It has previously not been possible to measure eardrum vibration of human subjects in the region of auditory threshold. It is proposed that such measurements should provide information about the status of the mechanical amplifier in the cochlea. It is this amplifier that is responsible for our extraordinary hearing sensitivity. Here, we present results from a laser Doppler vibrometer that we designed to noninvasively probe cochlear mechanics near auditory threshold. This device enables picometer-sized vibration measurements of the human eardrum in vivo. With this sensitivity, we found the eardrum frequency response to be linear down to at least a 20-dB sound pressure level (SPL). Nonlinear cochlear amplification was evaluated with the cubic distortion product of the otoacoustic emissions (DPOAEs) in response to sound stimulation with two tones. DPOAEs originate from mechanical nonlinearity in the cochlea. For stimulus frequencies, f 1 and f2, with f2/f1 ‫؍‬ 1.2 and f 2 ‫؍‬ 4 -9.5 kHz, and intensities L1 and L2, with L1 ‫؍‬ 0.4L2 ؉ 39 dB and L 2 ‫؍‬ 20 -65 dB SPL, the DPOAE displacement amplitudes were no more than 8 pm across subjects (n ‫؍‬ 20), with hearing loss up to 16 dB. DPOAE vibration was nonlinearly dependent on vibration at f 2. The dependence allowed the hearing threshold to be estimated objectively with high accuracy; the standard deviation of the threshold estimate was only 8.6 dB SPL. This device promises to be a powerful tool for differentially characterizing the mechanical condition of the cochlea and middle ear with high accuracy.cochlea ͉ hearing loss ͉ laser interferometer ͉ middle ear

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Introduction: Since 1996, the preferred approach for positioning the active middle-ear implant Vibrant Soundbridge© is a mastoidectomy and a posterior tympanotomy. With this device, placement of the floating mass transducer (FMT) on the long incus process is the standard method for treatment of mild-to-severe sensorineural hearing loss in the case of normal middle-ear anatomy. The aim of this study was to determine the vibrational effectiveness of FMT placement at the short incus process. Materials and Methods: An extended antrotomy and a posterior tympanotomy were performed in 5 fresh human temporal bones. As a control for normal middle-ear function, the tympanic membrane was stimulated acoustically and the vibration of the stapes footplate and the round-window (RW) membrane were (sequentially) measured by laser Doppler vibrometry. Vibration responses for coupling of an FMT to the long incus process (standard coupling) were compared to those for coupling to the short incus process. Results: Apart from narrow frequency bands near 3 and 9 kHz for the stapes footplate and RW membrane, respectively, the velocity responses presented no significant differences between standard coupling of the FMT and coupling to the short incus process. Conclusion: Coupling the FMT to the short incus process may be a viable alternative in cases where the surgical approach is limited to an extended antrotomy. A reliable technique for attachment to the short incus process has yet to be developed.

Distortion-product otoacoustic emissions (DPOAEs) emerge when presenting two primary tones with different frequencies f1 and f2 to the cochlea and are commonly used in diagnosis and research to evaluate the functional state of the cochlea. Optimal primary-tone stimulus levels accounting for the different level dependencies of the traveling-wave amplitudes of the two primary tones near the f2-tonotopic place on the basilar membrane are often used to maximize DPOAE amplitudes. However, parameters defining the optimal levels can be affected by wave interference between the nonlinear-distortion and coherent-reflection components of the DPOAE. Here, the components were separated in the time domain using a pulsed stimulus paradigm and optimal levels determined. Based on the amplitude dependence of the nonlinear-distortion components on primary-tone stimulus levels, level parameters yielding maximum DPOAE amplitudes were derived for six normal-hearing adults and compared to data recorded with continuous two-tone stimulation. The level parameters resulting from analysis of the nonlinear-distortion components show dependence on stimulus frequency and small standard deviations. DPOAE input/output functions derived for optimal levels exhibit larger slopes, wider dynamic range and less variability across subjects than those derived for conventional stimulus and analysis conditions, potentially increasing their reliability and sensitivity for assessing cochlea function.

Distortion-product otoacoustic emissions (DPOAEs) arise in the cochlea in response to two tones with frequencies f1 and f2 and mainly consist of two components, a nonlinear-distortion and a coherent-reflection component. Wave interference between these components limits the accuracy of DPOAEs when evaluating the function of the cochlea with conventional continuous stimulus tones. Here, DPOAE components are separated in the time domain from DPOAE signals elicited with short stimulus pulses. The extracted nonlinear-distortion components are used to derive estimated distortion-product thresholds (EDPTs) from semi-logarithmic input-output (I/O) functions for 20 normal-hearing and 21 hearing-impaired subjects. I/O functions were measured with frequency-specific stimulus levels at eight frequencies f2 = 1,…, 8 kHz (f2/f1 = 1.2). For comparison, DPOAEs were also elicited with continuous primary tones. Both acquisition paradigms yielded EDPTs, which significantly correlated with behavioral thresholds (p < 0.001) and enabled derivation of estimated hearing thresholds (EHTs) from EDPTs using a linear regression relationship. DPOAE-component separation in the time domain significantly reduced the standard deviation of EHTs compared to that derived from continuous DPOAEs (p < 0.01). In conclusion, using frequency-specific stimulus levels and DPOAE-component separation increases the reliability of DPOAE I/O functions for assessing cochlear function and estimating behavioral thresholds.