Role of the Tectorial Membrane Revealed by Otoacoustic Emissions Recorded from Wild-Type and Transgenic Tecta^ΔENT/ΔENT Mice

Distortion product otoacoustic emissions (DPOAE) were recorded from wild-type mice and mutant Tecta^ΔENT/ΔENT mice with detached tectorial membranes (TM) under combined ketamine/xylaxine anesthesia. In Tecta^ΔENT/ΔENT mice, DPOAEs could be detected above the noise floor only when the levels of the primary tones exceeded 65 dB SPL. DPOAE amplitude decreased with increasing frequency of the primaries in Tecta^ΔENT/ΔENT mice. This was attributed to hair cell excitation via viscous coupling to the surrounding fluid and not by interaction with the TM as in the wild-type mice. Local minima and corresponding phase transitions in the DPOAE growth functions occurred at higher DPOAE levels in wild-type than in Tecta^ΔENT/ΔENT mice. In less-sensitive Tecta^ΔENT/ΔENT mice, the position of the local minima varied nonsystematically with frequency or no minima were observed. A bell-like dependence of the DPOAE amplitude on the ratio of the primaries was recorded in both wild-type and Tecta^ΔENT/ΔENT mice. However, the pattern of this dependence was different in the wild-type and Tecta ^ΔENT/ΔENT mice, an indication that the bell-like shape of the DPOAE was produced by a combination of different mechanisms. A nonlinear low-frequency resonance, revealed by nonmonotonicity of the phase behavior, was seen in the wild-type but not in Tecta^ΔENT/ΔENT mice.