Review article: review of the literature on temporal resolution in listeners with cochlear hearing impairment: a critical assessment of the role of suprathreshold deficits

Perception of voice cues in school-age children with hearing aids

The just-noticeable differences (JNDs) of the voice cues of voice pitch (F0) and vocal-tract length (VTL) were measured in school-aged children with bilateral hearing aids and children and adults with normal hearing. The JNDs were larger for hearing-aided than normal-hearing children up to the age of 12 for F0 and into adulthood for all ages for VTL. Age was a significant factor for both groups for F0 JNDs, but only for the hearing-aided group for VTL JNDs. Age of maturation was later for F0 than VTL. Individual JNDs of the two groups largely overlapped for F0, but little for VTL. Hearing thresholds (unaided or aided, 500-400 Hz, overlapping with mid-range speech frequencies) did not correlate with the JNDs. However, extended low-frequency hearing thresholds (unaided, 125-250 Hz, overlapping with voice F0 ranges) correlated with the F0 JNDs. Hence, age and hearing status differentially interact with F0 and VTL perception, and VTL perception seems challenging for hearing-aided children. On the other hand, even children with profound hearing loss could do the task, indicating a hearing aid benefit for voice perception. Given the significant age effect and that for F0 the hearing-aided children seem to be catching up with age-typical development, voice cue perception may continue developing in hearing-aided children.

A Step Toward Precision Audiology: Individual Differences and Characteristic Profiles From Auditory Perceptual and Cognitive Abilities

Older adults with normal hearing or with age-related hearing loss face challenges when listening to speech in noisy environments. To better serve individuals with communication difficulties, precision diagnostics are needed to characterize individuals' auditory perceptual and cognitive abilities beyond pure tone thresholds. These abilities can be heterogenous across individuals within the same population. The goal of the present study is to consider the suprathreshold variability and develop characteristic profiles for older adults with normal hearing (ONH) and with hearing loss (OHL). Auditory perceptual and cognitive abilities were tested on ONH (n = 20) and OHL (n = 20) on an abbreviated test battery using portable automated rapid testing. Using cluster analyses, three main profiles were revealed for each group, showing differences in auditory perceptual and cognitive abilities despite similar audiometric thresholds. Analysis of variance showed that ONH profiles differed in spatial release from masking, speech-in-babble testing, cognition, tone-in-noise, and binaural temporal processing abilities. The OHL profiles differed in spatial release from masking, speech-in-babble testing, cognition, and tolerance to background noise performance. Correlation analyses showed significant relationships between auditory and cognitive abilities in both groups. This study showed that auditory perceptual and cognitive deficits can be present to varying degrees in the presence of audiometrically normal hearing and among listeners with similar degrees of hearing loss. The results of this study inform the need for taking individual differences into consideration and developing targeted intervention options beyond pure tone thresholds and speech testing.

Auditory enhancement in younger and older listeners with normal and impaired hearinga)

Auditory enhancement is a spectral contrast aftereffect that can facilitate the detection of novel events in an ongoing background. A single-interval paradigm combined with roved frequency content between trials can yield as much as 20 dB enhancement in young normal-hearing listeners. This study compared such enhancement in 15 listeners with sensorineural hearing loss with that in 15 age-matched adults and 15 young adults with normal audiograms. All groups were presented with stimulus levels of 70 dB sound pressure level (SPL) per component. The two groups with normal hearing were also tested at 45 dB SPL per component. The hearing-impaired listeners showed very little enhancement overall. However, when tested at the same high (70-dB) level, both young and age-matched normal-hearing listeners also showed substantially reduced enhancement, relative to that found at 45 dB SPL. Some differences in enhancement emerged between young and older normal-hearing listeners at the lower sound level. The results suggest that enhancement is highly level-dependent and may also decrease somewhat with age or slight hearing loss. Implications for hearing-impaired listeners may include a poorer ability to adapt to real-world acoustic variability, due in part to the higher levels at which sound must be presented to be audible.

The Influence of Sensorineural Hearing Loss on the Relationship Between the Perception of Speech in Noise and Dysarthric Speech

PURPOSE

The ability to understand speech under adverse listening conditions is highly variable across listeners. Despite this, studies have found that listeners with normal hearing display consistency in their ability to perceive speech across different types of degraded speech, suggesting that, for at least these listeners, global skills may be involved in navigating the ambiguity in speech signals. However, there are substantial differences in the perceptual challenges faced by listeners with normal and impaired hearing. This study examines whether listeners with sensorineural hearing loss demonstrate the same type of consistency as normal-hearing listeners when processing neurotypical (i.e., control) speech that has been degraded by external noise and speech that is neurologically degraded such as dysarthria.

METHOD

Listeners with normal hearing (n = 31) and listeners with sensorineural hearing loss (n = 36) completed an intelligibility task with neurotypical speech in noise and with dysarthric speech in quiet.

RESULTS

Findings were consistent with previous work demonstrating a relationship between the ability to perceive neurotypical speech in noise and dysarthric speech for listeners with normal hearing, albeit at a higher intelligibility level than previously observed. This relationship was also observed for listeners with hearing loss, although listeners with more severe hearing losses performed better with dysarthric speech than with neurotypical speech in noise.

CONCLUSIONS

This study demonstrated a high level of consistency in intelligibility performance for listeners across two different types of degraded speech, even when those listeners were further challenged by the presence of sensorineural hearing loss. Clinical implications for both listeners with hearing loss and their communication partners with dysarthria are discussed.

Temporal acuity is preserved in the auditory midbrain of aged mice

Impaired temporal resolution of the central auditory system has long been suggested to contribute to speech understanding deficits in the elderly. However, it has been difficult to differentiate between direct age-related central deficits and indirect effects of confounding peripheral age-related hearing loss on temporal resolution. To differentiate this, we measured temporal acuity in the inferior colliculus (IC) of aged CBA/J and C57BL/6 mice, as a model of aging with and without concomitant hearing loss. We used two common measures of auditory temporal processing: gap detection as a measure of temporal fine structure and amplitude-modulated noise as a measure of envelope sensitivity. Importantly, auditory temporal acuity remained precise in the IC of old CBA/J mice when no or only minimal age-related hearing loss was present. In contrast, temporal acuity was only indirectly reduced by the presence of age-related hearing loss in aged C57BL/6 mice, not by affecting the brainstem precision, but by affecting the signal-to-noise ratio of the neuronal activity in the IC. This demonstrates that indirect effects of age-related peripheral hearing loss likely remain an important factor for temporal processing in aging in comparison to 'pure' central auditory decline itself. It also draws attention to the issue that the threshold difference between 'nearly normal' or 'clinically normal' hearing aging subjects in comparison to normal hearing young subjects still can have indirect effects on central auditory neural representations of temporal processing.

Differences in Auditory Perception Between Young and Older Adults When Controlling for Differences in Hearing Loss and Cognition

This study was designed to examine age effects on various auditory perceptual skills using a large group of listeners (155 adults, 121 aged 60-88 years and 34 aged 18-30 years), while controlling for the factors of hearing loss and working memory (WM). All subjects completed 3 measures of WM, 7 psychoacoustic tasks (24 conditions) and a hearing assessment. Psychophysical measures were selected to tap phenomena thought to be mediated by higher-level auditory function and included modulation detection, modulation detection interference, informational masking (IM), masking level difference (MLD), anisochrony detection, harmonic mistuning, and stream segregation. Principal-components analysis (PCA) was applied to each psychoacoustic test. For 6 of the 7 tasks, a single component represented performance across the multiple stimulus conditions well, whereas the modulation-detection interference (MDI) task required two components to do so. The effect of age was analyzed using a general linear model applied to each psychoacoustic component. Once hearing loss and WM were accounted for as covariates in the analyses, estimated marginal mean thresholds were lower for older adults on tasks based on temporal processing. When evaluated separately, hearing loss led to poorer performance on roughly 1/2 the tasks and declines in WM accounted for poorer performance on 6 of the 8 psychoacoustic components. These results make clear the need to interpret age-group differences in performance on psychoacoustic tasks in light of cognitive declines commonly associated with aging, and point to hearing loss and cognitive declines as negatively influencing auditory perceptual skills.

Identification and Discrimination of Sound Textures in Hearing-Impaired and Older Listeners

Sound textures are a broad class of sounds defined by their homogeneous temporal structure. It has been suggested that sound texture perception is mediated by time-averaged summary statistics measured from early stages of the auditory system. The ability of young normal-hearing (NH) listeners to identify synthetic sound textures increases as the statistics of the synthetic texture approach those of its real-world counterpart. In sound texture discrimination, young NH listeners utilize the fine temporal stimulus information for short-duration stimuli, whereas they switch to a time-averaged statistical representation as the stimulus' duration increases. The present study investigated how younger and older listeners with a sensorineural hearing impairment perform in the corresponding texture identification and discrimination tasks in which the stimuli were amplified to compensate for the individual listeners' loss of audibility. In both hearing impaired (HI) listeners and NH controls, sound texture identification performance increased as the number of statistics imposed during the synthesis stage increased, but hearing impairment was accompanied by a significant reduction in overall identification accuracy. Sound texture discrimination performance was measured across listener groups categorized by age and hearing loss. Sound texture discrimination performance was unaffected by hearing loss at all excerpt durations. The older listeners' sound texture and exemplar discrimination performance decreased for signals of short excerpt duration, with older HI listeners performing better than older NH listeners. The results suggest that the time-averaged statistic representations of sound textures provide listeners with cues which are robust to the effects of age and sensorineural hearing loss.

Factors Underlying Individual Differences in Speech-Recognition Threshold (SRT) in Noise Among Older Adults

Many older adults have difficulty understanding speech in noisy backgrounds. In this study, we examined peripheral auditory, higher-level auditory, and cognitive factors that may contribute to such difficulties. A convenience sample of 137 volunteer older adults, 90 women, and 47 men, ranging in age from 47 to 94 years (M = 69.2 and SD = 10.1 years) completed a large battery of tests. Auditory tests included measures of pure-tone threshold, clinical and psychophysical, as well as two measures of gap-detection threshold and four measures of temporal-order identification. The latter included two monaural and two dichotic listening conditions. In addition, cognition was assessed using the complete Wechsler Adult Intelligence Scale-3rd Edition (WAIS-III). Two monaural measures of speech-recognition threshold (SRT) in noise, the QuickSIN, and the WIN, were obtained from each ear at relatively high presentation levels of 93 or 103 dB SPL to minimize audibility concerns. Group data, both aggregate and by age decade, were evaluated initially to allow comparison to data in the literature. Next, following the application of principal-components factor analysis for data reduction, individual differences in speech-recognition-in-noise performance were examined using multiple-linear-regression analyses. Excellent fits were obtained, accounting for 60-77% of the total variance, with most accounted for by the audibility of the speech and noise stimuli and the severity of hearing loss with the balance primarily associated with cognitive function.

Conductive hearing loss during development does not appreciably alter the sharpness of cochlear tuning

An increasing number of studies show that listeners often have difficulty hearing in situations with background noise, despite normal tuning curves in quiet. One potential source of this difficulty could be sensorineural changes in the auditory periphery (the ear). Signal in noise detection deficits also arise in animals raised with developmental conductive hearing loss (CHL), a manipulation that induces acoustic attenuation to model how sound deprivation changes the central auditory system. This model attributes perceptual deficits to central changes by assuming that CHL does not affect sensorineural elements in the periphery that could raise masked thresholds. However, because of efferent feedback, altering the auditory system could affect cochlear elements. Indeed, recent studies show that adult-onset CHL can cause cochlear synapse loss, potentially calling into question the assumption of an intact periphery in early-onset CHL. To resolve this issue, we tested the long-term peripheral effects of CHL via developmental bilateral malleus displacement. Using forward masking tuning curves, we compared peripheral tuning in animals raised with CHL vs age-matched controls. Using compound action potential measurements from the round window, we assessed inner hair cell synapse integrity. Results indicate that developmental CHL can cause minor synaptopathy. However, developmental CHL does not appreciably alter peripheral frequency tuning.

Changes in audiometric threshold and frequency selectivity correlate with cochlear histopathology in macaque monkeys with permanent noise-induced hearing loss

Exposure to loud noise causes damage to the inner ear, including but not limited to outer and inner hair cells (OHCs and IHCs) and IHC ribbon synapses. This cochlear damage impairs auditory processing and increases audiometric thresholds (noise-induced hearing loss, NIHL). However, the exact relationship between the perceptual consequences of NIHL and its underlying cochlear pathology are poorly understood. This study used a nonhuman primate model of NIHL to relate changes in frequency selectivity and audiometric thresholds to indices of cochlear histopathology. Three macaques (one Macaca mulatta and two Macaca radiata) were trained to detect tones in quiet and in noises that were spectrally notched around the tone frequency. Audiograms were derived from tone thresholds in quiet; perceptual auditory filters were derived from tone thresholds in notched-noise maskers using the rounded-exponential fit. Data were obtained before and after a four-hour exposure to a 50-Hz noise centered at 2 kHz at 141 or 146 dB SPL. Noise exposure caused permanent audiometric threshold shifts and broadening of auditory filters at and above 2 kHz, with greater changes observed for the 146-dB-exposed monkeys. The normalized bandwidth of the perceptual auditory filters was strongly correlated with audiometric threshold at each tone frequency. While changes in audiometric threshold and perceptual auditory filter widths were primarily determined by the extent of OHC survival, additional variability was explained by including interactions among OHC, IHC, and ribbon synapse survival. This is the first study to provide within-subject comparisons of auditory filter bandwidths in an animal model of NIHL and correlate these NIHL-related perceptual changes with cochlear histopathology. These results expand the foundations for ongoing investigations of the neural correlates of NIHL-related perceptual changes.

Contributions to Speech-Cue Weighting in Older Adults With Impaired Hearing

Purpose In a previous paper (Souza, Wright, Blackburn, Tatman, & Gallun, 2015), we explored the extent to which individuals with sensorineural hearing loss used different cues for speech identification when multiple cues were available. Specifically, some listeners placed the greatest weight on spectral cues (spectral shape and/or formant transition), whereas others relied on the temporal envelope. In the current study, we aimed to determine whether listeners who relied on temporal envelope did so because they were unable to discriminate the formant information at a level sufficient to use it for identification and the extent to which a brief discrimination test could predict cue weighting patterns. Method Participants were 30 older adults with bilateral sensorineural hearing loss. The first task was to label synthetic speech tokens based on the combined percept of temporal envelope rise time and formant transitions. An individual profile was derived from linear discriminant analysis of the identification responses. The second task was to discriminate differences in either temporal envelope rise time or formant transitions. The third task was to discriminate spectrotemporal modulation in a nonspeech stimulus. Results All listeners were able to discriminate temporal envelope rise time at levels sufficient for the identification task. There was wide variability in the ability to discriminate formant transitions, and that ability predicted approximately one third of the variance in the identification task. There was no relationship between performance in the identification task and either amount of hearing loss or ability to discriminate nonspeech spectrotemporal modulation. Conclusions The data suggest that listeners who rely to a greater extent on temporal cues lack the ability to discriminate fine-grained spectral information. The fact that the amount of hearing loss was not associated with the cue profile underscores the need to characterize individual abilities in a more nuanced way than can be captured by the pure-tone audiogram.

Auditory enhancement under forward masking in normal-hearing and hearing-impaired listeners

A target within a spectrally notched masker can be enhanced by a preceding copy of the masker. Enhancement can also increase the effectiveness of the target as a forward masker. Enhancement has been reported in hearing-impaired listeners under simultaneous but not forward masking. However, previous studies of enhancement under forward masking did not fully assess the potential effect of differences in sensation level or spectral resolution between the normal-hearing and hearing-impaired listeners. This study measured enhancement via forward masking in hearing-impaired and age-matched normal-hearing listeners with different spectral notches in the masker, to account for potential differences in frequency selectivity, and with levels equated by adding a background masking noise to equate both sensation level and sound pressure level or by reducing the sound pressure level of the stimuli to equate sensation level. Hearing-impaired listeners showed no significant enhancement, regardless of spectral notch width. Normal-hearing listeners showed enhancement at high levels, but showed less enhancement when sensation levels were reduced to match those of the hearing-impaired group, either by reducing sound levels or by adding a masking noise. The results confirm a lack of forward-masked enhancement in hearing-impaired listeners but suggest this may be partly due to reduced sensation level.

Two-Point Method for Measuring the Temporal Modulation Transfer Function

OBJECTIVE

The temporal modulation transfer function (TMTF) has been proposed to estimate the temporal resolution abilities of listeners with normal hearing and listeners with hearing loss. The TMTF data of patients would be useful for clinical diagnosis and for adjusting the hearing instruments at clinical and fitting sites. However, practical application is precluded by the long measurement time of the conventional method, which requires several measurement points. This article presents a new method to measure the TMTF that requires only two measurement points.

DESIGN

Experiments were performed to estimate the TMTF of normal listeners and listeners with hearing loss to demonstrate that the two-point method can estimate the TMTF parameter and the conventional method. Sixteen normal hearing and 21 subjects with hearing loss participated, and the difference between the estimated TMTF parameters and measurement time were compared.

RESULTS

The TMTF parameters (the peak sensitivity Lps and cutoff frequency fcutoff) estimated by the conventional and two-point methods showed significantly high correlations: the correlation coefficient for Lps was 0.91 (t(45) = 14.3; p < 10) and that for fcutoff was 0.89 (t(45) = 13.2; p < 10). There were no fixed and proportional biases. Therefore, the estimated values were in good agreement. Moreover, there was no systematic bias depending on the subject's profile. The measurement time of the two-point method was approximately 10 min, which is approximately one-third that of the conventional method.

CONCLUSION

The two-point method enables the introduction of TMTF measurement in clinical diagnosis.

Auditory temporal resolution in adaptive tasks. Gap detection investigation

OBJECTIVES

To study auditory temporal resolution skills using adaptive auditory tasks designed with a computer-based experimental program, and to provide normative valuesfor gap detection thresholds (GDTs) of young listeners in 3 listening conditions. Methods:  The GDTs were established under 3 stimulus conditions: 1) broadband noise (BBN), 2) narrowband noise within-channel (NBN WC) using similar leading and trailing markers centered at 1.0 KHz, and 3) narrowband noise across-channel (NBN AC) with the leading marker centered at 2.0 KHz and the trailing marker centered at 1.0 KHz. In within-subjects design, the GDTs were obtained from 27 normal hearing young Saudi adults at Speech and Hearing Laboratories, Department of Rehabilitation Health Sciences, College of Applied Medical Sciences, King Saud University, Riyadh, Saudi Arabia between April 2017 and April 2018. Results: The mean GDTs for the BBN condition was 3.19 millisecsond (msec), NBN WC was 14.53 msec, and NBN AC was 29.71 msec. Our findings for the GDTs measured in the 3 conditions were consistent with those of earlier investigations that used different instrumentations. Also, no correlations among the GDTs of the 3 stimulus conditions were found. Conclusion: The present study showed that experimental program is a reliable tool with clinical potential to estimate GDTs across different conditions. Also, the findings of no correlations in the GDTs across stimulus conditions suggest that different processes were involved in the perception of the temporal gap for different stimulus conditions.

Auditory enhancement under simultaneous masking in normal-hearing and hearing-impaired listeners

Auditory enhancement, where a target sound within a masker is rendered more audible by the prior presentation of the masker alone, may play an important role in auditory perception under variable everyday acoustic conditions. Cochlear hearing loss may reduce enhancement effects, potentially contributing to the difficulties experienced by hearing-impaired (HI) individuals in noisy and reverberant environments. However, it remains unknown whether, and by how much, enhancement under simultaneous masking is reduced in HI listeners. Enhancement of a pure tone under simultaneous masking with a multi-tone masker was measured in HI listeners and age-matched normal-hearing (NH) listeners as function of the spectral notch width of the masker, using stimuli at equal sensation levels as well as at equal sound pressure levels, but with the stimuli presented in noise to the NH listeners to maintain the equal sensation level between listener groups. The results showed that HI listeners exhibited some enhancement in all conditions. However, even when conditions were made as comparable as possible, in terms of effective spectral notch width and presentation level, the enhancement effect in HI listeners under simultaneous masking was reduced relative to that observed in NH listeners.

Comparison of Clinical and Traditional Gap Detection Tests

BACKGROUND

Temporal resolution is important for speech recognition and may contribute to variability in speech recognition among patients. Clinical tests of temporal resolution are available, but it is not clear how closely results of those tests correspond to results of traditional temporal resolution tests.

PURPOSE

The purpose of this study was to compare the Gaps-in-Noise (GIN) test to a traditional measure of gap detection.

STUDY SAMPLE

This study included older adults with hearing loss and younger adults with normal hearing.

DATA COLLECTION AND ANALYSIS

Participants completed one practice and two test blocks of each gap detection test, and a measure of speech-in-noise recognition. Individual data were correlated to examine the relationship between the tests.

RESULTS

The GIN and traditional gap detection were significantly, but not highly correlated. The traditional gap detection test contributed to variance in speech recognition in noise, while the GIN did not.

CONCLUSIONS

The brevity and ease of implementing the GIN in the clinic make it a viable test of temporal resolution. However, it differs from traditional measures in implementation, and as a result relies on different cognitive factors. The GIN thresholds should be interpreted carefully and not presumed to represent an approximation of traditional gap detection thresholds.

Psychoacoustic Characteristics of Tinnitus versus Temporal Resolution in Subjects with Normal Hearing Sensitivity

Introduction Cochlear or neural mechanisms of tinnitus generation may affect auditory temporal resolution in tinnitus patients even with normal audiometry. Thus, studying the correlation between tinnitus characteristics and auditory temporal resolution in subjects with tinnitus may help in proper modification of tinnitus management strategy.

Objective This study aims to examine the relationship between the psychoacoustic measures of tinnitus and the auditory temporal resolution in subjects with normal audiometry.

Methods Two normal hearing groups with ages ranging from 20 to 45 years were involved: control group of 15 adults (30 ears) without tinnitus and study group of 15 adults (24 ears) with tinnitus. Subjective scaling of annoyance and sleep disturbance caused by tinnitus, basic audiological evaluation, tinnitus psychoacoustic measures and Gaps in Noise test were performed. Data from both groups were compared using independent sample t-test. Psychoacoustic measures of tinnitus and Gaps in Noise test parameters of the tinnitus group were correlated with Pearson's correlation coefficient.

Results Significantly higher hearing threshold, higher approximate threshold and lower correct Gaps in Noise scores were observed in tinnitus ears. There was no significant correlation between psychoacoustic measures of tinnitus and Gaps in Noise test parameters of the tinnitus group.

Conclusion Auditory temporal resolution impairment was found in tinnitus patients, which could be attributed to cochlear impairment or altered neural firing within the auditory pathway. It is recommended to include temporal resolution testing in the tinnitus evaluation battery to provide a proper management planning.

Effect of conductive hearing loss on central auditory function

Introduction

It has been demonstrated that long-term Conductive Hearing Loss (CHL) may influence the precise detection of the temporal features of acoustic signals or Auditory Temporal Processing (ATP). It can be argued that ATP may be the underlying component of many central auditory processing capabilities such as speech comprehension or sound localization. Little is known about the consequences of CHL on temporal aspects of central auditory processing.

Objective

This study was designed to assess auditory temporal processing ability in individuals with chronic CHL.

Methods

During this analytical cross-sectional study, 52 patients with mild to moderate chronic CHL and 52 normal-hearing listeners (control), aged between 18 and 45 year-old, were recruited. In order to evaluate auditory temporal processing, the Gaps-in-Noise (GIN) test was used. The results obtained for each ear were analyzed based on the gap perception threshold and the percentage of correct responses.

Results

The average of GIN thresholds was significantly smaller for the control group than for the CHL group for both ears (right: p = 0.004; left: p < 0.001). Individuals with CHL had significantly lower correct responses than individuals with normal hearing for both sides (p < 0.001). No correlation was found between GIN performance and degree of hearing loss in either group (p > 0.05).

Conclusion

The results suggest reduced auditory temporal processing ability in adults with CHL compared to normal hearing subjects. Therefore, developing a clinical protocol to evaluate auditory temporal processing in this population is recommended.

ON and OFF inhibition as mechanisms for forward masking in the inferior colliculus: a modeling study

Masking effects of a preceding stimulus on the detection or perception of a signal have been found in several sensory systems in mammals, including humans and rodents. In the auditory system, it has been hypothesized that a central "OFF-inhibitory" mechanism, which is generated by neurons that respond after a sound is terminated, may contribute to the observed psychophysics. The present study constructed a systems model for the inferior colliculus that includes major ascending monaural and binaural auditory pathways. The fundamental characteristics of several neuron types along the pathways were captured by Hodgkin-Huxley models with specific membrane and synaptic properties. OFF responses were reproduced with a model of the superior paraolivary nucleus containing a hyperpolarization-activated h current and a T-type calcium current. When the gap between the end of the masker and the onset of the signal was large, e.g., >5 ms, OFF inhibition generated strong suppressive effects on the signal response. For smaller gaps, an additional inhibitory source, which was modeled as ON inhibition from the contralateral dorsal nucleus of the lateral lemniscus, showed the potential of explaining the psychophysics. Meanwhile, the effect of a forward masker on the binaural sensitivity to a low-frequency signal was examined, which was consistent with previous psychophysical findings related to sound localization.

The impact of degree of hearing loss on auditory brainstem response predictions of behavioral thresholds

OBJECTIVES

Diagnosis of hearing loss and prescription of amplification for infants and young children require accurate estimates of ear- and frequency-specific behavioral thresholds based on auditory brainstem response (ABR) measurements. Although the overall relationship between ABR and behavioral thresholds has been demonstrated, the agreement is imperfect, and the accuracy of predictions of behavioral threshold based on ABR may depend on degree of hearing loss. Behavioral thresholds are lower than ABR thresholds, at least in part due to differences in calibration interacting with the effects of temporal integration, which are manifest in behavioral measurements but not ABR measurements and depend on behavioral threshold. Listeners with sensory hearing loss exhibit reduced or absent temporal integration, which could impact the relationship between ABR and behavioral thresholds as degree of hearing loss increases. The present study evaluated the relationship between ABR and behavioral thresholds in infants and children over a range of hearing thresholds, and tested an approach for adjusting the correction factor based on degree of hearing loss as estimated by ABR measurements.

DESIGN

A retrospective review of clinical records was completed for 309 ears of 177 children with hearing thresholds ranging from normal to profound hearing loss and for whom both ABR and behavioral thresholds were available. Children were required to have the same middle ear status at both evaluations. The relationship between ABR and behavioral thresholds was examined. Factors that potentially could affect the relationship between ABR and behavioral thresholds were analyzed, including degree of hearing loss observed on the ABR, behavioral test method (visual reinforcement, conditioned play, or conventional audiometry), the length of time between ABR and behavioral assessments, and clinician-reported reliability of the behavioral assessment. Predictive accuracy of a correction factor based on the difference between ABR and behavioral thresholds as a function of ABR threshold was compared to the predictive accuracy achieved by two other correction approaches in current clinical use.

RESULTS

As expected, ABR threshold was a significant predictor of behavioral threshold. The agreement between ABR and behavioral thresholds varied as a function of degree of hearing loss. The test method, length of time between assessments, and reported reliability of the behavioral test results were not related to the differences between ABR and behavioral thresholds. A correction factor based on the linear relationship between the differences in ABR and behavioral thresholds as a function of ABR threshold resulted in more accurately predicted behavioral thresholds than other correction factors in clinical use.

CONCLUSIONS

ABR is a valid predictor of behavioral threshold in infants and children. A correction factor that accounts for the effect of degree of hearing loss on the differences between ABR and behavioral thresholds resulted in more accurate predictions of behavioral thresholds than methods that used a constant correction factor regardless of degree of hearing loss. These results are consistent with predictions based on previous research on temporal integration for listeners with hearing loss.

PsyAcoustX: A flexible MATLAB(®) package for psychoacoustics research

The demands of modern psychophysical studies require precise stimulus delivery and flexible platforms for experimental control. Here, we describe PsyAcoustX, a new, freely available suite of software tools written in the MATLAB(®) environment to conduct psychoacoustics research on a standard PC. PsyAcoustX provides a flexible platform to generate and present auditory stimuli in real time and record users' behavioral responses. Data are automatically logged by stimulus condition and aggregated in an exported spreadsheet for offline analysis. Detection thresholds can be measured adaptively under basic and complex auditory masking tasks and other paradigms (e.g., amplitude modulation detection) within minutes. The flexibility of the module offers experimenters access to nearly every conceivable combination of stimulus parameters (e.g., probe-masker relations). Example behavioral applications are highlighted including the measurement of audiometric thresholds, basic simultaneous and non-simultaneous (i.e., forward and backward) masking paradigms, gap detection, and amplitude modulation detection. Examples of these measurements are provided including the psychoacoustic phenomena of temporal overshoot, psychophysical tuning curves, and temporal modulation transfer functions. Importantly, the core design of PsyAcoustX is easily modifiable, allowing users the ability to adapt its basic structure and create additional modules for measuring discrimination/detection thresholds for other auditory attributes (e.g., pitch, intensity, etc.) or binaural paradigms.

Across-frequency processing of modulation phase differences in hearing-impaired listeners

Two experiments tested the influence of hearing impairment (HI) on representing across-frequency temporal coherence. In one experiment, HI listeners demonstrated similar abilities to normal-hearing listeners in detecting across-frequency differences in modulation phase. In another, spectral-shape discrimination was detrimentally affected by modulation phase disparities imposed on spectral components. Spectral-shape discrimination by HI listeners was less influenced by the disparities, suggesting that hearing loss alters the representation of envelope phase. Results suggest that multiple approaches may be necessary to determine alterations associated with hearing loss—detection tasks may not be sufficient to elucidate distortions to temporal envelope associated with hearing loss.

Auditory gap-in-noise detection behavior in ferrets and humans

The precise encoding of temporal features of auditory stimuli by the mammalian auditory system is critical to the perception of biologically important sounds, including vocalizations, speech, and music. In this study, auditory gap-detection behavior was evaluated in adult pigmented ferrets (Mustelid putorius furo) using bandpassed stimuli designed to widely sample the ferret's behavioral and physiological audiogram. Animals were tested under positive operant conditioning, with psychometric functions constructed in response to gap-in-noise lengths ranging from 3 to 270 ms. Using a modified version of this gap-detection task, with the same stimulus frequency parameters, we also tested a cohort of normal-hearing human subjects. Gap-detection thresholds were computed from psychometric curves transformed according to signal detection theory, revealing that for both ferrets and humans, detection sensitivity was worse for silent gaps embedded within low-frequency noise compared with high-frequency or broadband stimuli. Additional psychometric function analysis of ferret behavior indicated effects of stimulus spectral content on aspects of behavioral performance related to decision-making processes, with animals displaying improved sensitivity for broadband gap-in-noise detection. Reaction times derived from unconditioned head-orienting data and the time from stimulus onset to reward spout activation varied with the stimulus frequency content and gap length, as well as the approach-to-target choice and reward location. The present study represents a comprehensive evaluation of gap-detection behavior in ferrets, while similarities in performance with our human subjects confirm the use of the ferret as an appropriate model of temporal processing.

Tinnitus: animal models and findings in humans

Chronic tinnitus (ringing of the ears) is a medically untreatable condition that reduces quality of life for millions of individuals worldwide. Most cases are associated with hearing loss that may be detected by the audiogram or by more sensitive measures. Converging evidence from animal models and studies of human tinnitus sufferers indicates that, while cochlear damage is a trigger, most cases of tinnitus are not generated by irritative processes persisting in the cochlea but by changes that take place in central auditory pathways when auditory neurons lose their input from the ear. Forms of neural plasticity underlie these neural changes, which include increased spontaneous activity and neural gain in deafferented central auditory structures, increased synchronous activity in these structures, alterations in the tonotopic organization of auditory cortex, and changes in network behavior in nonauditory brain regions detected by functional imaging of individuals with tinnitus and corroborated by animal investigations. Research on the molecular mechanisms that underlie neural changes in tinnitus is in its infancy and represents a frontier for investigation.

Individual sensitivity to spectral and temporal cues in listeners with hearing impairment

PURPOSE

The present study was designed to evaluate use of spectral and temporal cues under conditions in which both types of cues were available.

METHOD

Participants included adults with normal hearing and hearing loss. We focused on 3 categories of speech cues: static spectral (spectral shape), dynamic spectral (formant change), and temporal (amplitude envelope). Spectral and/or temporal dimensions of synthetic speech were systematically manipulated along a continuum, and recognition was measured using the manipulated stimuli. Level was controlled to ensure cue audibility. Discriminant function analysis was used to determine to what degree spectral and temporal information contributed to the identification of each stimulus.

RESULTS

Listeners with normal hearing were influenced to a greater extent by spectral cues for all stimuli. Listeners with hearing impairment generally utilized spectral cues when the information was static (spectral shape) but used temporal cues when the information was dynamic (formant transition). The relative use of spectral and temporal dimensions varied among individuals, especially among listeners with hearing loss.

CONCLUSION

Information about spectral and temporal cue use may aid in identifying listeners who rely to a greater extent on particular acoustic cues and applying that information toward therapeutic interventions.

Speech perception in older hearing impaired listeners: benefits of perceptual training

Hearing aids (HAs) only partially restore the ability of older hearing impaired (OHI) listeners to understand speech in noise, due in large part to persistent deficits in consonant identification. Here, we investigated whether adaptive perceptual training would improve consonant-identification in noise in sixteen aided OHI listeners who underwent 40 hours of computer-based training in their homes. Listeners identified 20 onset and 20 coda consonants in 9,600 consonant-vowel-consonant (CVC) syllables containing different vowels (/ɑ/, /i/, or /u/) and spoken by four different talkers. Consonants were presented at three consonant-specific signal-to-noise ratios (SNRs) spanning a 12 dB range. Noise levels were adjusted over training sessions based on d’ measures. Listeners were tested before and after training to measure (1) changes in consonant-identification thresholds using syllables spoken by familiar and unfamiliar talkers, and (2) sentence reception thresholds (SeRTs) using two different sentence tests. Consonant-identification thresholds improved gradually during training. Laboratory tests of d’ thresholds showed an average improvement of 9.1 dB, with 94% of listeners showing statistically significant training benefit. Training normalized consonant confusions and improved the thresholds of some consonants into the normal range. Benefits were equivalent for onset and coda consonants, syllables containing different vowels, and syllables presented at different SNRs. Greater training benefits were found for hard-to-identify consonants and for consonants spoken by familiar than unfamiliar talkers. SeRTs, tested with simple sentences, showed less elevation than consonant-identification thresholds prior to training and failed to show significant training benefit, although SeRT improvements did correlate with improvements in consonant thresholds. We argue that the lack of SeRT improvement reflects the dominant role of top-down semantic processing in processing simple sentences and that greater transfer of benefit would be evident in the comprehension of more unpredictable speech material.

Gap detection and temporal modulation transfer function as behavioral estimates of auditory temporal acuity using band-limited stimuli in young and older adults

PURPOSE

Gap detection and the temporal modulation transfer function (TMTF) are 2 common methods to obtain behavioral estimates of auditory temporal acuity. However, the agreement between the 2 measures is not clear. This study compares results from these 2 methods and their dependencies on listener age and hearing status.

METHOD

Gap detection thresholds and the parameters that describe the TMTF (sensitivity and cutoff frequency) were estimated for young and older listeners who were naive to the experimental tasks. Stimuli were 800-Hz-wide noises with upper frequency limits of 2400 Hz, presented at 85 dB SPL. A 2-track procedure (Shen & Richards, 2013) was used for the efficient estimation of the TMTF.

RESULTS

No significant correlation was found between gap detection threshold and the sensitivity or the cutoff frequency of the TMTF. No significant effect of age and hearing loss on either the gap detection threshold or the TMTF cutoff frequency was found, while the TMTF sensitivity improved with increasing hearing threshold and worsened with increasing age.

CONCLUSION

Estimates of temporal acuity using gap detection and TMTF paradigms do not seem to provide a consistent description of the effects of listener age and hearing status on temporal envelope processing.

Prediction of consonant recognition in quiet for listeners with normal and impaired hearing using an auditory model

Consonant recognition was assessed in normal-hearing (NH) and hearing-impaired (HI) listeners in quiet as a function of speech level using a nonsense logatome test. Average recognition scores were analyzed and compared to recognition scores of a speech recognition model. In contrast to commonly used spectral speech recognition models operating on long-term spectra, a "microscopic" model operating in the time domain was used. Variations of the model (accounting for hearing impairment) and different model parameters (reflecting cochlear compression) were tested. Using these model variations this study examined whether speech recognition performance in quiet is affected by changes in cochlear compression, namely, a linearization, which is often observed in HI listeners. Consonant recognition scores for HI listeners were poorer than for NH listeners. The model accurately predicted the speech reception thresholds of the NH and most HI listeners. A partial linearization of the cochlear compression in the auditory model, while keeping audibility constant, produced higher recognition scores and improved the prediction accuracy. However, including listener-specific information about the exact form of the cochlear compression did not improve the prediction further.

Auditory and tactile gap discrimination by observers with normal and impaired hearing

Temporal processing ability for the senses of hearing and touch was examined through the measurement of gap-duration discrimination thresholds (GDDTs) employing the same low-frequency sinusoidal stimuli in both modalities. GDDTs were measured in three groups of observers (normal-hearing, hearing-impaired, and normal-hearing with simulated hearing loss) covering an age range of 21-69 yr. GDDTs for a baseline gap of 6 ms were measured for four different combinations of 100-ms leading and trailing markers (250-250, 250-400, 400-250, and 400-400 Hz). Auditory measurements were obtained for monaural presentation over headphones and tactile measurements were obtained using sinusoidal vibrations presented to the left middle finger. The auditory GDDTs of the hearing-impaired listeners, which were larger than those of the normal-hearing observers, were well-reproduced in the listeners with simulated loss. The magnitude of the GDDT was generally independent of modality and showed effects of age in both modalities. The use of different-frequency compared to same-frequency markers led to a greater deterioration in auditory GDDTs compared to tactile GDDTs and may reflect differences in bandwidth properties between the two sensory systems.

Nonadditivity of forward and simultaneous masking

The current study measured the additional masking obtained for combinations of forward and simultaneous maskers as a function of forward masker bandwidth, signal delay, and simultaneous masker level. The effects of the two individual maskers were equated in all conditions. Additional masking increased with increasing masker level, increasing signal delay, and decreasing masker bandwidth. The portion of the simultaneous masker that made the greater contribution to additional masking was the part that overlapped with the signal, not with the forward masker. The changes in additional masking observed as a function of forward masker bandwidth and the interaction between the effects of forward and simultaneous maskers call into question the use of additional masking as a measure of basilar membrane compression and present problems for the use of simultaneous noise to simulate hearing loss.

Effects of forward masking on sound localization in cats: basic findings with broadband maskers

Forward masking is traditionally measured with a detection task in which the addition of a preceding masking sound results in an increased signal-detection threshold. Little is known about the influence of forward masking on localization of free-field sound for human or animal subjects. Here we recorded gaze shifts of two head-unrestrained cats during localization using a search-coil technique. A broadband (BB) noise masker was presented straight ahead. A brief signal could come from 1 of the 17 speaker locations in the frontal hemifield. The signal was either a BB or a band-limited (BL) noise. For BB targets, the presence of the forward masker reduced localization accuracy at almost all target levels (20 to 80 dB SPL) along both horizontal and vertical dimensions. Temporal decay of masking was observed when a 15-ms interstimulus gap was added between the end of the masker and the beginning of the target. A large effect of forward masking was also observed for BL targets with low (0.2-2 kHz) and mid (2-7 kHz) frequencies, indicating that the interaural timing cue is susceptible to forward masking. Except at low sound levels, a small or little effect was observed for high-frequency (7-15 kHz) targets, indicating that the interaural level and the spectral cues in that frequency range remained relatively robust. Our findings suggest that different localization mechanisms can operate independently in a complex listening environment.

Musicians change their tune: how hearing loss alters the neural code

Individuals with sensorineural hearing loss have difficulty understanding speech, especially in background noise. This deficit remains even when audibility is restored through amplification, suggesting that mechanisms beyond a reduction in peripheral sensitivity contribute to the perceptual difficulties associated with hearing loss. Given that normal-hearing musicians have enhanced auditory perceptual skills, including speech-in-noise perception, coupled with heightened subcortical responses to speech, we aimed to determine whether similar advantages could be observed in middle-aged adults with hearing loss. Results indicate that musicians with hearing loss, despite self-perceptions of average performance for understanding speech in noise, have a greater ability to hear in noise relative to nonmusicians. This is accompanied by more robust subcortical encoding of sound (e.g., stimulus-to-response correlations and response consistency) as well as more resilient neural responses to speech in the presence of background noise (e.g., neural timing). Musicians with hearing loss also demonstrate unique neural signatures of spectral encoding relative to nonmusicians: enhanced neural encoding of the speech-sound's fundamental frequency but not of its upper harmonics. This stands in contrast to previous outcomes in normal-hearing musicians, who have enhanced encoding of the harmonics but not the fundamental frequency. Taken together, our data suggest that although hearing loss modifies a musician's spectral encoding of speech, the musician advantage for perceiving speech in noise persists in a hearing-impaired population by adaptively strengthening underlying neural mechanisms for speech-in-noise perception.

Temporal modulation transfer function for efficient assessment of auditory temporal resolution

Two common measures of auditory temporal resolution are the temporal modulation transfer function (TMTF) and the gap detection threshold (GDT). The current study addresses the lack of efficient psychophysical procedures for collecting TMTFs and the lack of literature on the comparisons of TMTF and GDT. Two procedures for efficient measurements of the TMTF are proposed: (1) A Bayesian procedure that adaptively chooses the stimulus modulation rate and depth to maximize the information gain from each trial and (2) a procedure that reduces the data collection to two adaptive staircase tracks. Results from experiments I and II showed that, for broadband carriers, these approaches provided similar results compared to TMTFs measured using traditional methods despite taking less than 10 min for data collection. Using these efficient procedures, TMTFs were measured from a large number of naive listeners and were compared to the gap detection thresholds collected from the same ears in experiment III. Results showed that the sensitivity parameter estimated from the TMTF measurements correlated well with the GDTs, whereas the cutoff rate is either uncorrelated or positively correlated with the gap detection threshold. These results suggest caution in interpreting a lower GDT as evidence for less sluggish temporal processing.

Word recognition for temporally and spectrally distorted materials: the effects of age and hearing loss

OBJECTIVES

The purpose of Experiment 1 was to measure word recognition in younger adults with normal hearing when speech or babble was temporally or spectrally distorted. In Experiment 2, older listeners with near-normal hearing and with hearing loss (for pure tones) were tested to evaluate their susceptibility to changes in speech level and distortion types. The results across groups and listening conditions were compared to assess the extent to which the effects of the distortions on word recognition resembled the effects of age-related differences in auditory processing or pure-tone hearing loss.

DESIGN

In Experiment 1, word recognition was measured in 16 younger adults with normal hearing using Northwestern University Auditory Test No. 6 words in quiet and the Words-in-Noise test distorted by temporal jittering, spectral smearing, or combined jittering and smearing. Another 16 younger adults were evaluated in four conditions using the Words-in-Noise test in combinations of unaltered or jittered speech and unaltered or jittered babble. In Experiment 2, word recognition in quiet and in babble was measured in 72 older adults with near-normal hearing and 72 older adults with hearing loss in four conditions: unaltered, jittered, smeared, and combined jittering and smearing.

RESULTS

For the listeners in Experiment 1, word recognition was poorer in the distorted conditions compared with the unaltered condition. The signal to noise ratio at 50% correct word recognition was 4.6 dB for the unaltered condition, 6.3 dB for the jittered, 6.8 dB for the smeared, 6.9 dB for the double-jitter, and 8.2 dB for the combined jitter-smear conditions. Jittering both the babble and speech signals did not significantly reduce performance compared with jittering only the speech. In Experiment 2, the older listeners with near-normal hearing and hearing loss performed best in the unaltered condition, followed by the jitter and smear conditions, with the poorest performance in the combined jitter-smear condition in both quiet and noise. Overall, listeners with near-normal hearing performed better than listeners with hearing loss by ~30% in quiet and ~6 dB in noise. In the quiet distorted conditions, when the level of the speech was increased, performance improved for the hearing loss group, but decreased for the older group with near-normal hearing. Recognition performance of younger listeners in the jitter-smear condition and the performance of older listeners with near-normal hearing in the unaltered conditions were similar. Likewise, the performance of older listeners with near-normal hearing in the jitter-smear condition and the performance of older listeners with hearing loss in the unaltered conditions were similar.

CONCLUSIONS

The present experiments advance our understanding regarding how spectral or temporal distortions of the fine structure of speech affect word recognition in older listeners with and without clinically significant hearing loss. The Speech Intelligibility Index was able to predict group differences, but not the effects of distortion. Individual differences in performance were similar across all distortion conditions with both age and hearing loss being implicated. The speech materials needed to be both spectrally and temporally distorted to mimic the effects of age-related differences in auditory processing and hearing loss.

Level-dependent changes in detection of temporal gaps in noise markers by adults with normal and impaired hearing

Compression in the basilar-membrane input-output response flattens the temporal envelope of a fluctuating signal when more gain is applied to lower level than higher level temporal components. As a result, level-dependent changes in gap detection for signals with different depths of envelope fluctuation and for subjects with normal and impaired hearing may reveal effects of compression. To test these assumptions, gap detection with and without a broadband noise was measured with 1, 000-Hz-wide (flatter) and 50-Hz-wide (fluctuating) noise markers as a function of marker level. As marker level increased, background level also increased, maintaining a fixed acoustic signal-to-noise ratio (SNR) to minimize sensation-level effects on gap detection. Significant level-dependent changes in gap detection were observed, consistent with effects of cochlear compression. For the flatter marker, gap detection that declines with increases in level up to mid levels and improves with further increases in level may be explained by an effective flattening of the temporal envelope at mid levels, where compression effects are expected to be strongest. A flatter effective temporal envelope corresponds to a reduced effective SNR. The effects of a reduction in compression (resulting in larger effective SNRs) may contribute to better-than-normal gap detection observed for some hearing-impaired listeners.

Behavioral estimates of basilar-membrane compression: additivity of forward masking in noise-masked normal-hearing listeners

Cochlear hearing loss is often associated with a loss of basilar-membrane (BM) compression, which in turn may contribute to degraded processing of suprathreshold stimuli. Behavioral estimates of compression may therefore be useful as long as they are valid over a wide range of levels and frequencies. Additivity of forward masking (AFM) may provide such a measure, but research to date lacks normative data from normal-hearing (NH) listeners at high sound levels, which is necessary to evaluate data from hearing-impaired (HI) listeners. The present study measured AFM in six NH listeners for signal frequencies of 500, 1500, and 4000 Hz in the presence of background noise, designed to elevate signal thresholds to levels similar to those experienced by HI listeners. Results consistent with compressive BM responses were found for all six listeners at 500 Hz, five listeners at 1500 Hz, but only two listeners at 4000 Hz. Further measurements in the absence of background noise also indicated a lack of consistent compression at 4000 Hz at higher signal levels, in contrast to earlier results collected at lower levels. A better understanding of this issue will be required before AFM can be used as a general behavioral estimate of BM compression.

Individual differences in behavioral estimates of cochlear nonlinearities

Psychophysical methods provide a mechanism to infer the characteristics of basilar membrane responses in humans that cannot be directly measured. Because these behavioral measures are indirect, the interpretation of results depends on several underlying assumptions. Ongoing uncertainty about the suitability of these assumptions and the most appropriate measurement and compression estimation procedures, and unanswered questions regarding the effects of cochlear hearing loss and age on basilar membrane nonlinearities, motivated this experiment. Here, estimates of cochlear nonlinearities using temporal masking curves (TMCs) were obtained in a large sample of adults of various ages whose hearing ranged from normal to moderate cochlear hearing loss (Experiment 1). A wide range of compression slopes was observed, even for subjects with similar ages and thresholds, which warranted further investigation (Experiment 2). Potential sources of variance contributing to these individual differences were explored, including procedural-related factors (test-retest reliability, suitability of the linear-reference TMC, probe sensation levels, and parameters of TMC fitting algorithms) and subject-related factors (age and age-related changes in temporal processing, strength of cochlear nonlinearities estimated with distortion-product otoacoustic emissions, estimates of changes in cochlear function from damage to outer hair cells versus inner hair cells). Subject age did not contribute significantly to TMC or compression slopes, and TMC slopes did not vary significantly with threshold. Test-retest reliability of TMCs suggested that TMC masker levels and the general shapes of TMCs did not change in a systematic way when re-measured many weeks later. Although the strength of compression decreased slightly with increasing hearing loss, the magnitude of individual differences in compression estimates makes it difficult to determine the effects of hearing loss and cochlear damage on basilar membrane nonlinearities in humans.

Temporal modulation transfer functions for listeners with real and simulated hearing loss

A functional simulation of hearing loss was evaluated in its ability to reproduce the temporal modulation transfer functions (TMTFs) for nine listeners with mild to profound sensorineural hearing loss. Each hearing loss was simulated in a group of three age-matched normal-hearing listeners through spectrally shaped masking noise or a combination of masking noise and multiband expansion. TMTFs were measured for both groups of listeners using a broadband noise carrier as a function of modulation rate in the range 2 to 1024 Hz. The TMTFs were fit with a lowpass filter function that provided estimates of overall modulation-depth sensitivity and modulation cutoff frequency. Although the simulations were capable of accurately reproducing the threshold elevations of the hearing-impaired listeners, they were not successful in reproducing the TMTFs. On average, the simulations resulted in lower sensitivity and higher cutoff frequency than were observed in the TMTFs of the hearing-impaired listeners. Discrepancies in performance between listeners with real and simulated hearing loss are possibly related to inaccuracies in the simulation of recruitment.

Characterizing auditory processing and perception in individual listeners with sensorineural hearing loss

This study considered consequences of sensorineural hearing loss in ten listeners. The characterization of individual hearing loss was based on psychoacoustic data addressing audiometric pure-tone sensitivity, cochlear compression, frequency selectivity, temporal resolution, and intensity discrimination. In the experiments it was found that listeners with comparable audiograms can show very different results in the supra-threshold measures. In an attempt to account for the observed individual data, a model of auditory signal processing and perception [Jepsen et al., J. Acoust. Soc. Am. 124, 422-438 (2008)] was used as a framework. The parameters of the cochlear processing stage of the model were adjusted to account for behaviorally estimated individual basilar-membrane input-output functions and the audiogram, from which the amounts of inner hair-cell and outer hair-cell losses were estimated as a function of frequency. All other model parameters were left unchanged. The predictions showed a reasonably good agreement with the measured individual data in the frequency selectivity and forward masking conditions while the variation of intensity discrimination thresholds across listeners was underestimated by the model. The model and the associated parameters for individual hearing-impaired listeners might be useful for investigating effects of individual hearing impairment in more complex conditions, such as speech intelligibility in noise.

Speech reception by listeners with real and simulated hearing impairment: effects of continuous and interrupted noise

The effects of audibility and age on masking for sentences in continuous and interrupted noise were examined in listeners with real and simulated hearing loss. The absolute thresholds of each of ten listeners with sensorineural hearing loss were simulated in normal-hearing listeners through a combination of spectrally-shaped threshold noise and multi-band expansion for octave bands with center frequencies from 0.25-8 kHz. Each individual hearing loss was simulated in two groups of three normal-hearing listeners (an age-matched and a non-age-matched group). The speech-to-noise ratio (S/N) for 50%-correct identification of hearing in noise test (HINT) sentences was measured in backgrounds of continuous and temporally-modulated (10 Hz square-wave) noise at two overall levels for unprocessed speech and for speech that was amplified with the NAL-RP prescription. The S/N in both continuous and interrupted noise of the hearing-impaired listeners was relatively well-simulated in both groups of normal-hearing listeners. Thus, release from masking (the difference in S/N obtained in continuous versus interrupted noise) appears to be determined primarily by audibility. Minimal age effects were observed in this small sample. Observed values of masking release were compared to predictions derived from intelligibility curves generated using the extended speech intelligibility index (ESII) [Rhebergen et al. (2006). J. Acoust. Soc. Am. 120, 3988-3997].

Effects of background noise level on behavioral estimates of basilar-membrane compression

Hearing-impaired (HI) listeners often show poorer performance on psychoacoustic tasks than do normal-hearing (NH) listeners. Although some such deficits may reflect changes in suprathreshold sound processing, others may be due to stimulus audibility and the elevated absolute thresholds associated with hearing loss. Masking noise can be used to raise the thresholds of NH to equal the thresholds in quiet of HI listeners. However, such noise may have other effects, including changing peripheral response characteristics, such as the compressive input-output function of the basilar membrane in the normal cochlea. This study estimated compression behaviorally across a range of background noise levels in NH listeners at a 4 kHz signal frequency, using a growth of forward masking paradigm. For signals 5 dB or more above threshold in noise, no significant effect of broadband noise level was found on estimates of compression. This finding suggests that broadband noise does not significantly alter the compressive response of the basilar membrane to sounds that are presented well above their threshold in the noise. Similarities between the performance of HI listeners and NH listeners in threshold-equalizing noise are therefore unlikely to be due to a linearization of basilar-membrane responses to suprathreshold stimuli in the NH listeners.

Effects on speech intelligibility of temporal jittering and spectral smearing of the high-frequency components of speech

In a previous study, we demonstrated that word recognition performance was reduced when the low-frequency components of speech (0-1.2 kHz) were distorted by temporal jittering, but not when they were distorted by spectral smearing (Pichora-Fuller et al., 2007). Temporal jittering distorts the fine structure of the speech signal with negligible alteration of either its long-term spectral or amplitude envelope characteristics. Spectral smearing simulates the effects of broadened auditory filters that occur with cochlear hearing loss (Baer and Moore, 1993). In the present study, the high-frequency components of speech (1.2-7 kHz) were distorted with jittering and smearing. Word recognition in noise for both distortion conditions was poorer than in the intact condition. However, unlike our previous study, no significant difference was found in word recognition performance in the two distorted conditions. Whereas temporal distortion seems to have a deleterious effect that cannot be attributed to spectral distortion when only the lower frequencies are distorted, when the higher frequencies are distorted both temporal and spectral distortion reduce speech intelligibility.