Tuning of harmonic components in cellular mechanical responses

The origin of mechanical harmonic distortion within the organ of Corti in living gerbil cochleae

Although auditory harmonic distortion has been demonstrated psychophysically in humans and electrophysiologically in experimental animals, the cellular origin of the mechanical harmonic distortion remains unclear. To demonstrate the outer hair cell-generated harmonics within the organ of Corti, we measured sub-nanometer vibrations of the reticular lamina from the apical ends of the outer hair cells in living gerbil cochleae using a custom-built heterodyne low-coherence interferometer. The harmonics in the reticular lamina vibration are significantly larger and have broader spectra and shorter latencies than those in the basilar membrane vibration. The latency of the second harmonic is significantly greater than that of the fundamental at low stimulus frequencies. These data indicate that the mechanical harmonics are generated by the outer hair cells over a broad cochlear region and propagate from the generation sites to their own best-frequency locations.

Recording depth of the heterodyne laser interferometer for cochlear vibration measurement

Measurement of the cochlear partition vibration as a function of the optical-axis (z-axis) position in the gerbil cochlea showed that the velocity distributes over a range of more than 300 microm, which is larger than the thickness of the cochlear partition. This finding suggests that the recording depth (RD) of the heterodyne interferometer probably is not as small as reported in the literature. In the current experiment, the RD of the heterodyne laser interferometer was studied by measuring the velocity of a vibrating mirror as a function of the z-axis position. Results demonstrate that the optical sectioning characteristic, measured by the intensity of the reflected laser beam as a function of the z-axis position, is not able to correctly estimate the RD of the heterodyne interferometer: the RD is much larger than optical sectioning, indicating a poor spatial resolution along the z axis.

Nonlinearity in the apical turn of living guinea pig cochlea

Mechanical vibrations were measured at the apical turn in living guinea pig cochlea, in response to sinusoidal acoustic stimuli, using heterodyne interferometry. The cochlea was sealed and the vibrations were measured at different cellular locations along a radial track at the level of reticular lamina and one point on the osseous spiral lamina. Averaged time waveforms were recorded at each test frequency. The nonlinearity in the apical turn is demonstrated by the distortion in the time waveforms and the richness of the harmonic components in their Fourier transforms. Tuning curves and input/output curves for the fundamental and harmonics components are shown. The fundamental component is essentially linear below about 90 dB SPL while the harmonics display strong nonlinearity and saturation. Negative feedback in the apical turn of the cochlea linearizes the response at the fundamental frequency.

Harmonic distortion on the basilar membrane in the basal turn of the guinea-pig cochlea

1. Mechanical responses to pure-tone stimuli were recorded from the basilar membrane in the basal turn of the guinea-pig cochlea using a displacement-sensitive laser interferometer. The harmonic content of the responses was evaluated using Fourier analysis. 2. Harmonic distortion products were observed in many of the basilar membrane responses. Response components locked to twice the frequency of the stimulus (i.e. 2F0) were the largest of the distortion products. 3. The second harmonic responses showed a bimodal frequency distribution at low to moderate sound pressure levels: one peak occurred around the preparation's best or most sensitive frequency (i.e. when F0 approximately 17 kHz), and another occurred around one-half of the best frequency (when F0 approximately 8.5 kHz). 4. The absolute levels of most distortion products increased progressively with increasing stimulus strength. When expressed with respect to the levels of the fundamental responses, however, the distortion levels usually decreased with increasing stimulus strength. 5. The levels of the distortion decreased (in both absolute and relative terms) with deterioration in the physiological condition of the cochlea. 6. Maximum second harmonic distortion levels amounted to approximately 3.5 and approximately 28 % of the fundamental responses to tones near and below the best frequency, respectively. 7. The above findings are shown to be consistent with a highly simplified model of cochlear mechanics which incorporates an asymmetric, saturating non-linearity in a positive feedback loop.