Quiet Thresholds Research Articles

Spectral and threshold effects on recognition of speech at higher-than-normal levels

To examine spectral and threshold effects for speech and noise at high levels, recognition of nonsense syllables was assessed for low-pass-filtered speech and speech-shaped maskers and high-pass-filtered speech and speech-shaped maskers at three speech levels, with signal-to-noise ratio held constant. Subjects were younger adults with normal hearing and older adults with normal hearing but significantly higher average quiet thresholds. A broadband masker was always present to minimize audibility differences between subject groups and across presentation levels. For subjects with lower thresholds, the declines in recognition of low-frequency syllables in low-frequency maskers were attributed to nonlinear growth of masking which reduced "effective" signal-to-noise ratio at high levels, whereas the decline for subjects with higher thresholds was not fully explained by nonlinear masking growth. For all subjects, masking growth did not entirely account for declines in recognition of high-frequency syllables in high-frequency maskers at high levels. Relative to younger subjects with normal hearing and lower quiet thresholds, older subjects with normal hearing and higher quiet thresholds had poorer consonant recognition in noise, especially for high-frequency speech in high-frequency maskers. Age-related effects on thresholds and task proficiency may be determining factors in the recognition of speech in noise at high levels.

Effects of age on speech understanding in normal hearing listeners: Relationship between the auditory efferent system and speech intelligibility in noise

Human listeners are able to listen to one voice in the midst of other conversations and background noise. Although the neural mechanisms for this process are not well understood, there is growing evidence that the medial olivocochlear (MOC) auditory efferent system located in the brainstem is involved in the detection of signals in noise, such as speech sounds, by modulating cochlear (inner ear) active physiological mechanisms. The present study examined MOC efferent (feedback) effects as revealed in distortion product otoacoustic emissions (DPOAEs) and effects of spatial separation for speech perception in background noise. Both spatial separation of speech in noise and contralateral suppression (CS) of DPOAE invoke neural mechanisms central to the inner ear. We sought to determine whether these tasks might be related and thereby represent involvement in the ability to listen to one voice in the midst of concurrent conversation and background noise; a situation in which elderly listeners have great difficulty. The Hearing in the Noise Test (HINT) [Nilsson, M., Soli, S.D., Sullivan, J.A., 1994. Development of the hearing in noise test for the measurement of speech reception thresholds in quiet and in noise. J. Acoust. Soc. Amer. 95, 1085–1099] was used to obtain release from masking (RFM) estimates of speech in noise. DPOAEs were used to measure effects of broadband noise introduced to the ear contralateral to the target ear (CS). Significant age effects were found in both domains. Significant correlations resulted between speech perception in noise (RFM) and the degree of CS of DPOAEs. RFM was significantly related only to age and CS at the DPOAE narrow-band frequency of 1–2 kHz; a frequency band critical for successful speech perception. These findings suggest that the MOC efferent system and neural mechanisms underlying RFM are related and contribute to sound source determination commonly referred to as the “cocktail party effect”.

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.SumarioLa 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.

A Swedish version of the Hearing In Noise Test (HINT) for measurement of speech recognition

A Swedish Hearing In Noise Test (HINT), consisting of everyday sentences to be used in an adaptive procedure to estimate the speech recognition thresholds in noise and quiet, has been developed. The material consists of 250 sentences, with a length of five to nine syllables, normalized for naturalness, difficulty and reliability. The sentences were recorded with a female speaker. From the sentences, 25 phonemically balanced lists were created. All lists fluctuate less than 1 dB of the overall mean. The standard deviation of the test-retest difference is 0.94 dB when testing with one list, and decreases to 0.68 dB and 0.56 dB for two and three lists, respectively. The average speech recognition thresholds in noise for the Swedish sentences were −3.0 dB signal/noise ratio (SD = 1.1 dB). The present study has resulted in a well-defined and internationally comparable set of sentences, which can be used in Swedish audiological rehabilitation and research to measure speech recognition in noise and quiet.

Psychophysical estimates of nonlinear cochlear processing in younger and older listeners

The primary goal of this project was to compare the performance of younger and older listeners on a number of psychophysical measures thought to be influenced by nonlinear cochlear processing. Younger (mean of 25.6 years) and older (mean of 63.8 years) listeners with normal hearing were matched (within 5 dB) according to their quiet thresholds at the two test frequencies of 1200 and 2400 Hz. They were similarly matched at the adjacent octave frequencies of 600 and 4800 Hz (within 5 dB at one and 9 dB at the other). Performance was compared on measures of auditory filter shape, psychophysical suppression, and growth of forward masking. There was no difference between the two age groups on these psychophysical estimates reflecting nonlinear processing, suggesting that aging per se does not affect the cochlear nonlinearity, at least for the ages sampled here. The results did, however, consistently demonstrate an age-related increase in the susceptibility to forward masking.

Binaurality in Middle Ear Implant Recipients Using Contralateral Digital Hearing Aids

For some patients, conventional hearing aids might have disadvantages that clearly limit the benefit of using them. The middle ear implant, Vibrant Soundbridge hearing prosthesis offers an approach to help such patients. Our study's objective was to identify the binaurality in a well-fitted digital hearing aid worn in the contralateral ear in recipients experienced with use of the Vibrant Soundbridge middle ear implant device. In a prospective study, warble-tone thresholds and stereophony were evaluated for the following conditions: (1) binaural unaided-with the middle ear implant inactive and the behind-the-ear hearing aid removed; (2) middle ear implant alone-middle ear implant active, behind-the-ear hearing aid removed; (3) middle ear implant plus behind-the-ear omnidirectional-middle ear implant active, behind-the-ear active; and (4) behind-the-ear omnidirectional alone-middle ear implant inactive, behind-the-ear omnidirectional active. Behind-the-ear omnidirectional and behind-the-ear is defined as the behind-the-ear active in the omnidirectional or directional response, respectively. Behind-the-ear is a contralateral digital hearing aid to the middle ear implant. Benefits such as improved sound detection, speech perception in quiet and in noise, and sound quality were investigated. The evaluation of subjective hearing benefit was based on the Abbreviated Profile of Hearing Aid Benefit (APHAB) test. Paired t tests (subject) were used to analyze the differences between mean thresholds for the different test conditions. Tertiary referral center. Eight adults (aged 45-68 yr) had undergone implantation with a single Vibrant Soundbridge at least 12 months before starting the study. These eight subjects presented no contraindication for contralateral hearing aid use. Patients were fitted 5 weeks before testing with a Siemens Signia digital behind-the-ear hearing aid in the side contralateral to the Vibrant Soundbridge. Five weeks after use of the contralateral hearing aid together with the middle ear implant, mean differences in warble-tone thresholds between Conditions 2 and 3 and between Conditions 3 and 4 were both statistically significant. The mean differences in speech reception thresholds were in line with the mean differences in average warble-tone thresholds. The mean speech reception threshold for the middle ear implant plus behind-the-ear omnidirectional condition was slightly worse (3 dB) than that for the middle ear implant alone condition; however, this difference was not statistically significant. Whereas speech reception threshold difference between Condition 1 and Condition 4 was not statistically significant (60 dB and 61 dB, respectively). The mean difference for the 0-degree azimuth alone was statistically significant (p < 0.05) for the conditions middle ear implant alone and middle ear implant plus behind-the-ear omnidirectional. The middle ear implant alone appears to give good sensitivity at 2 kHz for any direction. Increasing scores indicate greater percentages of problems. Percentages of problems were on average lower for the middle ear implant when used with the contralateral digital hearing aid based on the global score. The use of a middle ear implant Vibrant Soundbridge together with a contralateral digital hearing aid improved functional gain and speech perception thresholds in quiet, especially for the sound coming from the front of the patient. The use of a middle ear implant together with a contralateral digital hearing did not significantly improve hearing in noise.

Dependence of detection threshold estimates in rabbit on method of constant stimulus parameters

The goal of this study was to estimate behavioral detection thresholds in quiet and noise for subsequent physiological studies of responses to near-threshold stimuli. The difficulty in measuring detection thresholds is maintaining stimulus control of behavior over schedule control. Tracking paradigms are efficient, but the large proportion of trials below threshold weakens stimulus control. The Method of Constant Stimuli controls the proportion of trials below, near, and above the SPL for a given percent correct. Here, percentages of trials at different SPLs and zero intensity catch trials were varied to determine proportions that yielded satisfactory stimulus control by a 500-Hz tone in quiet. Trials were initiated by nose-poke observing responses; a nose-poke reporting response within a 3-s window after tone onset resulted in food reinforcement. Reporting responses in the absence of tones resulted in timeouts. A metric for stimulus control was the ratio of the number of correct detections to the number of responses in the absence of the tone. This ratio was highest when approximately 20% of trials were near the 50%-correct SPL and 80% of trials were 20 dB higher. The ratio was lowest when significant proportions of catch and/or below-threshold trials were included. [Work supported by NIDCD DC-001641.]

Speech understanding in noise: Contributions of compressive nonlinearities in the basilar-membrane response

The contribution of compressive nonlinearities in the basilar-membrane response was assessed by measuring speech recognition in noise as a function of noise level and growth of masking for tones. Consonant recognition was measured in interrupted noise at overall masker levels of 47–77 dB SPL. Additionally, thresholds for a 10-ms, 2.0-kHz tone were measured in a masker ranging in level from 40 to 85 dB SPL and centered at 1 kHz. Subjects were younger and older adults with normal hearing. With speech at a constant level and the masker level varying, it was hypothesized that the response of the basilar membrane to the masker would be linear at lower levels and compressed at medium to higher levels, resulting in less effective masking at higher masker levels. Consistent with the hypothesis, differences between observed and predicted consonant-recognition scores, determined using AI calculations, varied as a function of masker level, especially for younger subjects. At lower masker levels, observed scores declined as predicted as masker level increased; with further increases in masker level, scores declined less than predicted. For tonal growth-of-masking functions, breakpoints were correlated with quiet thresholds: both were significantly higher for older than younger subjects. [Work supported by NIH/NIDCD.]

Perceptual consequences of normal and abnormal peripheral compression: Potential links between psychoacoustics and speech perception

Cochlear damage can lead to a reduction in the overall amount of peripheral auditory compression, presumably due to outer hair cell (OHC) loss or dysfunction. The perceptual consequences of functional OHC loss include loudness recruitment and reduced dynamic range, poorer frequency selectivity, and poorer effective temporal resolution. These in turn may lead to a reduced ability to make use of spectral and temporal fluctuations in background noise when listening to a target sound, such as speech. We tested the effect of OHC function on speech reception in hearing-impaired listeners by comparing psychoacoustic measures of cochlear compression and sentence recognition in a variety of noise backgrounds. In line with earlier studies, we found weak (nonsignificant) correlations between the psychoacoustic tasks and speech reception thresholds in quiet or in steady-state noise. However, when spectral and temporal fluctuations were introduced in the masker, speech reception improved to an extent that was well predicted by the psychoacoustic measures. Thus, our initial results suggest a strong relationship between measures of cochlear compression and the ability of listeners to take advantage of spectral and temporal masker fluctuations in recognizing speech. [Work supported by NIH Grants Nos. R01DC03909, T32DC00038, and R01DC00117.]

Uncertainty and confusion in temporal masking

In a landmark study, Wright et al. [Nature 387, 176–178 (1997)] reported an apparent backward-masking deficit in language-impaired children. Subsequently, these controversial results have been influential in guiding treatments for childhood language problems. In this study we revisited Wright et al.’s temporal-masking paradigm to evaluate listener uncertainty effects. Masked detection was measured for 20-ms sinusoids (480, 1000, or 1680 Hz) presented at temporal positions before, during, or after a gated narrowband (W=600–1400 Hz) masker. Listener uncertainty was investigated by cueing various stimulus temporal properties with a 6000-Hz sinusoid presented either ipsi- or contra-lateral to the test ear or bilaterally. The primary cueing effect was measured in the backward-masking condition for a contralateral cue gated simultaneously with the on-frequency 1000-Hz signal. The resulting cued masked-detection threshold was reduced to quiet threshold. No significant cueing effects were obtained for other signal temporal positions in the masker nor for any off-frequency signal conditions. These results indicate that (1) uncertainty can be reduced or eliminated for on-frequency backward masking by cueing the signal and (2) the deficit reported by Wright et al. for language-impaired children may reflect uncertainty and confusion rather than a temporal-processing deficit per se. [Research supported by NIDCD.]

Speech recognition in fluctuating and continuous maskers: effects of hearing loss and presentation level.

Listeners with normal-hearing sensitivity recognize speech more accurately in the presence of fluctuating background sounds, such as a single competing voice, than in unmodulated noise at the same overall level. These performance differences are greatly reduced in listeners with hearing impairment, who generally receive little benefit from fluctuations in masker envelopes. If this lack of benefit is entirely due to elevated quiet thresholds and the resulting inaudibility of low-amplitude portions of signal + masker, then listeners with hearing impairment should derive increasing benefit from masker fluctuations as presentation levels increase. Listeners with normal-hearing (NH) sensitivity and listeners with hearing impairment (HI) were tested for sentence recognition at moderate and high presentation levels in competing speech-shaped noise, in competing speech by a single talker, and in competing time-reversed speech by the same talker. NH listeners showed more accurate recognition at moderate than at high presentation levels and better performance in fluctuating maskers than in unmodulated noise. For these listeners, modulated versus unmodulated performance differences tended to decrease at high presentation levels. Listeners with HI, as a group, showed performance that was more similar across maskers and presentation levels. Considered individually, only 2 out of 6 listeners with HI showed better overall performance and increasing benefit from masker fluctuations as presentation level increased. These results suggest that audibility alone does not completely account for the group differences in performance with fluctuating maskers; suprathreshold processing differences between groups also appear to play an important role. Competing speech frequently provided more effective masking than time-reversed speech containing temporal fluctuations of equal magnitude. This finding is consistent with "informational" masking resulting from competitive processing of words and phrases within the speech masker that would notoccur for time-reversed sentences.

Bilateral bone-anchored hearing aids (BAHAs): an audiometric evaluation.

Since the technique to implant bone-anchored hearing aids (BAHAs) with the use of osseointegrated implants was developed in 1977, more than 15,000 patients have been fitted with BAHAs worldwide. Although the majority have bilateral hearing loss, they are primarily fitted unilaterally. The main objective of this study was to reveal benefits and drawbacks of bilateral fitting of BAHAs in patients with symmetric or slight asymmetric bone-conduction thresholds. The possible effects were divided into three categories: hearing thresholds, directional hearing, and binaural hearing. Prospective study of 12 patients with bilateral BAHAs. Baseline audiometry, directional hearing, speech reception thresholds in quiet and in noise, and binaural masking level difference were tested when BAHAs were fitted unilaterally and bilaterally. Eleven of the 12 patients used bilateral BAHAs on a daily basis. Tests performed in the study show a significant improvement in sound localization with bilateral BAHAs; the results with unilateral fitting were close to the chance level. Furthermore, with bilateral application, the improvement of the speech reception threshold in quiet was 5.4 dB. An improvement with bilateral fitting was also found for speech reception in noise. Overall, the results with bilateral fitted BAHAs were better than with unilaterally fitted BAHA; the benefit is not only caused simply by bilateral stimulation but also, to some extent, by binaural hearing. Bilateral BAHAs should be considered for patients with bilateral hearing loss otherwise suitable for BAHAs.

Correlação entre limiares de reconhecimento de sentenças no silêncio e limiares tonais

INTRODUÇÃO: Testes de reconhecimento de sentenças no silêncio ainda são pouco utilizados na rotina clínica audiológica e até mesmo em pesquisas. Por isso, muitas vezes, ao realizar esta avaliação, o clínico não possui parâmetros para interpretar ou classificar os resultados. OBJETIVO: Assim, este estudo teve por objetivo investigar a correlação entre o limiar de reconhecimento de sentenças no silêncio (LRSS) e a média dos limiares tonais nas freqüências de 0,5, 1 e 2 kHz. MÉTODOS: Foram examinadas 200 orelhas de indivíduos normo-ouvintes, 50 homens e 50 mulheres (média 34,61 anos). Inicialmente, realizou-se anamnese, meatoscopia, audiometria tonal liminar e pesquisa do limiar de reconhecimento de fala e do índice percentual de reconhecimento de fala. Posteriormente, utilizando o teste Listas de Sentenças em Português, realizou-se a pesquisa dos LRSS. RESULTADOS: O LRSS médio obtido nas 200 orelhas examinadas foi 6,15 dBNA e a média dos limiares tonais para as freqüências de 0,5, 1 e 2 kHz foi de 8,55 dBNA. A análise estatística evidenciou correlação significante entre as variáveis em estudo. CONCLUSÕES: A média dos limiares tonais nas freqüências de 0,5, 1 e 2 kHz pode ser usada como referência para a análise dos resultados obtidos na pesquisa do LRSS. Além disso, pode-se esperar que os valores dos LRSS sejam melhores que os valores obtidos a partir das médias dos limiares tonais, pois as sentenças fornecem pistas acústicas e lingüísticas com significado.

The effects of hearing loss on the contribution of high- and low-frequency speech information to speech understanding.

The speech understanding of persons with "flat" hearing loss (HI) was compared to a normal-hearing (NH) control group to examine how hearing loss affects the contribution of speech information in various frequency regions. Speech understanding in noise was assessed at multiple low- and high-pass filter cutoff frequencies. Noise levels were chosen to ensure that the noise, rather than quiet thresholds, determined audibility. The performance of HI subjects was compared to a NH group listening at the same signal-to-noise ratio and a comparable presentation level. Although absolute speech scores for the HI group were reduced, performance improvements as the speech and noise bandwidth increased were comparable between groups. These data suggest that the presence of hearing loss results in a uniform, rather than frequency-specific, deficit in the contribution of speech information. Measures of auditory thresholds in noise and speech intelligibility index (SII) calculations were also performed. These data suggest that differences in performance between the HI and NH groups are due primarily to audibility differences between groups. Measures of auditory thresholds in noise showed the "effective masking spectrum" of the noise was greater for the HI than the NH subjects.

A PDE Based Two Level Model of the Masking Property of the Human Ear

Human ear has the masking property that certain audible sound becomes inaudible in the presence of another sound. Masking is quantified by the raised threshold from the absolute hearing threshold in quiet. It is of scientific and practical importance to compute masking thresholds. Empirical models on masking have applications in low bit rate digital music compression. A first principle based two level model is developed with partial differential equation (PDE) at the peripheral level and a similarity transform to represent all higher levels of auditory perception. Modeled masking thresholds of banded noise by tonal signals agree well with hearing data.

Afferent response parameters derived from postmasker probe-detection thresholds: ‘the decay of sensation’ revisited

The classical model of forward masking postulates that the detection threshold for a tone probe that follows a stimulus of similar frequency content is elevated relative to the quiet threshold because the probe must evoke a just-detectable increment in a decaying postmasker sensation. That postmasker decay is charted by probe-detection thresholds if the sensation increment is small and constant. This model was examined for a 2-kHz Gaussian-shaped probe and a 2-kHz forward masker, based on the model’s assumption that a just-detectable increment in sensation results from a just-detectable increment in level. Psychometric functions for detection were obtained at 2.5–30 ms postmasker. Their means and standard deviations generally decreased with delay. It was assumed that standard deviation is related to the putative just-detectable level increment by a simple monotonic transformation. Thus, if the standard deviation of the psychometric function for probe detection is neither small nor constant, then the corresponding just-detectable increment in level is neither small nor constant, and the just-detectable increment in sensation is neither small nor constant. The classical model also fails to allow for the variability of internal events. The concept of detection threshold as a sensation increment was preserved in a Signal Detection model, that does allow for internal variability. In this model the postmasker residual is the input to a probe detector. The new model produces an equation for the just-detectable level increment as a function of probe delay. Comparison data were generated by again assuming some relation between the standard deviation of the psychometric function for detection, and the just-detectable increment in level. The fit of equation to data yields robust values for the probe detector’s maximum firing rate, dynamic range, and spike-counting time. All that is required to account for the decay of sensation, for a pure tone, is a single neuron operating at some higher center.

Benefit of modulated maskers for speech recognition by younger and older adults with normal hearing.

To assess age-related differences in benefit from masker modulation, younger and older adults with normal hearing but not identical audiograms listened to nonsense syllables in each of two maskers: (1) a steady-state noise shaped to match the long-term spectrum of the speech, and (2) this same noise modulated by a 10-Hz square wave, resulting in an interrupted noise. An additional low-level broadband noise was always present which was shaped to produce equivalent masked thresholds for all subjects. This minimized differences in speech audibility due to differences in quiet thresholds among subjects. An additional goal was to determine if age-related differences in benefit from modulation could be explained by differences in thresholds measured in simultaneous and forward maskers. Accordingly, thresholds for 350-ms pure tones were measured in quiet and in each masker; thresholds for 20-ms signals in forward and simultaneous masking were also measured at selected signal frequencies. To determine if benefit from modulated maskers varied with masker spectrum and to provide a comparison with previous studies, a subgroup of younger subjects also listened in steady-state and interrupted noise that was not spectrally shaped. Articulation index (AI) values were computed and speech-recognition scores were predicted for steady-state and interrupted noise; predicted benefit from modulation was also determined. Masked thresholds of older subjects were slightly higher than those of younger subjects; larger age-related threshold differences were observed for short-duration than for long-duration signals. In steady-state noise, speech recognition for older subjects was poorer than for younger subjects, which was partially attributable to older subjects' slightly higher thresholds in these maskers. In interrupted noise, although predicted benefit was larger for older than younger subjects, scores improved more for younger than for older subjects, particularly at the higher noise level. This may be related to age-related increases in thresholds in steady-state noise and in forward masking, especially at higher frequencies. Benefit of interrupted maskers was larger for unshaped than for speech-shaped noise, consistent with AI predictions.

Speech recognition in continuous and fluctuating maskers by listeners with normal and impaired hearing: Effects of presentation level

Listeners with normal hearing show better speech recognition in the presence of fluctuating background sounds, such as a single competing voice, than in unmodulated noise at the same overall level. These performance differences are greatly reduced in listeners with hearing impairment who generally show little benefit from fluctuations in masker envelopes. If this lack of benefit is entirely due to elevated quiet thresholds and the resulting inaudibility of low-amplitude portions of signal+masker, hearing-impaired listeners should show an increasing benefit from masker fluctuations as the presentation levels increase. Normally hearing and hearing-impaired listeners were tested for sentence recognition at moderate and high presentation levels in a competing speech-shaped noise, in competing speech by a single talker, and in competing time-reversed speech by the same talker. Normal-hearing listeners showed more accurate recognition in the fluctuating maskers than in unmodulated noise with some evidence that modulated:unmodulated performance differences may decrease at high presentation levels. Hearing-impaired listeners showed more similar performance across maskers and presentation levels. The results suggest that audibility does not completely account for the group differences in performance with fluctuating maskers: supra-threshold processing differences between groups may also contribute to the disparity in performance. [Work supported by NIH.]

Frequency selectivity estimated using stimulus-frequency otoacoustic emissions and psychophysical masking

Psychophysical studies of frequency selectivity have concluded that the relative bandwidths of auditory filters decrease only slightly with increasing center frequency above 1 kHz. In contrast, estimates based on measurements of stimulus-frequency otoacoustic emissions (SFOAEs) in humans and other mammals suggest that frequency selectivity improves significantly with increasing center frequency. Psychophysical measures have mostly used simultaneous masking, which can be influenced by suppression. Since suppressive effects are stronger at high frequencies, these techniques may systematically overestimate filter bandwidths at high center frequencies. This study investigates whether psychophysical and physiological estimates of filter bandwidth can be reconciled if suppressive effects are eliminated by using forward masking. Filter bandwidths at center frequencies of 1, 2, 4, 6, and 8 kHz were measured using the notch-noise technique in simultaneous and forward masking with the signal level fixed at 10 and 30 dB above threshold in quiet. Relative bandwidths estimated in the same subjects using measurements of SFOAE group delay at 40-dB SPL were consistent with earlier results. The psychophysical and physiological estimates of relative bandwidth are generally consistent, suggesting that the relationship between center frequency and filter bandwidth found in previous psychophysical studies may have been influenced by suppression. [Work supported by NIDCD.]

Transient otoacoustic emissions and frequency selectivity

The relationship between transient evoked otoacoustic emissions (TEOAE) and frequency selectivity in the auditory system was examined. Three subjects with normal hearing sensitivity and TEOAE spectra with prominent peaks and valleys participated. Psychophysical tuning curves were measured at two pairs of frequencies: one where a robust emission occurs, and the other for an emission with a signal-to-noise ratio at least 6 dB SPL lower than the first. A forward masking paradigm with a 2AFC, 3 up – 1 down adaptive procedure was used to construct three point tuning curves. Quiet threshold for a given probe frequency was established first. The tip of the tuning curve was then obtained by determining the masker level for P(C)=79.1 with a probe at 10 dB SL. Finally, the Q—10 frequencies were acquired by adapting the frequency of the masker, which was set 10 dB above the tuning curve tip [Spetner and Olsho, 1990]. Preliminary results indicate that the Q—10 values for the tuning curves from the TEOAE peak frequencies are consistently greater than those of the tuning curves from valley frequencies. These results suggest that a relationship exists between frequency selectivity in the auditory system and the relative minima and maxima of TEOAE patterns. [Research supported by a grant from The Ohio State University College of Social and Behavioral Science.]