the attack) and 3 months after the attack. For the measurement of SVH, we used a device that has a red bar of light-emitting diodes with a head fixing frame. They also underwent vestibular evoked myogenic potentials (VEMPs) testing. For the recording of VEMPs, 95 dBnHL clicks were presented.

RESULTS

Patients with vestibular neurolabyrinthitis showed deviation of SVH toward the affected side-down at the early stage after the attack, irrespective of VEMP results. However, 3 months after the attack SVH was significantly more deviated toward the affected side-down in patients who showed absent VEMPs than those with VEMPs present (p<0.01 Mann-Whitney U test).

A multichannel semicircular canal neural prosthesis using electrical stimulation to restore 3-d vestibular sensation

Bilateral loss of vestibular sensation can be disabling. Those afflicted suffer illusory visual field movement during head movements, chronic disequilibrium and postural instability due to failure of vestibulo-ocular and vestibulo-spinal reflexes. A neural prosthesis that emulates the normal transduction of head rotation by semicircular canals could significantly improve quality of life for these patients. Like the three semicircular canals in a normal ear, such a device should at least transduce three orthogonal (or linearly separable) components of head rotation into activity on corresponding ampullary branches of the vestibular nerve. We describe the design, circuit performance and in vivo application of a head-mounted, semi-implantable multichannel vestibular prosthesis that encodes head movement in three dimensions as pulse-frequency-modulated electrical stimulation of three or more ampullary nerves. In chinchillas treated with intratympanic gentamicin to ablate vestibular sensation bilaterally, prosthetic stimuli elicited a partly compensatory angular vestibulo-ocular reflex in multiple planes. Minimizing misalignment between the axis of eye and head rotation, apparently caused by current spread beyond each electrode's targeted nerve branch, emerged as a key challenge. Increasing stimulation selectivity via improvements in electrode design, surgical technique and stimulus protocol will likely be required to restore AVOR function over the full range of normal behavior.

Latency and initiation of the human vestibuloocular reflex to pulsed galvanic stimulation

Cathodal galvanic currents activate primary vestibular afferents, whereas anodal currents inhibit them. Pulsed galvanic vestibular stimulation (GVS) was used to determine the latency and initiation of the human vestibuloocular reflex. Three-dimensional galvanic vestibuloocular reflex (g-VOR) was recorded with binocular dual-search coils in response to a bilateral bipolar 100-ms rectangular pulse of current at 0.9 (near-threshold), 2.5, 5.0, 7.5, and 10.0 mA in 11 normal subjects. The g-VOR consisted of three components: conjugate torsional eye rotation away from cathode toward anode; vertical divergence (skew deviation) with hypertropia of the eye on the cathodal and hypotropia of the eye on the anodal sides; and conjugate horizontal eye rotation away from cathode toward anode. The g-VOR was repeatable across all subjects, its magnitude a linear function of the current intensity, its latency about 9.0 ms with GVS of >or=2.5 mA, and was not suppressed by visual fixation. At 10-mA stimulation, the g-VOR [x, y, z] on the cathodal side was [0.77 +/- 0.10, -0.05 +/- 0.05, -0.18 +/- 0.06 degrees ] (mean +/- 95% confidence intervals) and on the anodal side was [0.79 +/- 0.10, 0.16 +/- 0.05, -0.19 +/- 0.06 degrees ], with a vertical divergence of 0.20 degrees . Although the horizontal g-VOR could have arisen from activation of the horizontal semicircular canal afferents, the vertical-torsional g-VOR resembled the vestibuloocular reflex in response to roll-plane head rotation about an Earth-horizontal axis and might be a result of both vertical semicircular canal and otolith afferent activations. Pulsed GVS is a promising technique to investigate latency and initiation of the human vestibuloocular reflex because it does not require a large mechanical apparatus nor does it pose problems of head inertia or slippage.