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Early speech development

Letn í Škola Lingvistiky, Dačice 2008. Early speech development. Mits Ota University of Edinburgh mits@ling.ed.ac.uk. Outline. Four basic questions about early speech development How do newborns perceive speech sounds?

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Early speech development

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  1. Letní Škola Lingvistiky, Dačice 2008 Early speech development Mits Ota University of Edinburgh mits@ling.ed.ac.uk

  2. Outline • Four basic questions about early speech development • How do newborns perceive speech sounds? • When and how do infants learn meaningful sound differences (phonemic contrasts) of the native language? • When do infants learn sound combinations (phonotactics) of the native language? • When and how do infants learn to pick out words from a speech stream?

  3. Part 1: How do newborns perceive speech sounds?

  4. Categorical perception We (= adults) perceive speech sounds as categories. Physically continuous acoustic differences are perceived as distinct groups of sounds. Category B Category A

  5. Voice onset time (VOT) and voicing 60 msec

  6. Categorical perception of VOT A VOT continuum is perceived as if it contains distincts sets of sounds. We hear the difference only around the boundary. D D 0ms 20ms D 20ms 40ms T T T 40ms 60ms % discriminated % identified as /d/ VOT VOT difference

  7. Infants’ categorical perception of VOT High-amplitude sucking procedure No change 20 ↔ 40ms 40 ↔ 60ms 1-, 4-mos reaction to changes in VOT (Eimas et al, 1971)

  8. Infants’ speech-discrimination capacities • Range of contrasts newborns and infants (≤ 2 months) can discriminate: • Voicing (e.g., /d/-/t/) • Place of articulation (e.g., /b/-/d/-/g/; Bertoncini et al., 1987) • Manner (e.g., /b/-/w/; Eimas & Miller, 1980) • Vowels (e.g., /a/-/i/; Trehub, 1973) • Stress (e.g., BAda-baDA; Jusczyk & Thompson, 1978) • Pitch contour (e.g., ame-ame; Nazzi et al. 1988)

  9. Infants’ speech-discrimination capacities • Even non-native contrasts can be discriminated: • Kikuyu 1- to 4-month-olds: [ba]-[pa] (Streeter, 1976) • English Canadian 1- to 4-month-olds: [pa]-[pã], [řa]-[za] (Trehub, 1976) • English Canadian 6-month-olds: [da]-[a], [‘ki]-[‘qi] (Werker & Tees, 1984) • English 4-½-month-olds: [u]-[y] (Polka & Werker, 1994) • Language experience is irrelevant  These speech discrimination capacities are part of the innate property of the infant’s auditory system.

  10. Categorical perception = Capacity specialised for language? • Arguments for specialised speech processing mechanisms • Duplex perception: Dichotically presented speech stimuli are categorically discriminated by 3- to 4-month-olds (Eimas & Miller, 1992).

  11. Categorical perception = Capacity shared with other types of processing? • Arguments against specialised speech processing mechanisms • Infants show categorical perception in non-speech sound stimuli: e.g., sequence of tone with different onset times (Jusczyk et al., 1980).

  12. Categorical perception = Capacity unique to humans? • Categorical perception of sounds occurs in non-human species. • Chinchillas (Kuhl & Miller, 1975) • Quails (křepelka) (Lotto et al., 1997) • But… • Non-humans need training to show the effect. • Non-humans need larger step-size in the stimulus.

  13. Summary of Part 1 • Newborns discriminate speech sounds categorically. • They do this for a wide range of sound contrasts including those that are not used in the native language. • Categorical perception of speech is an innate disposition of newborns. • It is not clear whether the underlying ability is specific to speech or humans, but compared to non-humans, human babies apply categorical perception more readily to human speech.

  14. Part 2: When and how do infants learn meaningful sound differences in the native language?

  15. Perceptual reorganization • Attunement to native contrasts • Sensitivity to nonnative contrasts declines between 8 and 10 months (Werker & Tees 1984).

  16. Learning lexical contrasts: Perception • Do children who discriminate native sounds (e.g., /b/-/d/) also know that the difference encodes a lexical contrast (e.g., big-dig)? • Testing lexical contrasts: the Switch Task (Stager & Werker, 1997) Habituation phase Test phase ‘Same’ ‘Switch’ ‘bih’ ‘dih’ ‘bih’ ‘dih’

  17. Learning lexical contrasts: Perception • 14-month-olds cannot perform the Switch Task on [b] - [d], even though they can discriminate the sounds (Stager & Werker, 1997).

  18. Learning lexical contrasts: Perception • 17-month-olds can perform the Switch Task on a native contrast (Werker et al., 2002). • Ability to map phonetic difference to new lexical contrast develops around 15-17 months. English /b/ - /d/ Hindi /da/ - /a/

  19. Learning lexical contrasts: Production • English children’s VOTs of attempted words in 1;6-2;1 (Macken & Barton, 1980)

  20. Learning lexical contrasts: Production Child’s contrast in development Adult’s contrast Phonetic difference No contrast Covert contrast Overshot contrast

  21. But how do they learn phonemic contrasts? • The problem: Children under 1;6 do not have a large enough vocabulary that contains critical minimal pairs: • dát vs. tát • An alternative explanation: Children are sensitive to the phonetic distribution of the sounds they hear (Maye, Werker, & Gerken, 2002).

  22. Summary of Part 2 • Infants lose perceptual sensitivities to nonnative contrasts around 6-9 months. • They become able to perceptually learn new words that involve native contrasts around 15-17 months. • Around the same time, they gradually become able to produce those contrasts with adultlike phonetic values. • They accomplish this without full lexical evidence (i.e., minimal pairs), presumably by observing the phonetic distribution of sounds produced in the environment.

  23. Part 3: When do infants learn sound combination patterns (phonotactics) in the native language?

  24. Prenatal ‘learning’ of native phonological patterns • Children are sensitive to certain characteristics of the ambient speech pattern even before they are born. • Two-day-olds respond to a passage read by the mother during the last 6 weeks of pregnancy (DeCasper and Spence, 1986). • 37-week-old fetuses respond to a passage read by the mother between the 33rd-37th weeks of gestation (DeCasper et al. 1994). • Memory of prosodic information (e.g., rhythm, pitch patterns).

  25. Head-turn preference procedure Experimenter Viewing hole Video camera Infant Loud speaker Loud speaker Parent

  26. Learning phonotactics: birth ~ 5 months • Newborns recognize global characteristics of utterances from parental language. • Must be typologically different languages: French vs. Russian, Spanish vs. English, French vs. Japanese (but not English vs. German) • Effects remain with low-pass filtered speech • Identification of global prosody • Typologically similar languages can be differentiated by 4-5 months if one of them is the native language. • Spanish vs. Catalan (Bosch & Sebastian-Galles 1997) • British vs. American English (Nazzi et al. 2000)

  27. Learning phonotactics: 6 ~ 9 months • Word-level sensitivity to prosodic patterns develops around 6-9 months. • American 6-month-olds prefer to listen to low-pass filtered English words over Norwegian words (Jusczyk et al., 1993). • Differences: Many Norwegian words have a pitch rise on the final syllable and low pitch on stressed syllables. • American 9-month-olds prefer to listen to low-pass filtered English words with strong/weak patterns (e.g., loony, pliant) than those with weak/strong patterns (e.g., abloom, comply) (Jusczyk, Cutler & Redanz, 1993). • Most disyllabic English words have a strong/weak pattern.

  28. Learning phonotactics: 6 ~ 9 months • Sensitivity to possible/plausible segmental sequences develops by 9 months. • American and Dutch 9-month-olds listen longer to their native words that feature language-specific segmental phonotactics (Jusczyk et al., 1993). • English: hubbub, aboard • Dutch: kalmpjes, opkomst, zweten • American 9-month-olds (but not 6-m-o) listen longer to high probability segmental sequences in English ([tn]) than to legal but low probability sequences ([tg]).

  29. Summary of Part 3 • Around, and even before, birth, babies learn some global prosodic characteristics of the native language utterances. • By 9 months, infants learn many prosodic and segmental characteristics of the native language words. • Infants keep track of whether and how often a particular sound pattern occurs in the ambient language(s).

  30. Part 4: When and how do infants pick out words in a stream of speech?

  31. The challenge of word segmentation • Identifying words in natural speech is not easy. • Only about 10-20% of parents’ speech to children consists of single word utterances (Ratner, 1996). • There are no consistent silences between words.

  32. Word segmentation: When? When do children begin to extract words from speech? • Jusczyk & Aslin (1995) • Presentation in isolation (15 times) “cup, cup, cup… cup” “feet, feet, feet… feet” • Presentation in passages (6 each) “The cup was bright and shiny” “A clown drank from the red cup” etc. “The feet were all different sizes” “The girl has very big feet” etc. Headturn results

  33. Segmentation mechanism (1): Transitional probabilities • Using the likelihood of one sound following another • 8-month-olds can keep track of frequently recurring patterns of syllable sequence (Saffran et al., 1996). • Familiarisation: “golabupabikututibubapapugolabugabupu …” • Testing: golabu > bupabi • Transitional probabilities • go-la, la-bu: 100% • pa-bi: 50% • bu-pa: 33% • Implication for real word learning: Probability that [pr] is followed by [ti] in child directed speech: 80%. Probability that [ti] is followed by [be]: 0.03%. Thus, [prtibebi] [prti]#[bebi]

  34. Segmentation mechanism (2): Prosodic cues • Using predominant prosodic patterns • Metrical Segmentation Strategy (MSS; Cutler & Norris 1988): Strong (stressed) syllable = Beginning of a word. • Evidence for Strong-weak parsing in English-learning 7½-month-olds (Jusczyk et al. 1999)

  35. Segmentation mechanism (3): Phonotactic cues • Using phonotactic possibilities Within word cluster Between word cluster nk nmk dfts dvts gktet gptet 9-month-olds’ preference: nk > nmk n k < nm k pause pause

  36. Segmentation mechanisms (4): Allophonic cues • Using allophonic variation • Children’s segmentation is influenced by phonetic differences between allophones: e.g., initial ([d]ice) vs final stop (weir[d]) (Mattys & Jusczyk 2001).

  37. Segmentation mechanisms: Sources of cues • How do children learn the segmentation cues without having the words to begin with? Some words Transitional probabilities Allophonic cues Global prosodic pattern Prosodic cues Phonotactic cues More words

  38. Summary of Part 4 • By 7.5 months, infants can pick up words in a stream of speech. • Several mechanisms for this process have been proposed, including the use of 1) transitional probabilities, 2) prosodic cues, 3) phonotactic cues, and 4) allophonic cues. Most likely, word segmentation employs all of these cues.

  39. General summary • Human babies are born with a disposition to perceive speech sounds categorically. • Within the first year, they: • lose perceptual sensitivity to acoustic differences not phonologically used in their native language • learn native phonotactics. • become able to extract wordforms out of speech streams. • They accomplish these by keeping track of the distribution and statistical probabilities of sounds and sound combinations they hear.

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