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Some individuals with noise-induced hearing loss (NIHL) also report balance problems. These accompanying vestibular complaints are not well understood. The present study used a rat model to examine the effects of noise exposure on the vestibular system. Rats were exposed to continuous broadband white noise (0–24 kHz) at an intensity of 116 dB sound pressure level (SPL) via insert ear phones in one ear for three hours under isoflurane anesthesia. Seven days after the exposure, a significant increase in ABR threshold (43.3 ± 1.9 dB) was observed in the noise-exposed ears, indicating hearing loss. Effects of noise exposure on vestibular function were assessed by three approaches. First, fluorescein-conjugated phalloidin staining was used to assess vestibular stereocilia following noise exposure. This analysis revealed substantial sensory stereocilia bundle loss in the saccular and utricular maculae as well as in the anterior and horizontal semicircular canal cristae, but not in the posterior semicircular canal cristae. Second, single unit recording of vestibular afferent activity was performed under pentobarbital anesthesia. A total of 548 afferents were recorded from 10 noise-treated rats and 12 control rats. Noise exposure produced a moderate reduction in baseline firing rates of regular otolith afferents and anterior semicircular canal afferents. Also a moderate change was noted in the gain and phase of the horizontal and anterior semicircular canal afferent's response to sinusoidal head rotation (1 and 2 Hz, 45°/s peak velocity). Third, noise exposure did not result in significant changes in gain or phase of the horizontal rotational and translational vestibulo-ocular reflex (VOR). These results suggest that noise exposure not only causes hearing loss, but also causes substantial damage in the peripheral vestibular system in the absence of immediate clinically measurable vestibular signs. These peripheral deficits, however, may lead to vestibular disorders over time. [ABSTRACT FROM AUTHOR]

Loss of vestibular function may be treatable with an implantable vestibular prosthesis that stimulates semicircular canal afferents with biphasic pulse trains. Several studies have demonstrated short-term activation of the vestibulo-ocular reflex (VOR) with electrical stimulation. Fewer long-term studies have been restricted to small numbers of animals and stimulation designed to produce adaptive changes in the electrically elicited response. This study is the first large consecutive series of implanted rhesus macaque to be studied longitudinally using brief stimuli designed to limit adaptive changes in response, so that the efficacy of electrical activation can be studied over time, across surgeries, canals and animals. The implantation of a vestibular prosthesis in animals with intact vestibular end organs produces variable responses to electrical stimulation across canals and animals, which change in threshold for electrical activation of eye movements and in elicited slow phase velocities over time. These thresholds are consistently lower, and the slow phase velocities higher, than those obtained in human subjects. The changes do not appear to be correlated with changes in electrode impedance. The variability in response suggests that empirically derived transfer functions may be required to optimize the response of individual canals to a vestibular prosthesis, and that this function may need to be remapped over time. This article is part of a Special Issue entitled . [ABSTRACT FROM AUTHOR]

• The variation tendency of horizontal or vertical nystagmus SPV is similar to the corresponding cupula response with the head's left-leaning angle changing. • Different head positions change the endolymphatic pressure gradient distribution. • The numerical model of bilateral SCCs could be used to explain the combined effects of each SCC's excitation or inhibition on human VOR under different head positions. The semicircular canals (SCCs) in the vestibular system can sense angular motion of the head, which performs a crucial role in maintaining the human's sense of balance. The different spatial orientations of the head affect the response of human SCCs to rotational movement. In this study, we combined the numerical model of bilateral human SCCs with vestibulo-ocular reflex experiments, and quantitatively investigated the responses of SCCs to constant angular acceleration when the head was in different left-leaning positions, including the head tilted 0°, 15°, 30°, 45°, 60°, 70°, 80°, and 90° to the left. The results showed that the vertical nystagmus slow-phase velocity (SPV) and the corresponding maximal cupula shear strain at the crista surface rose with an increase in the left-leaning angle of the head, reached a maximum at the position of the head tilted approximately 70° to the left, and then decreased gradually. Both the horizontal nystagmus SPV and the corresponding maximal cupula shear strain at the crista surface were the largest under the position of the head tilted 0° to the left, and decreased gradually as the left-leaning angle of the head increased. The numerical results of cupula shear strain at the crista surface in bilateral SCCs can quantitatively explain the combined effects of each SCC's excitation or inhibition on volunteers' nystagmus SPV under different head positions. In addition, a fluid-structure interaction investigation revealed that different left-leaning head positions changed the endolymphatic pressure gradient distribution in SCCs, which determined the transcupular pressure, cupula shear strain at the crista surface, and nystagmus SPV. [ABSTRACT FROM AUTHOR]

• Semicircular canal stimulation depends on the rotation centre. • A rotation centre further from the head stimulates the canal more strongly. • The rotation centre may be localised using the interaural stimulation difference. • Eccentric rotations lead to a stronger aVOR, compensating the larger viewing angle. • The rotation centre dependency is caused by the centrifugal and Euler forces. The vestibular system in the inner ear senses head movements, which is indispensable for balance during locomotion. The semicircular canals of the vestibular system are stimulated during head rotations, due to the interplay between its endolymph fluid and its cupula membrane. Until now, it has been assumed that the stimulation of the canals is independent of the rotation centre. However, Fluid-Structure Interaction investigation now shows that the horizontal canal is more strongly stimulated when the rotation centre lies further from the canal. Such eccentric rotations result in a different stimulation of the left and the right canal, because the distance to the rotation centre differs between both ears. The dissimilar stimulation enables localization of the rotation centre based on information from the canals alone, similar to sound localization based on the interaural time difference for hearing. Additionally, a rotation centre further away from the canal will probably elicit a larger reflexive eye movement. This is exactly what is required to compensate the larger change in viewing angle, and it corresponds to experimental findings. Two fictitious forces (d'Alembert forces), the centrifugal force and the Euler force, cause the rotation centre-dependency of the canal stimulation. Implications for (pre)clinical eccentric rotation experiments are discussed. Image, graphical abstract [ABSTRACT FROM AUTHOR]