Lexical viability constraints on speech segmentation by infants

The Possible Word Constraint limits the number of lexical candidates considered in speech recognition by stipulating that input should be parsed into a string of lexically viable chunks. For instance, an isolated single consonant is not a feasible word candidate. Any segmentation containing such a chunk is disfavored. Five experiments using the head-turn preference procedure investigated whether, like adults, 12-month-olds observe this constraint in word recognition. In Experiments 1 and 2, infants were familiarized with target words (e.g., rush), then tested on lists of nonsense items containing these words in "possible" (e.g., "niprush" [nip+rush]) or "impossible" positions (e.g., "prush" [p+rush]). The infants listened significantly longer to targets in "possible" versus "impossible" contexts when targets occurred at the end of nonsense items (rush in "prush"), but not when they occurred at the beginning (tan in "tance"). In Experiments 3 and 4, 12-month-olds were similarly familiarized with target words, but test items were real words in sentential contexts (win in "wind" versus "window"). The infants listened significantly longer to words in the "possible" condition regardless of target location. Experiment 5 with targets at the beginning of isolated real words (e.g., win in "wind") replicated Experiment 2 in showing no evidence of viability effects in beginning position. Taken together, the findings suggest that, in situations in which 12-month-olds are required to rely on their word segmentation abilities, they give evidence of observing lexical viability constraints in the way that they parse fluent speech.     

An interaction between prosody and statistics in the segmentation of fluent speech

Sensitivity to prosodic cues might be used to constrain lexical search. Indeed, the prosodic organization of speech is such that words are invariably aligned with phrasal prosodic edges, providing a cue to segmentation. In this paper we devise an experimental paradigm that allows us to investigate the interaction between statistical and prosodic cues to extract words from a speech stream. We provide evidence that statistics over the syllables are computed independently of prosody. However, we also show that trisyllabic sequences with high transition probabilities that straddle two prosodic constituents appear not to be recognized. Taken together, our findings suggest that prosody acts as a filter, suppressing possible word-like sequences that span prosodic constituents.     

Phonotactic probability influences speech production

Speakers are faster and more accurate at processing certain sound sequences within their language. Does this reflect the fact that these sequences are frequent or that they are phonetically less complex (e.g., easier to articulate)? It has been difficult to contrast these two factors given their high correlation in natural languages. In this study, participants were exposed to novel phonotactic constraints de-correlating complexity and frequency by subjecting the same phonological structure to varying degrees of probabilistic constraint. Participants' behavior was sensitive to variations in frequency, demonstrating that phonotactic probability influences speech production independent of phonetic complexity.     

Integration of multiple speech segmentation cues: a hierarchical framework

A central question in psycholinguistic research is how listeners isolate words from connected speech despite the paucity of clear word-boundary cues in the signal. A large body of empirical evidence indicates that word segmentation is promoted by both lexical (knowledge-derived) and sublexical (signal-derived) cues. However, an account of how these cues operate in combination or in conflict is lacking. The present study fills this gap by assessing speech segmentation when cues are systematically pitted against each other. The results demonstrate that listeners do not assign the same power to all segmentation cues; rather, cues are hierarchically integrated, with descending weights allocated to lexical, segmental, and prosodic cues. Lower level cues drive segmentation when the interpretive conditions are altered by a lack of contextual and lexical information or by white noise. Taken together, the results call for an integrated, hierarchical, and signal-contingent approach to speech segmentation.     

Newborns are sensitive to multiple cues for word segmentation in continuous speech

Before infants can learn words, they must identify those words in continuous speech. Yet, the speech signal lacks obvious boundary markers, which poses a potential problem for language acquisition (Swingley, Philos Trans R Soc Lond. Series B, Biol Sci 364 (1536), 3617–3632, 2009). By the middle of the first year, infants seem to have solved this problem (Bergelson & Swingley, Proc Natl Acad Sci 109 (9), 3253–3258, 2012; Jusczyk & Aslin, Cogn Psychol 29 , 1–23, 1995), but it is unknown if segmentation abilities are present from birth, or if they only emerge after sufficient language exposure and/or brain maturation. Here, in two independent experiments, we looked at two cues known to be crucial for the segmentation of human speech: the computation of statistical co‐occurrences between syllables and the use of the language's prosody. After a brief familiarization of about 3 min with continuous speech, using functional near‐infrared spectroscopy, neonates showed differential brain responses on a recognition test to words that violated either the statistical (Experiment 1) or prosodic (Experiment 2) boundaries of the familiarization, compared to words that conformed to those boundaries. Importantly, word recognition in Experiment 2 occurred even in the absence of prosodic information at test, meaning that newborns encoded the phonological content independently of its prosody. These data indicate that humans are born with operational language processing and memory capacities and can use at least two types of cues to segment otherwise continuous speech, a key first step in language acquisition.     

Converging measures of speech segmentation in preverbal infants

Two studies using novel extensions of the conditioned head-turning method examined contributions of rhythmic and distributional properties of syllable strings to 8-month-old infants' speech segmentation. The two techniques introduced exploit fundamental, but complementary, properties of representational units. The first involved assessment of discriminative response maintenance when simple training stimuli were embedded in more complex speech contexts; the second involved measurement of infants' latencies in detecting extraneous signals superimposed on speech stimuli. A complex pattern of results is predicted if infants succeed in grouping syllables into higher-order units. Across the two studies, the predicted pattern of results emerged, indicating that rhythmic properties of speech play an important role in guiding infants toward potential linguistically relevant units and simultaneously demonstrating that the techniques proposed here provide valid, converging measures of infants' auditory representational units.     

Treatment of stammering by reinforcement of fluent speech

A series of small experimental studies was conducted with three stammerers. The studies show that stammering may be controlled by positive reinforcement of fluent speech in a machine reading task. This new procedure for the treatment of stammering is convenient and effective in producing fluent speech in the laboratory. Evidence suggests some generalization of a stable kind to outside settings.

Stammering interrupts speech and disturbs communication. It has been treated by a variety of methods, ranging from physical assault on the speech organs, through procedures designed to establish new speech patterns, training in deliberate speech control, and masking of auditory feedback, to psychotherapy and behaviour therapy.

Negative practice has brought about improvement in up to a third of cases (Dunlap, 1932; Fishman. 333 1937; Sheehan, 1953; Lehner, 1954; Jones, 1955; Case, 1960). The other common form of behaviour therapy has usually involved negative reinforcement or punishment. Flanagan, Goldiamond and Azrin (1958) used a loud blast of noise every time subjects stammered. Stammering rate was markedly depressed during the aversive conditioning, but when the aversive conditions were discontinued, stammering rate showed a pronounced increase. More recently, Goldiamond (1965) used fluency to terminate a noxious stimulus and reported a reduction in stammering.     

Phonotactic regularities in the segmentation of spoken Italian

Five word-spotting experiments explored the role of consonantal and vocalic phonotactic cues in the segmentation of spoken Italian. The first set of experiments tested listeners’ sensitivity to phonotactic constraints cueing syllable boundaries. Participants were slower in spotting words in nonsense strings when target onsets were misaligned (e.g., lago in ri.blago) than when they were aligned (e.g., lago in rin.lago) with phonotactically determined syllabic boundaries. This effect held also for sequences that occur only word-medially (e.g., /tl/ in ri.tlago), and competition effects could not account for the disadvantage in the misaligned condition. Similarly, target detections were slower when their offsets were misaligned (e.g., cittá in cittáu.ba) than when they were aligned (e.g., cittá in cittá.oba) with a phonotactic syllabic boundary. The second set of experiments tested listeners’ sensitivity to phonotactic cues, which specifically signal lexical (and not just syllable) boundaries. Results corroborate the role of syllabic information in speech segmentation and suggest that Italian listeners make little use of additional phonotactic information that specifically cues word boundaries.     

Diminutives in child-directed speech supplement metric with distributional word segmentation cues

In two experiments, we explored whether diminutives (e.g.,birdie, Patty, bootie), which are characteristic of child-directed speech in many languages, aid word segmentation by regularizing stress patterns and word endings. In an implicit learning task, adult native speakers of English were exposed to a continuous stream of synthesized Dutch nonsense input comprising 300 randomized repetitions of six bisyllabic target nonwords. After exposure, the participants were given a forced choice recognition test to judge which strings had been present in the input. Experiment 1 demonstrated that English speakers used trochaic stress to isolate strings, despite being unfamiliar with Dutch phonotactics. Experiment 2 showed benefits from invariance introduced by affricates, which are typically found at onsets of final syllables in Dutch diminutives. Together, the results demonstrate that diminutives contain prosodic and distributional features that are beneficial for word segmentation.     

Phonotactic acquisition in healthy preterm infants

Previous work has shown that preterm infants are at higher risk for cognitive/language delays than full‐term infants. Recent studies, focusing on prosody (i.e. rhythm, intonation), have suggested that prosodic perception development in preterms is indexed by maturational rather than postnatal/listening age. However, because prosody is heard in‐utero, and preterms thus lose significant amounts of prenatal prosodic experience, both their maturation level and their prosodic experience (listening age) are shorter than that of full‐terms for the same postnatal age. This confound does not apply to the acquisition of phonetics/phonotactics (i.e. identity and order of consonants/vowels), given that consonant differences in particular are only perceived after birth, which could lead to a different developmental pattern. Accordingly, we explore the possibility that consonant‐based phonotactic perception develops according to listening age. Healthy French‐learning full‐term and preterm infants were tested on the perception of consonant sequences in a behavioral paradigm. The pattern of development for full‐term infants revealed that 7‐month‐olds look equally at labial‐coronal (i.e. /pat/) compared to coronal‐labial sequences (i.e. /tap/), but that 10‐month‐olds prefer the labial‐coronal sequences that are more frequent in the French lexicon. Preterm 10‐month‐olds (having 10 months of phonetic listening experience but 7 months of maturational age) behaved as full‐term 10‐month‐olds. These results establish that preterm developmental timing for consonant‐based phonotactic acquisition is based on listening age (experience with input). This questions the interpretation of previous results on prosodic acquisition in terms of maturational constraints, and raises the possibility that different constraints apply to the acquisition of different phonological subcomponents.     

Recurrent speech patterns as cues to the segmentation of multisyllabic sequences.

First and second language acquisition both require that speech be segmented into familiar, multiphonemic units (e.g., words and common phrases). The present research examines one segmentation cue that is of considerable theoretical interest: the repetition of fixed sequences of speech. On each trial, subjects heard repetitions ('pre-exposures') of two artificially-constructed, multisyllabic patterns that shared an embedded segment 1 or 2 syllables long (e.g., 2 shared syllables: [ga-li-SE] and [li-SE-stu]). There were 2 and 6, 4 and 4, or 6 and 2 repetitions of the two patterns, randomly ordered. Subjects were then to indicate the groupings they perceived within a subsequent, longer sequence containing both of the pre-exposed patterns (e.g., [ga-li-SE-stu]). Responses varied systematically with the size of the embedded segment, the repetition frequencies of the two pre-exposed patterns, and the serial position of each pre-exposure. The results illustrate how investigations of the processing of speech patterns may contribute to an understanding of some elementary aspects of language learning.     

Phonotactic regularities in the segmentation of spoken Italian

Five word-spotting experiments explored the role of consonantal and vocalic phonotactic cues in the segmentation of spoken Italian. The first set of experiments tested listeners' sensitivity to phonotactic constraints cueing syllable boundaries. Participants were slower in spotting words in nonsense strings when target onsets were misaligned (e.g., lago in ri.blago) than when they were aligned (e.g., lago in rin.lago) with phonotactically determined syllabic boundaries. This effect held also for sequences that occur only word-medially (e.g., /tl/ in ri.tlago), and competition effects could not account for the disadvantage in the misaligned condition. Similarly, target detections were slower when their offsets were misaligned (e.g., cittá in cittáu.ba) than when they were aligned (e.g., cittá in cittá.oba) with a phonotactic syllabic boundary. The second set of experiments tested listeners' sensitivity to phonotactic cues, which specifically signal lexical (and not just syllable) boundaries. Results corroborate the role of syllabic information in speech segmentation and suggest that Italian listeners make little use of additional phonotactic information that specifically cues word boundaries.     

Twenty-two-month-olds discriminate fluent from disfluent adult-directed speech

Deviation of real speech from grammatical ideals due to disfluency and other speech errors presents potentially serious problems for the language learner. While infants may initially benefit from attending primarily or solely to infant-directed speech, which contains few grammatical errors, older infants may listen more to adult-directed speech. In a first experiment, Post-verbal infants preferred fluent speech to disfluent speech, while Pre-verbal infants showed no preference. In a second experiment, Post-verbal infants discriminated disfluent and fluent speech even when lexical information was removed, showing that they make use of prosodic properties of the speech stream to detect disfluency. Because disfluencies are highly correlated with grammatical errors, this sensitivity provides infants with a means of filtering ungrammaticality from their input.     

Speech segmentation is facilitated by visual cues

Evidence from infant studies indicates that language learning can be facilitated by multimodal cues. We extended this observation to adult language learning by studying the effects of simultaneous visual cues (nonassociated object images) on speech segmentation performance. Our results indicate that segmentation of new words from a continuous speech stream is facilitated by simultaneous visual input that it is presented at or near syllables that exhibit the low transitional probability indicative of word boundaries. This indicates that temporal audio-visual contiguity helps in directing attention to word boundaries at the earliest stages of language learning. Off-boundary or arrhythmic picture sequences did not affect segmentation performance, suggesting that the language learning system can effectively disregard noninformative visual information. Detection of temporal contiguity between multimodal stimuli may be useful in both infants and second-language learners not only for facilitating speech segmentation, but also for detecting word–object relationships in natural environments.     

Effects of target monitoring on understanding fluent speech

Phoneme monitoring and word monitoring are two experimental tasks that have frequently been used to assess the processing of fluent speech. Each task is purported to provide an “online” measure of the comprehension process, and each requires listeners to pay conscious attention to some aspect or property of the sound structure of the speech signal. The present study is primarily a methodological one directed at the following question: Does the allocation of processing resources for conscious analysis of the sound structure of the speech signal affect ongoing comprehension processes or the ultimate level of understanding achieved for the content of the linguistic message? Our subjects listened to spoken stories. Then, to measure their comprehension, they answered multiple-choice questions about each story. During some stories, they were required to detect a specific phoneme; during other stories, they were required to detect a specific word; during still other stories, they were not required to monitor the utterance for any target. The monitoring results replicated earlier findings showing longer detection latencies for phoneme monitoring than for word monitoring. Somewhat surprisingly, the ancillary phoneme- and word-monitoring tasks did not adversely affect overall comprehension performance. This result undermines the specific criticism that on-line monitoring paradigms of this kind should not be used to study spoken language understanding because these tasks interfere with normal comprehension.     

Infant perception of audio-visual speech synchrony in familiar and unfamiliar fluent speech

We investigated the effects of linguistic experience and language familiarity on the perception of audio-visual (A-V) synchrony in fluent speech. In Experiment 1, we tested a group of monolingual Spanish- and Catalan-learning 8-month-old infants to a video clip of a person speaking Spanish. Following habituation to the audiovisually synchronous video, infants saw and heard desynchronized clips of the same video where the audio stream now preceded the video stream by 366, 500, or 666 ms. In Experiment 2, monolingual Catalan and Spanish infants were tested with a video clip of a person speaking English. Results indicated that in both experiments, infants detected a 666 and a 500 ms asynchrony. That is, their responsiveness to A-V synchrony was the same regardless of their specific linguistic experience or familiarity with the tested language. Compared to previous results from infant studies with isolated audiovisual syllables, these results show that infants are more sensitive to A-V temporal relations inherent in fluent speech. Furthermore, the absence of a language familiarity effect on the detection of A-V speech asynchrony at eight months of age is consistent with the broad perceptual tuning usually observed in infant response to linguistic input at this age.     

Auditory and phonetic coding of the cues for speech: Discrimination of the [r-l] distinction by young infants

Infants, 2 and 3 months of age, were found to discriminte stimuli along the acoustic continuum underlying the phonetic contrast [r] vs. [l] in a nearly categorical manner. For an approximately equal acoustic difference, discrimination, as measured by recovery from satiation or familiarization, was reliably better when the two stimuli were exemplars of different phonetic categories than when they were acoustic variations of the same phonetic category. Discrimination of the same acoustic information presented in a nonspeech mode was found to be continuous, that is, determined by acoustic rather than phonetic characteristics of the stimuli. The findings were discussed with reference to the nature of the mechanisms that may determine the processing of complex acoustic signals in young infants and with reference to the role of linguistic experience on the development of speech perception at the phonetic level.

     

Acoustic evidence of aberrant velocities in stutterers' fluent speech

Movement rates of formant frequencies and the extents of articulatory change were spectrographically analyzed in the fLuent (VCV) utterances of 20 stutterers and nonstutterers. The velocities of articulator movement throughout the first vowel and velocities into the second vowel were not significantly different for the two groups. These mean rates of movement, although nonsignificant, were slower in stutterers and slightly more variable, and the extent of articulator movement was comparable. These results do not support the contentions that stutterers use coarticulatory movements that are too rapid or that stutterers have a poorer competence for rapid coordination of speech movements. The rationales of rate-control treatment methods to slow coarticulatory movements in stutterers need to be reexamined.     

Perceptual comparisons of adolescent stutterers' and nonstutterers' fluent speech

Numerous investigators have reported that listeners are able to perceptually differentiate adult stutterers' and nonstutterers' fluent speech productions. However, findings from similar studies with children ranging in age from 3 to 9 yr have indicated that perceptual discrimination of child stutterers is difficult. A logical extension of this line of investigation would be to determine when during maturation from childhood to adulthood stutterers' fluent speech becomes perceptibly different than nonstutterers'. Therefore, in this study similar fluent speech samples from seven 12–16-yr-old adolescent male stutterers and seven matched nonstutterers were analyzed perceptually in a paired stimulus paradigm by 15 sophisticated listeners. Individual subject analyses using signal detection theory revealed that five of the seven stutterers were discriminated. When averaged for subject group comparison, these findings indicated that listeners successfully discriminated between the fluent speech of the two groups. Therefore, the perceptual difference in fluent speech production reported previously for adults appears to be present by adolescence.