Cochlear microphonics and otoacoustic emissions in chronically de-efferented chinchilla

The effects of eliminating the olivocochlear bundle (OCB) on cochlear elect romechanical properties were examined by measuring cochlear microphonics (C M) and distortion product otoacoustic emissions (DPOAEs) in chronically de- efferented chinchillas. The OCB fibers to the right ears were successfully sectioned in six out of 15 adult chinchillas via a posterior paraflocular f ossa approach. At the end of the experiment, these ears were histologically verified as being deprived of both lateral and medial OCB fibers. The oppo site (left) ears from the animals served as controls. Following de-efferent ation, changes of the inter-modulation distortion components (2f(1)-f(2), f (2)-f(1), 3f(1)-2f(2), 3f(2)-2f(1)) varied, depending on the frequencies an d levels of the stimuli. DPOAE amplitudes to low-level stimuli were within the 95% confidence intervals around mean DPOAE amplitudes of the control ea rs at all the frequencies (1-8 kHz). At high stimulus levels, DPOAE amplitu des increased by 5-20 dB at 1 and 2 kHz while remaining in the normal range at 4 and 8 kHz. In contrast, the CM input/output functions to stimuli from 1 to 8 kHz were significantly reduced by approximately 40-50% at all input levels. The results suggest that the OCB may play a role in modulating ele ctrical properties of the outer hair cells and in reducing the magnitude of cochlear distortion to high-level stimuli. (C) 2000 Elsevier Science B.V. All rights reserved.