Diana Turcanu

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

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, f1 and f2, with f2/f1 = 1.2 and f2 = 4–9.5 kHz, and intensities L1 and L2, with L1 = 0.4L2 + 39 dB and L2 = 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 f2. 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.

Extraction of sources of distortion product otoacoustic emissions by onset-decomposition.

The cubic component of the distortion product otoacoustic emission (DPOAE) in response to two tones of frequency f(1) and f(2) is generated by so-called primary- and secondary-source mechanisms in the cochlea. Interference between the resulting two source components can limit the usefulness of DPOAEs in assessing cochlear function. Although techniques are available for separating the source components, depending on the application, they can be either time-consuming or ineffective without a priori knowledge of optimal parameters. Here, we investigated, in humans, the possibility of separating the source components in the time-domain by sampling the onset and offset of the DPOAE-time signal at appropriate instants. Therefore, a DPOAE paradigm was developed in which the f(2) tone was periodically switched on during the continuous presence of the f(1) tone. F(2) was increased in 20-Hz steps from 1.5 to 2.5 kHz and the ratio f(2)/f(1) held constant at 1.2; measurements were made at six primary tone levels, ranging from L(2)=25 to 65 dB SPL. To investigate the possibility of separating the two sources by appropriate sampling, we developed an algorithm called onset-decomposition. The algorithm is based on the shape properties of DP-grams constructed from DPOAE responses at different time instants in the onset of the DPOAE signal. Thus, at each such time instant, the source components were extracted by time-windowing of the corresponding DP-gram. The time courses of the amplitude onsets of these separated primary- and secondary-source components provided evidence that the primary-source component attained its steady-state before the secondary-source component started to significantly influence the DPOAE by interference with the primary-source component. Consequently, in the final paradigm, the primary-source component is extracted by sampling the DPOAE signal at a single pre-defined time instant after the onset of the f(2) stimulus tone, before the secondary component begins to interfere. Based on the near-absence of interference maxima and minima in the DP-grams, the appropriate sampling instant was 8-10 ms for all frequencies and intensities in the stimulus set. Extracting the primary-source by onset sampling has the advantage that when individual source components for a given f(2) are to be investigated, there is no need to measure a DP-gram. In conclusion, it is shown that the technique can reliably and quickly separate the source components, making it an attractive paradigm for applications in basic research and clinical diagnosis.

Forward and reverse transfer functions of the middle ear based on pressure and velocity DPOAEs with implications for differential hearing diagnosis

Recently it was shown that distortion product otoacoustic emissions (DPOAEs) can be measured as vibration of the human tympanic membrane in vivo, and proposed to use these vibration DPOAEs to support a differential diagnosis of middle-ear and cochlear pathologies. Here, we investigate how the reverse transfer function (r-TF), defined as the ratio of DPOAE-velocity of the umbo to DPOAE-pressure in the ear canal, can be used to diagnose the state of the middle ear. Anaesthetized guinea pigs served as the experimental animal. Sound was delivered free-field and the vibration of the umbo measured with a laser Doppler vibrometer (LDV). Sound pressure was measured 2-3 mm from the tympanic membrane with a probe-tube microphone. The forward transfer function (f-TF) of umbo velocity relative to ear-canal pressure was obtained by stimulating with multi-tone pressure. The r-TF was assembled from DPOAE components generated in response to acoustic stimulation with two stimulus tones of frequencies f(1) and f(2); f(2)/f(1) was constant at 1.2. The r-TF was plotted as function of DPOAE frequencies; they ranged from 1.7 kHz to 23 kHz. The r-TF showed a characteristic shape with an anti-resonance around 8 kHz as its most salient feature. The data were interpreted with the aid of a middle-ear transmission-line model taken from the literature for the cat and adapted to the guinea pig. Parameters were estimated with a three-step fitting algorithm. Importantly, the r-TF is governed by only half of the 15 independent, free parameters of the model. The parameters estimated from the r-TF were used to estimate the other half of the parameters from the f-TF. The use of r-TF data - in addition to f-TF data - allowed robust estimates of the middle-ear parameters to be obtained. The results highlight the potential of using vibration DPOAEs for ascertaining the functionality of the middle ear and, therefore, for supporting a differential diagnosis of middle-ear and cochlear pathologies.

Accuracy of velocity distortion product otoacoustic emissions for estimating mechanically based hearing loss

Distortion product otoacoustic emissions (DPOAEs) measured as vibration of the human eardrum have been successfully used to estimate hearing threshold. The estimates have proved more accurate than similar methods using sound-pressure DPOAEs. Nevertheless, the estimation accuracy of the new technique might have been influenced by endogenous noise, such as heart beat, breathing and swallowing. Here, we investigate in an animal model to what extent the accuracy of the threshold estimation technique using velocity-DPOAEs might be improved by reducing noise sources. Velocity-DPOAE I/O functions were measured in normal and hearing-impaired anaesthetized guinea pigs. Hearing loss was either conductive or induced by furosemide injection. The estimated distortion product threshold (EDPT) obtained by extrapolation of the I/O function to the abscissa was found to linearly correlate with the compound action potential threshold at the f(2) frequency, provided that furosemide data were excluded. The standard deviation of the linear regression fit was 6 dB as opposed to 8 dB in humans, suggesting that this accuracy should be achievable in humans with appropriate improvement of signal-to-noise ratio. For the furosemide animals, the CAP threshold relative to the regression line provided an estimate of the functional loss of the inner hair cell system. For mechanical losses in the middle ear and/or cochlear amplifier, DPOAEs measured as velocity of the umbo promise an accuracy of hearing threshold estimation comparable to classical audiometry.

[Sound and velocity DPOAEs : Technology, methodology and perspectives].

Recent publications show that DPOAE measurements can generate a more accurate diagnosis, if (1) their fine structure is suppressed, and (2) if the calibration of the sound field is improved. Reduction of the fine structure is particularly important in the frequency range below 4 kHz in subjects with intact cochlear amplifier and can reduce the standard deviation of threshold estimations based on DPOAE-input/output functions from 11 dB to 6 dB. Improving the sound-field calibration has most impact in the frequency range above 4 kHz. Threshold estimations based on laserinterferometrically measured DPOAE input-output functions where the sound field was calibrated close to the tympanic membrane have been shown to reduce the standard deviation down to 8.6 dB in humans and 6.5 dB in guinea pigs. Compared with conventional DPOAE measures, such as amplitude or signal-to-noise ratio, threshold estimation based on DPOAE-I/O functions has the advantage that its slope provides additional information about the middle-ear; however, its specificity is limited. In the future, combined methods such as acoustic reflectance or laser vibrometry on the umbo promise a reliable assessment of the middle-ear contribution to DPOAE.

Schall- und Geschwindigkeits-DPOAE

Recent publications show that DPOAE measurements can generate a more accurate diagnosis, if (1) their fine structure is suppressed, and (2) if the calibration of the sound field is improved. Reduction of the fine structure is particularly important in the frequency range below 4 kHz in subjects with intact cochlear amplifier and can reduce the standard deviation of threshold estimations based on DPOAE-input/output functions from 11 dB to 6 dB. Improving the sound-field calibration has most impact in the frequency range above 4 kHz. Threshold estimations based on laserinterferometrically measured DPOAE input-output functions where the sound field was calibrated close to the tympanic membrane have been shown to reduce the standard deviation down to 8.6 dB in humans and 6.5 dB in guinea pigs. Compared with conventional DPOAE measures, such as amplitude or signal-to-noise ratio, threshold estimation based on DPOAE-I/O functions has the advantage that its slope provides additional information about the middle-ear; however, its specificity is limited. In the future, combined methods such as acoustic reflectance or laser vibrometry on the umbo promise a reliable assessment of the middle-ear contribution to DPOAE.

Laserinterferometrically measured distortion product otoacoustic emissions

Laserinterferometrically measured distortion product otoacoustic emissions provide a means of assessing hearing threshold with high accuracy. It is shown that together with the measurement of the vibration response of the tympanic membrane to sound, a model of the middle ear can be fitted automatically to a forward and a reverse transfer function. The model can be used to estimate the complete transfer function of the middle ear and thus provides a potential means for differential diagnosis of middle- and inner-ear pathologies.

Schall- und Geschwindigkeits-DPOAE@@@Sound and velocity DPOAEs: Technologie, Methodik und Perspektiven@@@Technology, methodology and perspectives

Recent publications show that DPOAE measurements can generate a more accurate diagnosis, if (1) their fine structure is suppressed, and (2) if the calibration of the sound field is improved. Reduction of the fine structure is particularly important in the frequency range below 4 kHz in subjects with intact cochlear amplifier and can reduce the standard deviation of threshold estimations based on DPOAE-input/output functions from 11 dB to 6 dB. Improving the sound-field calibration has most impact in the frequency range above 4 kHz. Threshold estimations based on laserinterferometrically measured DPOAE input-output functions where the sound field was calibrated close to the tympanic membrane have been shown to reduce the standard deviation down to 8.6 dB in humans and 6.5 dB in guinea pigs. Compared with conventional DPOAE measures, such as amplitude or signal-to-noise ratio, threshold estimation based on DPOAE-I/O functions has the advantage that its slope provides additional information about the middle-ear; however, its specificity is limited. In the future, combined methods such as acoustic reflectance or laser vibrometry on the umbo promise a reliable assessment of the middle-ear contribution to DPOAE.

Umbo vibration in response to two‐tone stimuli measured in human subjects

The aim of these experiments was to noninvasively investigate cochlear mechanics near auditory threshold in human subjects by measuring the vibration response of the umbo. The vibration measurement setup consists of a custom‐built laser Doppler vibrometer (LDV), employing a conventional heterodyne interferometer, for which all optical and electronic parts were tailored to the problem of measuring picometer‐sized vibrations in the presence of large, extraneous movements in the order of 0.1 mm (e.g., heart beat, breathing, or swallowing). The cubic distortion product (DP) of the otoacoustic emissions (OAEs) in response to sound stimulation with two tones was measured as vibration on the umbo. For stimulus frequencies, f1 and f2, with f2/f1=1.2 and f2=4–9.5 kHz, and intensities L1 and L2, with L1=0.4L2+39 dB and L2=20–65 dB SPL, the vibration amplitudes were 1–8 pm in subjects (N=20) with hearing loss no more than 25 dB SPL. Application of this vibration technique to a recently proposed method [P. Boege and ...