I Closure of the Cochlear Windows: Its Effect upon Air- and Bone-Conduction

Abstract

On AC stimulation, the cochlear fluid motion is purely translatory along its pathway from oval to round window. This rule needs to be modified only at higher driving intensities as above 60 db the bones of the skull start to respond directly to air-borne sound. Stapedial fixation impairs AC in proportion to the degree of fixation. Round-window closure impairs AC to a relatively small degree. The suggestion that there are auxiliary cochlear pressure outlets has not been proved yet with respect to AC. A non-rigid (elastic) closure of the round window may improve the response due to reflection from the (yielding) obstacle. The resulting standing-wave pattern along the pathway from window to window may increase the amplitude of the traveling wave along the cochlear partition to a small degree (theoretically up to 6 db) within a limited frequency range. Closure of both windows decreases AC conduction most severely, although there is no evidence of a complete elimination of the response. The experimental evidence available can only be explained by assuming a dual mode of BC stimulation: inertial and compressional. The inertial component is reduced either by stapedial fixation or by adding a mass to the incudostapedial portion of the ossicular chain, thus reducing its moment of inertia. The compressional component is reduced when the oval window is allowed to act as a shunt with respect to the pressure build-up upon contraction of the osseous cochlear capsule. Elimination of such a shunt will improve the effectiveness of the compressional component. Firm closure of the round window reduces BC responses because it affects adversely both the translatory motion due to the inertial component. However, the total effect was surprisingly small. Evidence was given by closing the cochlear aqueduct that, at least with respect to BC, the cochlea possesses auxiliary pressure outlets. Their presence tends to minimize the effect of round-window closure. Closure of both windows produced again a relatively small effect as to BC responses which was not larger than that produced by closure of the oval window alone, unless the cochlear aqueduct was closed off simultaneously.