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According to signal detection theory, the ability to detect a signal is limited only by internal noise, which comprises peripheral and central sources. Here, we develop a statistical approach to parse central from peripheral noise. Fifty-two Veterans (mean age = 47.8, range = 30–60) with normal or near-normal hearing performed AXB discrimination for several temporal processing tasks: gap duration discrimination, forward masking, frequency modulation detection, and interaural phase modulation detection. After training, a single adaptive run (40 reversals) was completed for each task. Subjects also completed speech-in-noise testing (“Theo-Victor-Michael") with four masker types (48 trials ea.): speech-shaped noise, speech-envelope modulated noise, one and two competing talkers. Composite speech performance was estimated using principal component analysis. Bayesian hierarchical regression was used to estimate two-parameter psychometric functions (threshold, slope) simultaneously for all temporal tasks and subjects. Crucially, fixed (group-level) thresholds were estimated per task but only a single random (subject-level) intercept was estimated (mean across-task deviation from the group thresholds). We assume central noise is the primary factor limiting across-task performance. The principal speech scores were entered as regressors on this “central threshold.” Indeed, central threshold was correlated with the principal speech scores, suggesting that central noise limits temporal processing and speech-in-noise.

The sound-induced loss of ribbon synapses connecting inner hair cells to auditory nerve fibers primarily exhibiting high thresholds has been the subject of numerous investigations. The condition resulting from this inner ear abnormality is commonly referred to as hidden hearing loss, a name that reflects the relative invulnerability of low threshold auditory nerve fibers that survive noise-exposure and function normally. Although auditory sensitivity recovers completely among animals experiencing hidden hearing loss, recovery of auditory brainstem response amplitudes is incomplete. The residual loss of function reflected in diminished response amplitudes to transient stimuli serves as a highly reliable indicator of the condition. The extent to which responses to sustained stimuli might serve as indicators of pathology is less clear. To that end, we will review findings related to differences in spectral magnitudes of envelope following responses to a battery of sinusoidally-amplitude modulated test conditions that include level dependent response growth, modulation transfer functions under a variety of carrier conditions, the influence of varying modulation depths, as well as the influence of maskers on responses acquired from control and noise-exposed animals. [Work supported by the Department of Defense Award #W81XWH-19-1-0862.]

Strelcyk et al. (2019) recently found that interaural phase discrimination in older hearing-impaired listeners was correlated with both visuospatial processing speed and interaural level discrimination. This suggests that temporal fine structure (TFS) processing relies on global processing speed and/or spatial cognition, though it is possible that, generally, complex auditory discrimination engages multiple cognitive domains. Here, 50 Veterans (mean age = 48.1, range = 30–60) with normal or near-normal hearing completed batteries of temporal processing and cognitive tests. Composite cognitive test scores reflecting processing speed/executive function (PS-EX) and working memory (WM) were obtained. Temporal processing tasks included measures of envelope (ENV; gap duration discrimination, forward masking) and TFS (frequency modulation detection, interaural phase modulation detection) processing. Bayesian hierarchical regression was used to fit psychometric functions simultaneously to all ENV and TFS tasks in all subjects. Fixed effects of PS-EX and WM on thresholds and slopes were estimated for the psychometric functions in each temporal task. In general, (i) PS-EX and WM influenced both TFS and ENV thresholds, but not slopes; and (ii) TFS thresholds were best explained by PS-EX scores while ENV thresholds were best explained by WM scores. These findings suggest a specific relation between global processing speed and TFS.

Recent studies suggest the brain tracks both attended and unattended speech streams. Here, we describe the cortical mechanisms that support active talker segregation by vocal gender. Thirty-three participants with normal or near-normal hearing performed a competing speech task during fMRI scanning. The target (competing) talker was female (male). Spectrotemporal modulation filtering was applied to stochastically modulate female and male vocal pitch across trials. Using the modulation-filter patterns as predictors, spectrotemporal receptive fields (STRFs) were obtained at each voxel using coordinate descent. STRF weights associated with female- (∼6 cyc/kHz) and male-talker (∼12 cyc/kHz) pitch were analyzed across subjects to identify pitch-sensitive voxels (logical OR, corrected p

Cochlear synaptopathy, otherwise known as hidden hearing loss, has been at least partially characterized in a number of mammalian species. Although well-established generally, the disorder is incompletely understood, particularly with regard to the extent of interspecies pathology associated with the condition. Furthermore, the extent to which recovery of lost function is achieved among species thus far studied is also incompletely understood. In this context, the existence of evidence suggesting that humans experience this form of synapse pathology calls for the development of a noninvasive protocol designed to further address the question. If confirmed, where on the disorder spectrum humans are positioned takes on a heightened sense of importance. To that end, a protocol that reliably identifies the disorder in a guinea pig model is under development and will, if successful, serve as the foundation for the development of an equivalent human protocol employing the same noninvasive electrophysiological strategy. In this report, preliminary findings from an ongoing study centered on a battery of electrophysiology and immunohistochemical studies supporting model development in guinea pigs will be reviewed with the goal of identifying response outcomes with diagnostic potential. [Work supported by the Department of Defense Award No. W81XWH-19-1-0862.]

Clinical speech-in-noise tests typically use materials without contextual constraint or balanced for linguistic properties like word/phoneme frequency. However, real-world linguistic context effects can be substantial and vary by listener and scenario. Here, 38 participants completed the Theo-Victor-Michael (TVM) speech test in four types of background: speech shaped noise (SSN), speech-envelope modulated noise (envSSN), one competing talker (1T), and two competing talkers (2T) (Helfer and Freyman, 2009). The TVM is a matrix test using keywords from a corpus of one- and two- syllable nouns that vary considerably in word frequency (FREQ) and phonological neighborhood density (DENS). Bayesian logistic regression was used to estimate the effects of FREQ/DENS on TVM performance. A multinomial model was used for 1T/2T to assess reporting of target and distractor keywords. Overall, percent-correct recognition increased with increasing keyword FREQ and decreased with increasing keyword DENS. Effects were larger in SSN/envSSN than 1T/2T. Statistically significant but small effects of FREQ/DENS were observed on distractor responses in 1T/2T. Adjusting performance for FREQ/DENS substantially shifted the distribution of scores but only for SSN/envSSN. Performance in 1T/2T may be dominated by non-linguistic factors, and/or less sensitive to FREQ/DENS due to higher difficulty or linguistic competition from the background talkers. [Work supported by VA RR&D Service.]

Individuals with hearing loss are thought to be less sensitive to the often subtle variations of acoustic information that support auditory stream segregation. Perceptual segregation can be influenced by differences in both the spectral and temporal characteristics of interleaved stimuli. The purpose of this study was to determine what stimulus characteristics support sequential stream segregation by normal-hearing and hearing-impaired listeners. Iterated rippled noises (IRNs) were used to assess the effects of tonality, spectral resolvability, and hearing loss on the perception of auditory streams in two pitch regions, corresponding to 250 and 1000 Hz. Overall, listeners with hearing loss were significantly less likely to segregate alternating IRNs into two auditory streams than were normally hearing listeners. Low pitched IRNs were generally less likely to segregate into two streams than were higher pitched IRNs. High-pass filtering was a strong contributor to reduced segregation for both groups. The tonality, or pitch strength, of the IRNs had a significant effect on streaming, but the effect was similar for both groups of subjects. These data demonstrate that stream segregation is influenced by many factors including pitch differences, pitch region, spectral resolution, and degree of stimulus tonality, in addition to the loss of auditory sensitivity.

Poor speech understanding in noise by hearing-impaired (HI) listeners is only partly explained by elevated audiometric thresholds. Suprathreshold-processing impairments such as reduced temporal or spectral resolution or temporal fine-structure (TFS) processing ability might also contribute. Although speech contains dynamic combinations of temporal and spectral modulation and TFS content, these capabilities are often treated separately. Modulation-depth detection thresholds for spectrotemporal modulation (STM) applied to octave-band noise were measured for normal-hearing and HI listeners as a function of temporal modulation rate (4-32 Hz), spectral ripple density [0.5-4 cycles/octave (c/o)] and carrier center frequency (500-4000 Hz). STM sensitivity was worse than normal for HI listeners only for a low-frequency carrier (1000 Hz) at low temporal modulation rates (4-12 Hz) and a spectral ripple density of 2 c/o, and for a high-frequency carrier (4000 Hz) at a high spectral ripple density (4 c/o). STM sensitivity for the 4-Hz, 4-c/o condition for a 4000-Hz carrier and for the 4-Hz, 2-c/o condition for a 1000-Hz carrier were correlated with speech-recognition performance in noise after partialling out the audiogram-based speech-intelligibility index. Poor speech-reception and STM-detection performance for HI listeners may be related to a combination of reduced frequency selectivity and a TFS-processing deficit limiting the ability to track spectral-peak movements.

Masked sentence perception by hearing-aid users is strongly correlated with three variables: (1) the ability to hear phonetic details as estimated by the identification of syllable constituents in quiet or in noise; (2) the ability to use situational context that is extrinsic to the speech signal; and (3) the ability to use inherent context provided by the speech signal itself. These conclusions are supported by the performance of 57 hearing-aid users in the identification of 109 syllable constituents presented in a background of 12-talker babble and the identification of words in naturally spoken sentences presented in the same babble. A mathematical model is offered that allows calculation of an individual listener's sentence scores from estimates of context utilization and the ability to identify syllable constituents. When the identification accuracy of syllable constituents is greater than about 55%, individual differences in context utilization play a minor role in determining the sentence scores. As syllable constituent scores fall below 55%, individual differences in context utilization play an increasingly greater role in determining sentence scores. When a listener's syllable constituent score is above about 71% in quiet, the listeners score in quiet will above about 55% in noise. [Watson and Miller are shareholders in Communication Disorders Technology, Inc., and may profit from sales of the software used in this study.] Masked sentence perception by hearing-aid users is strongly correlated with three variables: (1) the ability to hear phonetic details as estimated by the identification of syllable constituents in quiet or in noise; (2) the ability to use situational context that is extrinsic to the speech signal; and (3) the ability to use inherent context provided by the speech signal itself. These conclusions are supported by the performance of 57 hearing-aid users in the identification of 109 syllable constituents presented in a background of 12-talker babble and the identification of words in naturally spoken sentences presented in the same babble. A mathematical model is offered that allows calculation of an individual listener's sentence scores from estimates of context utilization and the ability to identify syllable constituents. When the identification accuracy of syllable constituents is greater than about 55%, individual differences in context utilization play a minor role in determining the sentence scores. A...

Contralateral suppression of otoacoustic emissions (OAEs) is frequently used to assess the medial olivocochlear (MOC) efferent system, and may have clinical utility. However, OAEs are weak or absent in hearing-impaired ears, so little is known about MOC function in the presence of hearing loss. A potential alternative measure is contralateral suppression of the auditory steady-state response (ASSR) because ASSRs are measurable in many hearing-impaired ears. This study compared contralateral suppression of both transient-evoked otoacoustic emissions (TEOAEs) and ASSRs in a group of ten primarily older adults with either normal hearing or mild sensorineural hearing loss. Responses were elicited using 75-dB peak sound pressure level clicks. The MOC was activated using contralateral broadband noise at 60 dB sound pressure level. Measurements were made concurrently to ensure a consistent attentional state between the two measures. The magnitude of contralateral suppression of ASSRs was significantly larger than contralateral suppression of TEOAEs. Both measures usually exhibited high test-retest reliability within a session. However, there was no significant correlation between the magnitude of contralateral suppression of TEOAEs and of ASSRs. Further work is needed to understand the role of the MOC in contralateral suppression of ASSRs.

The purpose of this study was to determine whether the perceived sensory dissonance of pairs of pure tones (PT dyads) or pairs of harmonic complex tones (HC dyads) is altered due to sensorineural hearing loss. Four normal-hearing (NH) and four hearing-impaired (HI) listeners judged the sensory dissonance of PT dyads geometrically centered at 500 and 2000 Hz, and of HC dyads with fundamental frequencies geometrically centered at 500 Hz. The frequency separation of the members of the dyads varied from 0 Hz to just over an octave. In addition, frequency selectivity was assessed at 500 and 2000 Hz for each listener. Maximum dissonance was perceived at frequency separations smaller than the auditory filter bandwidth for both groups of listners, but maximum dissonance for HI listeners occurred at a greater proportion of their bandwidths at 500 Hz than at 2000 Hz. Further, their auditory filter bandwidths at 500 Hz were significantly wider than those of the NH listeners. For both the PT and HC dyads, curves displaying dissonance as a function of frequency separation were more compressed for the HI listeners, possibly reflecting less contrast between their perceptions of consonance and dissonance compared with the NH listeners.

The ability to discriminate between sounds with different spectral shapes was evaluated for normal-hearing and hearing-impaired listeners. Listeners discriminated between a standard stimulus and a signal stimulus in which half of the standard components were decreased in level and half were increased in level. In one condition, the standard stimulus was the sum of six equal-amplitude tones (equal-SPL), and in another the standard stimulus was the sum of six tones at equal sensation levels re: audiometric thresholds for individual subjects (equal-SL). Spectral weights were estimated in conditions where the amplitudes of the individual tones were perturbed slightly on every presentation. Sensitivity was similar in all conditions for normal-hearing and hearing-impaired listeners. The presence of perturbation and equal-SL components increased thresholds for both groups, but only small differences in weighting strategy were measured between the groups depending on whether the equal-SPL or equal-SL condition was tested. The average data suggest that normal-hearing listeners may rely more on the central components of the spectrum whereas hearing-impaired listeners may have been more likely to use the edges. However, individual weighting functions were quite variable, especially for the HI listeners, perhaps reflecting difficulty in processing changes in spectral shape due to hearing loss. Differences in weighting strategy without changes in sensitivity suggest that factors other than spectral weights, such as internal noise or difficulty encoding a reference stimulus, also may dominate performance.

The ability of three species of birds to discriminate among selected harmonic complexes with fundamental frequencies varying from 50 to 1000 Hz was examined in behavioral experiments. The stimuli were synthetic harmonic complexes with waveform shapes altered by component phase selection, holding spectral and intensive information constant. Birds were able to discriminate between waveforms with randomly selected component phases and those with all components in cosine phase, as well as between positive and negative Schroeder-phase waveforms with harmonic periods as short as 1-2 ms. By contrast, human listeners are unable to make these discriminations at periods less than about 3-4 ms. Electrophysiological measures, including cochlear microphonic and compound action potential measurements to the same stimuli used in behavioral tests, showed differences between birds and gerbils paralleling, but not completely accounting for, the psychophysical differences observed between birds and humans. It appears from these data that birds can hear the fine temporal structure in complex waveforms over very short periods. These data show birds are capable of more precise temporal resolution for complex sounds than is observed in humans and perhaps other mammals. Physiological data further show that at least part of the mechanisms underlying this high temporal resolving power resides at the peripheral level of the avian auditory system.

The ability to discriminate between sounds with different spectral shapes was evaluated for normal-hearing and hearing-impaired listeners. Listeners detected a 920-Hz tone added in phase to a single component of a standard consisting of the sum of five tones spaced equally on a logarithmic frequency scale ranging from 200 to 4200 Hz. An overall level randomization of 10 dB was either present or absent. In one subset of conditions, the no-perturbation conditions, the standard stimulus was the sum of equal-amplitude tones. In the perturbation conditions, the amplitudes of the components within a stimulus were randomly altered on every presentation. For both perturbation and no-perturbation conditions, thresholds for the detection of the 920-Hz tone were measured to compare sensitivity to changes in spectral shape between normal-hearing and hearing-impaired listeners. To assess whether hearing-impaired listeners relied on different regions of the spectrum to discriminate between sounds, spectral weights were estimated from the perturbed standards by correlating the listener's responses with the level differences per component across two intervals of a two-alternative forced-choice task. Results showed that hearing-impaired and normal-hearing listeners had similar sensitivity to changes in spectral shape. On average, across-frequency correlation functions also were similar for both groups of listeners, suggesting that as long as all components are audible and well separated in frequency, hearing-impaired listeners can use information across frequency as well as normal-hearing listeners. Analysis of the individual data revealed, however, that normal-hearing listeners may be better able to adopt optimal weighting schemes. This conclusion is only tentative, as differences in internal noise may need to be considered to interpret the results obtained from weighting studies between normal-hearing and hearing-impaired listeners.

Reports using a variety of psychophysical tasks indicate that pitch perception by hearing-impaired listeners may be abnormal, contributing to difficulties in understanding speech and enjoying music. Pitches of complex sounds may be weaker and more indistinct in the presence of cochlear damage, especially when frequency regions are affected that form the strongest basis for pitch perception in normal-hearing listeners. In this study, the strength of the complex pitch generated by iterated rippled noise was assessed in normal-hearing and hearing-impaired listeners. Pitch strength was measured for broadband noises with spectral ripples generated by iteratively delaying a copy of a given noise and adding it back into the original. Octave-band-pass versions of these noises also were evaluated to assess frequency dominance regions for rippled-noise pitch. Hearing-impaired listeners demonstrated consistently weaker pitches in response to the rippled noises relative to pitch strength in normal-hearing listeners. However, in most cases, the frequency regions of pitch dominance, i.e., strongest pitch, were similar to those observed in normal-hearing listeners. Except where there exists a substantial sensitivity loss, contributions from normal pitch dominance regions associated with the strongest pitches may not be directly related to impaired spectral processing. It is suggested that the reduced strength of rippled-noise pitch in listeners with hearing loss results from impaired frequency resolution and possibly an associated deficit in temporal processing.

In humans, masking by harmonic complexes is dependent not only on the frequency content of the masker, but also its phase spectrum. Complexes that have highly modulated temporal waveforms due to the selection of their component phases usually provide less masking than those with flatter temporal envelopes. Moreover, harmonic complexes that are created with negative Schroeder phases (component phases monotonically decreasing with increasing harmonic frequency) may provide more masking than those created with positive Schroeder phases (monotonically increasing phase), even though both temporal envelopes are equally flat. To date, there has been little comparative work on the masking effectiveness of harmonic complexes. Using operant conditioning and the method of constant stimuli, masking of pure tones by harmonic complexes was examined in budgerigars at several different masker levels for complexes constructed with two different fundamental frequencies. In contrast to humans, thresholds in budgerigars differed very little for the two Schroeder-phase waveforms. Moreover, when there was a difference in masking by these two waveforms, the positive Schroeder was the more effective masker--the reverse of that described for humans. Control experiments showed that phase selection was relevant to the masking ability of harmonic complexes in budgerigars. Release from masking occurred when the components were in coherent phase, compared with a complex with random phases selected for each component. It is suggested that these psychoacoustic differences may emerge from structural and functional differences between the avian and mammalian peripheral auditory systems involving traveling wave mechanics and spectral tuning characteristics.

In a linear filterbank there is a simple trade‐off between filter bandwidth and duration of the impulse response. This suggests that hearing‐impaired listeners with broad auditory filters will have abnormally acute temporal sensitivity, and would be more sensitive than normal to phase manipulations in harmonic stimuli. However, repeated attempts to substantiate this prediction have largely failed. Linear models fail to account for the loss of gain associated with outer haircell damage in the impaired cochlea. In preparation for modeling phase discrimination with an active cochlea model, phase discrimination thresholds were measured at a number of stimulus levels for normal‐hearing and hearing‐impaired listeners in a standard/2AFC experiment. Stimuli were 21‐component harmonic complexes with a fundamental frequency of 125 Hz. The standard had all components in cosine phase. Test stimuli had odd‐numbered harmonics in cosine phase and even‐numbered harmonics shifted a fixed number of degrees as determined by...

The dynamic formant transitions present within consonant-vowel and vowel-consonant utterances may be poorly processed by hearing-impaired (HI) listeners and may, as a result, provide reduced segmental information to these listeners. Both the brief durations and rapid spectral change associated with these transitions have been proposed as contributing to these deficits in processing. To investigate HI listeners' processing of transitional stimuli, six HI and six normal-hearing (NH) listeners were asked to discriminate between frequency glides patterned after second formant transitions in English CV syllables. The influences of glide duration, rate, and frequency extent were examined for each group in quiet and in broadband noise. Reductions in glide duration and the presence of noise each led to significant increases in frequency difference limens for glide onset. The magnitudes of these effects were similar across groups, thereby failing to show increased susceptibility to noise or greater deficits in dealing with brief stimuli on the part of HI listeners. Contrary to the expectation that HI listeners would show greatest deficits (relative to the NH group) for the most rapidly changing glides, the only significant group differences were observed for gradual glides of limited frequency extent. This latter finding is discussed in terms of sensitivity to "cochlear dispersion" cues [Porter et al., J. Acoust. Soc. Am. 90, 1298-1308 (1991)]. It is hypothesized that HI listeners may not process these cues as effectively as NH listeners, perhaps as a result of impaired frequency selectivity.

Abnormal frequency resolution associated with sensorineural hearing impairment produces a smearing of spectral detail in the internal representation of complex acoustic stimuli. As a result, listeners with hearing loss may have difficulty locating spectral peaks (e.g., vowel formants) within stimuli which cue their identity. This study examined the relationship between frequency separation of peaks in a complex spectrum and the degree of spectral contrast preserved in the internal representations in normal and impaired auditory systems. Hearing-impaired and normal-hearing subjects discriminated a flat-spectrum bandpass stimulus from a stimulus containing a sinusoidal ripple across its frequency range. The peak-to-valley amplitude (in dB) necessary for detection of the ripple was measured for ripple frequencies ranging from 1 to 9 cycles/oct. Auditory filter characteristics were also measured at 1 and 3 kHz in order to examine the internal representations of the stimuli after cochlear processing. There were clear differences between groups in both auditory filter characteristics and spectral contrast detection. However, the amount of contrast in the internal representations predicted from these measurements was nearly the same for all subjects, suggesting that the reduced frequency resolution of the hearing-impaired group was largely responsible for differences in required peak-to-valley amplitude in the input spectra. Further, for all subjects, there was a trade-off between the absolute level of internal contrast necessary for ripple detection and the number of samples of this contrast available to the listener.

Exposure to high-intensity blasts is the most common cause of injury in recent U.S. military conflicts. Prior work indicates that blast-exposed Veterans report significantly more hearing handicap than non-blast-exposed Veterans, often in spite of clinically normal hearing thresholds. Our research explores the auditory effects of blast exposure using a combination of self-report, behavioral, and electrophysiological measures of auditory processing. Results of these studies clearly indicate that blast-exposed individuals are significantly more likely to perform poorly on tests requiring the use of binaural information and tests of pitch sequencing and temporal acuity compared to non-blast-exposed control subjects. Behavioral measures are corroborated by numerous objective electrophysiological measures, and are not necessarily attributable to peripheral hearing loss or impaired cognition. Thus, evidence indicates that blast exposure can lead to acquired deficits in central auditory processing (CAP) which may persist for at least 10 years following blast exposure. Future studies of these deficits in this and other adult populations are needed to address important issues such as individual susceptibility, anatomical, and physiological changes in auditory pathways which contribute to symptoms of these types of deficits, and development of effective evidence-based methods of rehabilitation in adult patients. [Work supported by the VA Rehabilitation Research & Development Service and the VA Office of Academic Affiliations.]

In some experimental situations, the psychometric function underlying performance may not be stable, but instead may shift along the stimulus axis in response to changes in attention, learning, or task difficulty. When this occurs, the measured threshold may be influenced and the slope of the measured function will be inaccurately shallow. With commonly used experimental procedures, it is difficult to know whether a shallow psychometric function slope is a true reflection of the sensory process, or is a result of "averaging" a highly variable underlying function. Here, a new method is described of estimating psychometric function slope from the variability in two interleaved adaptive tracks, consulted on alternate trials, that is resistant to the effects of shifting performance levels. Further, a mechanism is described for assessing the likelihood that a threshold was, in fact, stable over the course of its measurement. Computer simulations are reported as well as verification of the method in measurements of human performance on a psychophysical task. Several conditions of externally imposed variability were simulated to establish the ability of these procedures to identify unstable functions and produce accurate slope estimates. The procedures worked well for thresholds shifting by as little as 4 dB if the variation did not occur too rapidly. The procedure and associated analyses are recommended as a relatively "free" means of calculating slope and quantifying threshold reliability with little extra experimental effort.

Labeling of concurrently presented vowels generally improves with the F0 difference between vowels (up to a limit of 2 to 4 semitones difference). One interpretation of this finding is that the processing of F0 differences between vowels allows listeners to group spectral components according to a common fundamental and thereby improve identification. However, the benefit of F0 differences may be reduced for hearing‐impaired listeners to the extent that accuracy in processing F0 is compromised by the sensory impairment. In this study, normal‐hearing and hearing‐impaired listeners were tested to determine F0 difference limens for synthetic tokens of five steady‐state vowels. The same stimuli were then used in a concurrent‐vowel labeling task. Preliminary results indicate accurate processing of F0 differences between vowels, with thresholds generally below 2.0 Hz across vowels and listeners. On concurrent‐vowel data collected thus far, hearing‐impaired listeners showed an overall reduction in labeling accur...

The effect of onset asynchrony on discrimination of spectral shape was evaluated for hearing-impaired and normal-hearing listeners. Stimuli were the sum of four tones equally spaced on a logarithmic frequency scale. The standard stimulus had tones of equal amplitude, and the signal stimulus had two adjacent components increased in level, and the other two components decreased in level. Thresholds for discrimination between the standard and signal stimuli were measured as a function of an onset asynchrony among the components of 0, 50, and 200 ms. Hearing-impaired and normal-hearing listeners had similar thresholds when the stimulus components were widely spaced in frequency, but hearing-impaired listeners had much higher thresholds for narrowly spaced components. Excitation pattern analyses indicated that listeners may use spectral peaks in the stimulus rather than the change in excitation across the full stimulus bandwidth for spectral shape discrimination tasks. Increasing temporal asynchrony of components resulted in increased thresholds for both groups of listeners to a greater extent in the wide span than the narrow span. Reduced effects of onset asynchrony in the narrow span suggest that spectral resolvability of components plays an important role in the processing of onset asynchrony across frequency.

How do people extract a target speech signal from a chorus of other sounds including other speech sounds? And what are the central and peripheral auditory processes that make this possible, and that may fail in people with damaged auditory systems? These critical issues in speech and hearing science were of continuing interest to Sid Bacon throughout his career. As Sid has noted, the problem is multifaceted, and he chose to study many of the individual factors, as well as their interactions. In keeping with that approach, I will discuss how normal-hearing and hearing-impaired listeners use pitch to extract a target from a complex noise background. In one study, auditory stream segregation of iterated rippled noises (IRN) with varying pitch strengths was explored to understand the limits of tonality for separating two patterns of sounds. In a second study, voice pitch strength was investigated as a means to support perceptual separation of target and background speech, with a focus on either spectral or te...

The report in 1993 by Green [J. Acoust. Soc. Am. 93, 2096-2105 (1993)] describing the application of a new psychophysical method requiring few trials and little time to measure auditory thresholds has generated considerable interest among experimentalists. The procedure uses a single-interval stimulus presentation, requests a yes-no decision by subjects, and implements a maximum-likelihood calculation to determine the next trial stimulus level within an adaptive track, as well as the final threshold estimate. Data are presented here describing separate experiences with this procedure in two laboratories in both detection and discrimination tasks. Issues addressed include comparisons with more traditional psychophysical methods, variability in threshold estimates, experimental time required, and possible minor modifications to improve the basic procedure. Results using this procedure are comparable in terms of variability of estimates to those emerging from more lengthy procedures. However, because it may be difficult for some listeners to maintain a consistent criterion and because attentional lapses may be costly, experimenters must be willing to monitor performance closely and repeat some tracks in cases where excessively high variability is noted. Further, this procedure may not be suitable for tasks for which the form of the psychometric function is not well-established. Modifications allowing a variable slope parameter in the maximum-likelihood evaluations of psychometric functions may be of benefit.

Modulation detection and modulation rate discrimination thresholds were obtained at three different modulation rates (fm = 80, 160, and 320 Hz) and for three different ranges of modulation depths (m): full (100%), mid (70%-80%), and low (40%-60%) with both normal-hearing (NH) and hearing-impaired (HI) subjects. The results showed that modulation detection thresholds increased with modulation rate, but significantly more so for HI than for NH subjects. Similarly, rate discrimination thresholds (delta r) increased with increases in fm and decreases in modulation depth. When compared to NH subjects, rate discrimination thresholds for HI subjects were significantly worse for all rates and for all depths. At the fastest modulation rate with less than 100% modulation depth, most HI subjects could not discriminate any change in rate. When valid thresholds for rate discrimination were obtained for HI subjects, they ranged from 2.5 semitones (delta r = 12.7 Hz, fm = 80 Hz, m = 100%) to 8.7 semitones (delta r = 214.5 Hz, fm = 320 Hz, m = 100%). In contrast, average rate discrimination thresholds for NH subjects ranged from 0.9 semitones (delta r = 4.2 Hz, fm = 80 Hz, m = 100%) to 4.7 semitones (delta r = 103.5 Hz, fm = 320 Hz, m = 60%). Some of the differences in temporal processing between NH and HI subjects, especially those related to modulation detection, may be accounted for by differences in signal audibility, especially for high-frequency portions of the modulated noise. However, in many cases, HI subjects encountered great difficulty discriminating a change in modulation rate even though the modulation components of the standard and test stimuli were detectable.

Hearing-impaired and normal-hearing listeners labeled synthetic consonant-vowel stimuli (/bɑ/, /dɑ/, /gɑ/, /be/, /de/, /ge/) presented at moderate and high signal levels. First formant (F1) regions were synthesized at normal and at attenuated levels to test whether F1 attenuation might reduce upward spread of masking, making information contained in higher formant regions more available. Performance was tested in quiet and in broadband noise sufficient to mask initial release bursts. Although complete removal of F1 consistently reduced performance, F1 attenuation of up to 18 dB led to increased labeling accuracy, particularly in the /ɑ/ vowel context. Benefit associated with F1 attenuation was more consistently seen for hearing-impaired than for normal-hearing listeners and, in particular, for listeners with steep increases in audiometric thresholds between the first and second formant regions of the test stimuli. The availability of initial bursts as a source of place cues during testing in quiet did not reduce the benefit associated with F1 attenuation.

In mammals, brief rising frequency sweeps result in increased amplitudes for both auditory brainstem response (ABR) and compound action potential (CAP) recordings (Dau, 2000; Shore and Nuttall, 1985). The rising sweep is thought to result in increased synchronous activity. Changing the direction of the sweep exaggerated the delay of processing along the basilar membrane and decreased synchrony of neural responses. Here we recorded ABRs from budgerigars, canaries, and zebra finches to a variety of stimulus parameters, including rising and falling sweeps with different sweep rates, determined by changing duration and frequency range. Both linear and nonlinear sweeps in frequency over time were tested. Results show that rising sweeps produce larger peak amplitudes, shorter latencies and changes in wave morphology such as a narrower wave 1 width than falling sweeps, suggesting greater synchrony of response to sweeps moving from low to high frequency. These data are consistent with mammalian results, but with ...

Reduced frequency selectivity associated with sensorineural hearing loss may pose particular problems for hearing-impaired listeners in noisy environments. In these situations, broader-than-normal auditory filters may affect the perception of speech by reducing the contrast between spectral peaks and valleys in at least two ways. First, the peaks and valleys in the internal representation of the speech spectrum become smeared, resulting in less precise frequency analysis. Second, there may be a reduction in the signal-to-noise ratio (S/N) at the output of each auditory filter. In order to examine the relationship between frequency selectivity and identification of speechlike stimuli in noise, hearing-impaired and normal-hearing listeners were trained to assign vowel labels to four harmonic complexes which differed in the frequency locations of four elevated ("peak") harmonics. Peak harmonics were chosen to approximate first- and second-formant frequencies in four English vowels. Listeners were then tested to determine the spectral contrast necessary between peak and background components in order to maintain identification accuracy in the presence of various levels of broadband noise. Results indicated that for these stimuli, normal-hearing listeners required about 1 dB of additional spectral contrast for every doubling of the intensity of noise. The required increase in spectral contrast was generally greater for listeners with broader-than-normal auditory filters at 2000 Hz. This finding suggests indirectly that in the internal representations of speech sounds embedded in noise, the signal-to-noise ratio for listeners with abnormal frequency selectivity is poorer than for listeners with normal frequency selectivity. A poorer-than-normal internal S/N may be one factor underlying the common observation that noise often is more degrading to speech understanding by hearing-impaired listeners than by normal-hearing listeners.

Many factors contribute to successful perception of auditory stimuli in noise, including neural encoding in the central auditory system. Physiological measures such as event‐related potentials (ERPs) can provide a view of neural encoding that may inform our understanding of listeners’ abilities to perceive signals in the presence of background noise. We set out to determine the effect of stimulus type on neural encoding and perception of signals in noise. Nine young, normal‐hearing adults participated. ERPs were recorded (the N1‐P2 and P3), and a sentence‐in‐noise identification task was completed in quiet and in three noise conditions: (1) steady‐state speech spectrum noise, (2) interrupted noise, and (3) four‐talker babble. Stimuli were presented at a signal‐to‐noise ratio of −3 dB. Results demonstrated significant effects of noise type on both physiological and behavioral measures with the four‐talker babble condition showing the largest decrements in ERP and behavioral measures relative to testing in ...

Listeners with sensorineural hearing impairment typically exhibit auditory processing deficits such as reduced frequency and/or temporal resolution. Such deficits may represent separate sequela of auditory pathology or may result directly from the sensitivity loss and the requirement to listen at high levels. To assess the impact of increased thresholds on frequency resolution, auditory filter characteristics were determined for hearing-impaired and normal-hearing listeners at 500 and 2000 Hz in the presence of continuous broadband noise meant as a rough simulation of hearing loss. In the fitting procedure, the low-frequency skirt of the derived auditory filter was allowed to vary as a function of signal level, permitting different filter shapes to be estimated at high versus low signal levels. Listeners with moderate hearing losses at 2000 Hz demonstrated near-normal auditory filter shapes for lower signal levels, but increasingly broad and asymmetric filters as signal level was raised. At 500 Hz, where hearing losses were mild, filter bandwidths increased little at the higher signal levels. The presence of broadband noise had essentially no effect on filter shapes of either listener group. The filter shape abnormalities demonstrated by listeners with moderate hearing loss, which were not observed in normal-hearing listeners at the same signal levels, indicate that poor frequency resolution in these patients for high-intensity stimuli does not follow directly from decreased sensitivity, but instead reflects an independent pathology.

A listener’s ability to discriminate differences in amplitude spectra is generally thought to be independent of the temporal waveform shapes of the stimuli. However, there have been reports of enhanced contrast between spectral peaks and valleys of harmonic complexes represented in the temporal firing patterns of auditory‐nerve fibers for some combinations of component amplitudes and phases [e.g., Horst et al., J. Acoust. Soc. Am. 88, 2656–2681 (1990)]. The enhancement has been attributed to nonlinear processing of high‐amplitude peaked waveforms. To determine whether such enhancements are observed in psychoacoustic responses of human listeners, subjects were asked to discriminate between harmonic complexes that differed in the frequency location of three formantlike increments in harmonic amplitude. The stimuli were constructed to have either a very flat or a very peaked temporal envelope. Over a range of 1‐ to 10‐dB spectral contrast between elevated harmonics and background harmonics, discrimination ability was determined as a function of waveform shape and overall amplitude. Performance was measured for both normal‐hearing and hearing‐impaired listeners. Normal‐hearing listeners showed lower thresholds of spectral contrast for peaked waveforms at high intensities than for any of the other phase‐amplitude conditions. Although the effect was small (1 to 2 dB), it is consistent with an hypothesized enhancement due to nonlinear cochlear processing. Hearing‐impaired listeners did not demonstrate differential performance across phase or amplitude conditions, perhaps reflecting more linear processing in these damaged ears.

Suprathreshold distortions in auditory processing contribute to speech recognition deficits for hearing‐impaired (HI) listeners in noise. Outer hair cell damage and attendant reductions in frequency selectivity and peripheral compression may contribute to these deficits. Reduced sensitivity to spectral or temporal modulations or temporal fine structure (TFS) information may also play a role. Eight normal‐hearing and 18 HI listeners were tested in psychoacoustic tasks to assess frequency selectivity (notched‐noise), peripheral compression (temporal masking curves), TFS sensitivity [frequency modulation (FM) detection in the presence of random amplitude modulation], and spectrotemporal modulation (STM) sensitivity (spectral‐temporal “ripple” detection). Performance was examined at 500, 1000, 2000, and 4000 Hz at several presentation levels. Listeners were also tested on sentence recognition in stationary and modulated noise (92‐dB sound pressure level signal level; −6‐, −3‐, 0‐, and +3‐dB SNRs). HI listener...

Recent behavioral experiments on auditory masking and waveform discriminations using harmonic complexes as stimuli have shown significant differences in performance between birds and humans. In humans, but not in birds, Schroeder‐phase harmonic complexes constructed with monotonically increasing (positive Schroeder) or decreasing (negative Schroeder) component phases are differentially effective as maskers, even though they have identical temporal envelopes and long‐term spectra. The similarity in masking effectiveness of these harmonic complexes notwithstanding, birds can easily discriminate the fine‐structure differences between the positive and negative Schroeder waveforms even with harmonic periods as short as 2 ms. By contrast, humans are unable to make these discriminations unless the periods are greater than about 5–6 ms. These behavioral discrimination and masking results have now been replicated by measuring evoked‐potential responses in budgerigars. Amplitude and waveform shapes of the compound action potentials reflect the behavioral discriminations between the two Schroeder stimuli, but cochlear microphonics do not. The similarity in results between the behavioral and the electrophysiological studies, using the same stimuli, suggest differences in both temporal and spectral processing in avian versus mammalian cochleas. [Work supported by NIH R01 DC00198, DC00626, and NRSA DC00046.]

Previous work has demonstrated that iterated rippled noise (IRN) produces a weaker pitch in hearing‐impaired (HI) listeners than in normal‐hearing (NH) subjects (e.g., Leek and Summers, 2001). In this study, the hypothesis was tested that the weaker pitch is a result of an increased perceptual noisiness of IRN for HI subjects. The tone‐noise ratio (T/N) was developed by Patterson et al. (1996) to describe the relationship between the strength of the tone component and the strength of the noise component for a given IRN. The T/N was measured using an adaptive procedure to find the tone‐plus‐noise level that matched each IRN stimulus. Four NH and 4 HI subjects took part in the study. The stimuli were IRNs with delays ranging from 4 to 16 ms and with one to eight iterations. There was a significant effect of delay and of iteration number, with longer delays and more iterations producing greater T/N. There were no significant differences between subject groups. These findings suggest that the weaker complex pitch observed in listeners with hearing loss is not due to an abnormally noisy IRN percept but must be attributed to other impaired processing mechanisms. [Work supported by NIH.]

The effects of intrinsic and extrinsic distortions on vowel perception were evaluated. Potential sources of distortion were high presentation level, background noise, stimulus ambiguity, and hearing status. Vowel identifications were collected for three sets of synthesized vowels that varied in both F1 and F2 (front and back vowels), or F2 and F3 (central vowels). The systematic variation in formant frequencies produced stimulus sets that contained both clear, unambiguous category tokens as well as relatively ambiguous tokens that could be identified as more than one vowel category. As overall presentation level increased from 75–90 dB SPL, identification by normally hearing listeners became less consistent for ambiguous stimuli. Additional reductions in categorization precision occurred in the presence of background noise. The effects were highly dependent on stimulus set, with the largest effects observed for the central vowels. Data from hearing‐impaired listeners at high presentation levels and in noise will also be reported. Evaluation of response patterns in the presence of combinations of these sources of distortion will help explore the difficulty people with hearing loss have processing speech in adverse listening conditions. [Work supported by OHSU Medical Research Foundation, Portland, OR.]

Reduced ability to make use of temporal fine structure (TFS) information may contribute to deficits among hearing impaired (HI) listeners for speech recognition in competing background sounds. The broad tuning typically observed in HI listeners will produce more complex patterns of TFS as input to the auditory nerve, and these patterns may be more difficult to encode and/or interpret by higher auditory centers. In the current study, we tested HI listeners on a frequency‐modulation (FM) detection task thought to require the use of TFS cues, on tone detection in notched‐noise to assess frequency selectivity, and on speech recognition in continuous and modulated background sounds. Performance on the psychoacoustic tasks was tested at four frequencies (500, 1000, 2000, and 4000 Hz) at levels between 60 and 85 dB SPL (approximating the range of third‐octave band levels in the speech stimuli). HI listeners with similar audiometric thresholds showed clear differences in frequency tuning, FM detection thresholds, and speech performance. Relationships between the psychoacoustic measures in the four frequency regions and between these measures and speech performance will be discussed.

Anthropogenic noise (e.g., highway noise) that exceeds natural ambient sound levels potentially can mask important acoustic signals used by birds during communication. Signals degraded by masking can in turn adversely affect critical behaviors such as mate attraction, territorial defense, parent‐offspring communication, predator avoidance, and prey detection. The amount of masking depends not only on the physical characteristics of the noise (intensity, frequency, temporal pattern, etc.) but also the auditory capacity of the species, the acoustic structure of their vocal signals, the proximity of the birds to the noise source and each other, and the sound attenuation properties of the habitat. Here we present a quantitative model for predicting effective communication distances between birds subject to different levels and types of anthropogenic noise. Cross‐species analysis shows that communication distances are largely context/environment dependent and species specific. Extrapolation of noise impact distance estimates even within groups of similar species should be exercised with caution. This model should be useful in environmental impact analysis of anthropogenic noise on birds and other wildlife, particularly species of conservation concern.

For listeners with normal hearing, Schroeder‐phase harmonic complexes may vary in masking effectiveness by 20 dB or more depending on the temporal details of the stimuli. These masking differences are believed to be associated with curvature in cochlear phase response and compressive growth in the magnitude response for normal cochlear processing. Both curvature and compression may be greatly reduced in listeners with cochlear damage. In the current study, masker levels at masked threshold were determined for 1‐ and 4‐kHz tones at a range of fixed signal levels using harmonic maskers varying in phase curvature. A Bekesy‐tracking method, using a long‐duration harmonic masker that slowly varied in temporal structure, was used to measure detection thresholds continuously over a range of curvatures. The masker producing the least effective masking and the overall change in masker effectiveness with phase structure were used to estimate phase curvature and degree of compression in cochlear processing for norma...

Belgian Waterslager canaries (BWS), bred for a distinct low‐pitched song, have an inherited high‐frequency hearing loss related to hair cell abnormalities. Many hair cells are missing or damaged, and many of the remaining hair cells have abnormal stereocilia bundles. To investigate how the hair cell abnormalities affect frequency tuning along the BWS basilar papilla, we measured psychophysical tuning curves (PTCs) in BWS and normal‐hearing non‐BWS canaries using operant conditioning procedures. Non‐BWS canaries showed relatively symmetrical tuning curves that became more narrowly tuned with increasing center frequency. Despite near‐normal hearing at low frequencies, PTCs in BWS were elevated for all center frequencies tested. The sharpness of tuning was normal in BWS for PTCs with a center frequency of 1000 Hz and broader than normal at higher frequencies. The shapes of the PTCs were abnormal in BWS, particularly at higher frequencies. The abnormal tuning properties in the BWS are consistent with changes in the impedance characteristics of the basilar papilla due to structural damage. [Work supported by NIH DC01372 and DC04664.]

When a speaker utters a consonant‐vowel or vowel‐consonant syllable, the frequency transitions are critical to identification of the sound. However, an intensity contour is also superimposed on the frequency glides, as the mouth opens or closes. In this study, the interaction of frequency and intensity contours was explored using a frequency glide discrimination task. Offset frequency difference limens (offset DLs) were measured for upward and downward frequency glides in two frequency regions, and with three intensity contours, increasing from silence, decreasing to silence, and steady‐state. For both normal‐hearing (NH) and hearing‐impaired (HI) subjects, larger offset DLs were observed for the high frequency stimuli than for the lower frequencies, and for upward‐gliding stimuli than for downward frequency glides. Amplitude contour had little effect on the NH data, but did influence offset DLs for HI subjects. These findings indicate that for these stimuli, the interaction of frequency and amplitude contours plays only a small role in the perception of transition‐like glides for NH listeners, but may affect the perception of frequency transitions by HI listeners. [Work supported by NIDCD.]

Charles Watson’s studies of informational masking and the effects of stimulus uncertainty on auditory perception have had a profound impact on auditory research. His series of seminal studies in the mid‐1970s on the detection and discrimination of target sounds in sequences of brief tones with uncertain properties addresses the fundamental problem of extracting target signals from background sounds. As conceptualized by Chuck and others, informational masking results from more central (even ‘‘cogneetive’’) processes as a consequence of stimulus uncertainty, and can be distinguished from ‘‘energetic’’ masking, which primarily arises from the auditory periphery. Informational masking techniques are now in common use to study the detection, discrimination, and recognition of complex sounds, the capacity of auditory memory and aspects of auditory selective attention, the often large effects of training to reduce detrimental effects of uncertainty, and the perceptual segregation of target sounds from irrelevan...

The perceived dissonance of two simultaneous tones (a dyad) depends upon their frequency separation (in the case of two pure tones) or their fundamental frequency ratio (in the case of two harmonic complex tones). The purpose of this study was to determine whether the perceived dissonance of puretone and harmonic complex dyads is altered in the presence of sensorineural hearing (SNHL), and, if so, whether this can be explained by the reduced frequency selectivity typically associated with SNHL. Four normal‐hearing and four hearing‐impaired listeners evaluated the dissonance of puretone and harmonic complex dyads centered at 500 and 2000 Hz. Frequency selectivity was estimated at 500 and 2000 Hz for each listener. People with SNHL rated the dissonance of puretone dyads similarly to the normal‐hearing listeners, although auditory sensitivity and frequency selectivity differed considerably between the groups. However, their ratings of harmonic complex dyads did not show the pronounced differences in dissonance as a function of fundamental frequency ratio that were observed in the normal‐hearing group. The poorer frequency selectivity of these listeners may have allowed more extensive interactions to occur among harmonic components, resulting in a less clear separation of dissonance and consonance. [Work supported by NIH‐NIDCD.]

Vowel identification performance can be predicted reasonably well for normal‐hearing (NH) listeners based on principal components derived from excitation patterns [M. R. Molis, J. Acoust. Soc. Am. 111, 2433–2434 (2002)]. In this study, vowel categorization was measured in listeners with mild‐to‐moderate hearing loss. Stimuli were 54 synthesized vowels that varied orthogonally in F2 (1081–2120 Hz) and F3 (1268–2783 Hz) frequency in equal 0.8‐bark steps. Fundamental frequency contour, F1 (455 Hz), F4 (3250 Hz), F5 (3700 Hz), and duration (225 ms) were held constant. Hearing‐impaired (HI) listeners categorized the stimuli as the vowels /I/, /U/, or /ɝ/. Estimates of frequency resolution were also obtained, and excitation patterns were constructed for each listener. Categorization performance was more variable for HI listeners relative to NH listeners, both within and between listeners. Many subjects appeared to rely solely on F2 frequency and had particular difficulty with the /U/ vs /ɝ/ distinction. Excitat...

Hearing studies show that zebra finches, Taeniopygia guttata, small song birds that produce broadband, harmonic vocalizations are unusually sensitive to the mistuning of a single component in a harmonic complex [Lohr and Dooling, J. Comput. Psychol. 112, 36–47 (1998)]. Such a change in a harmonic stimulus creates both spectral and temporal cues. Previously it has been reported that these birds have very good temporal resolution of fine detail within harmonic complexes [Dooling et al., Hear. Res. 152, 159–172 (2001)]. Here the spectral resolving power is examined by measuring auditory patterns of masking by narrowband noises. Masking patterns were measured in zebra finches using operant conditioning and the method of constant stimuli for a 100‐Hz band noise masker centered at the birds frequency of best hearing, 2860 Hz. Probe frequencies were tones between 1000 Hz and 5700 Hz. Results demonstrate a symmetrical masking pattern with less upward spread of masking and a narrower resolution bandwidth in zebra finches than has been observed in humans. These findings are consistent with other measures of avian spectral resolution. [Work supported by NIH Grant DC‐00198 to RJD.]

Auditory spatial resolution in the horizontal plane was measured in five normal‐hearing subjects by determining the minimum audible angle (MAA) for brief 1.0‐kHz tones. Using a left–right discrimination paradigm, MAAs were determined for 22 reference azimuths ranging from −62° to +63°. The gross pattern of the mean data was similar to that reported previously for more sparsely sampled azimuth functions: spatial resolution tended to be best when the reference sound was near midline, becoming worse as the reference moved toward the periphery. However, there were remarkable asymmetries and nonmonotonicities in individual subject’s MAA versus azimuth functions, and these nonmonotonic features were highly subject‐dependent. Although such individual differences may be due partly to differences among subjects in head and pinna shape, it is possible that the highly individual and idiosyncratic MAA functions also reflect subject‐dependent nonmonotonicities in the underlying interaural time difference (ITD) discrim...