Simultaneous Intracochlear Pressure Measurements from Two Cochlear Locations: Propagation of Distortion Products in Gerbil

Abstract

Sound energy propagates in the cochlea through a forward-traveling or slow wave supported by the cochlear partition and fluid inertia. Additionally, cochlear models support traveling wave propagation in the reverse direction as the expected mechanism for conveying otoacoustic emissions out of the cochlea. Recently, however, this hypothesis has been questioned, casting doubt on the process by which otoacoustic emissions travel back out through the cochlea. The proposed alternative reverse travel path for emissions is directly through the fluids of the cochlea as a compression pressure in the form of a fast wave. In the present study, a custom-made micropressure sensor was used in vivo in the gerbil cochlea to map two-tone-evoked pressure responses at distinct longitudinal and vertical locations in both the scala tympani and scala vestibuli. Analyses of the magnitude and phase of intracochlear pressure responses at the primary tone and distortion product frequencies were used to distinguish between fast and slow waves in both the forward-and reverse-propagation directions. Results demonstrated that distortion products may travel in both forward and reverse directions postgeneration and the existence of both traveling and compression waves. The forward-traveling component appeared to duplicate the process of any external tone, tuned to the local characteristic-frequency place, as it increased compressively and nonlinearly with primary-tone levels. A compression wave was evidenced at frequencies above the cutoff of the recording site. In the opposite direction, a reversetraveling wave played the major role in driving the stapes reversely and contributed to the distortion product otoacoustic emission. The compression wave may also play a role in reverse propagation when distortion products are generated at a region close to the stapes.

Forward and Reverse Waves: Modeling Distortion Products in the Intracochlear Fluid Pressure

“…In this short wave region, the pressure becomes more 3D and localized around the BM. Both in model simulations and in experiments (11,33), the fluid pressure in the ST decays exponentially as the measurement location is moved away from the BM when f dp z CF. This is because the symmetric pressure (which is nearly uniform) has a much smaller amplitude than the antisymmetric pressure ( Fig.…”

“…A model of the gerbil cochlea was developed because measurements of the intracochlear DP pressure have been performed in this species (e.g., (11,25,33)). This model is adapted from the physiologically based 3D model of the guinea pig presented in a series of articles (35)(36)(37).…”

“…As in most of the intracochlear DP pressure measurements (11,33), the level of the primaries was chosen to be 80 dB SPL. For both primary ratios, the f 1 and f 2 responses have peaks of similar amplitude (Fig.…”

“…6, the effects of varying f 2 at a fixed position (20 kHz best place) for different f 2 /f 1 ratios are shown. This scheme is commonly used in experiments (e.g., (15,17,24,25,33)) to investigate intracochlear DP response and propagation. The magnitude and phase of the ratio of the pressure in the ST close to the BM to the pressure in the SV close to the stapes is compared with recent measurements from Dong (33).…”

Nonlinear Distortions and Parametric Amplification Generate Otoacoustic Emissions and Increased Hearing Sensitivity

“…However, a comparison with measured DPOAE requires a three-dimensional (3D) mechanical mathematical model of the cochlea because the generator of the DPOAE, which is the nonlinear stiffness of OHC, leads to high frequency mechanical waves in the surrounding fluid. These waves interfere and are measurable with a sensor in the cochlea [20] or in the ear canal [21]. As the fluid is not modeled as a dimensional continuum yet, it is not possible to evaluate the interfering components and produce realistic signals to be comparable with measured OAE.…”