Abstract
Tympanic membrane (TM) vibration under bone-conducted ultrasonic (BCU) stimulation was measured in four living human subjects using a laser Doppler vibrometer (LDV) to investigate the contributions of nonlinear distortions in the osseotympanic effects and/or the inertial effects of the middle-ear ossicles to ultrasonic perception in bone conduction. A signal processing algorithm to increase the signal-to-noise ratios of measured LDV signals by removing only optical spike noise components from the wave signals was presented in this study. Evidence of nonlinear distortions, especially the generation of audible subharmonics in the outer and middle ear, was then examined. We did not find any audible signals corresponding to the subjective pitch of a BCU tone in the TM vibrations. This suggests that nonlinear distortions in the osseotympanic and inertial effects do not contribute to BCU perception. Specific properties of perception may be related to mechanisms in the cochlea or afferent neural pathway. With this consideration, we discuss the possibility that the pitch perception of BCU does not relate to tonotopical motion of the basilar membrane corresponding to the subjective pitch, given that TM vibration can reflect the motion of cochlear fluid and hence the motion of the basilar membrane.
