Spectral Ripples in Round-Window Cochlear Microphonics: Evidence for Multiple Generation Mechanisms

Area of Science:

• Auditory Neuroscience
• Bioacoustics
• Otoacoustic Emissions

Background:

• The cochlear microphonic (CM) is generated by outer hair cell transduction currents.
• Classical theory posits CM is dominated by the basilar membrane's tail region, predicting minimal frequency-dependent variation.
• Observed spectral ripples in CM challenge this classical model.

Purpose of the Study:

• To investigate the origin of spectral ripples in CM.
• To understand the contribution of active cochlear processing to CM generation.
• To explore novel applications of CM measurements for cochlear health assessment.

Main Methods:

• Measurements of CM amplitude and phase-gradient delay in chinchillas using low-level tones.
• Analysis of spectral ripple reduction with a saturating tone.
• Time-domain analysis and windowing of CM components.
• Extension of the classical CM generation model.

Main Results:

• Striking, quasiperiodic spectral ripples were observed in CM, even at high frequencies.
• A saturating tone reduced spectral ripples, primarily affecting delayed CM energy.
• Time-domain analysis identified early, middle, and late CM components with distinct delays.
• Middle components linked to coherent summation and late components to internal reflection.

Conclusions:

• CM generation involves interference between components with different phase gradients, including active processing.
• Middle and late CM components are analogous to otoacoustic emission generation mechanisms.
• Low-level CM measurements, incorporating active processing contributions, can serve as sensitive assays of cochlear health.