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 search for correlates of age-related cochlear synaptopathy: Measures of temporal envelope processing and spatial release from speech-on-speech masking.

Older adults often experience difficulties understanding speech in adverse listening conditions. It has been suggested that for listeners with normal and near-normal audiograms, these difficulties may, at least in part, arise from age-related cochlear synaptopathy. The aim of this study was to assess if performance on auditory tasks relying on temporal envelope processing reveal age-related deficits consistent with those expected from cochlear synaptopathy. Listeners aged 20 to 66 years were tested using a series of psychophysical, electrophysiological, and speech-perception measures using stimulus configurations that promote coding by medium- and low-spontaneous-rate auditory-nerve fibers. Cognitive measures of executive function were obtained to control for age-related cognitive decline. Results from the different tests were not significantly correlated with each other despite a presumed reliance on common mechanisms involved in temporal envelope processing. Only gap-detection thresholds for a tone in noise and spatial release from speech-on-speech masking were significantly correlated with age. Increasing age was related to impaired cognitive executive function. Multivariate regression analyses showed that individual differences in hearing sensitivity, envelope-based measures, and scores from nonauditory cognitive tests did not significantly contribute to the variability in spatial release from speech-on-speech masking for small target/masker spatial separation, while age was a significant contributor.

Role of semantic context and talker variability in speech perception of cochlear-implant users and normal-hearing listeners.

This study assessed the impact of semantic context and talker variability on speech perception by cochlear-implant (CI) users and compared their overall performance and between-subjects variance with that of normal-hearing (NH) listeners under vocoded conditions. Thirty post-lingually deafened adult CI users were tested, along with 30 age-matched and 30 younger NH listeners, on sentences with and without semantic context, presented in quiet and noise, spoken by four different talkers. Additional measures included working memory, non-verbal intelligence, and spectral-ripple detection and discrimination. Semantic context and between-talker differences influenced speech perception to similar degrees for both CI users and NH listeners. Between-subjects variance for speech perception was greatest in the CI group but remained substantial in both NH groups, despite the uniformly degraded stimuli in these two groups. Spectral-ripple detection and discrimination thresholds in CI users were significantly correlated with speech perception, but a single set of vocoder parameters for NH listeners was not able to capture average CI performance in both speech and spectral-ripple tasks. The lack of difference in the use of semantic context between CI users and NH listeners suggests no overall differences in listening strategy between the groups, when the stimuli are similarly degraded.

Development and Validation of Sentences Without Semantic Context to Complement the Basic English Lexicon Sentences.

Purpose The goal of this study was to develop and validate a new corpus of sentences without semantic context to facilitate research aimed at isolating the effects of semantic context in speech perception. Method The newly developed corpus contains nonsensical sentences but is matched in vocabulary and syntactic structure to the existing Basic English Lexicon (BEL) corpus. It consists of 20 lists, with each list containing 25 sentences and each sentence having four keywords. Each new list contains the same keywords as the respective list in the original BEL corpus, but the keywords within each list are scrambled across sentences to eliminate semantic context within each sentence, while maintaining the original syntactic structure. All sentences in the original and nonsense BEL corpora were recorded by the same two male and two female talkers. Results Mean intelligibility scores for each list were estimated by calculating the mean proportion of correct keywords achieved by 40 normal-hearing listeners for one male and one female talker. Although small but significant differences were found between some pairs of lists, mean performance for all 20 lists fell within the 95% confidence intervals of the mean. Conclusions Lists in the newly developed nonsense corpus are reasonably well equated for difficulty and can be used interchangeably in a randomized experimental design. Both the original and nonsense BEL sentences, all recorded by the same four talkers, are publicly available. Supplemental Material https://doi.org/10.23641/asha.13022900.

The role of cochlear place coding in the perception of frequency modulation.

Natural sounds convey information via frequency and amplitude modulations (FM and AM). Humans are acutely sensitive to the slow rates of FM that are crucial for speech and music. This sensitivity has long been thought to rely on precise stimulus-driven auditory-nerve spike timing (time code), whereas a coarser code, based on variations in the cochlear place of stimulation (place code), represents faster FM rates. We tested this theory in listeners with normal and impaired hearing, spanning a wide range of place-coding fidelity. Contrary to predictions, sensitivity to both slow and fast FM correlated with place-coding fidelity. We also used incoherent AM on two carriers to simulate place coding of FM and observed poorer sensitivity at high carrier frequencies and fast rates, two properties of FM detection previously ascribed to the limits of time coding. The results suggest a unitary place-based neural code for FM across all rates and carrier frequencies.

Correction to: Speech Perception with Spectrally Non-overlapping Maskers as Measure of Spectral Resolution in Cochlear Implant Users.

An error in interpreting the statistical analysis output led to reporting errors in some of the effect sizes for the three-way repeated-measures ANOVAs in Experiment 1.

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.

Cognitive factors contribute to speech perception in cochlear-implant users and age-matched normal-hearing listeners under vocoded conditions.

This study examined the contribution of perceptual and cognitive factors to speech-perception abilities in cochlear-implant (CI) users. Thirty CI users were tested on word intelligibility in sentences with and without semantic context, presented in quiet and in noise. Performance was compared with measures of spectral-ripple detection and discrimination, thought to reflect peripheral processing, as well as with cognitive measures of working memory and non-verbal intelligence. Thirty age-matched and thirty younger normal-hearing (NH) adults also participated, listening via tone-excited vocoders, adjusted to produce mean performance for speech in noise comparable to that of the CI group. Results suggest that CI users may rely more heavily on semantic context than younger or older NH listeners, and that non-auditory working memory explains significant variance in the CI and age-matched NH groups. Between-subject variability in spectral-ripple detection thresholds was similar across groups, despite the spectral resolution for all NH listeners being limited by the same vocoder, whereas speech perception scores were more variable between CI users than between NH listeners. The results highlight the potential importance of central factors in explaining individual differences in CI users and question the extent to which standard measures of spectral resolution in CIs reflect purely peripheral processing.

Comparing Rapid and Traditional Forward-Masked Spatial Tuning Curves in Cochlear-Implant Users.

A rapid forward-masked spatial tuning curve measurement procedure, based on Bekesy tracking, was adapted and evaluated for use with cochlear implants. Twelve postlingually-deafened adult cochlear-implant users participated. Spatial tuning curves using the new procedure and using a traditional forced-choice adaptive procedure resulted in similar estimates of parameters. The Bekesy-tracking method was almost 3 times faster than the forced-choice procedure, but its test-retest reliability was significantly poorer. Although too time-consuming for general clinical use, the new method may have some benefits in individual cases, where identifying electrodes with poor spatial selectivity as candidates for deactivation is deemed necessary.

Speech Perception with Spectrally Non-overlapping Maskers as Measure of Spectral Resolution in Cochlear Implant Users.

Poor spectral resolution contributes to the difficulties experienced by cochlear implant (CI) users when listening to speech in noise. However, correlations between measures of spectral resolution and speech perception in noise have not always been found to be robust. It may be that the relationship between spectral resolution and speech perception in noise becomes clearer in conditions where the speech and noise are not spectrally matched, so that improved spectral resolution can assist in separating the speech from the masker. To test this prediction, speech intelligibility was measured with noise or tone maskers that were presented either in the same spectral channels as the speech or in interleaved spectral channels. Spectral resolution was estimated via a spectral ripple discrimination task. Results from vocoder simulations in normal-hearing listeners showed increasing differences in speech intelligibility between spectrally overlapped and interleaved maskers as well as improved spectral ripple discrimination with increasing spectral resolution. However, no clear differences were observed in CI users between performance with spectrally interleaved and overlapped maskers, or between tone and noise maskers. The results suggest that spectral resolution in current CIs is too poor to take advantage of the spectral separation produced by spectrally interleaved speech and maskers. Overall, the spectrally interleaved and tonal maskers produce a much larger difference in performance between normal-hearing listeners and CI users than do traditional speech-in-noise measures, and thus provide a more sensitive test of speech perception abilities for current and future implantable devices.

Mammalian behavior and physiology converge to confirm sharper cochlear tuning in humans.

Frequency analysis of sound by the cochlea is the most fundamental property of the auditory system. Despite its importance, the resolution of this frequency analysis in humans remains controversial. The controversy persists because the methods used to estimate tuning in humans are indirect and have not all been independently validated in other species. Some data suggest that human cochlear tuning is considerably sharper than that of laboratory animals, while others suggest little or no difference between species. We show here in a single species (ferret) that behavioral estimates of tuning bandwidths obtained using perceptual masking methods, and objective estimates obtained using otoacoustic emissions, both also employed in humans, agree closely with direct physiological measurements from single auditory-nerve fibers. Combined with human behavioral data, this outcome indicates that the frequency analysis performed by the human cochlea is of significantly higher resolution than found in common laboratory animals. This finding raises important questions about the evolutionary origins of human cochlear tuning, its role in the emergence of speech communication, and the mechanisms underlying our ability to separate and process natural sounds in complex acoustic environments.

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.

A Dynamically Focusing Cochlear Implant Strategy Can Improve Vowel Identification in Noise.

OBJECTIVES: The standard, monopolar (MP) electrode configuration used in commercially available cochlear implants (CI) creates a broad electrical field, which can lead to unwanted channel interactions. Use of more focused configurations, such as tripolar and phased array, has led to mixed results for improving speech understanding. The purpose of the present study was to assess the efficacy of a physiologically inspired configuration called dynamic focusing, using focused tripolar stimulation at low levels and less focused stimulation at high levels. Dynamic focusing may better mimic cochlear excitation patterns in normal acoustic hearing, while reducing the current levels necessary to achieve sufficient loudness at high levels. DESIGN: Twenty postlingually deafened adult CI users participated in the study. Speech perception was assessed in quiet and in a four-talker babble background noise. Speech stimuli were closed-set spondees in noise, and medial vowels at 50 and 60 dB SPL in quiet and in noise. The signal to noise ratio was adjusted individually such that performance was between 40 and 60% correct with the MP strategy. Subjects were fitted with three experimental strategies matched for pulse duration, pulse rate, filter settings, and loudness on a channel-by-channel basis. The strategies included 14 channels programmed in MP, fixed partial tripolar (σ = 0.8), and dynamic partial tripolar (σ at 0.8 at threshold and 0.5 at the most comfortable level). Fifteen minutes of listening experience was provided with each strategy before testing. Sound quality ratings were also obtained. RESULTS: Speech perception performance for vowel identification in quiet at 50 and 60 dB SPL and for spondees in noise was similar for the three tested strategies. However, performance on vowel identification in noise was significantly better for listeners using the dynamic focusing strategy. Sound quality ratings were similar for the three strategies. Some subjects obtained more benefit than others, with some individual differences explained by the relation between loudness growth and the rate of change from focused to broader stimulation. CONCLUSIONS: These initial results suggest that further exploration of dynamic focusing is warranted. Specifically, optimizing such strategies on an individual basis may lead to improvements in speech perception for more adult listeners and improve how CIs are tailored. Some listeners may also need a longer period of time to acclimate to a new program.

Speech intelligibility is best predicted by intensity, not cochlea-scaled entropy.

Cochlea-scaled entropy (CSE) is a measure of spectro-temporal change that has been reported to predict the contribution of speech segments to overall intelligibility. This paper confirms that CSE is highly correlated with intensity, making it impossible to determine empirically whether it is CSE or simply intensity that determines speech importance. A more perceptually relevant version of CSE that uses dB-scaled differences, rather than differences in linear amplitude, failed to predict speech intelligibility. Overall, a parsimonious account of the available data is that the importance of speech segments to overall intelligibility is best predicted by their relative intensity, not by CSE.

Assessing the Role of Place and Timing Cues in Coding Frequency and Amplitude Modulation as a Function of Age.

Natural sounds can be characterized by their fluctuations in amplitude and frequency. Ageing may affect sensitivity to some forms of fluctuations more than others. The present study used individual differences across a wide age range (20-79 years) to test the hypothesis that slow-rate, low-carrier frequency modulation (FM) is coded by phase-locked auditory-nerve responses to temporal fine structure (TFS), whereas fast-rate FM is coded via rate-place (tonotopic) cues, based on amplitude modulation (AM) of the temporal envelope after cochlear filtering. Using a low (500 Hz) carrier frequency, diotic FM and AM detection thresholds were measured at slow (1 Hz) and fast (20 Hz) rates in 85 listeners. Frequency selectivity and TFS coding were assessed using forward masking patterns and interaural phase disparity tasks (slow dichotic FM), respectively. Comparable interaural level disparity tasks (slow and fast dichotic AM and fast dichotic FM) were measured to control for effects of binaural processing not specifically related to TFS coding. Thresholds in FM and AM tasks were correlated, even across tasks thought to use separate peripheral codes. Age was correlated with slow and fast FM thresholds in both diotic and dichotic conditions. The relationship between age and AM thresholds was generally not significant. Once accounting for AM sensitivity, only diotic slow-rate FM thresholds remained significantly correlated with age. Overall, results indicate stronger effects of age on FM than AM. However, because of similar effects for both slow and fast FM when not accounting for AM sensitivity, the effects cannot be unambiguously ascribed to TFS coding.

Auditory Enhancement in Cochlear-Implant Users Under Simultaneous and Forward Masking.

Auditory enhancement is the phenomenon whereby the salience or detectability of a target sound within a masker is enhanced by the prior presentation of the masker alone. Enhancement has been demonstrated using both simultaneous and forward masking in normal-hearing listeners and may play an important role in auditory and speech perception within complex and time-varying acoustic environments. The few studies of enhancement in hearing-impaired listeners have reported reduced or absent enhancement effects under forward masking, suggesting a potentially peripheral locus of the effect. Here, auditory enhancement was measured in eight cochlear-implant (CI) users with direct stimulation. Masked thresholds were measured under simultaneous and forward masking as a function of the number of masking electrodes, and the electrode spacing between the maskers and the target. Evidence for auditory enhancement was obtained under simultaneous masking, qualitatively consistent with results from normal-hearing listeners. However, no significant enhancement was observed under forward masking, in contrast to earlier results with normal-hearing listeners. The results suggest that the normal effects of auditory enhancement are partially but not fully experienced by CI users. To the extent that the CI users' results differ from normal, it may be possible to apply signal processing to restore the missing aspects of enhancement.

Speech Masking in Normal and Impaired Hearing: Interactions Between Frequency Selectivity and Inherent Temporal Fluctuations in Noise.

Recent studies in normal-hearing listeners have used envelope-vocoded stimuli to show that the masking of speech by noise is dominated by the temporal-envelope fluctuations inherent in noise, rather than just overall power. Because these studies were based on vocoding, it was expected that cochlear-implant (CI) users would demonstrate a similar sensitivity to inherent fluctuations. In contrast, it was found that CI users showed no difference in speech intelligibility between maskers with and without inherent envelope fluctuations. Here, these initial findings in CI users were extended to listeners with cochlear hearing loss and the results were compared with those from normal-hearing listeners at either equal sensation level or equal sound pressure level. The results from hearing-impaired listeners (and in normal-hearing listeners at high sound levels) are consistent with a relative reduction in low-frequency inherent noise fluctuations due to broader cochlear filtering. The reduced effect of inherent temporal fluctuations in noise, due to either current spread (in CI users) or broader cochlear filters (in hearing-impaired listeners), provides a new way to explain the loss of masking release experienced in CI users and hearing-impaired listeners when additional amplitude fluctuations are introduced in noise maskers.

Loudness Context Effects in Normal-Hearing Listeners and Cochlear-Implant Users.

Context effects in loudness have been observed in normal auditory perception and may reflect a general gain control of the auditory system. However, little is known about such effects in cochlear-implant (CI) users. Discovering whether and how CI users experience loudness context effects should help us better understand the underlying mechanisms. In the present study, we examined the effects of a long-duration (1-s) intense precursor on the loudness relations between shorter-duration (200-ms) target and comparison stimuli. The precursor and target were separated by a silent gap of 50 ms, and the target and comparison were separated by a silent gap of 2 s. For normal-hearing listeners, the stimuli were narrowband noises; for CI users, all stimuli were delivered as pulse trains directly to the implant. Significant changes in loudness were observed in normal-hearing listeners, in line with earlier studies. The CI users also experienced some loudness changes but, in contrast to the results from normal-hearing listeners, the effect did not increase with increasing level difference between precursor and target. A "dual-process" hypothesis, used to explain earlier data from normal-hearing listeners, may provide an account of the present data by assuming that one of the two mechanisms, involving "induced loudness reduction," was absent or reduced in CI users.

A fast method for measuring psychophysical thresholds across the cochlear implant array.

A rapid threshold measurement procedure, based on Bekesy tracking, is proposed and evaluated for use with cochlear implants (CIs). Fifteen postlingually deafened adult CI users participated. Absolute thresholds for 200-ms trains of biphasic pulses were measured using the new tracking procedure and were compared with thresholds obtained with a traditional forced-choice adaptive procedure under both monopolar and quadrupolar stimulation. Virtual spectral sweeps across the electrode array were implemented in the tracking procedure via current steering, which divides the current between two adjacent electrodes and varies the proportion of current directed to each electrode. Overall, no systematic differences were found between threshold estimates with the new channel sweep procedure and estimates using the adaptive forced-choice procedure. Test-retest reliability for the thresholds from the sweep procedure was somewhat poorer than for thresholds from the forced-choice procedure. However, the new method was about 4 times faster for the same number of repetitions. Overall the reliability and speed of the new tracking procedure provides it with the potential to estimate thresholds in a clinical setting. Rapid methods for estimating thresholds could be of particular clinical importance in combination with focused stimulation techniques that result in larger threshold variations between electrodes.

Speech perception in tones and noise via cochlear implants reveals influence of spectral resolution on temporal processing.

Under normal conditions, human speech is remarkably robust to degradation by noise and other distortions. However, people with hearing loss, including those with cochlear implants, often experience great difficulty in understanding speech in noisy environments. Recent work with normal-hearing listeners has shown that the amplitude fluctuations inherent in noise contribute strongly to the masking of speech. In contrast, this study shows that speech perception via a cochlear implant is unaffected by the inherent temporal fluctuations of noise. This qualitative difference between acoustic and electric auditory perception does not seem to be due to differences in underlying temporal acuity but can instead be explained by the poorer spectral resolution of cochlear implants, relative to the normally functioning ear, which leads to an effective smoothing of the inherent temporal-envelope fluctuations of noise. The outcome suggests an unexpected trade-off between the detrimental effects of poorer spectral resolution and the beneficial effects of a smoother noise temporal envelope. This trade-off provides an explanation for the long-standing puzzle of why strong correlations between speech understanding and spectral resolution have remained elusive. The results also provide a potential explanation for why cochlear-implant users and hearing-impaired listeners exhibit reduced or absent masking release when large and relatively slow temporal fluctuations are introduced in noise maskers. The multitone maskers used here may provide an effective new diagnostic tool for assessing functional hearing loss and reduced spectral resolution.