The effects of crossed olivocochlear bundle section on transient evoked otoacoustic emissions

The purpose of this study was to investigate the effect of sectioning the crossed olivocochlear bundle (COCB) on transient evoked otoacoustic emissions (TEOAEs) in anesthetized adult chinchillas. Of particular interest is the role of cochlear efferents to the outer haircells (OHCs) and how they control mechanisms responsible for otoacoustic emissions. Specifically the experiment addressed whether a tonic level of inhibitory control is reduced by COCB section. The nonlinear component of TEOAEs was measured before and after COCB section. Analysis was made of the 1, 2, 3, 4, and 5 kHz frequency components and of the total emission, as quantified by fast Fourier transform (FFT) of the raw (time domain) response. After COCB section, the amplitude of the total response and of the 2, 3, 4, and 5 kHz components increased whereas the amplitude of the 1 kHz component decreased. The results indicate that COCB section reduces inhibitory control of the OHC mechanisms responsible for nonlinear TEOAE generation. It is not clear whether the nerve section eliminates a spontaneous level of activity in COCB efferents, or whether it results in the interruption of a stimulus-evoked feedback loop.