Infant Vowel Research Articles

Exploring infant talker bias: Insights from remote speech perception testing

Lab studies show that infants (4- to 7-month-olds) prefer to listen to vowels with infant-like f0 and formant frequencies over those of an adult female (Masapollo et al., 2015; Polka et al., 2021). This Infant Talker Bias may facilitate infants’ mapping of articulatory gestures to acoustic correlates. In this study, 4- to 12-month-olds completed a listening preference task on the Lookit online testing platform. Across eight trials, we presented synthesized infant and adult vowel sounds (/i/ and /a/) paired with a simple animation and recorded the infant’s response via the webcam. Infant looking time and vocalization to each vowel type were coded offline. Preliminary analyses (n = 91) show that listening time increased with age (p < 0.05), and all infants listened longer to infant vowels than to adult vowels (p < 0.01). Preliminary analyses (n = 62) also show an increase in infant vocalizations with age (p = 0.00) and a trend towards more and longer vocalizations in response to the adult vowels. These findings replicate and extend the infant talker bias to new vowel stimuli and to older infants, and support the use of remote testing in infant speech perception studies.

Relationship Between Parent Vowel Hyperarticulation in Infant-Directed Speech and Infant Phonetic Complexity on the Level of Conversational Turns.

When speaking to infants, parents typically use infant-directed speech, a speech register that in several aspects differs from that directed to adults. Vowel hyperarticulation, that is, extreme articulation of vowels, is one characteristic sometimes found in infant-directed speech, and it has been suggested that there exists a relationship between how much vowel hyperarticulation parents use when speaking to their infant and infant language development. In this study, the relationship between parent vowel hyperarticulation and phonetic complexity of infant vocalizations is investigated. Previous research has shown that on the level of subject means, a positive correlational relationship exists. However, the previous findings do not provide information about the directionality of that relationship. In this study the relationship is investigated on a conversational turn level, which makes it possible to draw conclusions on whether the behavior of the infant is impacting the parent, the behavior of the parent is impacting the infant, or both. Parent vowel hyperarticulation was quantified using the vhh-index, a measure that allows vowel hyperarticulation to be estimated for individual vowel tokens. Phonetic complexity of infant vocalizations was calculated using the Word Complexity Measure for Swedish. Findings were unexpected in that a negative relationship was found between parent vowel hyperarticulation and phonetic complexity of the immediately following infant vocalization. Directionality was suggested by the fact that no such relationship was found between infant phonetic complexity and vowel hyperarticulation of the immediately following parent utterance. A potential explanation for these results is that high degrees of vowel hyperarticulation either provide, or co-occur with, large amounts of phonetic and/or linguistic information, which may occupy processing resources to an extent that affects production of the next vocalization.

An acoustic perspective on 45 years of infant speech perception. II. Vowels and suprasegmentals

AbstractIn this two‐part review we examine the major results from infant consonant (Part 1), vowel, and suprasegmental (Part 2) discrimination research over the past 45 years from an acoustic perspective—an exegesis of the developmental perception literature that appeals to both acoustic aspects of speech contrasts and historically relevant typological facts about sound systems of the world's languages. We argue that infants’ speech discrimination abilities are best viewed through a lens that considers both synchronic and diachronic aspects of the particular speech contrast. The key to this approach is the notion that acoustic–perceptual salience, or the relative separation of speech categories along perceptually relevant acoustic dimensions and corresponding discrimination performance in adults, is reflected in both infant's perceptual performance and patterns observed in phonological typology and history. The present review highlights challenges offered by four decades of literature, identifies broad patterns in infant vowel perception according to the acoustic properties of speech contrasts, and offers linguistically motivated explanations and directions for future research into the nature of young infants’ discrimination abilities.

Cross-linguistic differences in the size of the infant vowel space

This study examined the effects of linguistic environmental input on babbling in cross-linguistic investigations of vowel space. Speech samples were collected from 10- to 18-month-old infants learning Arabic (N = 31). First (F1) and second (F2) formant frequencies were identified in the selected vowels and used to calculate the compact-diffuse (F2 − F1) and grave-acute ([F2 + F1]/2) values for each vowel and the size of the vowel space was calculated for each infant’s vowel space. These vowel space statistics were compared to similar data derived from vowels produced by English-learning infants (N = 20) and French-learning infants (N = 23) as previously described in Rvachew, Mattock, Polka, and Menard (2006). It was found that Arabic infants appeared to achieve a larger vowel space at a younger age compared to the English and French infants, which we attribute to the benefit of a less crowded vowel space in Arabic input compared to English and French input. Expansion of the vowel space toward the diffuse and grave corners was common to all three language groups, but the developmental trajectories for the mean F1 and mean F2 varied with language input. These findings suggest that the development of infant babbling is influenced by a complex interaction of endogenous and exogenous processes, which include the biological development of the vocal tract and language input from the ambient environment.

Fifty years of infant vowel discrimination research: what have we learned?

Fifty years of infant vowel discrimination research: what have we learned?

Fifty Years of Infant Vowel Discrimination Research: What Have We Learned?( Infants' Strategies for Acquisition of Speech)

Fifty Years of Infant Vowel Discrimination Research: What Have We Learned?( Infants' Strategies for Acquisition of Speech)

An online model for vowel imitation learning

When infants learn to pronounce speech sounds of their native language, they face the so-called correspondence problem – how to know which articulatory gestures lead to acoustic sounds that are recognized as native speech sounds by other speakers? Previous research suggests that infants might not learn to imitate their parents via autonomous babbling because direct evaluation of the acoustic similarity between the speech sounds of the two is not possible due to different spectral characteristics of the voices caused by differing vocal tract morphologies. We present a novel robust model of infant vowel imitation learning, following a hypothesis that an infant learns to match their productions to their caregiver's speech sounds when the caregiver imitates the infant's babbles. Adapting a cross-situational associative learning technique, evidently present in infant word learning, our simulated language learner can cope with ambiguity in caregiver's responses to babbling as well as with the imprecision of the articulatory gestures of the infant itself. Our fully online learning model also combines vocal exploration and imitative interaction into a single process. Learning performance is evaluated in experiments using Finnish adults as caregivers for a virtual infant, responding to the infant's babbles with lexical words and, after a learning stage, evaluating the quality of the vowels produced by the learner. After 1000 babble-response pairs, our virtual infant is seen to reach a satisfying vowel imitation accuracy of 70–80%.

The phonetic landscape in infant consonant perception is an uneven terrain

Previous research revealing universal biases in infant vowel perception forms the basis of the Natural Referent Vowel (NRV) framework (Polka & Bohn, 2011). To explore the feasibility of extending this framework to consonant manner perception, we investigated perception of the stop vs. fricative consonant contrast /b/-/v/ to test the hypothesis that young infants will display a perceptual bias grounded in the acoustic-phonetic properties of these sounds. We examined perception of stop-initial /bas/ and fricative-initial /vas/ syllables in English-learning and French-learning 5- to 6-month-olds. The /b/ and /v/ sounds distinguish words in English and French but have different distributional patterns; in spoken English /b/ occurs more frequently than /v/ whereas in spoken French /v/ occurs more frequently than /b/. A perceptual bias favoring /b/ over /v/ emerged in two experiments. In Experiment 1, a directional asymmetry was observed in discrimination; infants noticed when /vas/ changed to /bas/ but not when /bas/ changed to /vas/. In Experiment 2, a robust listening preference favoring stop-initial /bas/ was evident in responses from the same infants. This is the first study to show a perceptual bias related to consonant manner and to directly measure a consonant perception bias within the same infants. These data encourage further efforts to extend the NRV principles to perception of consonant manner. These findings indicate that we need to reform our view of infant speech perception to accommodate the fact that both discrimination abilities and biases shape speech perception during infancy.

Maternal Vocal Feedback to 9-Month-Old Infant Siblings of Children with ASD.

Infant siblings of children with autism spectrum disorder display differences in early language and social communication skills beginning as early as the first year of life. While environmental influences on early language development are well documented in other infant populations, they have received relatively little attention inside of the infant sibling context. In this study, we analyzed home video diaries collected prospectively as part of a longitudinal study of infant siblings. Infant vowel and consonant-vowel vocalizations and maternal language-promoting and non-promoting verbal responses were scored for 30 infant siblings and 30 low risk control infants at 9 months of age. Analyses evaluated whether infant siblings or their mothers exhibited differences from low risk dyads in vocalization frequency or distribution, and whether mothers' responses were associated with other features of the high risk context. Analyses were conducted with respect to both initial risk group and preliminary outcome classification. Overall, we found no differences in infants' consonant-vowel vocalizations, the frequency of overall maternal utterances, or the distribution of mothers' response types. Both groups of infants produced more vowel than consonant-vowel vocalizations, and both groups of mothers responded to consonant-vowel vocalizations with more language-promoting than non-promoting responses. These results indicate that as a group, mothers of high risk infants provide equally high quality linguistic input to their infants in the first year of life and suggest that impoverished maternal linguistic input does not contribute to high risk infants' initial language difficulties. Implications for intervention strategies are also discussed.

When infants talk, infants listen: pre‐babbling infants prefer listening to speech with infant vocal properties

To learn to produce speech, infants must effectively monitor and assess their own speech output. Yet very little is known about how infants perceive speech produced by an infant, which has higher voice pitch and formant frequencies compared to adult or child speech. Here, we tested whether pre-babbling infants (at 4-6 months) prefer listening to vowel sounds with infant vocal properties over vowel sounds with adult vocal properties. A listening preference favoring infant vowels may derive from their higher voice pitch, which has been shown to attract infant attention in infant-directed speech (IDS). In addition, infants' nascent articulatory abilities may induce a bias favoring infant speech given that 4- to 6-month-olds are beginning to produce vowel sounds. We created infant and adult /i/ ('ee') vowels using a production-based synthesizer that simulates the act of speaking in talkers at different ages and then tested infants across four experiments using a sequential preferential listening task. The findings provide the first evidence that infants preferentially attend to vowel sounds with infant voice pitch and/or formants over vowel sounds with no infant-like vocal properties, supporting the view that infants' production abilities influence how they process infant speech. The findings with respect to voice pitch also reveal parallels between IDS and infant speech, raising new questions about the role of this speech register in infant development. Research exploring the underpinnings and impact of this perceptual bias can expand our understanding of infant language development.

Who’s Talking Now? Infants’ Perception of Vowels With Infant Vocal Properties

Little is known about infants’ abilities to perceive and categorize their own speech sounds or vocalizations produced by other infants. In the present study, prebabbling infants were habituated to /i/ (“ee”) or /a/ (“ah”) vowels synthesized to simulate men, women, and children, and then were presented with new instances of the habituation vowel and a contrasting vowel on different trials, with all vowels simulating infant talkers. Infants showed greater recovery of interest to the contrasting vowel than to the habituation vowel, which demonstrates recognition of the habituation-vowel category when it was produced by an infant. A second experiment showed that encoding the vowel category and detecting the novel vowel required additional processing when infant vowels were included in the habituation set. Despite these added cognitive demands, infants demonstrated the ability to track vowel categories in a multitalker array that included infant talkers. These findings raise the possibility that young infants can categorize their own vocalizations, which has important implications for early vocal learning.

A virtual environment for modeling the acquisition of vowel normalization

Vowel normalization is a computation that is meant to account for the differences in the absolute direct (physical or psychophysical) representations of qualitatively equivalent vowel productions that arise due to differences in speaker properties such as body size types, age, gender, and other socially interpreted categories that are based on natural variation in vocal tract size and shape. We present a virtual environment for vocal learning which provides the means to model the acquisition of vowel normalization, along with other aspects of vocal learning. The environment consists of models of caretaker agents representing five different language communities—American English, Cantonese, Greek, Japanese, and Korean—derived from vowel category perception experiments (Munson et al., 2010, Plummer et al., 2013) and models of infant agents (Plummer, 2012, 2013) that “vocally interact” with their caretakers. Moreover, we develop a model of caretaker social and vocal signaling in response to infant vowel productions, and of an infant's internalization of these signals and the internal computations over them. More broadly, we model the acquisition of vowel normalization within a developmental framework encompassing a suite of vocal learning phenomena, including language-specific caretaker vocal exchanges, perceptual warping, and multisensory matching and narrowing.

Pre-babbling infants prefer listening to infant speech: Implications for vocal learning in humans

For human infants to engage in vocal learning, they must effectively monitor and assess their own self-produced speech, which entails perceiving speech produced by an infant. Yet, little is known about how infants respond to infant-produced speech. Here, we demonstrate that pre-babbling infants prefer listening to infant speech. Across four experiments, 3-to-6-month-olds were tested in a preferential listening procedure, using vowels synthesized to emulate productions by female adults and infants. In experiment 1, infants listened longer to vowels produced by infant than adult speakers. However, in experiment 2, infants failed to show any listening preference for infant versus adult vowels synthesized with matching, infant-appropriate pitch values, suggesting that infants were either attracted to higher voice pitch per se or to infant-like voice pitch. Failing to support a bias of the first type, infants in experiment 3 showed no listening preference when presented infant vowels with higher and lower infant-appropriate pitch values. Moreover, in experiment 4, infants showed a preference for infant versus adult vowels when synthesized with pitch values that are appropriate for a female using adult-directed speech; this suggests that infants are also attracted to infant vocal resonance properties. The implications of these results for speech development are discussed.

Perceptual attunement in vowels: A meta‐analysis

Although the majority of evidence on perceptual narrowing in speech sounds is based on consonants, most models of infant speech perception generalize these findings to vowels, assuming that vowel perception improves for vowel sounds that are present in the infant's native language within the first year of life, and deteriorates for non-native vowel sounds over the same period of time. The present meta-analysis contributes to assessing to what extent these descriptions are accurate in the first comprehensive quantitative meta-analysis of perceptual narrowing in infant vowel discrimination, including results from behavioral, electrophysiological, and neuroimaging methods applied to infants 0-14 months of age. An analysis of effect sizes for native and non-native vowel discrimination over the first year of life revealed that they changed with age in opposite directions, being significant by about 6 months of age.

Infants’ categorization of vowels with infant vocal properties

Prior research shows that infants can recognize phonetic equivalence among vowels produced by adult men, women, and children. It is unknown whether this ability extends to infant vowel productions, which have unique properties due to the size and geometry of the infant vocal tract. The present study was undertaken to determine whether infants recognize infant vowel productions as phonetically equivalent to vowels produced by adults and children. Infants (4–6 months) were tested in a look-to-listen procedure using isolated vowels, /i/ and /a/, synthesized to simulate productions by men, women, children and a 6-month-old. Infants were first habituated to diverse productions of the same vowel produced by several adult male, female, and child speakers while they fixated on a checkerboard. Following habituation, infants were then presented infant productions of the same vowel (familiar) and the other vowel (novel) in four test trials. A novelty effect (novel > familiar) was observed showing that infants recognized the familiar infant vowel to be similar to the habituation vowel. The findings are discussed in terms of the emergence of perceptual constancy in the development of vowel perception raising issues about how and when such knowledge is acquired in relation to the infant’s own productions.

Infant recognition of infant vocal signals

Most of the speech accessible to infants who are not yet babbling will not have infant vocal properties; yet their perception of infant vocal signals is critical for speech development. We report three experiments designed to assess whether 4- to 5-month-olds recognize and preferentially attend to vowels produced by infant talkers over vowels produced by adult talkers. Infants were tested in a sequential preferential looking procedure using isolated vowels synthesized by VLAM. In experiment 1, infants listened significantly longer to vowels produced by an infant talker than an adult (female). In experiment 2, infants failed to show any listening preference for infant versus adult vowels synthesized with matching (infant-appropriate) f0 values, suggesting that infants either recognized the un-natural pairing of f0 and formant structure in these adult vowels or are attracted to high f0 values. Failing to support the latter interpretation, infants in experiment 3 showed no listening preference when presented infant vowels with different (infant-appropriate) f0 values. Together, these findings suggest that young infants recognize the converging vocal (source and filter) properties that specify an adult and an infant talker. These recognition skills appear to be available prior to babbling, and thus are available to support early vocal learning.

A Synthetic Investigation of the Caregiver's Auto-Mirroring that Guides Infant Vowel Development

A Synthetic Investigation of the Caregiver's Auto-Mirroring that Guides Infant Vowel Development

A Constructivist Investigation of the Caregiver's Auto-mirroring that Guides Infant Vowel Development

A Constructivist Investigation of the Caregiver's Auto-mirroring that Guides Infant Vowel Development

Natural Referent Vowel (NRV) framework: An emerging view of early phonetic development

The aim of this paper is to provide an overview of an emerging new framework for understanding early phonetic development—the Natural Referent Vowel (NRV) framework. The initial support for this framework was the finding that directional asymmetries occur often in infant vowel discrimination. The asymmetries point to an underlying perceptual bias favoring vowels that fall closer to the periphery of the F1/F2 vowel space. In Polka and Bohn (2003) we reviewed the data on asymmetries in infant vowel perception and proposed that certain vowels act as natural referent vowels and play an important role in shaping vowel perception. In this paper we review findings from studies of infant and adult vowel perception that emerged since Polka and Bohn (2003), from other labs and from our own work, and we formally introduce the NRV framework. We outline how this framework connects with linguistic typology and other models of speech perception and discuss the challenges and promise of NRV as a conceptual tool for advancing our understanding of phonetic development.

How Caregiver's Anticipation Shapes Infant's Vowel Through Mutual Imitation

The mechanism of infant vowel development is a fundamental issue of human cognitive development that includes perceptual and behavioral development. This paper models the mechanism of imitation underlying caregiver-infant interaction by focusing on potential roles of the caregiver's imitation in guiding infant vowel development. Proposed imitation mechanism is constructed with two kinds of the caregiver's possible biases in mind. The first is what we call ?sensorimotor magnets,? in which a caregiver perceives and imitates infant vocalizations as more prototypical ones as mother-tongue vowels. The second is based on what we call ?automirroring bias,? by which the heard vowel is much closer to the expected vowel because of the anticipation being imitated. Computer simulation results of caregiver-infant interaction show the sensorimotor magnets help form small clusters and the automirroring bias shapes these clusters to become clearer vowels in association with the sensorimotor magnets.