Abstract:   In 2024, the Veterans Benefits Administration reported that tinnitus and hearing loss are some of the most prevalent service-connected disabilities, accounting for 12% of all veteran compensations. Service members are often exposed to varying types of damaging sounds, which may be present continuously (e.g., aircraft carriers) or just briefly (e.g., improvised explosive devices, IEDs). It is imperative to understand how exposure to different sounds affects auditory processing across the auditory pathway, especially at moderate sound levels considered to be non-damaging. Awake chinchillas (n = 18, 9 female) were exposed to noise mimicking aircraft-carrier conditions experienced by U.S. Navy service members. Animals were exposed for four consecutive weeks (40 hours/week) at 87.5 dBA using chinchilla middle-ear absorbance to determine sound-level weighting. Auditory assessment was performed pre- and post-exposure (1, 2, and 4 weeks from the start of noise exposure) using the following measures: tympanometry, wide-band middle-ear muscle reflex (WB-MEMR), otoacoustic emissions (OAEs; swept distortion product, DP; swept stimulus frequency, SF; transient evoked, TE), auditory brainstem response (ABR), and envelope frequency response (EFR) to modulated sounds. Sedated measures (ABR and EFR) were performed under anesthesia while awake measures (tympanometry, WB-MEMR, and OAEs) were collected with the animals restrained and fully conscious. DPOAEs: Reduced amplitudes at mid-to-high frequencies (5-20 dB shift at 2-12 kHz) as soon as 1-week post-exposure with no sign of recovery or worsening up to 4 weeks. SFOAEs: Progressive amplitude reduction at mid-to-high frequencies (5-15 dB shift at 3-8 kHz) with no recovery. TEOAEs: Mixed effects were observed across all frequencies. WB-MEMR: Absorbed power was progressively reduced up to 2 weeks post-exposure, but partially recovered by week 4. ABR: Hearing thresholds were elevated across all frequencies resulting in acute mild hearing loss with no signs of recovery. EFR: Reduced neural coding of middle harmonics was evident at 1-week post-exposure with no signs of recovery. Our multi-metric auditory framework has highlighted potential hearing deficits and how these deficits develop over time due to continuous noise exposure up to 4 weeks post-exposure. Both peripheral and central deficits appeared within one week of noise exposure and persisted up to 4 weeks. These findings will help elucidate the relative contributions of peripheral and central damage to the overall development of hearing loss due to continuous noise exposure. The second part of the project will evaluate auditory deficits following blast-induced injury using the same auditory framework.

This year’s SHRP schedule is available here: https://purdue.edu/TPAN/hearing/shrp_schedule