Developmental changes in speech discrimination in infants

Development of amplitude modulation, voice onset time, and consonant identification in noise and reverberation

Children's speech understanding is vulnerable to indoor noise and reverberation: e.g., from classrooms. It is unknown how they develop the ability to use temporal acoustic cues, specifically amplitude modulation (AM) and voice onset time (VOT), which are important for perceiving distorted speech. Through three experiments, we investigated the typical development of AM depth detection in vowels (experiment I), categorical perception of VOT (experiment II), and consonant identification (experiment III) in quiet and in speech-shaped noise (SSN) and mild reverberation in 6- to 14-year-old children. Our findings suggested that AM depth detection using a naturally produced vowel at the rate of the fundamental frequency was particularly difficult for children and with acoustic distortions. While the VOT cue salience was monotonically attenuated with increasing signal-to-noise ratio of SSN, its utility for consonant discrimination was completely removed even under mild reverberation. The reverberant energy decay in distorting critical temporal cues provided further evidence that may explain the error patterns observed in consonant identification. By 11-14 years of age, children approached adult-like performance in consonant discrimination and identification under adverse acoustics, emphasizing the need for good acoustics for younger children as they develop auditory skills to process distorted speech in everyday listening environments.

Emergence of the cortical encoding of phonetic features in the first year of life

Even prior to producing their first words, infants are developing a sophisticated speech processing system, with robust word recognition present by 4-6 months of age. These emergent linguistic skills, observed with behavioural investigations, are likely to rely on increasingly sophisticated neural underpinnings. The infant brain is known to robustly track the speech envelope, however previous cortical tracking studies were unable to demonstrate the presence of phonetic feature encoding. Here we utilise temporal response functions computed from electrophysiological responses to nursery rhymes to investigate the cortical encoding of phonetic features in a longitudinal cohort of infants when aged 4, 7 and 11 months, as well as adults. The analyses reveal an increasingly detailed and acoustically invariant phonetic encoding emerging over the first year of life, providing neurophysiological evidence that the pre-verbal human cortex learns phonetic categories. By contrast, we found no credible evidence for age-related increases in cortical tracking of the acoustic spectrogram.

The acquisition of speech categories: Beyond perceptual narrowing, beyond unsupervised learning and beyond infancy

An early achievement in language is carving a variable acoustic space into categories. The canonical story is that infants accomplish this by the second year, when only unsupervised learning is plausible. I challenge this view, synthesizing five lines of developmental, phonetic and computational work. First, unsupervised learning may be insufficient given the statistics of speech (including infant-directed). Second, evidence that infants "have" speech categories rests on tenuous methodological assumptions. Third, the fact that the ecology of the learning environment is unsupervised does not rule out more powerful error driven learning mechanisms. Fourth, several implicit supervisory signals are available to older infants. Finally, development is protracted through adolescence, enabling richer avenues for development. Infancy may be a time of organizing the auditory space, but true categorization only arises via complex developmental cascades later in life. This has implications for critical periods, second language acquisition, and our basic framing of speech perception.

Acoustic Change Complex and Visually Reinforced Infant Speech Discrimination Measures of Vowel Contrast Detection

OBJECTIVES

To measure the effect of stimulus rate and vowel change direction on the acoustic change complex (ACC) latencies and amplitudes and compare ACC metrics to behavioral measures of vowel contrast detection for infants tested under the age of 1 year. We tested the hypothesis that the direction of spectral energy shift from a vowel change would result in differences in the ACC, owing to the sensitivity of cortical neurons to the direction of frequency change. We evaluated the effect of the stimulus rate (1/s versus 2/s) on the infants' ACC. We evaluated the ACC amplitude ratio's sensitivity (proportion of ACCs present for each change trial) and compared it to perceptual responses obtained using a visually reinforced infant speech discrimination paradigm (VRISD). This report provides normative data from infants for the ACC toward the ultimate goal of developing a clinically useful index of neural capacity for vowel discrimination.

DESIGN

Twenty-nine infants, nine females, 4.0 to 11.8 months of age, participated. All participants were born at full term and passed their newborn hearing screens. None had risk factors for hearing or neurologic impairment. Cortical auditory evoked potentials were obtained in response to synthesized vowel tokens /a/, /i/, /o/, and /u/ presented at a rate of 1- or 2/s in an oddball stimulus paradigm with a 25% probability of the deviant stimulus. All combinations of vowel tokens were tested at the two rates. The ACC was obtained in response to the deviant stimulus. The infants were also tested for vowel contrast detection using a VRISD paradigm with the same combinations of vowel tokens used for the ACC. The mean age at the time of the ACC test was 5.4 months, while the mean age at the behavioral test was 6.8 months.

RESULTS

Variations in ACC amplitude and latency occurred as a function of the initial vowel token and the contrast token. However, the hypothesis that the direction of vowel (spectral) change would result in significantly larger change responses for high-to-low spectral changes was not supported. The contrasts with /a/ as the leading vowel of the contrast pair resulted in the largest ACC amplitudes than other conditions. Significant differences in the ACC presence and amplitude were observed as a function of rate, with 2/s resulting in ACCs with the largest amplitude ratios. Latency effects of vowel contrast and rate were present, but not systematic. The ACC amplitude ratio's sensitivity for detecting a vowel contrast was greater for the 2/s rate than the 1/s rate. For an amplitude ratio criterion of ≥1.5, the sensitivity was 93% for ACC component P2-N2 at 2/s, whereas at 1/s sensitivity was 70%. VRISD tests of vowel-contrast detection had a 71% hit and a 21% false-positive rate. Many infants who could not reach performance criteria for VRISD had ACC amplitude ratios of ≥2.0.

CONCLUSIONS

The ACC for vowel contrasts presented at a rate of 2/s is a robust index of vowel-contrast detection when obtained in typically developing infants under the age of 1 year. The ACC is present in over 90% of infants tested at this rate when an amplitude ratio criterion of ≥1.5 is used to define a response. The amplitude ratio appears to be a sensitive metric for the difference between a control and contrast condition. The ACC can be obtained in infants who do not yet exhibit valid behavioral responses for vowel change contrasts and may be useful for estimating neural capacity for discriminating these sounds.

Phonetic discrimination, phonological awareness, and pre-literacy skills in Spanish-English dual language preschoolers

The current study explores variation in phonemic representation among Spanish-English dual language learners (DLLs, n = 60) who were dominant in English or in Spanish. Children were given a phonetic discrimination task with speech sounds that: 1) occur in English and Spanish, 2) are exclusive to English, and 3) are exclusive to Russian, during Fall (age m = 57 months) and Spring (age m = 62 months, n = 42). In Fall, English-dominant DLLs discriminated more accurately than Spanish-dominant DLLs between English-Spanish phones and English-exclusive phones. Both groups discriminated Russian phones at or close to chance. In Spring, however, groups no longer differed in discriminating English-exclusive phones and both groups discriminated Russian phones above chance. Additionally, joint English-Spanish and English-exclusive phonetic discrimination predicted children's phonological awareness in both groups. Results demonstrate plasticity in early childhood through diverse language exposure and suggest that phonemic representation begins to emerge driven by lexical restructuring.

Relationship between Behavioral Infant Speech Perception and Hearing Age for Children with Hearing Loss

(1) Background: Research has demonstrated that early intervention for children who are hard-of-hearing (CHH) facilitates improved language development. Early speech perception abilities may impact CHH outcomes and guide future intervention. The objective of this study was to examine the use of a conditioned head turn (CHT) task as a measure of speech discrimination in CHH using a clinically feasible protocol. (2) Methods: Speech perception was assessed for a consonant and vowel contrast among 57 CHH and 70 children with normal hearing (CNH) aged 5–17 months using a CHT paradigm. (3) Results: Regardless of hearing status, 74% of CHH and 77% of CNH could discriminate /a-i/, and 55% of CHH and 56% of CNH could discriminate /ba-da/. Regression models revealed that both CHH and CNH performed better on /ba-da/ at 70 dBA compared to 50 dBA. Performance by hearing age showed no speech perception differences for CNH and children with mild hearing loss for either contrast. However, children with hearing losses ≥ 41 dB HL performed significantly poorer than CNH for /a-i/. (4) Conclusions: This study demonstrates the clinical feasibility of assessing early speech perception in infants with hearing loss and replicates previous findings of speech perception abilities among CHH and CNH.

Introducing BITTSy: The Behavioral Infant & Toddler Testing System

This manuscript introduces BITTSy, the Behavioral Infant & Toddler Testing System. This software system is capable of running the headturn preference procedure, preferential looking, conditioned headturn, and visual fixation/habituation procedures. It uses only commercial-off-theshelf (COTS) hardware to implement the procedures in an affordable and space-efficient setup. The software package, example protocols and data sets, and manual are freely available and downloadable from go.umd.edu/BITTSy, making this entire set of procedures available to resource-limited labs. Researchers can easily use BITTSy at multiple sites in a uniform manner, resulting in a standardized, powerful research tool that can enhance cross-site research collaborations.

6-month-olds are sensitive to English morphology

Each language has its unique way to mark grammatical information such as gender, number and tense. For example, English marks number and tense/aspect information with morphological suffixes (e.g., -s or -ed). These morphological suffixes are crucial for language acquisition as they are the basic building blocks of syntax, encode relationships, and convey meaning. Previous research shows that English-learning infants recognize morphological suffixes attached to nonce words by the end of the first year, although even 8-month-olds recognize them when they are attached to known words. These results support an acquisition trajectory where discovery of meaning guides infants' acquisition of morphological suffixes. In this paper, we re-evaluated English-learning infants' knowledge of morphological suffixes in the first year of life. We found that 6-month-olds successfully segmented nonce words suffixed with -s, -ing, -ed and a pseudo-morpheme -sh. Additionally, they related nonce words suffixed with -s, but not -ing, -ed or a pseudo-morpheme -sh and stems. By 8-months, infants were also able to relate nonce words suffixed with -ing and stems. Our results show that infants demonstrate knowledge of morphological relatedness from the earliest stages of acquisition. They do so even in the absence of access to meaning. Based on these results, we argue for a developmental timeline where the acquisition of morphology is, at least, concurrent with the acquisition of phonology and meaning.

Stem similarity modulates infants' acquisition of phonological alternations

Phonemes have variant pronunciations depending on context. For instance, in American English, the [t] in pat [pæt] and the [d] in pad [pæd] are both realized with a tap [ɾ] when the -ing suffix is attached, [pæɾɪŋ]. We show that despite greater distributional and acoustic support for the [t]-tap alternation, 12-month-olds successfully relate taps to stems with a perceptually-similar final [d], not the dissimilar final-[t]. Thus, distributional learning of phonological alternations is constrained by infants' preference for the alternation of perceptually-similar segments. Further, the ability to relate variant surface forms emerges between 8- and 12-months. Our findings of biased learning provide further empirical support for a role for perceptual similarity in the acquisition of linguistically-relevant categories. We discuss the implications of our findings for phonological theory, language acquisition and models of the mental lexicon.

Auditory perception skills in children receiving simultaneous bilateral cochlear implants: early speech-discrimination results

PURPOSE

The objective of the present study is to evaluate early speech-perception abilities using VRISD in toddlers who received simultaneous bilateral CIs between 12 and 36 months of age and to compare them with the findings in NH infants and to monitor the development of speech-perception skills within 6 months after cochlear implantation.

METHODS

The VRISD test was performed using video visual reinforcement to assess speech-discrimination ability in the CI and NH groups. Four stimuli were used for testing in the present study: /a/, /i/, /ba/, and /da/. The two contrasts used for the research were /a-i/ and /ba-da/. Auditory and listening skills in the CI group were assessed using the IT-MAIS.

RESULTS

The responses to the /a-i/ and /ba-da/ phoneme contrast were found to be similar in the NH and CI groups. No statistically significant difference was found between the groups (p > 0.05). VRISD test result and the IT-MAIS score were highly correlated in CI group (p = 0.001, r = 0.822).

CONCLUSION

The VRISD test can be effectively used to evaluate the development of speech-discrimination skills in hearing-impaired babies before and after CI. This research suggests that the development of speech-perception ability with CI is seriously influenced by environmental exposure and sound access. To the best of our knowledge, this is the first study to evaluate the speech-perception skills in toddlers with simultaneous bilateral CI.

Phonemic contrasts under construction? Evidence from Basque

Attunement theories of speech perception development suggest that native-language exposure is one of the main factors shaping infants' phonemic discrimination capacity within the second half of their first year. Here, we focus on the role of acoustic-perceptual salience and language-specific experience by assessing the discrimination of acoustically subtle Basque sibilant contrasts. We used the infant-controlled version of the habituation procedure to assess discrimination in 6- to 7-month and 11- to 12-month-old infants who varied in their amount of exposure to Basque and Spanish. We observed no significant variation in the infants' discrimination behavior as a function of their linguistic experience. Infants in both age-groups exhibited poor discrimination, consistent with Basque adults finding these contrasts more difficult than some others. Our findings are in agreement with previous research showing that perceptual discrimination of subtle speech sound contrasts may follow a different developmental trajectory, where increased native-language exposure seems to be a requisite.

Assessing Fine-Grained Speech Discrimination in Young Children With Bilateral Cochlear Implants

HYPOTHESIS

Children of 2 to 3 years old with cochlear implants can perform consonant discriminations using fine-grained acoustic cues.

BACKGROUND

Children born with severe-to-profound deafness are provided with early cochlear implantation (<2 yr) to maximize oral communication outcomes. Little is known regarding their abilities to discriminate consonant contrasts for accurately identifying speech sounds.

METHODS

Using a Reaching for Sound paradigm to collect behavioral responses, consonant contrast discrimination was measured in 13 children with bilateral cochlear implants (BiCIs; aged 28-37 mo), and 13 age-matched normal-hearing (NH) children. Four contrast pairs were tested: 1) place + voicing, 2) place, 3) voicing, and 4) reduced voice-onset-time cue. Using standard processing strategies, electrodograms showing pulsatile stimulation patterns were created retrospectively to assess the spectral-temporal cues delivered through the clinical speech processors.

RESULTS

As a group, children with BiCIs were able to discriminate all consonant contrasts at a level that was above chance, but their performance was poorer than NH children. Larger individual variability in discrimination performance was found in children with BiCIs. Stepwise regression revealed that, in the place contrast, chronological age was correlated with improved discrimination performance among children with BiCIs.

CONCLUSION

Children with BiCIs were able to discriminate consonant contrasts using fine-grained spectral-temporal cues above chance level but more poorly than their NH peers. Electrodogram analysis confirmed the access to spectral-temporal cues in the consonant contrasts through clinical speech processors. However, the cue saliency might not have be enough for children with BiCIs to achieve the same discrimination accuracy as NH children.

The development of auditory functions

Typical development and maturation of the auditory system, at both the peripheral and central levels, is essential for the acquisition of speech, language, and auditory skills. The peripheral system generally encodes three basic parameters associated with auditory stimuli-time, frequency, and intensity. These acoustic cues are subsequently processed by the central auditory structures to reach and be perceived by the cerebral cortex. Observations of the human fetal and neonatal ear indicate that the peripheral auditory system is structurally and functionally adult-like at birth. In contrast, the central auditory system exhibits progressive anatomical and physiologic changes until early adulthood. Enriched experience with sound is fundamental and critical to auditory development. The absence of early and prolonged acoustic stimulation delays neuronal maturation, affecting the central auditory nervous system, in particular, and leading to atypical development. The present chapter reviews the various stages of development of the auditory system structures, especially the embryology of the human ear, before briefly presenting the trajectories of typical development of auditory abilities from infants to school-aged children.

Social visual stimuli increase infants suck response: A preliminary study

This study investigated whether visual stimuli (FACES vs. CARS) combined with the presence of maternal scent can influence suck patterning in healthy infants. Fifteen healthy full-term infants (six months and younger) were exposed to their mother’s scent during a visual preference paradigm consisting of FACES vs. CARS stimuli while sucking on a custom research pacifier. Infants looked significantly longer to the FACES compared to CARS, p = .041. Repeated Measures ANOVA revealed a significant main effect for non-nutritive suck (NNS) bursts and visual stimuli (p = .001) with the largest differences evident between FACES and when the infant looked away from the visual stimuli (p = 0.008) as well as between FACES and CARS (p = 0.026). These preliminary findings suggest that infants have more suck attempts when looking at FACES in the presence of maternal scent thereby indicating potent links between visual preference and suck behavior.

Development of fricative sound perception in Korean infants: The role of language experience and infants' initial sensitivity

In this paper, we report data on the development of Korean infants' perception of a rare fricative phoneme distinction. Korean fricative consonants have received much interest in the linguistic community due to the language's distinct categorization of sounds. Unlike many fricative contrasts utilized in most of the world's languages, Korean fricatives (/s*/-/s/) are all voiceless. Moreover, compared with other sound categories, fricatives have received very little attention in the speech perception development field and no studies thus far have examined Korean infants' development of native phonology in this domain. Using a visual habituation paradigm, we tested 4‒6-month-old and 7‒9-month-old Korean infants on their abilities to discriminate the Korean fricative pair in the [a] vowel context, /s*a/-/sa/, which can be distinguished based on acoustic cues, such as the durations of aspiration and frication noise. Korean infants older than 7 months were able to reliably discriminate the fricative pair but younger infants did not show clear signs of such discrimination. These results add to the growing evidence that there are native sound contrasts infants cannot discriminate early on without a certain amount of language exposure, providing further data to help delineate the specific nature of early perceptual capacity.

Young infants' discrimination of subtle phonetic contrasts

It is generally accepted that infants initially discriminate native and non-native contrasts and that perceptual reorganization within the first year of life results in decreased discrimination of non-native contrasts, and improved discrimination of native contrasts. However, recent findings from Narayan, Werker, and Beddor (2010) surprisingly suggested that some acoustically subtle native-language contrasts might not be discriminated until the end of the first year of life. We first provide countervailing evidence that young English-learning infants can discriminate the Filipino contrast tested by Narayan et al. when tested in a more sensitive paradigm. Next, we show that young infants learning either English or French can also discriminate comparably subtle non-native contrasts from Tamil. These findings show that Narayan et al.'s null findings were due to methodological choices and indicate that young infants are sensitive to even subtle acoustic contrasts that cue phonetic distinctions cross-linguistically. Based on experimental results and acoustic analyses, we argue that instead of specific acoustic metrics, infant discrimination results themselves are the most informative about the salience of phonetic distinctions.

Refining Stimulus Parameters in Assessing Infant Speech Perception Using Visual Reinforcement Infant Speech Discrimination: Sensation Level

BACKGROUND

Speech perception measures have long been considered an integral piece of the audiological assessment battery. Currently, a prelinguistic, standardized measure of speech perception is missing in the clinical assessment battery for infants and young toddlers. Such a measure would allow systematic assessment of speech perception abilities of infants as well as the potential to investigate the impact early identification of hearing loss and early fitting of amplification have on the auditory pathways.

PURPOSE

To investigate the impact of sensation level (SL) on the ability of infants with normal hearing (NH) to discriminate /a-i/ and /ba-da/ and to determine if performance on the two contrasts are significantly different in predicting the discrimination criterion.

RESEARCH DESIGN

The design was based on a survival analysis model for event occurrence and a repeated measures logistic model for binary outcomes. The outcome for survival analysis was the minimum SL for criterion and the outcome for the logistic regression model was the presence/absence of achieving the criterion. Criterion achievement was designated when an infant's proportion correct score was >0.75 on the discrimination performance task.

STUDY SAMPLE

Twenty-two infants with NH sensitivity participated in this study. There were 9 males and 13 females, aged 6-14 mo.

DATA COLLECTION AND ANALYSIS

Testing took place over two to three sessions. The first session consisted of a hearing test, threshold assessment of the two speech sounds (/a/ and /i/), and if time and attention allowed, visual reinforcement infant speech discrimination (VRISD). The second session consisted of VRISD assessment for the two test contrasts (/a-i/ and /ba-da/). The presentation level started at 50 dBA. If the infant was unable to successfully achieve criterion (>0.75) at 50 dBA, the presentation level was increased to 70 dBA followed by 60 dBA. Data examination included an event analysis, which provided the probability of criterion distribution across SL. The second stage of the analysis was a repeated measures logistic regression where SL and contrast were used to predict the likelihood of speech discrimination criterion.

RESULTS

Infants were able to reach criterion for the /a-i/ contrast at statistically lower SLs when compared to /ba-da/. There were six infants who never reached criterion for /ba-da/ and one never reached criterion for /a-i/. The conditional probability of not reaching criterion by 70 dB SL was 0% for /a-i/ and 21% for /ba-da/. The predictive logistic regression model showed that children were more likely to discriminate the /a-i/ even when controlling for SL.

CONCLUSIONS

Nearly all normal-hearing infants can demonstrate discrimination criterion of a vowel contrast at 60 dB SL, while a level of ≥70 dB SL may be needed to allow all infants to demonstrate discrimination criterion of a difficult consonant contrast.

Innateness, Experience, and Music Perception

Musical acculturation from infancy to adulthood was studied by testing the abilities of Western 6-month-olds and adults to notice mistunings in melodies based on native Western major, native Western minor, and non-native Javanese pelog scales. Results indicated that infants were similarly able to perceive native and non-native scales. Adults, however, were generally better perceivers of native than non-native scales. These findings suggest that infants are born with an equipotentiality for the perception of scales from a variety of cultures and that subsequent culturally specific experience substantially influences music perception.

Articulating What Infants Attune to in Native Speech

To become language users, infants must embrace the integrality of speech perception and production. That they do so, and quite rapidly, is implied by the native-language attunement they achieve in each domain by 6-12 months. Yet research has most often addressed one or the other domain, rarely how they interrelate. Moreover, mainstream assumptions that perception relies on acoustic patterns whereas production involves motor patterns entail that the infant would have to translate incommensurable information to grasp the perception-production relationship. We posit the more parsimonious view that both domains depend on commensurate articulatory information. Our proposed framework combines principles of the Perceptual Assimilation Model (PAM) and Articulatory Phonology (AP). According to PAM, infants attune to articulatory information in native speech and detect similarities of nonnative phones to native articulatory patterns. The AP premise that gestures of the speech organs are the basic elements of phonology offers articulatory similarity metrics while satisfying the requirement that phonological information be discrete and contrastive: (a) distinct articulatory organs produce vocal tract constrictions and (b) phonological contrasts recruit different articulators and/or constrictions of a given articulator that differ in degree or location. Various lines of research suggest young children perceive articulatory information, which guides their productions: discrimination of between- versus within-organ contrasts, simulations of attunement to language-specific articulatory distributions, multimodal speech perception, oral/vocal imitation, and perceptual effects of articulator activation or suppression. We conclude that articulatory gesture information serves as the foundation for developmental integrality of speech perception and production.

Infant cortical electrophysiology and perception of vowel contrasts

Cortical auditory evoked potentials (CAEPs) were obtained for vowel tokens presented in an oddball stimulus paradigm. Perceptual measures of vowel discrimination were obtained using a visually-reinforced head-turn paradigm. The hypothesis was that CAEP latencies and amplitudes would differ as a function of vowel type and be correlated with perceptual performance. Twenty normally hearing infants aged 4-12 months were evaluated. CAEP component amplitudes and latencies were measured in response to the standard, frequent token /a/ and for infrequent, deviant tokens /i/, /o/ and /u/, presented at rates of 1 and 2 tokens/s. The perceptual task required infants to make a behavioral response for trials that contained two different vowel tokens, and ignore those in which the tokens were the same. CAEP amplitudes were larger in response to the deviant tokens, when compared to the control condition in which /a/ served as both standard and deviant. This was also seen in waveforms derived by subtracting the response to standard /a/ from the responses to deviant tokens. CAEP component latencies in derived responses at 2/s also demonstrated some sensitivity to vowel contrast type. The average hit rate for the perceptual task was 68.5%, with a 25.7% false alarm rate. There were modest correlations of CAEP amplitudes and latencies with perceptual performance. The CAEP amplitude differences for vowel contrasts could be used as an indicator of the underlying neural capacity to encode spectro-temporal differences in vowel sounds. This technique holds promise for translation to clinical methods for evaluating speech perception.

Object labeling influences infant phonetic learning and generalization

Different kinds of speech sounds are used to signify possible word forms in every language. For example, lexical stress is used in Spanish (/'be.be/, 'he/she drinks' versus /be.'be/, 'baby'), but not in French (/'be.be/ and /be.'be/ both mean 'baby'). Infants learn many such native language phonetic contrasts in their first year of life, likely using a number of cues from parental speech input. One such cue could be parents' object labeling, which can explicitly highlight relevant contrasts. Here we ask whether phonetic learning from object labeling is abstract-that is, if learning can generalize to new phonetic contexts. We investigate this issue in the prosodic domain, as the abstraction of prosodic cues (like lexical stress) has been shown to be particularly difficult. One group of 10-month-old French-learners was given consistent word labels that contrasted on lexical stress (e.g., Object A was labeled /'ma.bu/, and Object B was labeled /ma.'bu/). Another group of 10-month-olds was given inconsistent word labels (i.e., mixed pairings), and stress discrimination in both groups was measured in a test phase with words made up of new syllables. Infants trained with consistently contrastive labels showed an earlier effect of discrimination compared to infants trained with inconsistent labels. Results indicate that phonetic learning from object labeling can indeed generalize, and suggest one way infants may learn the sound properties of their native language(s).

Vowel discrimination by hearing infants as a function of number of spectral channels

Reduced spectral resolution negatively impacts speech perception, particularly perception of vowels and consonant place. This study assessed impact of number of spectral channels on vowel discrimination by 6-month-old infants with normal hearing by comparing three listening conditions: Unprocessed speech, 32 channels, and 16 channels. Auditory stimuli (/ti/ and /ta/) were spectrally reduced using a noiseband vocoder and presented to infants with normal hearing via visual habituation. Results supported a significant effect of number of channels on vowel discrimination by 6-month-old infants. No differences emerged between unprocessed and 32-channel conditions in which infants looked longer during novel stimulus trials (i.e., discrimination). The 16-channel condition yielded a significantly different pattern: Infants demonstrated no significant difference in looking time to familiar vs novel stimulus trials, suggesting infants cannot discriminate /ti/ and /ta/ with only 16 channels. Results support effects of spectral resolution on vowel discrimination. Relative to published reports, young infants need more spectral detail than older children and adults to perceive spectrally degraded speech. Results have implications for development of perception by infants with hearing loss who receive auditory prostheses.

Cognitive and linguistic sources of variance in 2-year-olds’ speech-sound discrimination: a preliminary investigation

PURPOSE

This preliminary investigation explored potential cognitive and linguistic sources of variance in 2-year-olds’ speech-sound discrimination by using the toddler change/ no-change procedure and examined whether modifications would result in a procedure that can be used consistently with younger 2-year-olds.

METHOD

Twenty typically developing 2-year-olds completed the newly modified toddler change/no-change procedure. Behavioral tests and parent report questionnaires were used to measure several cognitive and linguistic constructs. Stepwise linear regression was used to relate discrimination sensitivity to the cognitive and linguistic measures. In addition, discrimination results from the current experiment were compared with those from 2-year-old children tested in a previous experiment.

RESULTS

Receptive vocabulary and working memory explained 56.6% of variance in discrimination performance. Performance was not different on the modified toddler change/no-change procedure used in the current experiment from in a previous investigation, which used the original version of the procedure.

CONCLUSIONS

The relationship between speech discrimination and receptive vocabulary and working memory provides further evidence that the procedure is sensitive to the strength of perceptual representations. The role for working memory might also suggest that there are specific subject-related, nonsensory factors limiting the applicability of the procedure to children who have not reached the necessary levels of cognitive and linguistic development.

Investigating the role of articulatory organs and perceptual assimilation of native and non-native fricative place contrasts

The perceptual assimilation model (PAM; Best, C. T. [1995]. A direct realist view of cross-language speech perception. In W. Strange (Ed.), Speech perception and linguistic experience: Issues in cross-language research (pp. 171-204). Baltimore, MD: York Press.) accounts for developmental patterns of speech contrast discrimination by proposing that infants shift from untuned phonetic perception at 6 months to natively tuned perceptual assimilation at 11-12 months, but the model does not predict initial discrimination differences among contrasts. To address that issue, we evaluated the Articulatory Organ Hypothesis, which posits that consonants produced using different articulatory organs are initially easier to discriminate than those produced with the same articulatory organ. We tested English-learning 6- and 11-month-olds' discrimination of voiceless fricative place contrasts from Nuu-Chah-Nulth (non-native) and English (native), with one within-organ and one between-organ contrast from each language. Both native and non-native contrasts were discriminated across age, suggesting that articulatory-organ differences do not influence perception of speech contrasts by young infants. The results highlight the fact that a decline in discrimination for non-native contrasts does not always occur over age.

The Acoustic Change Complex in Young Children with Hearing Loss: A Preliminary Study

The acoustic change complex (ACC) is a cortical auditory evoked potential elicited in response to a change in an ongoing sound. The ACC may have promise for assessing speech perception in infants and toddlers. In this preliminary study, the ACC was elicited in adults and young children in response to changes in speech stimuli representing vowel height /u/-/a/ and vowel place /u/-/i/ contrasts. The participants were adults with normal hearing (n = 3), children with normal hearing (n = 5), and children with mild to moderately severe bilateral sensorineural hearing loss (n = 5). The children with hearing loss were hearing aid users. The ages ranged from 2 years 3 months to 6 years 3months for the children and 44 to 55 years for the adults. Robust P1-N1-P2 responses were present for the adults and P1-N2 responses were present for all but the youngest child with hearing loss. The ACC response for the vowel place contrast was less robust than that for the vowel height contrast in one child with substantial hearing loss. The findings from this preliminary study support the conclusion that the ACC can be used successfully to assess auditory resolution in most young children.

Studies in pediatric hearing loss at the House Research Institute

Three clinical research projects are described that are relevant to pediatric hearing loss. The three projects fall into two distinct areas. The first area emphasizes clinical studies that track developmental outcomes in children with hearing loss; one project is specific to cochlear implants and the other to hearing aids. The second area addresses speech perception test development for very young children with hearing loss. Although these two lines of research are treated as separate areas, they begin to merge as new behavioral tests become useful in developing protocols for contemporary studies that address longitudinal follow-up of children with hearing loss.

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.

Effects of the distribution of acoustic cues on infants' perception of sibilants

A current theoretical view proposes that infants converge on the speech categories of their native language by attending to frequency distributions that occur in the acoustic input. To date, the only empirical support for this statistical learning hypothesis comes from studies where a single, salient dimension was manipulated. Additional evidence is sought here, by introducing a less salient pair of categories supported by multiple cues. We exposed English-learning infants to a multi-cue bidimensional grid between retroflex and alveolopalatal sibilants in prevocalic position. This contrast is substantially more difficult according to previous cross-linguistic and perceptual research, and its perception is driven by cues in both the consonantal and the following vowel portions. Infants heard one of two distributions (flat, or with two peaks), and were tested with sounds varying along only one dimension. Infants' responses differed depending on the familiarization distribution, and their performance was equally good for the vocalic and the frication dimension, lending some support to the statistical hypothesis even in this harder learning situation. However, learning was restricted to the retroflex category, and a control experiment showed that lack of learning for the alveolopalatal category was not due to the presence of a competing category. Thus, these results contribute fundamental evidence on the extent and limitations of the statistical hypothesis as an explanation for infants' perceptual tuning.

Modified spectral tilt affects older, but not younger, infants' native-language fricative discrimination

PURPOSE

It is important to ensure that hearing aid fitting strategies for infants take into account the infant's developing speech perception system. As a way of exploring this issue, this study examined how 6- and 9-month-olds with normal hearing perceive native-language speech in which the natural spectral shape was altered to emphasize either high-frequency (positive spectral tilt) or low-frequency (negative spectral tilt) information.

METHOD

Discrimination was tested using a visual habituation procedure. Forty-eight 6-month-olds and forty-eight 9-month-olds were presented with a fricative contrast, /f/-/s/, in 1 of 3 conditions: (a) as unmodified speech; (b) with a -6 dB/octave tilt; or (c) with a +6 dB/octave tilt.

RESULTS

Six-month-olds showed evidence of discriminating /f/-/s/ in all 3 conditions, but 9-month-olds showed such evidence only in the unmodified condition.

CONCLUSIONS

The findings suggest that the perceptual reorganization that emerges for consonants at the end of the first year affects 9-month-olds' discrimination of native speech sounds. Perceptual reorganization is usually indexed by a decline in the ability to discriminate nonnative speech sounds. In this study, 6-month-olds demonstrated an acoustic-based sensitivity to both modified and unmodified native speech sounds, but 9-month-olds were most sensitive to the unmodified speech sounds that adhered to the native spectral profile.

The interaction between acoustic salience and language experience in developmental speech perception: evidence from nasal place discrimination

Previous research suggests that infant speech perception reorganizes in the first year: young infants discriminate both native and non-native phonetic contrasts, but by 10-12 months difficult non-native contrasts are less discriminable whereas performance improves on native contrasts. In the current study, four experiments tested the hypothesis that, in addition to the influence of native language experience, acoustic salience also affects the perceptual reorganization that takes place in infancy. Using a visual habituation paradigm, two nasal place distinctions that differ in relative acoustic salience, acoustically robust labial-alveolar [ma]-[na] and acoustically less salient alveolar-velar [na]-[ enga], were presented to infants in a cross-language design. English-learning infants at 6-8 and 10-12 months showed discrimination of the native and acoustically robust [ma]-[na] (Experiment 1), but not the non-native (in initial position) and acoustically less salient [na]-[ enga] (Experiment 2). Very young (4-5-month-old) English-learning infants tested on the same native and non-native contrasts also showed discrimination of only the [ma]-[na] distinction (Experiment 3). Filipino-learning infants, whose ambient language includes the syllable-initial alveolar (/n/)-velar (/ eng/) contrast, showed discrimination of native [na]-[ enga] at 10-12 months, but not at 6-8 months (Experiment 4). These results support the hypothesis that acoustic salience affects speech perception in infancy, with native language experience facilitating discrimination of an acoustically similar phonetic distinction [na]-[ enga]. We discuss the implications of this developmental profile for a comprehensive theory of speech perception in infancy.

Infant Discrimination of a Morphologically Relevant Word-Final Contrast

Six-, 12-, and 18-month-old English-hearing infants were tested on their ability to discriminate nonword forms ending in the final stop consonants /k/ and /t/ from their counterparts with final /s/ added, resulting in final clusters /ks/ and /ts/, in a habituation-dishabituation, looking time paradigm. Infants at all 3 ages demonstrated an ability to discriminate this type of contrast, a contrast that constitutes one phonetic cue for the English morphological concepts of plural, possession, and person. These results suggest that across a significant portion of the development of infants' speech perception, this type of final contrast is discriminable.

The human auditory system: A timeline of development

This review traces the structural maturation of the human auditory system, and compares the timeline of anatomical development with cotemporaneous physiological and behavioral events. During the embryonic period, there is formation of basic structure at all levels of the system, i.e. the inner ear, the brainstem pathway, and the cortex. The second trimester is a time of rapid growth and development, and by the end of this period, the cochlea has acquired a very adult-like configuration. During the perinatal period, the brainstem reaches a mature state, and brainstem activity is reflected in behavioral responses to sound, including phonetic discrimination, and in evoked brainstem and early middle latency responses. The perinatal period is also the time of peak development of brainstem input to the cortex through the marginal layer, and of the long latency cortical potentials, the N(2) and mismatch negativity. In early childhood, from the sixth post-natal month to age five, there is progressive maturation of the thalamic projections to the cortex and of the longer latency Pa and P(1) evoked potentials. Later childhood, from six to twelve years, is the time of maturation of the superficial cortical layers and their intracortical connections, accompanied by appearance of the N(1) potential and improved linguistic discriminative abilities. Some consideration is given to the potential negative effects of deafness-induced sound deprivation during the perinatal period and childhood.

Assessing speech pattern contrast perception in infants: early results on VRASPAC

OBJECTIVE

To assess perception of specific speech pattern contrasts in infants with normal hearing (NH) and infants with sensorineural hearing loss (HL).

STUDY DESIGN

Prospective longitudinal and cross-sectional study.

SETTING

Tertiary referral center.

PATIENTS

Nine infants with NH and 11 infants with HL ranging from mild to profound.

METHODS

Speech pattern contrast perception was evaluated in infants with NH and HL using a new behavioral test called Visual Reinforcement Assessment of the Perception of Speech Pattern Contrasts (VRASPAC). VRASPAC assesses the infant's ability to distinguish the speech features of vowel height and place and consonant voicing, continuance, and place. Detection of a phonetic change is indicated by a conditioned head-turn response. Performance is reported as the percent confidence that responses are not random.

RESULTS

The infants with NH attained high confidence scores for the vowel height and place contrasts used in this study. The infants with HL achieved high scores for the vowel height contrast regardless of degree of hearing loss. High scores were attained for the vowel place contrast by infants with losses less than 60 dB HL. Performance for the consonant contrasts was variable for all of the infants. Results from 1 infant assessed precochlear and postcochlear implantation indicated substantial improvement in speech pattern contrast perception with the implant.

CONCLUSION

Perception of speech pattern contrasts in infants, particularly for the vowel contrasts, can be evaluated with a high level of confidence using VRASPAC. Consonant contrast perception is much more variable than vowel contrast perception and requires further investigation.

Phonetic learning as a pathway to language: new data and native language magnet theory expanded (NLM-e)

Infants' speech perception skills show a dual change towards the end of the first year of life. Not only does non-native speech perception decline, as often shown, but native language speech perception skills show improvement, reflecting a facilitative effect of experience with native language. The mechanism underlying change at this point in development, and the relationship between the change in native and non-native speech perception, is of theoretical interest. As shown in new data presented here, at the cusp of this developmental change, infants' native and non-native phonetic perception skills predict later language ability, but in opposite directions. Better native language skill at 7.5 months of age predicts faster language advancement, whereas better non-native language skill predicts slower advancement. We suggest that native language phonetic performance is indicative of neural commitment to the native language, while non-native phonetic performance reveals uncommitted neural circuitry. This paper has three goals: (i) to review existing models of phonetic perception development, (ii) to present new event-related potential data showing that native and non-native phonetic perception at 7.5 months of age predicts language growth over the next 2 years, and (iii) to describe a revised version of our previous model, the native language magnet model, expanded (NLM-e). NLM-e incorporates five new principles. Specific testable predictions for future research programmes are described.

Statistical phonetic learning in infants: facilitation and feature generalization

Over the course of the first year of life, infants develop from being generalized listeners, capable of discriminating both native and non-native speech contrasts, into specialized listeners whose discrimination patterns closely reflect the phonetic system of the native language(s). Recent work by Maye, Werker and Gerken (2002) has proposed a statistical account for this phenomenon, showing that infants may lose the ability to discriminate some foreign language contrasts on the basis of their sensitivity to the statistical distribution of sounds in the input language. In this paper we examine the process of enhancement in infant speech perception, whereby initially difficult phonetic contrasts become better discriminated when they define two categories that serve a functional role in the native language. In particular, we demonstrate that exposure to a bimodal statistical distribution in 8-month-old infants' phonetic input can lead to increased discrimination of difficult contrasts. In addition, this exposure also facilitates discrimination of an unfamiliar contrast sharing the same phonetic feature as the contrast presented during familiarization, suggesting that infants extract acoustic/phonetic information that is invariant across an abstract featural representation.

Developmental effects of multiple looks in speech sound discrimination

PURPOSE

The change/no-change procedure (J. E. Sussman & A. E. Carney, 1989), which assesses speech discrimination, has been used under the assumption that the number of stimulus presentations does not influence performance. Motivated by the tenets of the multiple looks hypothesis (N. F. Viemeister & G. H. Wakefield, 1991), work by R. F. Holt and A. E. Carney (2005) called this assumption into question (at least for adults): Nonsense syllable discrimination improved with more stimulus presentations. This study investigates the nature of developmental differences and the effects of multiple stimulus presentations in the change/no-change procedure.

METHOD

Thirty normal-hearing children, ages 4.0-5.9 years, were tested on 3 consonant-vowel contrasts at various signal-to-noise ratios using combinations of 2 and 4 standard and comparison stimulus repetitions.

RESULTS

Although performance fell below that which is predicted by the multiple looks hypothesis in most conditions, discrimination was enhanced with more stimulus repetitions for 1 speech contrast. The relative influence of standard and comparison stimulus repetitions varied across the speech contrasts in a manner different from that of adults.

CONCLUSION

Despite providing no additional sensory information, multiple stimulus repetitions enhanced children's discrimination of 1 set of nonsense syllables. The results have implications for models of developmental speech perception and assessing speech discrimination in children.

Assessing auditory capabilities in young children

OBJECTIVE

Early detection of hearing loss in infants and toddlers has created a need for age-appropriate tests that assess auditory perceptual capabilities. This article describes a progressive test battery we have developed to evaluate phonetic contrast perception, phoneme recognition, and word recognition in children 6 months to 5 years. This battery is part of a clinical research protocol designed to track auditory development in this population.

METHODS

The progressive test battery originated from a model of auditory perceptual development to assess phonetic discrimination and word recognition. Phonetic discrimination is evaluated using the Battery of Auditory Speech Perception Tests for Infants and Toddlers (BATIT). The BATIT is composed of four measures (VRASPAC, PLAYSPAC, OLIMSPAC, and VIDSPAC) intended to assess the child's ability to distinguish between phonologically significant contrasts using developmentally appropriate tasks. Designed for children aged 6 months and up, performance is represented either by percent correct or by the level of confidence that the child's responses are not random. Phoneme and word recognition are assessed in children 4 years and older using lists of consonant-vowel-consonant (CVC) phonemes in words and lexically controlled words both in and out of sentence context (LEXSEN).

RESULTS

Cross-sectional data show that children with normal hearing may be assessed by the age of 7 months on VRASPAC; by 3 years on PLAYSPAC and OLIMSPAC; and by 4-5 years on VIDSPAC, CVC phonemes in words, and LEXSEN words in isolation and in sentences. Data from infants with hearing loss show that VRASPAC is sensitive to degree of hearing loss, but performance with normally hearing children declines after 12 months of age.

CONCLUSION

Assessment of phonetic discrimination and word recognition is, for the most part, attainable in young children using a progressive test battery, but none of the tests used here is effective between 1 and 3 years of age. Continued development will be required to fill this gap and to separate auditory from non-auditory influences on performance.

Perception of native and non-native affricate-fricative contrasts: cross-language tests on adults and infants

Previous studies have shown improved sensitivity to native-language contrasts and reduced sensitivity to non-native phonetic contrasts when comparing 6-8 and 10-12-month-old infants. This developmental pattern is interpreted as reflecting the onset of language-specific processing around the first birthday. However, generalization of this finding is limited by the fact that studies have yielded inconsistent results and that insufficient numbers of phonetic contrasts have been tested developmentally; this is especially true for native-language phonetic contrasts. Three experiments assessed the effects of language experience on affricate-fricative contrasts in a cross-language study of English and Mandarin adults and infants. Experiment 1 showed that English-speaking adults score lower than Mandarin-speaking adults on Mandarin alveolo-palatal affricate-fricative discrimination. Experiment 2 examined developmental change in the discrimination of this contrast in English- and Mandarin-leaning infants between 6 and 12 months of age. The results demonstrated that native-language performance significantly improved with age while performance on the non-native contrast decreased. Experiment 3 replicated the perceptual improvement for a native contrast: 6-8 and 10-12-month-old English-learning infants showed a performance increase at the older age. The results add to our knowledge of the developmental patterns of native and non-native phonetic perception.