Chronic low-level noise exposure alters distortion product otoacoustic emissions

Chen et al. (1995) recently reported an altered response to the application of ATP in outer hair cells (OHC) isolated from guinea pigs continuously exposed for 10 or 11 days to a 65 dB SPL (A-scale) narrow-band noise (1.1-2.0 kHz). The primary goal of the present study was to test the hypothesis that the continuous low-level noise used by Chen et al. (1995) alters cochlear function. Cubic (2f1-f2) and quadratic (f2-f1) DPOAEs, as well as, the amount of contralateral suppression of DPOAE amplitudes were chosen for study. Responses were recorded in urethane-anesthetized guinea pigs with sectioned middle ear muscles. The animals had either been exposed to the low-level noise for 3 or 11 days or not exposed at all (n = 13 animals per group). Results demonstrate that this noise induces frequency-dependent and very localized reductions in 2f1-f2 DPOAE input/output (I/O) functions. However, the f2-f1 DPOAE I/O functions appear to be insensitive to the noise exposure. No noise-related changes were found in the amount of contralateral suppression between the different exposure groups, with the exception of one unexplainable data point (f2-f1 DPOAE = 0.5 kHz; day 3) where it was reduced. The 2f1-f2 DPOAE amplitude alterations lend support to the conclusions of Chen et al. (1995) that chronic low-level noise exposure induces molecular changes in the OHCs which may, in turn, alter cochlear function.