Middle-ear and inner-ear contribution to bone conduction in chinchilla: The development of Carhart's notch

While the cochlea is considered the primary site of the auditory response to bone conduction (BC) stimulation, the paths by which vibratory energy applied to the skull (or other structures) reaches the inner ear are a matter of continued investigation. We present acoustical measurements of sound in the inner ear that separate out the components of BC stimulation that excite the inner ear via ossicular motion (compression of the walls of the ear canal or ossicular inertia) from the components that act directly on the cochlea (cochlear compression or inertia, and extra-cochlear 'third-window' pathways). The results are consistent with our earlier suggestion that the inner-ear mechanisms play a large role in bone-conduction stimulation in the chinchilla at all frequencies. However, the data also suggest the pathways that conduct vibration to the inner ear via ossicular-motion make a significant contribution to the response to BC stimulation in the 1-3 kHz range, such that interruption of these path leads to a 5 dB reduction in total stimulation in that frequency range. The mid-frequency reduction produced by ossicular manipulations is similar to the 'Carhart's notch' phenomenon observed in otology and audiology clinics in cases of human ossicular disorders. We also present data consistent with much of the ossicular-conducted sound in chinchilla depending on occlusion of the ear canal.