The relative weighting of acoustic properties in the perception of [s] + stop clusters by children and adults

Influence of Audibility and Distortion on Recognition of Reverberant Speech for Children and Adults with Hearing Aid Amplification

BACKGROUND

Adults and children with sensorineural hearing loss (SNHL) have trouble understanding speech in rooms with reverberation when using hearing aid amplification. While the use of amplitude compression signal processing in hearing aids may contribute to this difficulty, there is conflicting evidence on the effects of amplitude compression settings on speech recognition. Less clear is the effect of a fast release time for adults and children with SNHL when using compression ratios derived from a prescriptive procedure.

PURPOSE

The aim of the study is to determine whether release time impacts speech recognition in reverberation for children and adults with SNHL and to determine if these effects of release time and reverberation can be predicted using indices of audibility or temporal and spectral distortion.

RESEARCH DESIGN

This is a quasi-experimental cohort study. Participants used a hearing aid simulator set to the Desired Sensation Level algorithm m[i/o] for three different amplitude compression release times. Reverberation was simulated using three different reverberation times.

PARTICIPANTS

Participants were 20 children and 16 adults with SNHL.

DATA COLLECTION AND ANALYSES

Participants were seated in a sound-attenuating booth and then nonsense syllable recognition was measured. Predictions of speech recognition were made using indices of audibility, temporal distortion, and spectral distortion and the effects of release time and reverberation were analyzed using linear mixed models.

RESULTS

While nonsense syllable recognition decreased in reverberation release time did not significantly affect nonsense syllable recognition. Participants with lower audibility were more susceptible to the negative effect of reverberation on nonsense syllable recognition.

CONCLUSION

We have extended previous work on the effects of reverberation on aided speech recognition to children with SNHL. Variations in release time did not impact the understanding of speech. An index of audibility best predicted nonsense syllable recognition in reverberation and, clinically, these results suggest that patients with less audibility are more susceptible to nonsense syllable recognition in reverberation.

Factors responsible for remote-frequency masking in children and adults

Susceptibility to remote-frequency masking in children and adults was evaluated with respect to three stimulus features: (1) masker bandwidth, (2) spectral separation of the signal and masker, and (3) gated versus continuous masker presentation. Listeners were 4- to 6-year-olds, 7- to 10-year-olds, and adults. Detection thresholds for a 500-ms, 2000-Hz signal were estimated in quiet or presented with a band of noise in one of four frequency regions: 425-500 Hz, 4000-4075 Hz, 8000-8075 Hz, or 4000-10 000 Hz. In experiment 1, maskers were gated on in each 500-ms interval of a three-interval, forced-choice adaptive procedure. Masking was observed for all ages in all maskers, but the greatest masking was observed for the 4000-4075 Hz masker. These findings suggest that signal/masker spectral proximity plays an important role in remote-frequency masking, even when peripheral excitation associated with the signal and masker does not overlap. Younger children tended to have more masking than older children or adults, consistent with a reduced ability to segregate simultaneous sounds and/or listen in a frequency-selective manner. In experiment 2, detection thresholds were estimated in the same noises, but maskers were presented continuously. Masking was reduced for all ages relative to gated conditions, suggesting improved segregation and/or frequency-selective listening.

Speech sound learning depends on individuals' ability, not just experience

AIMS

The goal of this study was to investigate if phonetic experience with two languages facilitated the learning of novel speech sounds or if general perceptual abilities independent of bilingualism played a role in this learning.

METHOD

The underlying neural mechanisms involved in novel speech sound learning were observed in groups of English monolinguals (n = 20), early Spanish-English bilinguals (n = 24), and experimentally derived subgroups of individuals with advanced ability to learn novel speech sound contrasts (ALs, n = 28) and individuals with non-advanced ability to learn novel speech sound contrasts (non-ALs, n = 16). Subjects participated in four consecutive sessions of phonetic training in which they listened to novel speech sounds embedded in Hungarian pseudowords. Participants completed two fMRI sessions, one before training and another one after training. While in the scanner, participants passively listened to the speech stimuli presented during training. A repeated measures behavioral analysis and ANOVA for fMRI data were conducted to investigate learning after training.

RESULTS AND CONCLUSIONS

The results showed that bilinguals did not significantly differ from monolinguals in the learning of novel sounds behaviorally. Instead, the behavioral results revealed that regardless of language group (monolingual or bilingual), ALs were better at discriminating pseudowords throughout the training than non-ALs. Neurally, region of interest (ROI) analysis showed increased activity in the superior temporal gyrus (STG) bilaterally in ALs relative to non-ALs after training. Bilinguals also showed greater STG activity than monolinguals. Extracted values from ROIs entered into a 2×2 MANOVA showed a main effect of performance, demonstrating that individual ability exerts a significant effect on learning novel speech sounds. In fact, advanced ability to learn novel speech sound contrasts appears to play a more significant role in speech sound learning than experience with two phonological systems.

Phonetic categorisation and cue weighting in adolescents with Specific Language Impairment (SLI)

This study investigates phonetic categorisation and cue weighting in adolescents and young adults with Specific Language Impairment (SLI). We manipulated two acoustic cues, vowel duration and F1 offset frequency, that signal word-final stop consonant voicing ([t] and [d]) in English. Ten individuals with SLI (14.0-21.4 years), 10 age-matched controls (CA; 14.6-21.9 years) and 10 non-matched adult controls (23.3-36.0 years) labelled synthetic CVC non-words in an identification task. The results showed that the adolescents and young adults with SLI were less consistent than controls in the identification of the good category representatives. The group with SLI also assigned less weight to vowel duration than the adult controls. However, no direct relationship between phonetic categorisation, cue weighting and language skills was found. These findings indicate that some individuals with SLI have speech perception deficits but they are not necessarily associated with oral language skills.

A sensitive period for shibboleths: the long tail and changing goals of speech perception over the course of development

It is clear that the ability to learn new speech contrasts changes over development, such that learning to categorize speech sounds as native speakers of a language do is more difficult in adulthood than it is earlier in development. There is also a wealth of data concerning changes in the perception of speech sounds during infancy, such that infants quite rapidly progress from language-general to more language-specific perceptual biases. It is often suggested that the perceptual narrowing observed during infancy plays a causal role in the loss of plasticity observed in adulthood, but the relationship between these two phenomena is complicated. Here I consider the relationship between changes in sensitivity to speech sound categorization over the first 2 years of life, when they appear to reorganize quite rapidly, to the "long tail" of development throughout childhood, in the context of understanding the sensitive period for speech perception.

Assessing toddlers' speech-sound discrimination

OBJECTIVE

Valid and reliable methods for assessing speech perception in toddlers are lacking in the field, leading to conspicuous gaps in understanding how speech perception develops and limited clinical tools for assessing sensory aid benefit in toddlers. The objective of this investigation was to evaluate speech-sound discrimination in toddlers using modifications to the Change/No-Change procedure [1].

METHODS

Normal-hearing 2- and 3-year-olds' discrimination of acoustically dissimilar ("easy") and similar ("hard") speech-sound contrasts were evaluated in a combined repeated measures and factorial design. Performance was measured in d'. Effects of contrast difficulty and age were examined, as was test-retest reliability, using repeated measures ANOVAs, planned post hoc tests, and correlation analyses.

RESULTS

The easy contrast (M=2.53) was discriminated better than the hard contrast (M=1.72) across all ages (p<.0001). The oldest group of children (M=3.13) discriminated the contrasts better than youngest (M=1.04; p<.0001) and the mid-age children (M=2.20; p=.037), who in turn discriminated the contrasts better than the youngest children (p=.010). Test-retest reliability was excellent (r=.886, p<.0001). Almost 90% of the children met the teaching criterion. The vast majority demonstrated the ability to be tested with the modified procedure and discriminated the contrasts. The few who did not were 2.5 years of age and younger.

CONCLUSIONS

The modifications implemented resulted, at least preliminarily, in a procedure that is reliable and sensitive to contrast difficulty and age in this young group of children, suggesting that these modifications are appropriate for this age group. With further development, the procedure holds promise for use in clinical populations who are believed to have core deficits in rapid phonological encoding, such as children with hearing loss or specific language impairment, children who are struggling to read, and second-language learners.

Cue weighting in auditory categorization: implications for first and second language acquisition

The ability to integrate and weight information across dimensions is central to perception and is particularly important for speech categorization. The present experiments investigate cue weighting by training participants to categorize sounds drawn from a two-dimensional acoustic space defined by the center frequency (CF) and modulation frequency (MF) of frequency-modulated sine waves. These dimensions were psychophysically matched to be equally discriminable and, in the first experiment, were equally informative for accurate categorization. Nevertheless, listeners' category responses reflected a bias for use of CF. This bias remained even when the informativeness of CF was decreased by shifting distributions to create more overlap in CF. A reversal of weighting (MF over CF) was obtained when distribution variance was increased for CF. These results demonstrate that even when equally informative and discriminable, acoustic cues are not necessarily equally weighted in categorization; listeners exhibit biases when integrating multiple acoustic dimensions. Moreover, changes in weighting strategies can be affected by changes in input distribution parameters. This methodology provides potential insights into acquisition of speech sound categories, particularly second language categories. One implication is that ineffective cue weighting strategies for phonetic categories may be alleviated by manipulating variance of uninformative dimensions in training stimuli.

Temporal processing deficits in the pre-senescent auditory system

This study tested the hypothesis that temporal processing deficits are evident in the pre-senescent (middle-aged) auditory system for listening tasks that involve brief stimuli, across-frequency-channel processing, and/or significant processing loads. A gap duration discrimination (GDD) task was employed that used either fixed-duration gap markers (experiment 1) or random-duration markers (experiment 2). Independent variables included standard gap duration (0, 35, and 250 ms), marker frequency (within- and across-frequency), and task complexity. A total of 18 young and 23 middle-aged listeners with normal hearing participated in the GDD experiments. Middle age was defined operationally as 40-55 years of age. The results indicated that middle-aged listeners performed more poorly than the young listeners in general, and that this deficit was sometimes, but not always, exacerbated by increases in task complexity. A third experiment employed a categorical perception task that measured the gap duration associated with a perceptual boundary. The results from 12 young and 12 middle-aged listeners with normal hearing indicated that the categorical boundary was associated with shorter gaps in the young listeners. The results of these experiments indicate that temporal processing deficits can be observed relatively early in the aging process, and are evident in middle age.

The role of early language experience in the development of speech perception and phonological processing abilities: evidence from 5-year-olds with histories of otitis media with effusion and low socioeconomic status

This study tested the hypothesis that early language experience facilitates the development of language-specific perceptual weighting strategies believed to be critical for accessing phonetic structure. In turn, that structure allows for efficient storage and retrieval of words in verbal working memory, which is necessary for sentence comprehension. Participants were forty-nine 5-year-olds, evenly distributed among four groups: those with chronic otitis media with effusion (OME), low socio-economic status (low-SES), both conditions (both), or neither condition (control). All children participated in tasks of speech perception and phonological awareness. Children in the control and OME groups participated in additional tasks examining verbal working memory, sentence comprehension, and temporal processing. The temporal-processing task tested the hypothesis that any deficits observed on the language-related tasks could be explained by temporal-processing deficits. Children in the three experimental groups demonstrated similar results to each other, but different from the control group for speech perception and phonological awareness. Children in the OME group differed from those in the control group on tasks involving verbal working memory and sentence comprehension, but not temporal processing. Overall these results supported the major hypothesis explored, but failed to support the hypothesis that language problems are explained to any extent by temporal-processing problems.

LEARNING OUTCOMES

As a result of this activity, the participant will be able to (1) Explain the relation between language experience and the development of mature speech perception strategies, phonological awareness, verbal working memory, and syntactic comprehension. (2) Name at least three populations of individuals who exhibit delays in the development of mature speech perception strategies, phonological awareness, verbal working memory, and syntactic comprehension, and explain why these delays exist for each group. (3) Point out why perceptual strategies for speech are different for different languages. (4) Describe Baddeley's model [A.D. Baddeley, The development of the concept of working memory: implications and contributions of neuropsychology, in: G. Vallar, T. Shallice (Eds.), Neuropsychological Impairments of Short-term Memory, Cambridge University Press, New York, 1990, p. 54] of verbal working memory.

The role of temporal and dynamic signal components in the perception of syllable-final stop voicing by children and adults

Adults whose native languages permit syllable-final obstruents, and show a vocalic length distinction based on the voicing of those obstruents, consistently weight vocalic duration strongly in their perceptual decisions about the voicing of final stops, at least in laboratory studies using synthetic speech. Children, on the other hand, generally disregard such signal properties in their speech perception, favoring formant transitions instead. These age-related differences led to the prediction that children learning English as a native language would weight vocalic duration less than adults, but weight syllable-final transitions more in decisions of final-consonant voicing. This study tested that prediction. In the first experiment, adults and children (eight and six years olds) labeled synthetic and natural CVC words with voiced or voiceless stops in final C position. Predictions were strictly supported for synthetic stimuli only. With natural stimuli it appeared that adults and children alike weighted syllable-offset transitions strongly in their voicing decisions. The predicted age-related difference in the weighting of vocalic duration was seen for these natural stimuli almost exclusively when syllable-final transitions signaled a voiced final stop. A second experiment with adults and children (seven and five years old) replicated these results for natural stimuli with four new sets of natural stimuli. It was concluded that acoustic properties other than vocalic duration might play more important roles in voicing decisions for final stops than commonly asserted, sometimes even taking precedence over vocalic duration.

Coherence in children's speech perception

Studies with adults have demonstrated that acoustic cues cohere in speech perception such that two stimuli cannot be discriminated if separate cues bias responses equally, but oppositely, in each. This study examined whether this kind of coherence exists for children's perception of speech signals, a test that first required that a contrast be found for which adults and children show similar cue weightings. Accordingly, experiment 1 demonstrated that adults, 7-, and 5-year-olds weight F2-onset frequency and gap duration similarly in "spa" versus "sa" decisions. In experiment 2, listeners of these same ages made "same" or "not-the-same" judgments for pairs of stimuli in an AX paradigm when only one cue differed, when the two cues were set within a stimulus to bias the phonetic percept towards the same category (relative to the other stimulus in the pair), and when the two cues were set within a stimulus to bias the phonetic percept towards different categories. Unexpectedly, adults' results contradicted earlier studies: They were able to discriminate stimuli when the two cues conflicted in how they biased phonetic percepts. Results for 7-year-olds replicated those of adults, but were not as strong. Only the results of 5-year-olds revealed the kind of perceptual coherence reported by earlier studies for adults. Thus, it is concluded that perceptual coherence for speech signals is present from an early age, and in fact listeners learn to overcome it under certain conditions.

Effect of stimulus bandwidth on the perception of /s/ in normal- and hearing-impaired children and adults

Recent studies with adults have suggested that amplification at 4 kHz and above fails to improve speech recognition and may even degrade performance when high-frequency thresholds exceed 50-60 dB HL. This study examined the extent to which high frequencies can provide useful information for fricative perception for normal-hearing and hearing-impaired children and adults. Eighty subjects (20 per group) participated. Nonsense syllables containing the phonemes /s/, /f/, and /O/, produced by a male, female, and child talker, were low-pass filtered at 2, 3, 4, 5, 6, and 9 kHz. Frequency shaping was provided for the hearing-impaired subjects only. Results revealed significant differences in recognition between the four groups of subjects. Specifically, both groups of children performed more poorly than their adult counterparts at similar bandwidths. Likewise, both hearing-impaired groups performed more poorly than their normal-hearing counterparts. In addition, significant talker effects for /s/ were observed. For the male talker, optimum performance was reached at a bandwidth of approximately 4-5 kHz, whereas optimum performance for the female and child talkers did not occur until a bandwidth of 9 kHz.

Challenging the notion of innate phonetic boundaries

Numerous studies of infants' speech perception abilities have demonstrated that these young listeners have access to acoustic detail in the speech signal. Because these studies have used stimuli that could be described in terms of adult-defined phonetic categories, authors have concluded that infants innately recognize stimuli as members of these categories, as adults do. In fact, the predominant, current view of speech perception holds that infants are born with sensitivities for the universal set of phonetic boundaries, and that those boundaries supported by the ambient language are maintained, while those not supported by the ambient language dissolve. In this study, discrimination abilities of 46 infants and 75 3-year-olds were measured for several phonetic contrasts occurring in their native language, using natural and synthetic speech. The proportion of children who were able to discriminate any given contrast varied across contrasts, and no one contrast was discriminated by anything close to all of the children. While these results did not differ from those reported by others, the interpretation here is that we should reconsider the notion of innate phonetic categories and/or boundaries. Moreover, success rates did not differ for natural and synthetic speech, and so a minor conclusion was that children are not adversely affected by the use of synthetic stimuli in speech experiments.

Diverse acoustic cues at consonantal landmarks

The consonantal segments that underlie an utterance are manifested in the acoustic signal by abrupt discontinuities or dislocations in the spectral pattern. There are potentially two such discontinuities for each consonant, corresponding to the formation and release of a constriction in the oral cavity by the lips, the tongue blade, or the tongue body. Acoustic cues for the various consonant features of place, voicing and nasality reside in the signal in quite different forms on the two sides of each acoustic discontinuity. Examples of these diverse cues and their origin in acoustic theory are reviewed, with special attention to place features and features related to the laryngeal state and to nasalization. A listener appears to have the ability to integrate these diverse, brief acoustic cues for the features of consonants, although the mechanism for this integration process is unclear.

The effect of segmental order on fricative labeling by children and adults

We examined whether children modify their perceptual weighting strategies for speech on the basis of the order of segments within a syllable, as adults do. To this end, fricative-vowel (FV) and vowel-fricative (VF) syllables were constructed with synthetic noises from an/[symbol: see text]/-to-/s/continuum combined with natural/a/and/u/portions with transitions appropriate for a preceding or a following /[symbol: see text]/or/s/. Stimuli were played in their original order to adults and children (ages of 7 and 5 years) in Experiment 1 and in reversed order in Experiment 2. The results for adults and, to a lesser extent, those for 7-year-olds replicated earlier results showing that adults assign different perceptual weights to acoustic properties, depending on segmental order. In contrast, results for 5-year-olds suggested that these listeners applied the same strategies during fricative labeling, regardless of segmental order. Thus, the flexibility to modify perceptual weighting strategies for speech according to segmental order apparently emerges with experience.

Examining the role of auditory sensitivity in the developmental weighting shift

Studies comparing children's and adults' labeling of speech stimuli have repeatedly shown that children's phonological decisions are more strongly related to portions of the signal that involve rapid spectral change (i.e., formant transitions) and less related to other signal components than are adults' decisions. Such findings have led to a model termed the Developmental Weighting Shift, which suggests that children initially assign particularly strong weight to formant transitions to help delimit individual words in the continuous speech stream but gradually modify these strategies to be more like those of adults as they learn about word-internal structure. The goal of the current study was to test a reasonable alternative: that these apparent age-related differences in perceptual weighting strategies for speech are instead due to age-related differences in auditory sensitivity. To this end, difference limens (DLs) were obtained from children (ages 5 and 7 years) and adults for three types of acoustic properties: dynamic-spectral, static-spectral, and temporal. Two testable hypotheses were offered: Labeling results could reflect either absolute differences in sensitivity between children and adults or relative differences in sensitivity within each group. Empirical support for either hypothesis would indicate that apparent developmental changes in perceptual weighting strategies are actually due to developmental changes in auditory sensitivity to acoustic properties. Results of this study contradicted predictions of both hypotheses, sustaining the suggestion that children's perceptual weighting strategies for speech-relevant acoustic properties change as they gain experience with a native language.