Sarosh Kapadia

ARE NORMAL HEARING THRESHOLDS A SUFFICIENT CONDITION FOR CLICK-EVOKED OTOACOUSTIC EMISSIONS?

Transiently evoked otoacoustic emissions (TEOAE) have been reported in several studies as absent in a small minority of normal ears. Other studies have reported TEOAEs in all normal ears. Differences between studies may arise directly from criteria for TEOAE identification, criteria for selection of normals, or statistically due to limited sample sizes. In order to understand and model cochlear processes involved in TEOAE generation, it needs to be known whether the presence of normal hearing leads automatically to generation of TEOAEs. The present study set out to establish in a large sample if any ears could be found that lacked TEOAEs despite normal hearing threshold levels (HTL). A total of 397 ears from highly cooperative adult subjects were examined under laboratory conditions. Using cross correlation between replicate nonlinear waveforms as the criterion, TEOAEs were present in 99.2% of the sample (lower CI 98.1%). However, careful visual assessment of the recorded waveforms for the remaining ears ...

Modification of the Threshold Equalising Noise (TEN) test for cochlear dead regions for use with steeply sloping high-frequency hearing loss

Steeply sloping high-frequency hearing loss is often associated with cochlear dead regions. These can be identified by measuring pure-tone thresholds in quiet and in Threshold-Equalising Noise (TEN). However, many patients cannot be adequately tested because the low frequencies in the TEN lead to uncomfortable loudness. We investigated the effect of high-pass filtering on the TEN-test results and the loudness of the TEN. Twenty-four normally hearing subjects and 35 subjects with steeply sloping high-frequency hearing loss were tested, using the standard TEN (TENs), and TEN high-pass filtered at 0.5 kHz (TEN0.5) or 1 kHz (TEN1). For both groups, masked thresholds did not differ across noise types for frequencies above 1 kHz. Over 50% of the hearing-impaired ears tested met the criteria for a dead region at 4 kHz, using all three noise types. However, masked thresholds and the prevalence of positive TEN-test results at 1 kHz were both lower with the TEN1. The TEN1 was judged the most comfortable noise by 68% of the hearing-impaired subjects. We conclude that high-pass filtering would allow testing at higher TEN levels for patients with steeply sloping hearing loss. Sumario La hipoacusia de perfil abruptamente descendente en frecuencias agudas se asocia frecuentemente con regiones cocleares muertas. Esto se puede identificar midiendo los umbrales tonales en silencio y en ruido ecualizado al umbral (TEN). Sin embargo, algunos pacientes no pueden ser estudiados adecuadamente porque las frecuencias bajas en el TEN, les produce molestia. Investigamos el efecto del filtro pasa-alto en los resultados de las pruebas TEN y la intensidad subjetiva del TEN. Se examinaron 24 sujetos normoyentes y 35 con hipoacusia de perfil abruptamente descendente en frecuencias agudas, utilizando el TEN habitual y el TEN con filtro pasa/alto a 0.5 kHz (TEN0.5) o 1 kHz (TEN1). Los umbrales de enmascaramiento no difirieron con los diferentes tipos de ruido para las frecuencias por arriba de 1 kHz en ambos grupos. Más del 50% de los oídos hipoacúsicos cumplieron con los criterios para considerar una zona muerta a 4 kHz, con los tres tipos de ruido. Sin embargo, los umbrales de enmascaramiento y la prevalencia de una prueba TEN positiva a 1 kHZ fueron menores con el TEN1. El 68% de los hipoacúsicos consideraron al TEN1 como el ruido más confortable. Concluimos que el filtro pasa-alto puede permitir realizar pruebas con mayor intensidad TEN en pacientes con hipoacusia de perfil abruptamente descendente en frecuencias agudas.

Reduced àudiogram ripple' in normally-hearing subjects with weak otoacoustic emissions.

Correspondence between spectral patterns in otoacoustic emissions (OAE) and the fine structure of the pure-tone audiogram has often been noted, but the link is by no means clearly understood nor complete in all subjects. This paper presents part of a broader study focusing on individuals with normal hearing but anomalously weak OAEs, the primary purpose of which was to determine the reasons for the weak OAEs. Subjects were selected from an exhaustive search of some 400 ears of highly co-operative adults, and comprised a test group of subjects with normal hearing thresholds but weak OAEs, and a control group of normals from the same sample. Reported here are data on audiogram fine structure measured in the two groups of subjects. The basic finding is that the subjects with weak OAEs also exhibited significantly less audiogram fine structure than the controls, as evaluated by analysing the periodicity in the respective threshold curves as well as by identifying and quantifying individual peaks in the curves. These findings first provide further evidence of an underlying link between the fine structure of the audiogram and OAEs, as proposed by Kemp in his original work. Second, assuming that the degree of fine structure would be largely unaffected by minor middle ear alterations, our findings suggest that predominantly cochlear rather than middle ear factors are responsible for the low levels of OAEs in the normal subjects of our test group. Finally, the results presented suggest that, like OAEs, audiogram fine structure measurements provide information on the auditory system that is not available in the conventional pure-tone audiogram.

Reassessment of cochlear dead regions in hearing-impaired teenagers with severe-to-profound hearing loss

The aim of this study was to reassess cochlear dead regions after an interval of twelve months, using the Threshold Equalising Noise (TEN) test. Thirty-four ears of 24 teenagers (mean age of 14 years) with longstanding severe-to-profound sensorineural hearing impairment were tested. Testing was repeated after an interval of 12 months using the same experimental set-up. A total of eight (23.5%) out of 34 ears changed category on retest: this decreased to two (7.1%) out of 27 ears when the inconclusive category was removed from the analysis. In both of these ears (of the same participant) the criteria were met at a single frequency, and the masked threshold was only 10 dB above the TEN level per ERBN. When all of the data were examined on a frequency-by-frequency basis, the instances that changed category ranged from 15 to 51%. The range decreased to between 4 and 34% when the inconclusive category was removed from the analysis.

Effect of aspirin on phase gradient of 2F1–F2 distortion product otoacoustic emissions

It is well known that aspirin consumption temporarily reduces overall otoacoustic emission (OAE) amplitude in humans. However, little is known about changes in the separate components of distortion product otoacoustic emissions (DPOAE), which may be distinguished by examining phase gradients. The effects of aspirin on the phase gradient of the DPOAE 2F1-F2 obtained with fixed frequency ratio sweeps were studied longitudinally in a group of twelve subjects in whom a temporary hearing loss was induced by aspirin consumption. DPOAE were recorded daily for two days pre-aspirin consumption, during the three days of aspirin consumption and two days afterwards. DP-grams were recorded over a restricted frequency range centered on 2,3,4 and 6 kHz with the following stimulus levels: L1/L2 of 60/50-80/70 in 10-dB steps. The effects of aspirin on the phase gradients varied between the subjects and across frequency: the general trend was that the phase gradient became steeper across successive sessions for the higher frequencies, while no significant effect was found at the lower frequencies. These results suggest that aspirin may have more persistent effects on cochlear function than are disclosed by measurements of hearing threshold level or DPOAE amplitude. Particularly, DPOAE phase gradient appears to be increased by aspirin consumption and has not recovered two days after cessation of aspirin intake, despite almost complete recovery of DPOAE amplitude and hearing threshold levels. These findings may suggest differential effects on the distortion and reflection mechanisms considered to underlie DPOAE generation.

Dynamic nonlinear cochlear model predictions of click-evoked otoacoustic emission suppression

A comprehensive set of results from 2-click suppression experiments on otoacoustic emissions (OAEs) have been presented by Kapadia and Lutman [Kapadia, S., Lutman, M.E., 2000a. Nonlinear temporal interactions in click-evoked otoacoustic emissions. I. Assumed model and polarity-symmetry. Hear. Res. 146, 89-100]. They found that the degree of suppression of an OAE evoked by a test click varied systematically with the timing and the level of a suppressor click, being greatest for suppressor clicks occurring some time before the test click, particularly at lower levels of suppression. Kapadia and Lutman also showed that although the general shape of the graph of suppression against suppressor click timing could be predicted by a static power law model, this did not predict the asymmetry with respect to the timing of the suppressor click. A generalised automatic gain control (AGC) is presented as a simple example of a dynamic nonlinear system. Its steady state nonlinear behaviour, as quantified by its level curve, and its dynamic behaviour, as quantified by its transient response, can be independently set by the feedback gain law and detector time constant, respectively. The previously reported suppression results, with the asymmetry in the timing, are found to be predicted better by such an AGC having a level curve with a slope of about 0.5 dB/dB, and a detector time constant of about twice the period at the characteristic frequency. Although this gives adequate predictions for high suppression levels, it under predicts the suppression and the asymmetry for lower levels. Further research is required to establish whether simple peripheral feedback models can explain OAE suppression of this type.

Static input-output non-linearity as the source of non-linear effects in maximum length sequence click-evoked OAEs.

Abstract The application of the maximum length sequence (MLS) technique to the recording of click-evoked otoacoustic emissions (CEOAEs) allows for a reduction in test time by one to two orders of magnitude. This is because the technique permits the use of extremely high click rales, as inter-click intervals are not constrained to be greater than the duration of the response. However, increasing the click rate also causes a progressive reduction in amplitude, or ‘suppression’, of the CEOAE. The origin of this suppression is unclear, with diverse suggestions in the literature as to its nature and mechanism. This paper presents a simple model of the well-known compressive non-linearity of the CEOAE level function, based on a static amplitude non-linearity within each of a number of narrowband frequency channels. The response of the model to MLS stimulation demonstrates suppression broadly of the form and magnitude previously reported in experimental studies. Furthermore, the model exhibits the generation of additional non-linear components that have been speculated on in connection with CEOAE recordings using the MLS technique. It is concluded that the MLS suppression phenomenon is derived largely, if not entirely, from the static non-linearity of the CEOAE level function. The approach to modelling the phenomenon as described here also bears promise for understanding various aspects of non-linearity in MLS-based CEOAE recordings.

Does static ear canal pressure influence pure tone pitch perception

Objective That static ear canal air pressure (ECP) influences the frequency of spontaneous otoacoustic emissions (SOAEs) suggests that it may influence intracochlear, in addition to middle ear, processes. A previous study suggested that ECP influences pure tone pitch perception at 1,000 Hz, which was interpreted as indicating an effect on the cochlear place-frequency map. The present study extended investigations of this effect to 500 and 4,000 Hz. Materials and Methods Nine normal-hearing listeners performed interaural pitch matching with monaural ECPs of 0 and -300 daPa. Results Some indications of a small downward pitch shift (mean 0.2%) at 500 Hz were observed, which were marginally statistically significant at the 5% level. No pitch shifts were observed at 4,000 Hz. Conclusions ECP does not influence pitch to the extent suggested previously or by SOAE frequency shifts. No evidence was found to support the notion that ECP influences the place-frequency map.