OBSERVING AUDITORY MECHANICS WITH PRESSURE MEASUREMENTS

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The proposed project explores the mechanics of the cochlea and middle ear via measurements of sound pressure in the cochlea, the ear canal and the middle ear cavity. The experiments and interpretation are direct, basic probes of auditory mechanics. The majority of the research effort will be spent on the cochlear study; the middle ear work is a related, smaller study. The cochlea is selective to individual frequencies of sound pressure, as sensitive as physical limits allow, and instantaneously adaptive to a wide range of stimulus levels. At the heart of cochlear operation is a fluid/tissue -and- pressure/motion wave which transports sound energy down the cochlea to frequency-dependent locations on the organ of Corti. Many questions about cochlear mechanics remain. These questions concern fundamental unknowns, such as the physical basis for frequency mapping and tuning, as well as more refined issues, such as the basis for nonlinearity. The proposed experiments examine both the tissue and fluid components of the cochlear traveling wave by using pressure maps to simultaneously measure the wave's pressure and motion components. Pressure will be mapped in the fluid close to the basilar membrane while stimulating the cochlea with sound delivered to the ear canal, or with electric current (at levels which activate the cochlea's natural electro-mechanical transduction). The results will be used to quantify and explore elements central to tuning, frequency mapping and nonlinearity: the wave's effective fluid mass and the mechanical impedance of the organ of Corti, mode changes in the motion of the basilar membrane and energy injection into the traveling wave. Understanding the mechanics of the cochlea is a vital and elusive goal. The progress of many researchers, and technical and computing innovations are bringing this goal within reach. Better understanding the cochlea's mechanical operation will impact on deafness prevention and treatment, especially the design of digital hearing aids and cochlear implants. Accumulating evidence indicates that sound is transmitted through the middle ear as a traveling wave. For example, the phase-vs- frequency behavior of the sound pressure inside the cochlea at the stapes (the output of the middle ear), relative to that in the ear canal (the input to the middle ear) is delay-like between 2 and 40 kHz. The gain (cochlear pressure/ear canal pressure) is nearly flat over these frequencies. Thus, both the temporal and the frequency information in sound is transmitted by the middle ear to the cochlea with high fidelity. How does the middle ear do it? To address this question, sound will be delivered to the ear canal and pressure measurements will be made in the ear canal, the cochlea's scala vestibuli, and the middle ear space. These pressures, and their changes following reversible and irreversible manipulations to the ear will be analyzed to understand how the tympanic membrane and ossicles deliver sound to the cochlea. These results will impact on the treatment of the middle ear and the design of middle ear prostheses.