Cochlear Boundary Motion During Bone Conduction Stimulation: Implications for Inertial and Compressional Excitation of the Cochlea

• It is well accepted that the perception of bone conducted (BC) sound in the human relies on multiple pathways. Of these pathways, the inertial forces in the cochlear fluid and compression and expansion of the cochlear space have been suggested to be the most important. However, the frequency ranges where these two pathways dominate have not been clarified. This was investigated here using a box-model of the inner ear to estimate wall motion for a one-dimensional BC longitudinal skull vibration. Based on the dimensions of the inner ear and a BC wave speed of 400 m/s, the magnitude of the inertial motion of the cochlea was almost identical to the magnitude of the BC excitation except at the highest frequencies investigated (10 kHz). The compression (differential motion) was almost 100 times smaller than the inertial motion at 100 Hz but increased with frequency and at 5.9 kHz and above, the compression motion was greater than the inertial motion of the cochlea. This data was further analyzed in an impedance model of the cochlea and vestibule where the cochlear fluid, basilar membrane, oval window, round window, and cochlear and vestibular aqueducts were represented by acoustical impedances. That analysis showed that for a normal cochlea, the inertial excitation dominates the basilar membrane excitation for the whole frequency range investigated (0.1 to 10 kHz). However, when the oval window was immobilized simulating otosclerosis, the inertial effect diminished and the compressional excitation mode improved resulting in a dominant excitation from the compression of the cochlea at frequencies of 1.2 kHz and above. Also, the simulated BC hearing losses with otosclerosis according to this model were almost identical to the proposed Carharts notch seen clinically.

Subject headings

MEDICIN OCH HÄLSOVETENSKAP  -- Klinisk medicin (hsv//swe)

MEDICAL AND HEALTH SCIENCES  -- Clinical Medicine (hsv//eng)

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kon (subject category)