Abstract
The present study examined the efficacy of 5 MHz low-intensity focused ultrasound (LiFU) as a stimulus to remotely activate inner ear vestibular otolith organs. The otolith organs are the primary sensory apparati responsible for detecting orientation of the head relative to gravity and linear acceleration in three-dimensional space. These organs also respond to loud sounds and vibration of the temporal bone. The oyster toadfish, Opsanus tau, was used to facilitate unobstructed acoustic access to the otolith organs in vivo. Single-unit responses to amplitude-modulated LiFU were recorded in afferent neurons identified as innervating the utricle or the saccule. Neural responses were equivalent to direct mechanical stimulation, and arose from the nonlinear acoustic radiation force acting on the otolithic mass. The magnitude of the acoustic radiation force acting on the otolith was measured ex vivo. Results demonstrate that LiFU stimuli can be tuned to mimic directional forces occurring naturally during physiological movements of the head, loud air conducted sound, or bone conducted vibration.
Highlights
Available methods used to stimulate vestibular otolith organs can be divided grossly into high-frequency and lowfrequency regimes
Raw action potentials are shown along with the instantaneous firing rate. This saccular afferent phaselocked at winding ratios from k 1⁄4 4
Unlike responses to low-frequency sinusoidal acceleration generated with mechanical stimulation, responses to continuous-wave LiFU (cwLiFU) (Fig. 2) were rectified and responded preferentially during one phase of the stimulus [a direct comparison of mechanical and cwLiFU is shown in Figs. 7(A), 7(B), and 7(D)]
Summary
Available methods used to stimulate vestibular otolith organs can be divided grossly into high-frequency and lowfrequency regimes. In the high-frequency regime, pulsed packets ($1–10 ms) of auditory frequency air conducted sound or bone conducted vibration are used to evoke compensatory otolith-driven motor outputs. Vestibular evoked myogenic potentials (VEMPs) are measured clinically in the cervical sternocleidomastoid muscles (cVEMP) and/or extraocular muscles (oVEMP) in response the high-frequency stimulus (Curthoys et al, 2014; Curthoys and Grant, 2015). VEMPs reflect function of the otolith organs as well as the neural circuits responsible for the muscle activation (Naranjo et al, 2016).
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