Suppression of Low-Frequency Tones in the Organ of Corti Vibrations of the Basal Turn in the Mongolian Gerbil Cochlea.

Abstract

Purpose: The basilar membrane (BM) motion evoked by single or two-tone stimuli shows nonlinearity largely confined to the region of the traveling wave peak(s) with a passive and linear response basal to the peak. For the same stimuli, nonlinear two-tone interactions in the ear canal pressure and cochlear microphonics appear to originate in a region that extends considerably basal to the peak of the BM traveling wave. Recent measurements from the reticular lamina region (RL) of the organ of Corti in the mouse apex exhibit active gain and broadly tuned two-tone suppression of the response to a lower-frequency probe stimulus that is not measured in the BM at the same location. These results suggest that suppressible active gain is evident in the RL region motion in the region basal to the characteristic frequency (CF) place of the probe tone. The purpose of this study is to explore the spatial extent of active gain and nonlinearity of the cochlear partition by measuring two-tone suppression in the RL region and BM responses to a probe two octaves lower in frequency than the CF of the recording location.

Methods: In the current study in the basal turn of the Mongolian gerbil cochlea (15 animals), we used a sensitive custom-built scanning low-coherence heterodyne interferometer to measure two-tone interactions between a suppressor tone near the local CF (~ 20 kHz) and a probe tone two octaves below CF in the vibrations of the RL region and BM.

Results: In sensitive animals, we demonstrate prominent two-tone suppression of the response in the motion of the RL region to a 5 kHz 40 dB SPL probe tone for suppressor tones near the ~ 20 kHz CF at levels as low as 40 dB SPL. Suppression of the probe response became more pronounced as the suppressor level was increased. Responses to the 40 dB SPL probe could not be measured in BM where responses were in the noise. When the probe level was raised to 60 dB SPL, such that the BM response was well above the system noise, we did not measure any change in BM vibrations for suppressor levels up to 70 dB SPL.

Conclusions: Both active gain and nonlinearity in cochlear mechanics originate in the organ of Corti at a location that extends far basal to the place of the probe tone and not in the BM. Together with the previous reports by others, we conclude that the nonlinear acoustic interaction in the ear canal between a probe tone and tones much higher than the probe frequency is caused by the reduction (suppression) of mechanical gain actively generated by outer hair cells (OHC) in the organ of Corti. There was no indication that the suppressor generated a response at the probe frequency that was not already present in the response to the probe tone presented alone. Our results support the hypothesis that stimulus-frequency otoacoustic emissions (SFOAEs) measured in the ear canal sound pressure originate in a large region extending basal to the place of the probe tone. The BM does not appear to be the site of either generation or propagation of SFOAEs.