Sensorineural hearing loss alters auditory discrimination of natural soundscapes.

OBJECTIVE: The ability to discriminate natural soundscapes recorded in a temperate terrestrial biome was measured in 15 hearing-impaired (HI) listeners with bilateral, mild to severe sensorineural hearing loss and 15 normal-hearing (NH) controls. DESIGN: Soundscape discrimination was measured using a three-interval oddity paradigm and the method of constant stimuli. On each trial, sequences of 2-second recordings varying the habitat, season and period of the day were presented diotically at a nominal SPL of 60 or 80 dB. RESULTS: Discrimination scores were above chance level for both groups, but they were poorer for HI than NH listeners. On average, the scores of HI listeners were relatively well accounted for by those of NH listeners tested with stimuli spectrally-shaped to match the frequency-dependent reduction in audibility of individual HI listeners. However, the scores of HI listeners were not significantly correlated with pure-tone audiometric thresholds and age. CONCLUSIONS: These results indicate that the ability to discriminate natural soundscapes associated with changes in habitat, season and period of the day is disrupted but it is not abolished. The deficits of the HI listeners are partly accounted for by reduced audibility. Supra-threshold auditory deficits and individual listening strategies may also explain differences between NH and HI listeners.

Human Auditory Ecology: Extending Hearing Research to the Perception of Natural Soundscapes by Humans in Rapidly Changing Environments.

Research in hearing sciences has provided extensive knowledge about how the human auditory system processes speech and assists communication. In contrast, little is known about how this system processes "natural soundscapes," that is the complex arrangements of biological and geophysical sounds shaped by sound propagation through non-anthropogenic habitats [Grinfeder et al. (2022). Frontiers in Ecology and Evolution. 10: 894232]. This is surprising given that, for many species, the capacity to process natural soundscapes determines survival and reproduction through the ability to represent and monitor the immediate environment. Here we propose a framework to encourage research programmes in the field of "human auditory ecology," focusing on the study of human auditory perception of ecological processes at work in natural habitats. Based on large acoustic databases with high ecological validity, these programmes should investigate the extent to which this presumably ancestral monitoring function of the human auditory system is adapted to specific information conveyed by natural soundscapes, whether it operate throughout the life span or whether it emerges through individual learning or cultural transmission. Beyond fundamental knowledge of human hearing, these programmes should yield a better understanding of how normal-hearing and hearing-impaired listeners monitor rural and city green and blue spaces and benefit from them, and whether rehabilitation devices (hearing aids and cochlear implants) restore natural soundscape perception and emotional responses back to normal. Importantly, they should also reveal whether and how humans hear the rapid changes in the environment brought about by human activity.

Auditory discrimination of natural soundscapes.

A previous modelling study reported that spectro-temporal cues perceptually relevant to humans provide enough information to accurately classify "natural soundscapes" recorded in four distinct temperate habitats of a biosphere reserve [Thoret, Varnet, Boubenec, Ferriere, Le Tourneau, Krause, and Lorenzi (2020). J. Acoust. Soc. Am. 147, 3260]. The goal of the present study was to assess this prediction for humans using 2 s samples taken from the same soundscape recordings. Thirty-one listeners were asked to discriminate these recordings based on differences in habitat, season, or period of the day using an oddity task. Listeners' performance was well above chance, demonstrating effective processing of these differences and suggesting a general high sensitivity for natural soundscape discrimination. This performance did not improve with training up to 10 h. Additional results obtained for habitat discrimination indicate that temporal cues play only a minor role; instead, listeners appear to base their decisions primarily on gross spectral cues related to biological sound sources and habitat acoustics. Convolutional neural networks were trained to perform a similar task using spectro-temporal cues extracted by an auditory model as input. The results are consistent with the idea that humans exclude the available temporal information when discriminating short samples of habitats, implying a form of a sub-optimality.

The Influence of Noise Type and Semantic Predictability on Word Recall in Older Listeners and Listeners With Hearing Impairment.

PURPOSE: A dual-task paradigm was implemented to investigate how noise type and sentence context may interact with age and hearing loss to impact word recall during speech recognition. METHOD: Three noise types with varying degrees of temporal/spectrotemporal modulation were used: speech-shaped noise, speech-modulated noise, and three-talker babble. Participant groups included younger listeners with normal hearing (NH), older listeners with near-normal hearing, and older listeners with sensorineural hearing loss. An adaptive measure was used to establish the signal-to-noise ratio approximating 70% sentence recognition for each participant in each noise type. A word-recall task was then implemented while matching speech-recognition performance across noise types and participant groups. Random-intercept linear mixed-effects models were used to determine the effects of and interactions between noise type, sentence context, and participant group on word recall. RESULTS: The results suggest that noise type does not significantly impact word recall when word-recognition performance is controlled. When data from noise types were pooled and compared with quiet, and recall was assessed: older listeners with near-normal hearing performed well when either quiet backgrounds or high sentence context (or both) were present, but older listeners with hearing loss performed well only when both quiet backgrounds and high sentence context were present. Younger listeners with NH were robust to the detrimental effects of noise and low context. CONCLUSIONS: The general presence of noise has the potential to decrease word recall, but type of noise does not appear to significantly impact this observation when overall task difficulty is controlled. The presence of noise as well as deficits related to age and/or hearing loss appear to limit the availability of cognitive processing resources available for working memory during conversation in difficult listening environments. The conversation environments that impact these resources appear to differ depending on age and/or hearing status.

Effect of Dual-Carrier Processing on the Intelligibility of Concurrent Vocoded Sentences.

Purpose: The goal of this study was to examine the role of carrier cues in sound source segregation and the possibility to enhance the intelligibility of 2 sentences presented simultaneously. Dual-carrier (DC) processing (Apoux, Youngdahl, Yoho, & Healy, 2015) was used to introduce synthetic carrier cues in vocoded speech. Method: Listeners with normal hearing heard sentences processed either with a DC or with a traditional single-carrier (SC) vocoder. One group was asked to repeat both sentences in a sentence pair (Experiment 1). The other group was asked to repeat only 1 sentence of the pair and was provided additional segregation cues involving onset asynchrony (Experiment 2). Results: Both experiments showed that not only is the "target" sentence more intelligible in DC compared with SC, but the "background" sentence intelligibility is equally enhanced. The participants did not benefit from the additional segregation cues. Conclusions: The data showed a clear benefit of using a distinct carrier to convey each sentence (i.e., DC processing). Accordingly, the poor speech intelligibility in noise typically observed with SC-vocoded speech may be partly attributed to the envelope of independent sound sources sharing the same carrier. Moreover, this work suggests that noise reduction may not be the only viable option to improve speech intelligibility in noise for users of cochlear implants. Alternative approaches aimed at enhancing sound source segregation such as DC processing may help to improve speech intelligibility while preserving and enhancing the background.

The noise susceptibility of various speech bands.

The degrading influence of noise on various critical bands of speech was assessed. A modified version of the compound method [Apoux and Healy (2012) J. Acoust. Soc. Am. 132, 1078-1087] was employed to establish this noise susceptibility for each speech band. Noise was added to the target speech band at various signal-to-noise ratios to determine the amount of noise required to reduce the contribution of that band by 50%. It was found that noise susceptibility is not equal across the speech spectrum, as is commonly assumed and incorporated into modern indexes. Instead, the signal-to-noise ratio required to equivalently impact various speech bands differed by as much as 13 dB. This noise susceptibility formed an irregular pattern across frequency, despite the use of multi-talker speech materials designed to reduce the potential influence of a particular talker's voice. But basic trends in the pattern of noise susceptibility across the spectrum emerged. Further, no systematic relationship was observed between noise susceptibility and speech band importance. It is argued here that susceptibility to noise and band importance are different phenomena, and that this distinction may be underappreciated in previous works.

Speech-material and talker effects in speech band importance.

Band-importance functions created using the compound method [Apoux and Healy (2012). J. Acoust. Soc. Am. 132, 1078-1087] provide more detail than those generated using the ANSI technique, necessitating and allowing a re-examination of the influences of speech material and talker on the shape of the band-importance function. More specifically, the detailed functions may reflect, to a larger extent, acoustic idiosyncrasies of the individual talker's voice. Twenty-one band functions were created using standard speech materials and recordings by different talkers. The band-importance functions representing the same speech-material type produced by different talkers were found to be more similar to one another than functions representing the same talker producing different speech-material types. Thus, the primary finding was the relative strength of a speech-material effect and weakness of a talker effect. This speech-material effect extended to other materials in the same broad class (different sentence corpora) despite considerable differences in the specific materials. Characteristics of individual talkers' voices were not readily apparent in the functions, and the talker effect was restricted to more global aspects of talker (i.e., gender). Finally, the use of multiple talkers diminished any residual effect of the talker.

The Effect of Remote Masking on the Reception of Speech by Young School-Age Children.

Purpose: Psychoacoustic data indicate that infants and children are less likely than adults to focus on a spectral region containing an anticipated signal and are more susceptible to remote masking of a signal. These detection tasks suggest that infants and children, unlike adults, do not listen selectively. However, less is known about children's ability to listen selectively during speech recognition. Accordingly, the current study examines remote masking during speech recognition in children and adults. Method: Adults and 7- and 5-year-old children performed sentence recognition in the presence of various spectrally remote maskers. Intelligibility was determined for each remote-masker condition, and performance was compared across age groups. Results: It was found that speech recognition for 5-year-olds was reduced in the presence of spectrally remote noise, whereas the maskers had no effect on the 7-year-olds or adults. Maskers of different bandwidth and remoteness had similar effects. Conclusions: In accord with psychoacoustic data, young children do not appear to focus on a spectral region of interest and ignore other regions during speech recognition. This tendency may help account for their typically poorer speech perception in noise. This study also appears to capture an important developmental stage, during which a substantial refinement in spectral listening occurs.

Shifting Fundamental Frequency in Simulated Electric-Acoustic Listening: Effects of F0 Variation.

OBJECTIVE: Shifting the mean fundamental frequency (F0) of target speech down in frequency may be a way to provide the benefits of electric-acoustic stimulation (EAS) to cochlear implant (CI) users whose limited residual hearing precludes a benefit typically, even with amplification. However, previous study showed a decline in the amount of benefit at the greatest downward frequency shifts, and the authors hypothesized that this might be related to F0 variation. Thus, in the present study, the authors sought to determine the relationship between mean F0, F0 variation, and the benefits of combining electric stimulation from a CI with low-frequency residual acoustic hearing. DESIGN: The authors measured speech intelligibility in normal-hearing listeners using an EAS simulation consisting of a sine vocoder combined either with speech low-pass filtered at 500 Hz, or with a pure tone representing target F0. The authors used extracted target voice pitch information to modulate the tone, and manipulated both the frequency of the carrier (mean F0), as well as the standard deviation of the voice pitch information (F0 variation). RESULTS: A decline in EAS benefit was observed at the lowest mean F0 tested, but this decline disappeared when F0 variation was reduced to be proportional to the amount of the shift in frequency (i.e., when F0 was shifted logarithmically instead of linearly). CONCLUSION: Lowering mean F0 by shifting the frequency of a pure tone carrying target voice pitch information can provide as much EAS benefit as an unshifted tone, at least in the current simulation of EAS. These results may have implications for CI users with extremely limited residual acoustic hearing.

Dual-carrier processing to convey temporal fine structure cues: Implications for cochlear implants.

Speech intelligibility in noise can be degraded by using vocoder processing to alter the temporal fine structure (TFS). Here it is argued that this degradation is not attributable to the loss of speech information potentially present in the TFS. Instead it is proposed that the degradation results from the loss of sound-source segregation information when two or more carriers (i.e., TFS) are substituted with only one as a consequence of vocoder processing. To demonstrate this segregation role, vocoder processing involving two carriers, one for the target and one for the background, was implemented. Because this approach does not preserve the speech TFS, it may be assumed that any improvement in intelligibility can only be a consequence of the preserved carrier duality and associated segregation cues. Three experiments were conducted using this "dual-carrier" approach. All experiments showed substantial sentence intelligibility in noise improvements compared to traditional single-carrier conditions. In several conditions, the improvement was so substantial that intelligibility approximated that for unprocessed speech in noise. A foreseeable and potentially promising implication for the dual-carrier approach involves implementation into cochlear implant speech processors, where it may provide the TFS cues necessary to segregate speech from noise.

Speech-cue transmission by an algorithm to increase consonant recognition in noise for hearing-impaired listeners.

Consonant recognition was assessed following extraction of speech from noise using a more efficient version of the speech-segregation algorithm described in Healy, Yoho, Wang, and Wang [(2013) J. Acoust. Soc. Am. 134, 3029-3038]. Substantial increases in recognition were observed following algorithm processing, which were significantly larger for hearing-impaired (HI) than for normal-hearing (NH) listeners in both speech-shaped noise and babble backgrounds. As observed previously for sentence recognition, older HI listeners having access to the algorithm performed as well or better than young NH listeners in conditions of identical noise. It was also found that the binary masks estimated by the algorithm transmitted speech features to listeners in a fashion highly similar to that of the ideal binary mask (IBM), suggesting that the algorithm is estimating the IBM with substantial accuracy. Further, the speech features associated with voicing, manner of articulation, and place of articulation were all transmitted with relative uniformity and at relatively high levels, indicating that the algorithm and the IBM transmit speech cues without obvious deficiency. Because the current implementation of the algorithm is much more efficient, it should be more amenable to real-time implementation in devices such as hearing aids and cochlear implants.

Evidence for independent time-unit processing of speech using noise promoting or suppressing masking release (L).

The relative independence of time-unit processing during speech reception was examined. It was found that temporally interpolated noise, even at very high levels, had little effect on sentence recognition using masking-release conditions similar to those of Kwon et al. [(2012). J. Acoust. Soc. Am. 131, 3111-3119]. The current data confirm the earlier conclusions of Kwon et al. involving masking release based on the relative timing of speech and noise. These data also indicate substantial levels of independence in the time domain, which has implications for current theories of speech perception in noise.

Role and relative contribution of temporal envelope and fine structure cues in sentence recognition by normal-hearing listeners.

The present study investigated the role and relative contribution of envelope and temporal fine structure (TFS) to sentence recognition in noise. Target and masker stimuli were added at five different signal-to-noise ratios (SNRs) and filtered into 30 contiguous frequency bands. The envelope and TFS were extracted from each band by Hilbert decomposition. The final stimuli consisted of the envelope of the target/masker sound mixture at x dB SNR and the TFS of the same sound mixture at y dB SNR. A first experiment showed a very limited contribution of TFS cues, indicating that sentence recognition in noise relies almost exclusively on temporal envelope cues. A second experiment showed that replacing the carrier of a sound mixture with noise (vocoder processing) cannot be considered equivalent to disrupting the TFS of the target signal by adding a background noise. Accordingly, a re-evaluation of the vocoder approach as a model to further understand the role of TFS cues in noisy situations may be necessary. Overall, these data are consistent with the view that speech information is primarily extracted from the envelope while TFS cues are primarily used to detect glimpses of the target.

A glimpsing account of the role of temporal fine structure information in speech recognition.

Many behavioral studies have reported a significant decrease in intelligibility when the temporal fine structure (TFS) of a sound mixture is replaced with noise or tones (i.e., vocoder processing). This finding has led to the conclusion that TFS information is critical for speech recognition in noise. How the normal -auditory system takes advantage of the original TFS, however, remains unclear. Three -experiments on the role of TFS in noise are described. All three experiments measured speech recognition in various backgrounds while manipulating the envelope, TFS, or both. One experiment tested the hypothesis that vocoder processing may artificially increase the apparent importance of TFS cues. Another experiment evaluated the relative contribution of the target and masker TFS by disturbing only the TFS of the target or that of the masker. Finally, a last experiment evaluated the -relative contribution of envelope and TFS information. In contrast to previous -studies, however, the original envelope and TFS were both preserved - to some extent - in all conditions. Overall, the experiments indicate a limited influence of TFS and suggest that little speech information is extracted from the TFS. Concomitantly, these experiments confirm that most speech information is carried by the temporal envelope in real-world conditions. When interpreted within the framework of the glimpsing model, the results of these experiments suggest that TFS is primarily used as a grouping cue to select the time-frequency regions -corresponding to the target speech signal.

Band importance for sentences and words reexamined.

Band-importance functions were created using the "compound" technique [Apoux and Healy, J. Acoust. Soc. Am. 132, 1078-1087 (2012)] that accounts for the multitude of synergistic and redundant interactions that take place among speech bands. Functions were created for standard recordings of the speech perception in noise (SPIN) sentences and the Central Institute for the Deaf (CID) W-22 words using 21 critical-band divisions and steep filtering to eliminate the influence of filter slopes. On a given trial, a band of interest was presented along with four other bands having spectral locations determined randomly on each trial. In corresponding trials, the band of interest was absent and only the four other bands were present. The importance of the band of interest was determined by the difference between paired band-present and band-absent trials. Because the locations of the other bands changed randomly from trial to trial, various interactions occurred between the band of interest and other speech bands which provided a general estimate of band importance. Obtained band-importance functions differed substantially from those currently available for identical speech recordings. In addition to differences in the overall shape of the functions, especially for the W-22 words, a complex microstructure was observed in which the importance of adjacent frequency bands often varied considerably. This microstructure may result in better predictive power of the current functions.

Use of a compound approach to derive auditory-filter-wide frequency-importance functions for vowels and consonants.

Speech recognition in noise presumably relies on the number and spectral location of available auditory-filter outputs containing a relatively undistorted view of local target signal properties. The purpose of the present study was to estimate the relative weight of each of the 30 auditory-filter wide bands between 80 and 7563 Hz. Because previous approaches were not compatible with this goal, a technique was developed. Similar to the "hole" approach, the weight of a given band was assessed by comparing intelligibility in two conditions differing in only one aspect-the presence or absence of the band of interest. In contrast to the hole approach, however, random gaps were also created in the spectrum. These gaps were introduced to render the auditory system more sensitive to the removal of a single band and their location was randomized to provide a general view of the weight of each band, i.e., irrespective of the location of information elsewhere in the spectrum. Frequency-weighting functions derived using this technique confirmed the main contribution of the 400-2500 Hz frequency region. However, they revealed a complex microstructure, contrasting with the "bell curve" shape typically reported.

On the mechanisms involved in the recovery of envelope information from temporal fine structure.

Three experiments were designed to provide psychophysical evidence for the existence of envelope information in the temporal fine structure (TFS) of stimuli that were originally amplitude modulated (AM). The original stimuli typically consisted of the sum of a sinusoidally AM tone and two unmodulated tones so that the envelope and TFS could be determined a priori. Experiment 1 showed that normal-hearing listeners not only perceive AM when presented with the Hilbert fine structure alone but AM detection thresholds are lower than those observed when presenting the original stimuli. Based on our analysis, envelope recovery resulted from the failure of the decomposition process to remove the spectral components related to the original envelope from the TFS and the introduction of spectral components related to the original envelope, suggesting that frequency- to amplitude-modulation conversion is not necessary to recover envelope information from TFS. Experiment 2 suggested that these spectral components interact in such a way that envelope fluctuations are minimized in the broadband TFS. Experiment 3 demonstrated that the modulation depth at the original carrier frequency is only slightly reduced compared to the depth of the original modulator. It also indicated that envelope recovery is not specific to the Hilbert decomposition.

Relative contribution of target and masker temporal fine structure to the unmasking of consonants in noise.

The present study assessed the relative contribution of the "target" and "masker" temporal fine structure (TFS) when identifying consonants. Accordingly, the TFS of the target and that of the masker were manipulated simultaneously or independently. A 30 band vocoder was used to replace the original TFS of the stimuli with tones. Four masker types were used. They included a speech-shaped noise, a speech-shaped noise modulated by a speech envelope, a sentence, or a sentence played backward. When the TFS of the target and that of the masker were disrupted simultaneously, consonant recognition dropped significantly compared to the unprocessed condition for all masker types, except the speech-shaped noise. Disruption of only the target TFS led to a significant drop in performance with all masker types. In contrast, disruption of only the masker TFS had no effect on recognition. Overall, the present data are consistent with previous work showing that TFS information plays a significant role in speech recognition in noise, especially when the noise fluctuates over time. However, the present study indicates that listeners rely primarily on TFS information in the target and that the nature of the masker TFS has a very limited influence on the outcome of the unmasking process.

Relative contribution of off- and on-frequency spectral components of background noise to the masking of unprocessed and vocoded speech.

The present study examined the relative influence of the off- and on-frequency spectral components of modulated and unmodulated maskers on consonant recognition. Stimuli were divided into 30 contiguous equivalent rectangular bandwidths. The temporal fine structure (TFS) in each "target" band was either left intact or replaced with tones using vocoder processing. Recognition scores for 10, 15 and 20 target bands randomly located in frequency were obtained in quiet and in the presence of all 30 masker bands, only the off-frequency masker bands, or only the on-frequency masker bands. The amount of masking produced by the on-frequency bands was generally comparable to that produced by the broadband masker. However, the difference between these two conditions was often significant, indicating an influence of the off-frequency masker bands, likely through modulation interference or spectral restoration. Although vocoder processing systematically lead to poorer consonant recognition scores, the deficit observed in noise could often be attributed to that observed in quiet. These data indicate that (i) speech recognition is affected by the off-frequency components of the background and (ii) the nature of the target TFS does not systematically affect speech recognition in noise, especially when energetic masking and/or the number of target bands is limited.

On the number of auditory filter outputs needed to understand speech: further evidence for auditory channel independence.

The number of auditory filter outputs required to identify phonemes was estimated in two experiments. Stimuli were divided into 30 contiguous equivalent rectangular bandwidths (ERB(N)) spanning 80-7563Hz. Normal-hearing listeners were presented with limited numbers of bands having frequency locations determined randomly from trial to trial to provide a general view, i.e., irrespective of specific band location, of the number of 1-ERB(N)-wide speech bands needed to identify phonemes. The first experiment demonstrated that 20 such bands are required to accurately identify vowels, and 16 are required to identify consonants. In the second experiment, speech-shaped noise or time-reversed speech was introduced to the non-speech bands at various signal-to-noise ratios. Considerably elevated noise levels were necessary to substantially affect phoneme recognition, confirming a high degree of channel independence in the auditory system. The independence observed between auditory filter outputs supports current views of speech recognition in noise in which listeners extract and combine pieces of information randomly distributed both in time and frequency. These findings also suggest that the ability to partition incoming sounds into a large number of narrow bands, an ability often lost in cases of hearing impairment or cochlear implantation, is critical for speech recognition in noise.