THE INFLUENCE OF THE COCHLEAR EFFERENT SYSTEM ON CHRONIC ACOUSTIC TRAUMA

The role of the olivocochlear bundle (OCB) in modulating noise-induced permanent injury to the auditory periphery was studied by completely sectioning the OCB fibers in chinchillas and exposing the animals whil e awake to a broad-band noise at 105 dB SPL for 6 h. Outer hair cell ( OHC) function was assessed by measuring 2f(1)-f(2) distortion product otoacoustic emissions (DPOAE) at frequencies from 1.2 to 9.6 kHz and c ochlear microphonics (CM) at frequencies from 1 to 8 kHz. As a result of de-efferentation, the CM was decreased but the DPOAEs were unchange d in de-efferented ears as compared with efferented control and sham-o perated ears. Following noise exposure, the ears that were de-efferent ed showed significantly more depression of DPOAE input/output function s and greater decrement of CM amplitude. The differences between de-ef ferented and efferent-innervated ears were evident across all the freq uencies. The cochlear lesions of the OHCs reflected by traditional cyt ocochleograms, however, were minimal in both efferented and de-efferen ted ears. The results indicate that cochlear de-efferentation decrease s the CM in chinchilla and increases the ear's susceptibility to noise -induced permanent hearing damage. More importantly, de-efferentation increases susceptibility at low frequencies as well as high frequencie s.