Distortion product otoacoustic emissions measured as vibration on the eardrum of human subjects

doi:10.1073/pnas.0610185103

Distortion product otoacoustic emissions measured as vibration on the eardrum of human subjects

Department of Otolaryngology, Tübingen Hearing Research Centre, Section of Physiological Acoustics and Communication, University of Tübingen, Elfriede-Aulhorn-Strasse 5, 72076 Tübingen, Germany

Communicated by Jozef J. Zwislocki, Syracuse University, Syracuse, NY, November 23, 2006 (received for review June 14, 2006)

Abstract

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 f ₂, with f ₂/f ₁ = 1.2 and f ₂ = 4–9.5 kHz, and intensities L ₁ and L ₂, with L ₁ = 0.4L ₂ + 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.

Footnotes

*To whom correspondence should be addressed. E-mail: anthony.gummer{at}uni-tuebingen.de
Author contributions: E.D. and D.T. contributed equally to this work; E.D., D.T., and A.W.G. designed research; E.D. and D.T. performed research; E.D. contributed new reagents/analytic tools; E.D., D.T., H.-P.Z., and A.W.G. analyzed data; E.D. designed and built the laser interferometer; D.T. carried out all measurements on human subjects; H.-P.Z. provided clinical advice; and A.W.G. compared DPOAEs on the basilar membrane and umbo.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/cgi/content/full/0610185103/DC1.
↵ ^†An alternative hypothesis proposes that the forward wave derives from reflection of a backward wave from the stapes (11). However, the exact mechanisms are not important for our present purposes.
↵ ^‡By convention, the reference sound pressure magnitude for defining dB SPL is 20 μPa. Correspondingly, we could have defined an umbo velocity level as 20log(umbo velocity/reference velocity), where the reference velocity = 1 μm/s. However, we have refrained from introducing this definition, using instead the clumsy logarithmic notation, because it only occurs in this paragraph.
↵ ^§Statistical significance is set at the 95% confidence level and above.
↵ ^¶The model in ref. 37 suggests <0.4 dB nonlinearity for SPL from 10 to 90 dB, using nonlinear cochlear amplification of 40 dB above 2 kHz, decreasing to 10 dB by 100 Hz (A. Vetešnik, personal communication). Clearly, this difference is not detectable by the LDV (Fig. 1).
Abbreviations:

DP,

distortion product;

OAE,

otoacoustic emission;

DPOAE,

DP of the OAE;

EDPT,

estimated DP threshold;

v-EDPT,

velocity EDPT;

I/O,

input/output;

LDV,

laser Doppler vibrometer;

SNR,

signal-to-noise ratio;

SPL,

sound pressure level.
Freely available online through the PNAS open access option.