Experimental Study of Bone Conduction in Ears with Mechanical Impairment of the Ossicles

Abstract

The cochlear-microphonic potential of guinea pig was recorded during stimulation by bone-conducted sounds, and the amplitude and the phase of the response were measured while the ossicles were altered in various ways. At low and middle frequencies, fixation of the ossicular chain produces a decrease of amplitude and a phase advance of about 70[degree], while an increase of the mass yields an increase of amplitude and a phase lag greater than 180[degree]. A progressive increase of the mass, produced by introducing paraffin oil in the middle-ear cavity, yields a progressive variation of amplitude and phase and, at a particular moment, a complete disappearance of the response. Symmetrical results were obtained by altering the intrabullar pressure. The results are interpreted as showing the participation of several mechanisms, contributing to the whole response. At low frequencies a translational mechanism involves 2 components: one related to the motion of the ossicular chain and the other related to the motion of the perilymphatic fluid. These 2 components contribute to the whole response according to their amplitude and phase. At higher frequencies a compressional mode of bone conduction, independent of the motion of the ossicles, is responsible for the stimulation. These findings agree with the alteration of bone conduction observed in clinical cases and provide an explanation for the lateralization of the sound source in an ear affected by conductive deafness, as shown in the Weber test. This lateralization may be accounted for by the observed phase shifts.