Eye Movements Evoked by Pulsed Infrared Radiation of the Rat Vestibular System

Abstract

Light at infrared wavelengths has been demonstrated to modulate the pattern of neural signals transmitted from the angular motion sensing semicircular canals of the vestibular system to the brain. In the present study, we have characterized physiological eye movements evoked by focused, pulsed infrared radiation (IR) stimuli directed at an individual semicircular canal in a mammalian model. Pulsed IR (1863 nm) trains were directed at the posterior semicircular canal in a rat using 200-400 µm optical fibers. Evoked bilateral eye movements were measured using a custom-modified video-oculography system. The activation of vestibulo-ocular motor pathways by frequency modulated pulsed IR directed at single posterior semicircular canals evoked significant, characteristic bilateral eye movements. In this case, the resulting eye movements were disconjugate with ipsilateral eye moving upwards with a rotation towards the stimulated ear and the contralateral eye moving downwards. The eye movements were stable through several hours of repeated stimulation and could be maintained with 30 + minutes of continuous, frequency-modulated IR stimulation. Following the measurements, the distance of the fiber from target structures and orientation of the beam relative to vestibular structures were determined using micro-computed tomography. Results highlight the spatial selectivity of optical stimulation. Our results demonstrate a novel strategy for direct optical stimulation of the vestibular pathway in rodents and lays the groundwork for future applications of optical neural stimulation in inner ear research and therapeutic applications.

Pulsed Infrared Stimulation of Vertical Semicircular Canals Evokes Cardiovascular Changes in the Rat

“…Activation of the targeted end organ-PC or AC-during either infrared or electrical stimulation was confirmed using eye movements (37,41) that were observed and recorded in some animals using a video-oculography system (ETL-200; ISCAN Inc., Figure 1B and lower traces in Figure 1C). The system located and tracked the center of the pupil and corneal reflections of both eyes.…”

“…In the vestibular system, pulsed IR has been shown to rapidly alter the rate of transmitter release from inner ear hair cells and modulate the discharge rate of primary afferent fibers (34)(35)(36). In addition, our group recently reported that pulsed IR can be used to selectively activate the PC in anesthetized rats (37). In the present study, we measured eye movements, BP, HR, and heart rate variability (HRV) during unilateral pulsed IR stimulation of the PC or AC.…”

“…Note that the bony labyrinth covering the crista ampullaris was not opened since IR directed at the canal crista through the thin bone was sufficient to evoke physiological responses. Confirmation that the end organs were accurately targeted by the IR stimulation was based on evoked eye movements and by post-mortem microtomography (37). For the microtomography, following the terminal experiment the inner ears were fixed in 4% paraformaldehyde in 0.1 M phosphate buffer with the optical fiber fixed in place using dental acrylic.…”

“…The system located and tracked the center of the pupil and corneal reflections of both eyes. The linear positions of the pupil and corneal reflection were converted to angular rotation in the horizontal and vertical directions using previously detailed methods (37). Sinusoidally-modulated IR stimulation of the PC evoked frequency-matched torsion in the ipsilateral eye and a vertical downward movement in the contralateral eye whereas that of the AC evoked primarily upward movements ipsilaterally and upward extorsion contralaterally.…”

Histamine H1 Receptor Contributes to Vestibular Compensation

“…Spontaneous nystagmus scoring. Spontaneous nystagmus was recorded using a video eye-tracking system for rodents (ETL-200, ISCAN;Dyhrfjeld-Johnsen et al, 2013;Jiang and Rajguru, 2018). Spontaneous nystagmus with fast phase was scored with 6 -10 points, with 1 point for every 60 beats per minute (bpm; Bergquist et al, 2008;Günther et al, 2015).…”