Bone-conducted Vibration Stimuli Research Articles

Effects of bone-conducted vibration stimulation of various frequencies on the vertical vection

Illusory self-motion (“vection”) has been used to present a sense of movement in virtual reality (VR) and other similar applications. It is crucial in vection research to present a stronger sense of movement. Bone-conducted vibration (BCV) is a small and generally acceptable method for enhancing the sense of movement in VR. However, its effects on vection have not been extensively studied. Here, we conducted two experiments to investigate the effect of BCV on the vection, which generates an upward sensation under the hypothesis that BCV stimulation to the mastoid processes causes noise in the vestibular system and enhances visually-induced self-motion perception. The experiments focused on the effects of BCV stimuli of different frequencies on the vection experience. The results suggested that 500 Hz BCV was more effective as noise to the vestibular system than other frequency BCVs and improved self-motion sensation. This study examines the effects of BCV with different frequencies on the vection experience and designs a theory for using BCV in VR.

A mathematical model for mechanical activation and compound action potential generation by the utricle in response to sound and vibration.

Calyx bearing vestibular afferent neurons innervating type I hair cells in the striolar region of the utricle are exquisitely sensitive to auditory-frequency air conducted sound (ACS) and bone conducted vibration (BCV). Here, we present experimental data and a mathematical model of utricular mechanics and vestibular compound action potential generation (vCAP) in response to clinically relevant levels of ACS and BCV. Vibration of the otoconial layer relative to the sensory epithelium was simulated using a Newtonian two-degree-of-freedom spring-mass-damper system, action potential timing was simulated using an empirical model, and vCAPs were simulated by convolving responses of the population of sensitive neurons with an empirical extracellular voltage kernel. The model was validated by comparison to macular vibration and vCAPs recorded in the guinea pig, in vivo. Transient stimuli evoked short-latency vCAPs that scaled in magnitude and timing with hair bundle mechanical shear rate for both ACS and BCV. For pulse BCV stimuli with durations <0.8 ms, the vCAP magnitude increased in proportion to temporal bone acceleration, but for pulse durations >0.9 ms the magnitude increased in proportion to temporal bone jerk. Once validated using ACS and BCV data, the model was applied to predict blast-induced hair bundle shear, with results predicting acute mechanical damage to bundles immediately upon exposure. Results demonstrate the switch from linear acceleration to linear jerk as the adequate stimulus arises entirely from mechanical factors controlling the dynamics of sensory hair bundle deflection. The model describes the switch in terms of the mechanical natural frequencies of vibration, which vary between species based on morphology and mechanical factors.

A Single Fast Test for Semicircular Canal Dehiscence-oVEMP n10 to 4000 Hz-Depends on Stimulus Rise Time.

As previously reported, a single test measuring oVEMP n10 to 4000 Hz stimuli (bone-conducted vibration (BCV) or air-conducted sound (ACS)) provides a definitive diagnosis of semicircular canal dehiscence (SCD) in 22 CT-verified patients, with a sensitivity of 1.0 and specificity of 1.0. This single short screening test has great advantages of speed, minimizing testing time, and the exposure of patients to stimulation. However, a few studies of the 4000 Hz test for SCD have reported sensitivity and specificity values which are slightly less than reported previously. We hypothesized that the rise time of the stimulus is important for detecting the oVEMP n10 to 4000 Hz, similarly to what we had shown for 500 and 750 Hz BCV. We measured oVEMP n10 in 15 patients with CT-verified SCD in response to 4000 Hz ACS or BCV stimuli with rise times of 0, 1, and 2 ms. As a result, increasing the rise time of the stimulus reduced the oVEMP n10 amplitude. This outcome is expected from the physiological evidence of guinea pig primary vestibular afferents, which are activated by sound or vibration. Therefore, for clinical VEMP testing, short rise times are optimal (preferably 0 ms).

Activation of Guinea Pig Irregular Semicircular Canal Afferents by 100 Hz Vibration: Clinical Implications for Vibration-induced Nystagmus and Vestibular-evoked Myogenic Potentials.

Bone-conducted vibration (BCV) at 100 Hz causes endolymph displacement at hair cell stereocilia in semicircular canal (SCC) ducts of the intact bony labyrinth resulting in activation of irregularly discharging afferent neurons. Suprathreshold 100 Hz BCV is employed in the clinic to evoke skull vibration-induced nystagmus, an indicator for peripheral vestibular asymmetry. Recently, this stimulus has also been used in vestibular-evoked myogenic potentials, a selective test for otolithic function. We performed extracellular recordings from utricular and SCC afferents in guinea pigs during application of suprathreshold BCV stimuli (100-500 Hz) to the animal's skull. Vibration was administered in a way that the animal, the vibrator, and the recording electrode moved as one. In summary, 19 of 43 recorded SCC afferents displayed a stimulus- and phase-locked increase in firing during stimulation at 100 Hz BCV with no perstimulatory adaptation and no poststimulatory silencing. All of the 19 activated SCC afferents had an irregular resting discharge. Neuronal activation of SCC afferents was less pronounced at 200 Hz and largely absent at 500 Hz. On the contrary, a stimulus- and phase-locked increase in firing was observed for irregularly discharging utricular neurons at all frequencies tested. At intensities usually applied in the clinic, 500 Hz BCV is a largely selective otolithic stimulus, while 100 Hz BCV can activate both otolith and SCC afferents. Therefore, while 100 Hz BCV is ideally suited for evoking skull vibration-induced nystagmus in peripheral vestibular asymmetry, it is not recommended for vestibular-evoked myogenic potentials, as it lacks otolithic specificity.

In vivo recording of the vestibular microphonic in mammals

BackgroundThe Vestibular Microphonic (VM) has only featured in a handful of publications, mostly involving non-mammalian and ex vivo models. The VM is the extracellular analogue of the vestibular hair cell receptor current, and offers a tool to monitor vestibular hair cell activity in vivo. ObjectiveTo characterise features of the VM measured in vivo in guinea pigs, using a relatively simple experimental setup. MethodsThe VM, evoked by bone-conducted vibration (BCV), was recorded from the basal surface of either the utricular or saccular macula after surgical removal of the cochlea, in 27 guinea pigs. ResultsThe VM remained after vestibular nerve blockade, but was abolished following end-organ destruction or death. The VM reversed polarity as the recording electrode tracked across the utricular or saccular macula surface, or through the utricular macula. The VM could be evoked by BCV stimuli of frequencies between 100 Hz and 5 kHz, and was largest to vibrations between 600 Hz and 800 Hz. Experimental manipulations demonstrated a reduction in the VM amplitude with maculae displacement, or rupture of the utricular membrane. ConclusionsResults mirror those obtained in previous ex vivo studies, and further demonstrate that vestibular hair cells are sensitive to vibrations of several kilohertz. Changes in the VM with maculae displacement or rupture suggest utricular hydrops may alter vestibular hair cell sensitivity due to either mechanical or ionic changes.

Superior canal dehiscence reveals concomitant unilateral utricular loss (UUL)

Conclusion: We report enhanced symmetrical cervical vestibular evoked myogenic potential (cVEMP) but asymmetrical ocular VEMP (oVEMP) responses in a patient with CT-verified bilateral superior semicircular canal dehiscence (SCD) but with acute vestibular syndrome. This implies that absence of unilateral utricular macula function alone is sufficient to cause symptoms of acute vertigo. Acute vertigo should not automatically be presumed to originate from semicircular canal dysfunction. Objectives: To identify the cause of an acute vertigo attack in a patient with bilateral SCD. Methods: The functional state of all peripheral vestibular sense organs was tested using the video head impulse test (vHIT) for all semicircular canals and VEMPs to air-conducted sound (ACS) or bone-conducted vibration (BCV) to test all otolith organs. The cVEMP tested mainly saccular function and the oVEMP mainly utricular function. Results: All semicircular canals showed normal function. The cVEMPs showed enhanced, but symmetrical saccular function. In contrast, oVEMPs showed an enhanced but asymmetric n10 component – it was greatly reduced beneath the left eye, implying decreased function in the right utricular macula. That result was confirmed using very high frequency stimuli which are effective in SCD: 4000 Hz BCV stimuli showed that oVEMP n10 was present beneath the right eye but absent beneath the left eye.

Selective effects of head posture on ocular vestibular-evoked myogenic potential (oVEMP) by bone-conducted vibration

ObjectiveBy altering head postures from sitting, supine to head hanging, this study investigated the effects of gravitational force on ocular vestibular-evoked myogenic potential (oVEMP) via either air-conducted sound (ACS) or bone-conducted vibration (BCV) stimuli. MethodsTwenty healthy volunteers underwent the oVEMP test via ACS or BCV stimuli with the sitting, supine, and head hanging positions on the same day in a randomized order. ResultsAll subjects had clear BCV oVEMPs in the three head postures. No significant differences existed in terms of mean nI and pI latencies, the nI–pI interval, and asymmetry ratio regardless of various positions. However, the mean nI–pI amplitude with the head hanging position (15.9±6.4μV) was significantly larger than that with the sitting position (13.8±6.0μV), but not significantly larger than that with the supine position (14.7±6.1μV). Nevertheless, such a difference in reflex amplitude does not exist in oVEMPs elicited by ACS stimuli. With the sitting position, mean linear acceleration at the mastoids in response to BCV stimuli was −0.06±0.02, 0.20±0.04 and −0.04±0.02g along the x-, y-, and z-axis, respectively, which did not differ significantly from those with the head hanging position. ConclusionBy altering head postures from sitting to head hanging, gravitational force can exert a selective effect on the reflex amplitude of oVEMPs elicited by BCV stimuli, but not by ACS stimuli. SignificanceCompared to ACS mode, BCV mode can provoke higher response rate, generate earlier and larger waveforms, and be influenced by both dynamic shearing force and static gravitational force to enlarge the reflex amplitude of oVEMPs.

Influence of facial paresis on ocular vestibular-evoked myogenic potentials

Conclusion: The ocular vestibular-evoked myogenic potential (oVEMP) test with eyes up condition may help differentiate facial paresis with and without vestibular nerve deficit, while the oVEMP test with the eyes closed condition is not reliable for testing patients with facial paresis. Objective: This study investigated the influence of facial paresis on oVEMPs via the eyes up versus eyes closed conditions. Methods: Twenty patients with unilateral facial paresis underwent a battery of audiovestibular function tests. The oVEMP test was performed via bone-conducted vibration stimuli with the eyes up and eyes closed conditions used in a randomized order. Results: Under the eyes up condition, the rate of abnormal oVEMP tests in the lesion ear was 30%, significantly higher than the 0% in the opposite healthy ear, indicating that the lesion ear not the recorded eye is responsible for abnormal oVEMPs. However, abnormal oVEMP test results of lesion ears under the eyes closed condition did not significantly differ from those of opposite healthy ears. Analysis of caloric and oVEMP test results indicated that both tests obtained normal responses from 10 ears, and abnormal responses from 5 ears. The caloric and oVEMP test results were significantly correlated.

An Indicator of Probable Semicircular Canal Dehiscence: Ocular Vestibular Evoked Myogenic Potentials to High Frequencies

The n10 component of the ocular vestibular evoked myogenic potential (oVEMP) to sound and vibration stimuli is a crossed response that has enhanced amplitude and decreased threshold in patients with CT-verified superior semicircular canal dehiscence (SSCD). However, demonstrating enhanced VEMP amplitude and reduced VEMP thresholds requires multiple trials and can be very time consuming and tiring for patients, so a specific indicator of probable SCD that is fast and not tiring would be preferred. Here we report a 1-trial indicator: that the oVEMP n10 in response to a very high frequency stimulus (4000 Hz), either air-conducted sound (ACS) or bone conducted vibration (BCV), is such a fast indicator of probable SCD. In 22 healthy subjects, oVEMP n10 at 4000 Hz was not detectable; however, in all 22 CT-verified SSCD patients tested, oVEMP n10 responses were clearly present to 4000 Hz to either ACS or BCV stimuli.

Potential application of ocular and cervical vestibular‐evoked myogenic potentials in meniere's disease: A review

By stimulating the ear with air-conducted sound or bone-conducted vibration stimuli, vestibular-evoked myogenic potential (VEMP) can be recorded on the contracted neck muscles, termed cervical VEMP (cVEMP), and on the extraocular muscles, termed ocular VEMP (oVEMP). These two electrophysiological tests expand the test battery for clinicians to explore the dynamic otolithic function, adding a potential usefulness to the sacculocollic reflex and vestibulo-ocular reflex, respectively. The inner ear test battery, including audiometry, and cVEMP, oVEMP and caloric tests, is designed for complete evaluation of the inner ear function, namely, the cochlea, saccule, utricle, and semicircular canals, respectively. Using this test battery to study the localization and prevalence of hydrops formation reveals that the declining function in the cochlea, saccule, utricle, and semicircular canals mimics the declining sequence of hydrops formation in temporal bone studies. This study reviewed the physiological results in Meniere's patients via the inner ear test battery, especially the potential application of the oVEMP and cVEMP tests, to correlate with the histopathological findings of Meniere's disease.

Correlation between acceleration magnitude and ocular vestibular-evoked myogenic potential

This study combined bone-conducted vibration (BCV) stimulation with triaxial accelerometry to correlate the acceleration magnitudes of BCV stimuli with ocular vestibular-evoked myogenic potential (oVEMP) test results. Fourteen healthy volunteers underwent oVEMP test using BCV stimuli with simultaneous monitoring the triaxial acceleration. All (100%) subjects exhibited clear oVEMPs in response to BCV stimuli from a vibrator. The lowest acceleration magnitudes for eliciting oVEMPs along the x-, y- and z-axes were 0.05±0.01g, 0.16±0.08g, and 0.04±0.01g, respectively, exhibiting significantly higher acceleration magnitude along the y-axis than those along the x- and z-axes. In addition, significantly positive correlations were noted between the acceleration magnitude along each axis and the oVEMP amplitude. In conclusion, measuring the acceleration magnitude throughout oVEMP testing revealed a significant correlation between linear acceleration and oVEMP responses. Restated, increasing acceleration magnitude may have more synchronization of firing of vestibular afferents, resulting in more synchronized evoked potentials and greater oVEMP amplitude.

Augmentation of Ocular Vestibular‐Evoked Myogenic Potentials via Bone‐Conducted Vibration Stimuli in Ménière Disease

The asymmetry ratio of ocular vestibular-evoked myogenic potential (oVEMP) >40% is interpreted as augmented or reduced response depending on whether the nI-pI amplitude of the lesion ear is larger or smaller than that of the opposite ear, respectively. This study compared unilateral Ménière disease patients with augmented oVEMPs vs reduced oVEMPs to elucidate the mechanism of augmented oVEMP. Case series with chart review. University hospital. Forty patients with unilateral definite Ménière disease were enrolled in this study, including 20 patients with augmented oVEMPs and another 20 patients with reduced oVEMPs in the hydropic side. All patients underwent audiometry, caloric test, and oVEMP and cervical VEMP (cVEMP) tests via bone-conducted vibration stimuli. Then, the oVEMP and cVEMP test results were compared with the stage of Ménière disease, respectively. The augmented group had earlier nI and pI latencies and larger nI-pI amplitude of oVEMPs compared with the reduced group. Caloric test also revealed a significant difference in abnormal responses between the augmented and reduced groups. However, both groups did not differ significantly in the abnormal percentage of cVEMP test results. A significant trend to decline in the prevalence of augmented oVEMPs was noted from stages I to III-IV but not in that of abnormal cVEMPs. The augmented oVEMPs have earlier latencies and larger amplitudes compared with the reduced oVEMPs, indicating that a relatively larger population of intact utricular afferents is activated during the early stage of Ménière disease.

VESTIBULAR AFFERENTS POPULATION ACTIVATED BY VARIOUS MODES FOR ELICITING OCULAR AND CERVICAL VESTIBULAR-EVOKED MYOGENIC POTENTIALS

This study compared cervical and ocular vestibular-evoked myogenic potentials (cVEMPs and oVEMPs) between air-conducted sound (ACS) and bone-conducted vibration (BCV) modes to determine whether these two stimulation modes activate the same population of primary vestibular afferents. Fifteen healthy subjects underwent cVEMP and oVEMP tests using ACS stimuli at 127 dB pe SPL and BCV stimuli at 128 dB force level. The characteristic parameters of cVEMPs and oVEMPs were compared between ACS and BCV modes. The mean p13 and n23 latencies of ACS-cVEMPs were significantly longer than those of BCV-cVEMPs. Likewise, the mean nI and pI latencies for ACS-oVEMPs were also significantly longer than those for BCV-oVEMPs. There was no significant difference in the mean amplitude of cVEMPs between the ACS and BCV modes. However, comparing the oVEMP amplitude, a relationship: (Amplitude of BCV-oVEMP) = 2.3 x (Amplitude of ACS-oVEMP) was demonstrated. In conclusion, the population of primary vestibular afferents activated by ACS and BCV stimuli is similar for cVEMPs. In contrast with oVEMPs, BCV mode activates more number of primary vestibular afferents than ACS mode does. In interpreting oVEMP and cVEMP results, stimulation mode should be checked first.

Comparison of Bone‐Conducted Vibration for Eliciting Ocular Vestibular‐Evoked Myogenic Potentials

This study compared bone-conducted vibration (BCV) stimuli at forehead (Fz) and mastoid sites for eliciting ocular vestibular-evoked myogenic potentials (oVEMPs). Prospective study. University hospital. Twenty healthy subjects underwent oVEMP testing via BCV stimuli at Fz and mastoid sites. Another 50 patients with unilateral Meniere's disease also underwent oVEMP testing. All healthy subjects showed clear oVEMPs via BCV stimulation regardless of the tapping sites. The right oVEMPs stimulated by tapping at the right mastoid had earlier nI and pI latencies and a larger nI-pI amplitude compared with those stimulated by tapping at the Fz and left mastoid. Similar trends were also observed in left oVEMPs. However, the asymmetry ratio did not differ significantly between the ipsilateral mastoid and Fz sites. Clinically, tapping at the Fz revealed absent oVEMPs in 28% of Meniere's ears, which decreased to 16% when tapping at the ipsilesional (hydropic) mastoid site, exhibiting a significant difference. Tapping at the ipsilateral mastoid site elicits earlier oVEMP latencies and larger oVEMP amplitudes when compared with tapping at the Fz site. Thus, tapping at the Fz site is suggested to screen for the otolithic function, whereas tapping at the ipsilesional mastoid site is suitable for evaluating residual otolithic function.

Differentiating cerebellar and brainstem lesions with ocular vestibular-evoked myogenic potential test

This study applied both ocular vestibular-evoked myogenic potential (oVEMP) and cervical VEMP (cVEMP) tests in patients with cerebellar disorders to determine whether VEMP test can differentiate between cerebellar and brainstem lesions. A total of 12 patients with cerebellar disorder, including extended cerebellar lesion (involving the brainstem) in 8 and localized cerebellar lesion (excluding the brainstem) in 4, were enrolled in this study. All patients underwent caloric, visual suppression, and oVEMP and cVEMP tests via bone-conducted vibration stimuli. The abnormal rates for the caloric, visual suppression, and oVEMP and cVEMP tests were 62, 83, 88 and 75% in patients with extended cerebellar lesion and 0, 25, 0 and 0% in those with localized cerebellar lesion, respectively. The rate of abnormal oVEMP results significantly differed between the two groups, but caloric, visual suppression and cVEMP test results did not differ. In another ten healthy subjects, characteristic parameters of oVEMPs obtained under light and dark conditions did not significantly differ. In conclusion, ocular VEMP test can differentiate between cerebellar and brainstem lesions. Abnormal oVEMPs in patients with cerebellar disorder may indicate adjacent brainstem involvement.

The vestibular evoked-potential profile of Ménière’s disease

Objective To define the ocular and cervical vestibular evoked myogenic potential (oVEMP and cVEMP) profile in Ménière’s Disease (MD), we studied air-conducted (AC) sound and bone-conducted vibration (BCV)-evoked responses in 77 patients and 35 controls. Methods oVEMPs were recorded from unrectified infra-orbital surface electromyography (EMG) during upward gaze. cVEMPs were recorded from rectified and unrectified sternocleidomastoid EMG during head elevation against gravity. Responses to AC clicks delivered via headphones and BC forehead taps delivered with a mini-shaker (bone-conduction vibrator) and a triggered tendon-hammer were recorded. Results In clinically definite unilateral MD ( n = 60), the prevalence of unilateral VEMP abnormalities was 50.0%, 10.2% and 11.9% for click, minitap and tendon-hammer evoked oVEMPs, 40.0%, 22.8% and 10.7% for click, minitap and tendon-hammer evoked cVEMPs. The most commonly observed profile was abnormality to AC stimulation alone (33.3%), followed by abnormalities to both AC and BCV stimuli (26.7%). Isolated abnormalities to BCV stimuli were rare (5%) and limited to the minitap cVEMP. The prevalence of abnormalities for each of the AC VEMPs was significantly higher than for any one BCV VEMP. For click cVEMP, click oVEMP and minitap cVEMP, average Reflex Asymmetry Ratios (AR) were significantly higher in MD compared with controls. Test results for AC cVEMP, AC oVEMP, minitap cVEMP and caloric asymmetry were significantly correlated with hearing loss. Conclusions Predominance of abnormalities in oVEMP and cVEMP responses to AC sound is characteristic of MD and indicative of saccular involvement. Significance This pattern of VEMP abnormalities may enable separation of Ménière’s disease from other peripheral vestibulopathies.

An animal model of ocular vestibular-evoked myogenic potential in guinea pigs

This study aimed to establish an animal model of ocular vestibular-evoked myogenic potential (oVEMP) in guinea pigs. Ten healthy and 10 gentamicin-treated guinea pigs underwent oVEMP test using a hand-held bone-conducted vibrator placed on the animal's forehead. All 10 healthy animals exhibited bilateral oVEMPs at the stimulus intensity of 139 dB force level (FL), with a mean threshold and latencies of peak nI and pI of 130 +/- 4 dBFL, 3.17 +/- 0.37 ms and 4.72 +/- 0.38 ms, respectively. Similar to response rate, the nI-pI amplitude decreased markedly in magnitude as stimulus intensity decreased. Another 10 animals administered with gentamicin (2 mg) on the left ear 1 week after surgery had 100% clear oVEMPs beneath the left eye (ipsilateral to the lesion side), whereas oVEMPs were absent and reduced beneath the right eye (opposite to the lesion side) in 7 and 3 animals, respectively. Morphological study of animals with absent oVEMPs identified substantial damage to the hair cells of the utricular macula. Quantitative analysis revealed that histological density of intact hair cells of the utricular macula from control and lesion ears were 194 +/- 15 and 66 +/- 9 per 130 x 130 microm(2) field, respectively, showing a 68% reduction in the latter. Further, the stereocilia of the residual hair cells were either fused or deformed, and pointed outward randomly. In conclusion, this study establishes the animal model of oVEMP in guinea pigs using bone-conducted vibration stimuli, which sets the stage for investigating the pathophysiology of the utricular disorders.

Ocular vestibular evoked myogenic potentials to bone conducted vibration of the midline forehead at Fz in healthy subjects

Objective To provide the empirical basis for using ocular vestibular evoked myogenic potentials (oVEMPS) in response to Fz bone conducted vibration (BCV) stimulation to indicate vestibular function in human subjects. To show the generality of the response by testing a large number of unselected healthy subjects across a wide age range and the repeatability of the response within subjects. To provide evidence that the response depends on otolithic function. Methods The early negative component (n10) of the oVEMP to brief BCV of the forehead, in the midline at the hairline (Fz) is recorded by surface EMG electrodes just beneath the eyes. We used a Bruel and Kjaer 4810 Mini-Shaker or a light tap with a tendon hammer to provide adequate BCV stimuli to test a large number (67) of unselected healthy people to quantify the individual differences in n10 magnitude, latency and symmetry to Fz BCV. A Radioear B-71 bone oscillator at Fz is not adequate to elicit a reliable n10 response. Results The n10 oVEMP response showed substantial differences in amplitude between subjects, but is repeatable within subjects. n10 is of equal magnitude in both eyes with an average asymmetry around 11%. The average n10 amplitude for Mini Tone Burst BCV is 8.47 μV ± 4.02 (sd), the average latency is 10.35 ms ± 0.63 (sd). The amplitude of n10 decreases and its latency increases with age. Conclusions oVEMPs are a new reliable, repeatable test to indicate vestibular and probably otolithic function. Significance This study shows the optimum conditions for recording oVEMPs and provides baseline values for individual differences and asymmetry. oVEMPs can be measured in senior subjects without difficulty.

Bone conducted vibration selectively activates irregular primary otolithic vestibular neurons in the guinea pig

The main objective of this study was to determine whether bone-conducted vibration (BCV) is equally effective in activating both semicircular canal and otolith afferents in the guinea pig or whether there is preferential activation of one of these classes of vestibular afferents. To answer this question a large number (346) of single primary vestibular neurons were recorded extracellularly in anesthetized guinea pigs and were identified by their location in the vestibular nerve and classed as regular or irregular on the basis of the variability of their spontaneous discharge. If a neuron responded to angular acceleration it was classed as a semicircular canal neuron, if it responded to maintained roll or pitch tilts it was classified as an otolith neuron. Each neuron was then tested by BCV stimuli-either clicks, continuous pure tones (200-1,500 Hz) or short tone bursts (500 Hz lasting 7 ms)-delivered by a B-71 clinical bone-conduction oscillator cemented to the guinea pig's skull. All stimulus intensities were referred to that animal's own auditory brainstem response (ABR) threshold to BCV clicks, and the maximum intensity used was within the animal's physiological range and was usually around 70 dB above BCV threshold. In addition two sensitive single axis linear accelerometers cemented to the skull gave absolute values of the stimulus acceleration in the rostro-caudal direction. The criterion for a neuron being classed as activated was an audible, stimulus-locked increase in firing rate (a 10% change was easily detectable) in response to the BCV stimulus. At the stimulus levels used in this study, semicircular canal neurons, both regular and irregular, were insensitive to BCV stimuli and very few responded: only nine of 189 semicircular canal neurons tested (4.7%) showed a detectable increase in firing in response to BCV stimuli up to the maximum 2 V peak-to-peak level we delivered to the B-71 oscillator (which produced a peak-to-peak skull acceleration of around 6-8 g and was usually around 60-70 dB above the animal's own ABR threshold for BCV clicks). Regular otolithic afferents likewise had a poor response; only 14 of 99 tested (14.1%) showed any increase in firing rate up to the maximum BCV stimulus level. However, most irregular otolithic afferents (82.8%) showed a clear increase in firing rate in response to BCV stimuli: of the 58 irregular otolith neurons tested, 48 were activated, with some being activated at very low intensities (only about 10 dB above the animal's ABR threshold to BCV clicks). Most of the activated otolith afferents were in the superior division of the vestibular nerve and were probably utricular afferents. That was confirmed by evidence using juxtacellular injection of neurobiotin near BCV activated neurons to trace their site of origin to the utricular macula. We conclude there is a very clear preference for irregular otolith afferents to be activated selectively by BCV stimuli at low stimulus levels and that BCV stimuli activate some utricular irregular afferent neurons. The BCV generates compressional and shear waves, which travel through the skull and constitute head accelerations, which are sufficient to stimulate the most sensitive otolithic receptor cells.