Direction-dependent excitatory and inhibitory ocular vestibular-evoked myogenic potentials (oVEMPs) produced by oppositely directed accelerations along the midsagittal axis of the head

Oppositely directed displacements of the head need oppositely directed vestibulo-ocular reflexes (VOR), i.e. compensatory responses. Ocular vestibular-evoked myogenic potentials (oVEMPs) mainly reflect the synchronous extraocular muscle activity involved in the process of generating the VOR. The oVEMPs recorded beneath the eyes when looking up represent electro-myographic responses mainly of the inferior oblique muscle. We aimed: (1) to study the properties of these responses as they were produced by head acceleration impulses to the forehead and to the back of the head; (2) to investigate the relationships between these responses and the 3-D linear head accelerations that might reflect the true stimulus that acts on the vestibular hair cells. We produced backward- and forward-directed acceleration stimuli in four conditions (positive and negative head acceleration impulses to the hairline and to the inion) in 16 normal subjects. The oVEMPs produced by backward- and forward-directed accelerations of the head showed consistent differences. They were opposite in the phase. The responses produced by backward accelerations of the head began with an initial negativity, n11; conversely, those produced by accelerations directed forward showed initially a positive response, p11. There was a high inter-subject correlation of head accelerations along the head anteroposterior and transverse axes, but almost no correlation of accelerations along the vertical axis of the head. We concluded that backward-directed head accelerations produced an initial excitatory response, and forward-directed accelerations of the head were accompanied by an initial inhibitory response. These responses showed dependence on acceleration direction in the horizontal plane of the head. This could be consistent with activation of the utricle

Short latency responses in the averaged electro-oculogram elicited by vibrational impulse stimuli applied to the skull: could they reflect vestibulo-ocular reflex function?

Objectives: To investigate whether vibrational impulse stimuli applied to the skull can be used to evoke the vestibulo-ocular reflex (VOR) and detect vestibular lesions. Methods: Twenty four patients with unilateral vestibular loss (UVD), five with bilateral vestibular loss, two with ocular palsies, and 10 healthy subjects participated. Vibrations of the skull were induced with head taps and with a single period of 160 Hz tone burst on the inion, vertex, and the mastoids while the patients viewed a distant target. Several patients were also examined while viewing a near target, with eccentric gaze and in tilted postures. Responses were recorded by EOG. Results: Responses occurred between 5 ms and 20 ms and seemed to be compensatory to the second phase of the sine wave of vibration impulse and were greatly diminished/absent in patients with bilateral VD and ocular palsies. The patients with UVD had asymmetrical responses in the vertical EOG with stimuli applied on the inion and vertex, with enhancement of the response amplitude on the side of vestibular loss and/or diminution on the healthy side. The asymmetry ratios between the healthy subjects and patients with UVD, and among patients with UVD were statistically significant. Some gaze and positional influences could be demonstrated consistent with otolithic reflexes. Conclusion: If the asymmetric responses to skull vibration in UVD result from passive oscillatory movements of the orbital tissues they may reflect the otolith mediated sustained skew torsion. Conversely, if generated by active eye movements, their likely origin is a phasic VOR

Relative diagnostic value of ocular vestibular evoked potentials and the subjective visual vertical during tilt and eccentric rotation

OBJECTIVE: We compared vibration-induced ocular vestibular evoked myogenic potentials (OVEMPs) with the visual vertical during whole-body roll tilt and eccentric rotation in healthy subjects and patients with unilateral vestibular loss, to determine which test was most sensitive in discriminating impaired utricle function. METHODS: OVEMPs and the visual vertical were measured in 11 patients and 11 healthy subjects. Visual vertical was measured during roll tilts between -9.6 degrees and 9.6 degrees , and during rotation at 400 degrees /s with the head upright and the vertical rotation axis located between +/-3.5cm from the head center. RESULTS: OVEMPs in patients were strikingly asymmetric, whereas they were approximately symmetric in healthy subjects. Patients showed impaired visual vertical gain during eccentric rotation and increased errors for both roll tilt and eccentric rotation tests. OVEMPs were superior at discriminating between patients and healthy subjects, although eccentric rotation performed nearly as well. CONCLUSIONS: OVEMPs provide a powerful test for discriminating between healthy subjects and patients with chronic unilateral vestibular loss, and testing the visual vertical testing during eccentric rotation was superior to testing during whole-body roll tilt. SIGNIFICANCE: OVEMPs are easier to administer, less demanding on patients, and in general are more effective at identifying chronic unilateral vestibular loss than visual vertical measurements

Acoustic Clicks Activate both the Canal and Otolith Vestibulo-Ocular Reflex Pathways in Behaving Monkeys

Acoustic activation of the vestibular system has been well documented in humans and animal models. In the past decade, sound-evoked myogenic potentials in the sternocleidomastoid muscle (cVEMP) and the extraocular muscles (oVEMP) have been extensively studied, and their potentials as new tests for vestibular function have been widely recognized. However, the extent to which sound activates the otolith and canal pathways remains controversial. In the present study, we examined this issue in a recently developed nonhuman primate model of acoustic activation of the vestibular system, i.e., sound-evoked vestibulo-ocular reflexes (VOR) in behaving monkeys. To determine whether the canal and otolith VOR pathways are activated by sound, we analyzed abducens neurons' responses to clicks that were delivered into either ear. The main finding was that clicks evoked short-latency excitatory responses in abducens neurons on both sides. The latencies of the two responses, however, were different. The mean latency of the contralateral and ipsilateral abducens neurons was 2.44 ± 0.4 and 1.65 ± 0.28 ms, respectively. A further analysis of the excitatory latencies, in combination with the known canal and otolith VOR pathways, suggests that the excitatory responses of the contralateral abducens neurons were mediated by the contralateral disynaptic VOR pathways that connect the lateral canal to the contralateral abducens neurons, and the excitatory responses of the ipsilateral abducens neurons were mediated by the ipsilateral monosynaptic VOR pathways that connect the utricle to the ipsilateral abducens neurons. These results provide new insights into the understanding of the neural basis for sound-evoked vestibular responses, which is essential for developing new tests for both canal and otolith functions in humans