Evidence of inner-ear mechanisms in bone conduction in chinchillas

Abstract

While much is known about the process of how airborne sound is conducted to the inner-ear via the outer ear and middle ear, so-called air conduction (AC), the mechanisms by which vibrations of the head and body, so-called bone conduction (BC), produce an auditory response are not well understood. It is clear that the inner ear is the sensory site of auditory stimulation by bone conduction, and that the resultant activation of the inner ear has many features in common with air-conduction stimulation; however, bone conduction is known to stimulate the inner ear through multiple pathways. The relative significance and frequency dependence of these different pathways have not been well defined. Our previous work on bone conduction in chinchillas suggested inner-ear mechanisms are the dominant sources in BC. This thesis builds upon the early work by investigating inner ear mechanisms with stapes fixation and ear canal occlusion. Results of stapes fixation show a decrease in scala vestibuli sound pressure Psv and little change in scala tympani sound pressure PST in bone conduction. Ear canal occlusion produces an increase in ear canal sound pressure PEC with a similar amount of increase in Psv, but almost no change in Pst. We attributed the differences in the change between Psv and PST in bone conduction after these manipulations to the existence of compressible cochlear structures or third window pathways, e.g. the cochlear aqueduct. While ear canal compression and middle ear inertia sources may contribute to the total bone conduction response (a 10 dB decrease in Psv after middle ear interruption and stapes fixation, and a 10 dB increase after ear canal occlusion), inner ear mechanisms are still the most significant sources in bone conduction because the changes in Psv and Pst in BC are much smaller than the changes in AC.