A second cochlear-frequency map that correlates distortion product and neural tuning measurements

Area of Science:

• Auditory Neuroscience
• Bioacoustics
• Otoacoustic Emissions

Background:

• Acoustic distortion products (DPs) arise from nonlinearities in the cochlea's basilar membrane (BM) motion.
• DPs measured in the ear canal are influenced by the frequency ratio of primary tones (f2/f1).
• DPs exhibit a maximum magnitude at a specific frequency ratio (f2/f1 ≈ 1.1–1.4).

Purpose of the Study:

• To investigate the underlying mechanism responsible for the frequency-specific maximum of acoustic distortion products.
• To propose a model explaining the generation and propagation of DPs within the cochlea.
• To correlate DP maxima with cochlear micromechanical properties.

Main Methods:

• Analysis of acoustic intermodulation distortion products (DPs) generated by nonlinear BM motion.
• Examination of the relationship between primary tone frequencies (f1, f2) and DP frequency (fd(n)).
• Hypothesizing the role of micromechanical resonances in DP frequency tuning.

Main Results:

• All odd-order DPs (fd(n) = f1 -n(f2-f1)) show a frequency maximum independent of their order.
• This DP maximum frequency coincides with the frequency where neural tuning curve "tip" meets the "tail".
• The findings suggest the basilar membrane has two resonant impedances, not one.

Conclusions:

• Cochlear micromechanical resonances are responsible for the frequency-specific maximum of DPs.
• A second, lower-tuned resonant impedance acts as a shunt across inner hair cells.
• This shunt resonance enhances DP feedback and transmission, explaining frequency-dependent DP increases.