Differential Intracochlear Sound Pressure Measurements in Normal Human Temporal Bones

Abstract

We present the first simultaneous sound pressure measurements in scala vestibuli and scala tympani of the cochlea in human cadaveric temporal bones. The technique we employ, which exploits microscale fiberoptic pressure sensors, enables the study of differential sound pressure at the cochlear base. This differential pressure is the input to the cochlear partition, driving cochlear waves and auditory transduction. In our results, the sound pressure in scala vestibuli (P (SV)) was much greater than scala tympani pressure (P (ST)), except for very low and high frequencies where P (ST) significantly affected the input to the cochlea. The differential pressure (P (SV) - P (ST)) is a superior measure of ossicular transduction of sound compared to P (SV) alone: (P (SV)-P (ST)) was reduced by 30 to 50 dB when the ossicular chain was disarticulated, whereas P (SV) was not reduced as much. The middle ear gain P (SV)/P (EC) and the differential pressure normalized to ear canal pressure (P (SV) - P (ST))/P (EC) were generally bandpass in frequency dependence. At frequencies above 1 kHz, the group delay in the middle ear gain is about 83 micros, over twice that of the gerbil. Concurrent measurements of stapes velocity produced estimates of cochlear input impedance, the differential impedance across the partition, and round window impedance. The differential impedance was generally resistive, while the round window impedance was consistent with compliance in conjunction with distributed inertia and damping. Our technique of measuring differential pressure can be used to study inner ear conductive pathologies (e.g., semicircular dehiscence), as well as non-ossicular cochlear stimulation (e.g., round window stimulation and bone conduction)--situations that cannot be completely quantified by measurements of stapes velocity or scala vestibuli pressure by themselves.