Speech perception as information processing.

The Reflections series takes a look back on historical articles from The Journal of the Acoustical Society of America that have had a significant impact on the science and practice of acoustics.

Learning and change in a dual lexicon model of speech production.

Speech motor processes and phonological forms influence one another because speech and language are acquired and used together. This hypothesis underpins the Computational Core (CC) model, which provides a framework for understanding the limitations of perceptually-driven changes to production. The model assumes a lexicon of motor and perceptual wordforms linked to concepts and whole-word production based on these forms. Motor wordforms are built up with speech practice. Perceptual wordforms encode ambient language patterns in detail. Speech production is the integration of the two forms. Integration results in an output trajectory through perceptual-motor space that guides articulation. Assuming successful communication of the intended concept, the output trajectory is incorporated into the existing motor wordform for that concept. Novel word production exploits existing motor wordforms to define a perceptually-acceptable path through motor space that is further modified by the perceptual wordform during integration. Simulation results show that, by preserving a distinction between motor and perceptual wordforms in the lexicon, the CC model can account for practice-based changes in the production of known words and for the effect of expressive vocabulary size on production accuracy of novel words.

Prosodic Development During the Early School-Age Years.

PURPOSE: This study used a cross-sequential design to identify developmental changes in narrative speech rhythm and intonation. The aim was to provide a robust, clinically relevant characterization of normative changes in speech prosody across the early school-age years. METHOD: Structured spontaneous narratives were elicited annually from 60 children over a 3-year period. Children were aged 5-7 years at study outset and then were aged 7-9 years at study offset. Articulation rate, prominence spacing, and intonational phrase length and duration were calculated for each narrative to index speech rhythm; measures of pitch variability and pitch range indexed intonation. Linear mixed-effects (LME) models tested for cohort-based and within-subject longitudinal change on the prosodic measures; linear regression was used to test for the simple effect of age-in-months within year on the measures. RESULTS: The LME analyses indicated systematic longitudinal changes in speech rhythm across all measures except phrase duration; there were no longitudinal changes in pitch variability or pitch range across the school-age years. Linear regression results showed an increase in articulation rate with age; there were no systematic differences between age cohorts across years in the study. CONCLUSIONS: The results indicate that speech rhythm continues to develop during the school-age years. The results also underscore the very strong relationship between the rate and rhythm characteristics of speech and so suggest an important influence of speech motor skills on rhythm production. Finally, the results on pitch variability and pitch range are interpreted to suggest that these are inadequate measures of typical intonation development during the school-age years.

Acoustic correlates and listener ratings of function word reduction in child versus adult speech.

The present study investigated "the" reduction in phrase-medial Verb-the-Noun sequences elicited from 5-year-old children and young adults (18-22 yr). Several measures of reduction were calculated based on acoustic measurement of these sequences. Analyses on the measures indicated that the determiner vowel was reduced in both child and adult speech relative to content word vowels, but it was reduced less in child speech compared to adult speech. Listener ratings on the sequences indicated a preference for adult speech over children's speech. Acoustic measures of reduction also predicted goodness ratings. Listeners preferred sequences with shorter and lower amplitude determiner vowels relative to content word vowels. They also preferred a more neutral schwa over more coarticulated versions. In sequences where ratings differed by age group, the effect of coarticulation was limited to adult speech and the effect of relative schwa duration was limited to child speech. The results are discussed with reference to communicative pressures on speech, including the rhythmic and semantic pressures towards reduction versus the pressure to convey adequate information in the acoustic signal. It is argued that these competing pressures on production may delay the acquisition of adult-like function word reduction.

Measurement Properties of Mean Length of Utterance in School-Age Children.

PURPOSE: Mean length of utterance (MLU) is one of the most widely reported measures of syntactic development in the developmental literature, but its responsiveness in young school-age children's language has been questioned, and it has been shown to correlate with nonsyntactic measures. This study tested the extent to which MLU shows measurement properties of responsiveness and construct validity when applied to language elicited from elementary school children. METHOD: Thirty-two typically developing children in two age groups (5 and 8 years) provided four short language samples each. Language samples were elicited in a question-answer context and a narrative context. MLU was calculated with both morpheme and word counts. Other established measures of syntactic complexity (clausal density [CD], developmental level [D-Level], mean length of clause [MLC]) and lexical diversity (lexical density, moving-average type-token ratio, number of different words) were also calculated. RESULTS: Linear mixed-effects analyses revealed that MLU varied systematically with discourse context and children's age group. The syntactic measures, CD and MLC, were found to vary systematically with MLU. None of the lexical diversity measures varied systematically with MLU. CONCLUSION: Results suggest that MLU is a responsive and valid measure of children's syntactic development across age and discourse context during the early school-age years.

A Cross-Sectional Age Group Study of Coarticulatory Resistance: The Case of Late-Acquired Voiceless Fricatives in English.

PURPOSE: As a class, fricatives are more "resistant" to consonant-vowel coarticulation than other English sounds. This study investigates the relative coarticulatory resistance of /θ, s, ʃ/ in child and adult speech to better understand the acquisition of individuated speech sounds. METHOD: Ten 5-year-old children, seven 8-year-old children, and nine college-age adults produced [əFV] sequences in carrier phrases, where F was /θ/, /s/, or /ʃ/ and V was /æ/, /i/, or /u/. In Experiment 1, coarticulation was perceptually indexed: 65 adults predicted the target stressed vowel based on forward-gated audiovisual speech samples for a subset of four speakers from each age group. In Experiment 2, dynamic spectral measures of the /əFV/ sequences were analyzed using smoothing spline analysis of variance to again test for vowel effects on fricative articulation across age groups. RESULTS: The perceptual results indicated that fricatives blocked vowel-vowel coarticulation across speaker age groups. Contrary to expectation, vowels were most accurately predicted when F was /s/ and not when it was /ʃ/ or /θ/ across age groups. Acoustic results indicated the expected biomechanically motivated /ʃ/ > /s/ > /θ/ coarticulatory resistance hierarchy in adults' speech. By contrast, /ʃ/ > /s/ were similarly influenced by context in 8-year-olds' speech, and the results from 5-year-olds' speech suggested an influence of order of acquisition in that /θ/ was surprisingly resistant to coarticulation. CONCLUSION: The study results are taken to suggest that a temporal constraint on fricative articulation interacts with biomechanical constraints during development to influence patterns of coarticulation in school-age children's speech.

The Acquisition of Articulatory Timing for Liquids: Evidence From Child and Adult Speech.

Purpose Liquids are among the last sounds to be acquired by English-speaking children. The current study considers their acquisition from an articulatory timing perspective by investigating anticipatory posturing for /l/ versus /ɹ/ in child and adult speech. Method In Experiment 1, twelve 5-year-old, twelve 8-year-old, and 11 college-aged speakers produced carrier phrases with penultimate stress on monosyllabic words that had /l/, /ɹ/, or /d/ (control) as singleton onsets and /æ/ or /u/ as the vowel. Short-domain anticipatory effects were acoustically investigated based on schwa formant values extracted from the preceding determiner (= the) and dynamic formant values across the /ə#LV/ sequence. In Experiment 2, long-domain effects were perceptually indexed using a previously validated forward-gated audiovisual speech prediction task. Results Experiment 1 results indicated that all speakers distinguished /l/ from /ɹ/ along F3. Adults distinguished /l/ from /ɹ/ with a lower F2. Older children produced subtler versions of the adult pattern; their anticipatory posturing was also more influenced by the following vowel. Younger children did not distinguish /l/ from /ɹ/ along F2, but both liquids were distinguished from /d/ in the domains investigated. Experiment 2 results indicated that /ɹ/ was identified earlier than /l/ in gated adult speech; both liquids were identified equally early in 5-year-olds' speech. Conclusions The results are interpreted to suggest a pattern of early tongue-body retraction for liquids in /ə#LV/ sequences in children's speech. More generally, it is suggested that children must learn to inhibit the influence of vowels on liquid articulation to achieve an adultlike contrast between /l/ and /ɹ/ in running speech.

Clause-initial AND usage in a cross-sectional and longitudinal corpus of school-age children's narratives.

Young children adopt an event-chaining strategy when storytelling, frequently linking clauses with and. The current study tested whether age-related changes in clause-initial and usage might index narrative structure development in the Eugene Children's Story Corpus (ECSC), which includes 180 structured spontaneous narratives elicited yearly for three years from 60 children, aged five to seven at study onset. The narratives were segmented into clauses to quantify clause-initial and usage. Adult judgments of narrative coherence and cohesiveness were elicited as measures of narrative structure. Mean length of utterance (MLU) and clause (MLC) were used as measures of language complexity. Results indicated developmental increases in all measures, but only and-connected dependent clause usage increased with cross-sectional and longitudinal age. Only MLC predicted the relative frequency of clause-initial and regardless of children's age. These results suggest children's frequent use of and to connect events reflects immature language; its association with flat narrative structure is likely epiphenomenal.

A psycholinguistic method for measuring coarticulation in child and adult speech.

A perceiver's ability to accurately predict target sounds in a forward-gated AV speech task indexes the strength and scope of anticipatory coarticulation in adult speech (Redford et al., JASA, 144, 2447-2461, 2018). This suggests a perception-based method for studying coarticulation in populations who may poorly tolerate the more invasive or restrictive techniques used to measure speech movements directly. But the use of perception to measure production begs the question of confounding influences on perceiver performance and thus on the reliability and generalizability of the proposed method. The present study was therefore designed to test whether a gated AV speech method for measuring coarticulation provides reliable results across different study populations (child versus adult), different task environments (in-lab versus online), and different coarticulatory directions (forward/anticipatory versus backward/carryover). The results indicated excellent measurement reliability across age groups in the forward/anticipatory measurement direction, though more perceivers are needed to achieve the same levels of agreement and consistency when the task is completed online. Accuracy was lower in the backward/carryover direction, and although agreement and consistency were still reasonably high across perceivers, the effect of age group differed between the laboratory and online environments, suggesting measurement error in one or both environments. Overall, the results support using in-lab or online perceptual judgments to measure anticipatory coarticulation in developmental studies of speech production. Further validation study is needed before the method can be extended to measure carryover coarticulation.

The Emergence of Discrete Perceptual-Motor Units in a Production Model That Assumes Holistic Phonological Representations.

Intelligible speakers achieve specific vocal tract constrictions in rapid sequence. These constrictions are associated in theory with speech motor goals. Adult-focused models of speech production assume that discrete phonological representations, sequenced into word-length plans for output, define these goals. This assumption introduces a serial order problem for speech. It is also at odds with children's speech. In particular, child phonology and timing control suggest holistic speech plans, and so the hypothesis of whole word production. This hypothesis solves the serial order problem by avoiding it. When the same solution is applied to adult speech the problem becomes how to explain the development of highly intelligible speech. This is the problem addressed here. A modeling approach is used to demonstrate how perceptual-motor units of production emerge over developmental time with the perceptual-motor integration of holistic speech plans that are also phonological representations; the specific argument is that perceptual-motor units are a product of trajectories (nearly) crossing in motor space. The model, which focuses on the integration process, defines the perceptual-motor map as a set of linked pairs of experienced perceptual and motor trajectories. The trajectories are time-based excursions through speaker-defined perceptual and motor spaces. By hypothesis, junctures appear where motor trajectories near or overlap one another in motor space when the shared (or extremely similar) articulatory configurations in these regions are exploited to combine perceptually-linked motor paths along different trajectories. Junctures form in clusters in motor space. These clusters, along with their corresponding (linked) perceptual points, represent perceptual-motor units of production, albeit at the level of speech motor control only. The units serve as pivots in motor space during speaking; they are points of transition from one motor trajectory to another along perceptually-linked paths that are selected to produce best approximations of whole word targets.

LISTENER PREFERENCE IS FOR REDUCED DETERMINERS THAT ANTICIPATE THE FOLLOWING NOUN.

This study examines the effects of determiner reduction and coarticulation on the perceived naturalness of resynthesized shock-the-geek (V-the-N) sequences. The determiner, equally spaced between monosyllabic V and N, was manipulated in 3 experiments along a 7-step continuum: (1) duration varied from 0.25x the original duration to 4x this duration; (2) amplitude varied from 55 dB to 85 dB; (3) schwa formants varied from completely overlapped with the vowel in V to completely overlapped with the vowel in N. Listeners rated V-the-N sequences with reduced duration and intensity and more anticipatory coarticulation more favourably than sequences with increased duration and intensity and more preservatory coarticulation. These results are consistent with a listener preference for the production of supralexical chunks that adhere to morphosyntactic rather than metrical structure.

Liquid coarticulation in child and adult speech.

Although liquids are mastered late, English-speaking children are said to have fully acquired these segments by age 8. The aim of this study was to test whether liquid coarticulation was also adult-like by this age. 8-year-old productions of /əLa/ and /əLu/ sequences were compared to 5-year-old and adult productions of these sequences. SSANOVA analyses of formant frequency trajectories indicated that, while adults contrasted rhotics and laterals from the onset of the vocalic sequence, F2 trajectories for rhotics and lateral were overlapped at the onset of the /əLa/ sequence in 8-year-old productions and across the entire /əLu/ sequence. The F2 trajectories for rhotics and laterals were even more overlapped in 5-year olds' productions. Overall, the study suggests that whereas younger children have difficulty coordinating the tongue body/root gesture with the tongue tip gesture, older children still struggle with the intergestural timing associated with liquid production.

CHARACTERIZING THE COORDINATION OF SPEECH PRODUCTION AND BREATHING.

Previous studies have concluded that breath intake patterns during speech emerge as a function of planning processes. Little work has tested for similar effects of respiratory recovery on these patterns. Moreover, previous work has relied on one-by-one elicitation of read sentences which incorporates a direct cue to upcoming length, allowing for anticipatory effects to emerge but prohibiting a test of preceding material on intakes. The current study investigated the relative influences of recovery and anticipatory factors on breath intakes in a connected speech task that better approximates spontaneous production. Participants (N = 6) were asked to recite a passage of 20 unrelated sentences from memory. Results revealed a significant effect of preceding utterance length on presence of breath intakes during pauses, but not of following utterance length. It is concluded that respiratory recovery drives breath intakes in connected speech.

Speech Production From a Developmental Perspective.

Purpose Current approaches to speech production aim to explain adult behavior and so make assumptions that, when taken to their logical conclusion, fail to adequately account for development. This failure is problematic if adult behavior can be understood to emerge from the developmental process. This problem motivates the proposal of a developmentally sensitive theory of speech production. The working hypothesis, which structures the theory, is that feedforward representations and processes mature earlier than central feedback control processes in speech production. Method Theoretical assumptions that underpin the 2 major approaches to adult speech production are reviewed. Strengths and weaknesses are evaluated with respect to developmental patterns. A developmental approach is then pursued. The strengths of existing theories are borrowed, and the ideas are resynthesized under the working hypothesis. The speech production process is then reimagined in developmental stages, with each stage building on the previous one. Conclusion The resulting theory proposes that speech production relies on conceptually linked representations that are information-reduced holistic perceptual and motoric forms, constituting the phonological aspect of a system that is acquired with the lexicon. These forms are referred to as exemplars and schemas, respectively. When a particular exemplar and schema are activated with the selection of a particular lexical concept, their forms are used to define unique trajectories through an endogenous perceptual-motor space that guides implementation. This space is not linguistic, reflecting its origin in the prespeech period. Central feedback control over production emerges with failures in communication and the development of a self-concept.

Leveraging audiovisual speech perception to measure anticipatory coarticulation.

A noninvasive method for accurately measuring anticipatory coarticulation at experimentally defined temporal locations is introduced. The method leverages work in audiovisual (AV) speech perception to provide a synthetic and robust measure that can be used to inform psycholinguistic theory. In this validation study, speakers were audio-video recorded while producing simple subject-verb-object sentences with contrasting object noun rhymes. Coarticulatory resistance of target noun onsets was manipulated as was metrical context for the determiner that modified the noun. Individual sentences were then gated from the verb to sentence end at segmental landmarks. These stimuli were presented to perceivers who were tasked with guessing the sentence-final rhyme. An audio-only condition was included to estimate the contribution of visual information to perceivers' performance. Findings were that perceivers accurately identified rhymes earlier in the AV condition than in the audio-only condition (i.e., at determiner onset vs determiner vowel). Effects of coarticulatory resistance and metrical context were similar across conditions and consistent with previous work on coarticulation. These findings were further validated with acoustic measurement of the determiner vowel and a cumulative video-based measure of perioral movement. Overall, gated AV speech perception can be used to test specific hypotheses regarding coarticulatory scope and strength in running speech.

Coarticulatory effects on "the" production in child and adult speech.

If prosodic words are the principle units of speech planning and production, then the production of an unstressed grammatical word should be especially influenced by the adjacent context word with which it is chunked. The current study tested this prediction in child and adult speech to investigate the development of the speech plan. Anticipatory and perseveratory influences on determiner vowel production were investigated in simple SVO sentences produced by 5-year-old children and college-aged adults. Although children's productions indicated greater perseveratory influences than adults' productions, anticipatory effects were consistently stronger than perseveratory effects across age groups. The results suggest that, by age 5 years, children chunk determiners along morphosyntactic boundaries just like adults.

Grammatical Word Production Across Metrical Contexts in School-Aged Children's and Adults' Speech.

Purpose: The purpose of this study is to test whether age-related differences in grammatical word production are due to differences in how children and adults chunk speech for output or to immature articulatory timing control in children. Method: Two groups of 12 children, 5 and 8 years old, and 1 group of 12 adults produced sentences with phrase-medial determiners. Preceding verbs were varied to create different metrical contexts for chunking the determiner with an adjacent content word. Following noun onsets were varied to assess the coherence of determiner-noun sequences. Determiner vowel duration, amplitude, and formant frequencies were measured. Results: Children produced significantly longer and louder determiners than adults regardless of metrical context. The effect of noun onset on F1 was stronger in children's speech than in adults' speech; the effect of noun onset on F2 was stronger in adults' speech than in children's. Effects of metrical context on anticipatory formant patterns were more evident in children's speech than in adults' speech. Conclusion: The results suggest that both immature articulatory timing control and age-related differences in how chunks are accessed or planned influence grammatical word production in school-aged children's speech. Future work will focus on the development of long-distance coarticulation to reveal the evolution of speech plan structure over time.

Lay Listener Classification and Evaluation of Typical and Atypical Children's Speech.

Verbal children with autism spectrum disorder (ASD) often also have atypical speech. In the context of the many challenges associated with ASD, do speech sound pattern differences really matter? The current study addressed this question. Structured spontaneous speech was elicited from 34 children: 17 with ASD, whose clinicians reported unusual speech prosody; and 17 typically-developing, age-matched controls. Multiword utterances were excerpted from each child's speech sample and presented to young adult listeners, who had no clinical training or experience. In Experiment 1, listeners classified band pass filtered and unaltered excerpts as "typical" or "disordered". Children with ASD were only distinguished from typical children based on unaltered speech, but the analyses indicated unique contributions from speech sound patterns. In Experiment 2, listeners provided likeability ratings on the filtered and unaltered excerpts. Again, lay listeners only distinguished children with ASD from their typically-developing peers based on unaltered speech, with typical children rated as more likeable than children with ASD. In Experiment 3, listeners evaluated the unaltered speech along several perceptual dimensions. High overlap between the dimensions of articulation, clearness, and fluency was captured by an emergent dimension: intelligibility. This dimension predicted listeners' likeability ratings nearly as well as it predicted their judgments of disorder. Overall, the results show that lay listeners can distinguish atypical from typical children outside the social-interactional context based solely on speech, and that they attend to speech intelligibility to do this. Poor intelligibility also contributes to listeners' negative social evaluation of children, and so merits assessment and remediation.

The Representation and Execution of Articulatory Timing in First and Second Language Acquisition.

The early acquisition of language-specific temporal patterns relative to the late development of speech motor control suggests a dissociation between the representation and execution of articulatory timing. The current study tested for such a dissociation in first and second language acquisition. American English-speaking children (5- and 8-year-olds) and Korean-speaking adult learners of English repeatedly produced real English words in a simple carrier sentence. The words were designed to elicit different language-specific vowel length contrasts. Measures of absolute duration and variability in single vowel productions were extracted to evaluate the realization of contrasts (representation) and to index speech motor abilities (execution). Results were mostly consistent with a dissociation. Native English-speaking children produced the same language-specific temporal patterns as native English-speaking adults, but their productions were more variable than the adults'. In contrast, Korean-speaking adult learners of English typically produced different temporal patterns than native English-speaking adults, but their productions were as stable as the native speakers'. Implications of the results are discussed with reference to different models of speech production.

Sound categories or phonemes?

Vihman emphasizes the importance of early word production to the emergence of phonological knowledge. This emphasis, consistent with the generative function of phonology, provides insight into the concurrent representation of phonemes and words. At the same time, Vihman's focus on phonology leads her to possibly overstate the influence of early word acquisition on the emergence of sound categories that are probably purely phonetic in nature at the outset of learning.