Functional Correlates of Noise-Induced Damage to the Vestibular Periphery

Despite the accumulating evidence that noise exposure damages the vestibular periphery (for review, Stewart et al., 2020), it is often overlooked as a potential source of vestibular damage. Reduced vestibular function plays a significant role in elevated fall risk (Agrawal et al., 2012); however, diagnosis of vestibular disorders can be difficult. Symptoms are often nonspecific and can be attributed to disturbances of other senses, such as vision, hearing or proprioception (Agrawal et al. 2009). Simple measures of otolith organ dysfunction are of considerable value in detection and tracking of noise-induced vestibular loss in human subjects, while rodent studies provide direct measures of physiological and cellular damage with controlled noise exposure dosing. To understand the impact of degree of noise exposure on otolith organ dysfunction, rat and human vestibular data were correlated with degree of noise exposure. In rats, vestibular short-latency evoked potentials (VsEPs) were used to evaluate central and peripheral vestibular activity arising from the otolith organs. In humans, ability to judge static orientation using the subjective visual vertical task was used as a measure of otolith organ function (Alberts et al., 2019). 
Rat VsEP loss was related to degree of noise exposure (110-120 dB, 2-6 hrs). While lesser noise exposures produced transient attenuation of VsEP responses, more intense noise exposures produced loss of responses to small head-jerk stimuli (0.32-1.1 g/ms) and significant, persistent attenuation of responses to larger head-jerk stimuli (2.2-5.5 g/ms).
In a preliminary human study, we recruited 4 individuals with a history of noise exposure (3F, age: 35-49) and 4 age-matched controls (1F, age: 36-71). We assessed degree of noise exposure using the Noise Exposure Structured Interview (NESI, Guest et al. 2018). All participants viewed a monitor with the head upright while seated in complete darkness. A bar (length=12º, width=0.22º visual angle) was flashed for 17 ms and observers indicated if it was rotated clockwise or counterclockwise of vertical in a 2AFC design. Participants performed 120 trials (6 orientations, 2 directions, 10 repetitions), after a set of suprathreshold practice trials. In our pilot data we observed decreased sensitivities and greater bias in orientation judgements of individuals with a history of noise exposure across age groups.
Taken together, our data suggest that noise-induced otolith organ dysfunction is dependent on degree of noise exposure, providing a potential link between physiological effects of noise exposure on the vestibular periphery and functional outcomes to be probed further.