Günther Scholz

Low-frequency modulation of the 2f1−f2 distortion product otoacoustic emissions in the human ear

Low-frequency masking is a recent clinical procedure for the differential diagnosis of sensory hearing loss. Currently this requires the recording of the phase-dependent masked subjective threshold, which is time consuming and not always accurate. As an objective method, the recording of modulated distortion product otoacoustic emissions (DPOAEs) can be performed continuously, and with better frequency specificity. Results of measurements of the low-frequency modulated two-tone DPOAE 2f1-f2 in the human ear, and its dependence on various acoustic parameters, are presented here for the first time. Similar to the masked hearing threshold, the pattern of the phase-dependent modulated DPOAEs displayed two minima, at the phases of maximal rarefaction and condensation, respectively, with a latency of about 4 ms (suppressor frequency 32.8 Hz). The smaller dip, at maximal condensation, appeared only for a high suppressor level, and for a low level of the primary tone f2. The modulating effect measured for the primary frequencies f1 = 2.5 kHz and f2 = 3 kHz, decreased for 4 and 4.8 kHz, and vanished for 5 and 6 kHz. The results are discussed using a cubic distortion model based on the Boltzmann function for mechano-electrical transduction of the hair cells. The saturation behavior of the increase of the DPOAE level at different phases is compared with the growth rates of the DPOAE level in normal hearing and in sensory hearing loss.

Diagnosis of endolymphatic hydrops by low-frequency masking

Low-frequency masking is a new method for the diagnosis of endolymphatic hydrops. A short acoustic stimulus and a low-frequency masker tone are applied to the same ear in an adjustable phase relationship. We recorded phase-dependent masked thresholds from normal-hearing subjects, and patients with Ménières disease and sensory hearing loss without vertigo. In normal hearing, there is a mean maximal difference in masking (modulation depth) of 28 dB between the phase delays of 0 degree and 270 degrees. In patients with sensory hearing loss without vertigo, modulation depth is reduced due to recruitment. In Ménière cases, the phase dependence may be totally absent and varies as the disease progresses. Therefore, repeated measurements of masking are required: patients and subjects with normal hearing were tested for a period of 1 year. Also, modulation depth is significantly reduced in the contralateral nonsymptomatic ears of Ménière patients. The results indicate that low-frequency masking is a quick, noninvasive and relevant method for the diagnosis of endolymphatic hydrops.

Masking and pitch shift of tone bursts and clicks by low-frequency tones.

From experiments in animals and investigations in humans it is known that the normally phase-dependent masking of a short stimulus by a low-frequency continuous tone does not occur in the case of endolymphatic hydrops. The recording of the masked threshold of short tone stimuli in a loud tone of 30 Hz is to be evaluated for the clinical diagnostics of Ménières disease. To this purpose, the main parameters of the measurements (type, frequency, duration of the stimulus, and intensity of the masker) and their effect of phase-dependent masking and pitch-shift are investigated. Stimuli above 2 kHz are masked less than those of lower frequencies. Wide-band stimuli are less useful, since only the low-frequency component of their spectrum is masked. The tone stimuli should be short (1 - 2 ms) in order to make the measurement of the phase dependence more accurate. With increasing masker level the masking at phase 0 degree corresponds to the increase in level, at phase 270 degrees the amount is twice as much. The pitch shift which is perceived in low-tone masking depends on the phase of the stimulus, and on the levels of the stimulus and the masking tone. The use of brain stem recordings in the investigation of phase-dependent low tone masking is problematic since well-synchronizing stimuli with high frequency spectral components are masked poorly.

Low-frequency masking for detection of endolymphatic hydrops in patients with glaucoma

The coincidence of various eye and ear abnormalities has been described in the literature. Some authors discuss the possible existence of endolymphatic hydrops in patients with glaucoma. Whereas the current diagnostic tests for glaucoma are well-defined and evident, those for endolymphatic hydrops are not so reliable. This has made it difficult to accurately study the coincidence of endolymphatic hydrops and glaucoma. For better detection of endolymphatic hydrops, we performed low-frequency masking tests in 23 patients with primary open-angle glaucoma without signs of Ménìeres disease. The phase dependent sensitivity of the organ of Corti to a short test stimulus can be measured by applying a low-frequency masker tone to determine the modulation depth. Whereas the modulation depth in the normal hearing population is around 20-35 dB, the modulation depth in patients with Ménières disease may be significantly decreased (5-10 dB), depending on the stage of disease. A decreased modulation depth was found in at least one ear in 19 of our 23 patients with glaucoma. Correlations between homeostatic mechanisms and their histological characteristics, e.g. melanocytes and their hormonal and enzymatic regulation, will be discussed. If the common pathogenesis of eye and ear lesions is better understood, it may be possible to develop new and more effective strategies for prevention and therapy.

Phase-dependent Suppression of Transient Evoked and Distortion Product Otoacoustic Emissions by a Low-frequency Tone

The subjective recording of the masked threshold of short acoustical stimuli with a loud tone of 30 Hz (phase audiogram) has been used for the clinical diagnosis of endolymphatic hydrops (EH). In normally-hearing subjects, a marked modulation of the threshold was found, depending on the phase of the low-frequency tone. A very small dependence was found in patients with Menières disease, due to the micromechanical changes in the basilar membrane (BM). The same phase relationship becomes apparent in low-frequency suppression of otoacoustic emissions. The amplitudes of TEOAEs are controlled by the phase-dependent displacement of the BM. The suppressed TEOAEs have to be measured separately in each phase relationship. During recording of suppressed DPOAEs, the low-frequency suppressor is permanently superimposed on the pair of primary tones. After time averaging and a moving short-time FFT, the spectral values of the DPOAEs are obtained depending on the phase of the low-frequency tone. Modulation depends also on the masker level, the levels of the primary tones, and on their frequency range. The method of low-frequency suppressed DPOAEs is an objective method to diagnose EH and could be a useful tool in human inner ear research.

Modulation of distortion product otoacoustic emissions (DPOAE) by low‐frequency masker tones in human ears

Biasing of the basilar membrane displacement by a loud low‐frequency tone causes changes of the DPOAEs depending on the phase of the masker. A corresponding model helps to understand the nonlinear behavior of the cochlea and to improve the diagnosis of inner ear diseases. Up to now, short time Fourier transform has been used to measure the DPOAE suppression pattern. This method has inherent time and frequency resolution limits. Therefore a new approach is presented, where the primaries and all frequencies generated by the process are harmonics of the masker. A rectangular time window with length according to the masker period is used. The part of the spectrum representing the DPOAE signal is regarded as modulated tone, consisting of a DPOAE frequency (2f1−f2) and spectral side frequencies. The time course of the DPOAE suppression pattern is computed by superposition of these complex components. The results are compared with a simulation of the distortions of the outer hair cells’ mechano‐electrical transf...