formal regularity of the falling tone in children’s early meaningful speech

Journal of Phonetics (1995) 23 , 387 – 405

Formal regularity of the falling tone in children’s early meaningful speech

David Snow* Uni y ersity of Washington , Seattle , WA , U .S .A .

Recei y ed 2 9 th July 1 9 9 3 , and in re y ised form 1 0 th May 1 9 9 5

Previous research has shown that adults’ intonation is relatively unaf fected by small to moderate dif ferences in utterance length . To address conflicting findings that have been reported in developmental studies , this investigation was designed to determine whether children’s nuclear tones are independent of the length of the phonetic strings in which the tones are expressed . Nine children were studied from the one-word period (18 months of age) until three months after the onset of syntax (24 months) . The children’s falling pitch contours were analyzed by using a schematic continuum that depicts accent range and complexity as gradients within bipolar directional classes . The results indicated that contour complexity was variable during the period of early meaningful speech . However , accent range tended to be uniform in phonetic strings that dif fered in number of syllables . This formal regularity associated with accent range preceded the onset of word combinations in the children’s language development . The findings suggest that accent range in falling tones is one of the first expressive features of the grammar that one-year-old children acquire . ÷ 1995 Academic Press Limited

1 . Introduction

A significant milestone in children’s language development is the transition from single-word utterances such as baby and hat to two-word combinations such as baby hat . Traditionally , word order has been viewed as the first grammatical feature that children control (Schlesinger , 1971) . For this reason , the child’s first word combinations are usually interpreted as the onset of syntax .

Intonation , or the pitch pattern of utterances , is also an early-developing feature . At the heart of intonation in English is a set of ‘‘tunes’’ or ‘‘nuclear tones’’ , that is , melody sequences that begin on the word bearing the main phrase stress (Vanderslice & Ladefoged , 1972 ; Liberman , 1979 ; Cruttenden , 1986) . Some children use adultlike nuclear tones in the late babbling period (Lenneberg , 1967) . Others do not use these canonical intonation patterns until they begin producing their first meaningful words (Leopold , 1947) . In spite of such individual dif ferences , children seem to acquire one or more of the basic nuclear tones by the end of the one-word period (Scollon , 1976 ; Crystal , 1986) . Thus , nuclear tones constitute a class of

* Currently at the Child Language Center , Department of Speech & Hearing Sciences , University of Arizona , Tucson , AZ 85721 , U . S . A .

0095-4470 / 95 / 040387 1 19 $12 . 00 / 0 ÷ 1995 Academic Press Limited

D . Snow 388

features that may begin to stabilize in child speech before the first appearance of word combinations (Crystal , 1975) .

1 . 1 . Pitch direction and pitch range

As an example of children’s early grammatical use of intonation , Galligan (1987) reported that English-speaking children by about 17 months of age appropriately used falling versus rising tones to signal statements versus requests in single-word utterances . Even at earlier stages of development , cross-language dif ferences in the frequency of rising versus falling contours in infant babbling suggest that children in the pre-speaking period may be sensitive to the prevailing pattern of contour direction in the ambient language (Halle , de Boysson-Bardies , & Vihman , 1991 ; Whalen , Levitt , & Wang , 1991) .

The magnitude of pitch change in falling tones has also been studied , although this tonal feature , sometimes called ‘‘accent range’’ , has received less attention to date than the contrast between rises and falls . Marcos (1987) , for example , found that children early in the single-word period varied accent range to express dif ferences in emphasis . Other data , mostly owing to diary studies , support Marcos’ findings and suggest that accent range may be a source of intonational or lexical tone contrast that develops even earlier than the directional contrast (Crystal , 1986) .

1 . 2 . Length of utterance and pitch

Both Galligan’s and Marcos’ studies showed that children at the single-word stage of language development vary the form of nuclear tones to express dif ferent discoursal or attitudinal meanings . The present study focused on another aspect of children’s control of tonal patterns , namely , the regularity with which they express tones of the same meaning in utterances of dif ferent length .

The relationship between utterance length and vocal pitch has been a topic of research interest in studies of adults as well as children . Some of these research interests stem from the idea that physiologically-based constraints related to utterance length may af fect the acoustic correlate of pitch , i . e ., fundamental frequency (f 0 ) , and may even compete with the intentional , meaningful use of intonation . Accordingly , such constraints might be reflected in marked variability of intonational form across sentences varying in length . Contrary to this expectation , studies of adult speech revealed that one global measure of intonation—the downdrift in f 0 throughout utterances—was relatively stable in sentences of dif ferent length (Pierrehumbert , 1979) . Similarly , Robb , Saxman , & Grant (1989) found no evidence to suggest that the f 0 variability of one- and two-year-old children dif fered in monosyllabic versus disyllabic utterances .

Other studies , however , suggest that children’s control of intonational form is not completely adultlike even at a more advanced stage of language development than that observed by Robb et al . For example , Allen & Hawkins (1980) studied three-year-olds’ marking of the nuclear accent , a basic part of the nuclear tone pattern (Vanderslice & Ladefoged , 1972 ; Beckman & Ayers , 1993) . They noted that toddlers used a smaller pitch obtrusion on phrase-final monosyllables such as dog than on the nonfinal nuclear accented syllable of a longer word or compound like pirate dog . These and related findings , which Allen & Hawkins interpreted as

Children ’ s falling tones 389

departures from or exaggerations of adult characteristics , suggest that at least some aspects of children’s intonation may vary in utterances of dif ferent length .

Data that would help to resolve the questions posed by these conflicting findings would have implications for our understanding of early prosodic development . For example , if English-speaking children in the single-word period were found to treat pitch as partly orthogonal to utterance length (i . e ., in a manner similar to adult speech) , this would be consistent with other evidence cited in the preceding paragraphs that the child’s presyntactic use of tone demonstrates some of the formal properties and key regularities of the target intonational system . Such evidence would be compatible with the view that children’s early pitch patterns reflect the linguistic influences of intonation .

The apparent discrepancies between the Allen & Hawkins and Robb et al . investigations may be due to dif ferences in the age range of the children and dif ferent methods of analyzing intonation . The nuclear accent , which was the focus of Allen & Hawkins’ research , corresponds to the onset of the nuclear tone . Thus , their unit of analysis was the portion of three-year-olds’ sentences that coincided with all or part of the nuclear tone contour . In contrast , Robb et al . studied one- and two-syllable utterances of younger children . The accent pattern on the disyllabic utterances was not specified and therefore may have varied . Thus , the unit of analysis in their study corresponded to complete utterances , which , in all likelihood , were sometimes coextensive with nuclear tones and sometimes were not .

To reconcile the dif ferent approaches of the Robb et al . and Allen & Hawkins investigations , the present study focused on children’s development of intonation from 18 to 24 months (part of the age range observed by Robb et al . ) and adapted Allen & Hawkins’ nuclear accentuation framework for the purpose of analyzing whole nuclear tones . The study extended an investigation of accented syllables in child speech reported by Snow (1994) . The database used in that study was expanded to include both monosyllables and disyllables that children produced within a falling nuclear tone envelope . The primary research goal was to determine whether children’s production of falling nuclear tones is af fected by the number of syllables in which the tones are expressed .

2 . Methods

2 . 1 . Participants

Nine children participated in the study . The participating families were recruited by an advertisement in a local newsletter . Participants met the following selection criteria : (1) first-born girls from monolingual English-speaking homes , (2) productive vocabulary between 30 and 70 words , (3) single-word utterances only (no word combinations yet) , and (4) normal hearing as assessed by VRA procedures using warbled tones of 500 , 1000 , 2000 , and 4000 Hz (eight of the nine participants responded to all tones at a screening level of 20 dB HL ; the other child responded to the 500 Hz tone at 25 dB HL and to the other tones at 20 dB HL) .

D . Snow 390

2 . 2 . Procedures

Data collection began when the participants had a productive vocabulary of at least 30 words according to parent report using the MacArthur Communicative Develop- ment Inventories (Fenson , Dale , Reznick , Thal , Bates , Hartung , Pethick , & Reilly , 1991) . Each child’s age and expressive vocabulary at the beginning of the study are listed in Table 1 .

There were four data collection sessions spaced three months apart . Data collection was carried out at the University of Washington’s Speech and Hearing Clinic . Speech samples were elicited in semi-structured play activities involving the child , her mother , and the investigator . The sessions , which were about 35 to 45 minutes in length , were audiotaped via a lapel microphone clipped to a cloth vest worn by the child . The vest contained a wireless transmitter for relaying the signals to an Aiwa S30 tape recorder and to the audio track of a Panasonic camcorder in an adjacent room .

The data were organized relative to the children’s transition to combinatorial speech . All of the children made this transition in chronological session 2 or 3 . The criterion for determining the onset of combinatorial speech was that the language sample of 100 utterances contain at least 3 utterances using lexical word combinations (e . g ., baby bottle , need sock ) . For each child , the session which marked the onset of two-word phrases was defined as relative session 0 . This paper reports data for the three relative sessions in which all nine children are represented , that is , relative session 2 1 (late one-word period) , 0 (beginning of two-word combinations) , and 1 (the end of the two-word period (Cruttenden , 1986)) . Table 2 lists the children’s mean length of utterance (Chapman , 1981) and mean age by relative session .

2 . 3 . Analysis procedures

2 . 3 . 1 . The database

The children’s utterances were transcribed orthographically . Phrase-final stressed words were also transcribed phonetically . Words or phrases that were candidates for acoustic analysis were nonimitated , free from interfering background noise or overlapping speech , and produced without a voice quality that would preclude an analysis of f 0 . Candidates for analysis also met the following criteria .

1 . All were single-syllable words or two-syllable trochaic words or phrases occurring in isolation or in phrase-final position .

2 . If selected from a multiword utterance , the stressed syllable was the nuclear accent of the utterance , or main phrase stress (Vanderslice & Ladefoged , 1972) .

3 . The phrase-final word was followed by a pause of at least 400 ms before the next speech-like vocalization of the child .

4 . The words or phrases were in utterances perceived to function as statements or question-word questions . Acoustically , the utterances had a falling intonation contour , that is , a nuclear tone whose overall magnitude of decline was at least equal to the amount of rise in f 0 , within one semitone .

Examples of the analyzed sample are the underlined portions of the following


T ABLE I . Age and expressive vocabulary of the participants within 15 days before chronological session 1 (‘‘No . of words’’ 5 expressive vocabulary reported by the parents on the MacArthur Communicative Development Inventory (CDI) ; ‘‘Percentile’’ 5 percentile ranking based on the MacArthur CDI norms for expressive vocabulary size , gender , and age)

Child Age

(months) No . of words Percentile

AD EJ

NBL LH MN AS MB AC AK

12 13 13 15 16 17 18 19 20

33 58 31 42 42 54 68 38 58

95 95 85 55 40 45 30 5

10

T ABLE II . Participants’ mean age and mean length of utterance (MLU) by relative session ( n 5 sample size—first 50 or 100 utterances)

MLU (morphemes)

Session Age (months) n 5 50 n 5 100

2 1 0 1

18 . 52 21 . 62 24 . 52

1 . 10 1 . 47 2 . 30

1 . 12 1 . 52 2 . 22

single- and multi-word utterances (nuclear accented syllables are in capitals) : ‘‘baby SOCK’’ , ‘‘need CUP’’ , ‘‘put SOCK on’’ , ‘‘bottle BAby’’ , ‘‘ BOOK’’ .

Because syllable duration may influence some aspects of intonation (Ashby , 1978) , the data selection criteria also controlled for the phonetic length of accented vocalic nuclei used by the children , that is , monosyllables and disyllables were matched in terms of the length of the accented syllable . Only two length categories are discussed in this paper : short and long . Based on studies of adult speech (Black , 1949 ; Klatt , 1975) , short nuclei consisted of any of the vowels [ I , E , v , V ) . Long nuclei contained any other vowel or a vowel and a sonorant consonant .

To control for af fect level (e . g ., see Brazil , 1975) , all utterances selected as candidates for analysis were categorized according to the perceived ‘‘emphasis’’ or af fective in y ol y ement expressed by the speaker . Three category levels were used : 1) low , 2) normal , and 3) high . In orthographic representation , words or phrases spoken with emphasis level 3 would probably be written with an exclamation mark , for example , ‘‘It’s a baby!’’ Level 1 utterances were faint , sometimes whispered , and conveyed a sense of hesitation or tentativeness . Items in emphasis level 1 were not selected , because words spoken with little af fect are dif ficult to analyze and

D . Snow 392

interpret acoustically . Thus , the sample consisted of words or phrases at level 2 (normal af fect) and level 3 (high af fect) .

Monosyllables were randomly matched with disyllabic units of the same accented syllable length category and emphasis level . For example CUP might be paired with PUppy or KItty . SHOE ! might be paired with BAby ! or BOttle ! The database for this paper consisted of 192 matched pairs (384 words) , or about 7 pairs for each child and session .

2 . 3 . 2 . Instrumental measures All words were analyzed instrumentally to determine their duration and fundamental frequency contour . The acoustic procedures used the PC-based Interactive Laboratory Systems (ILS-PC) signal analysis system . The data were digitized at a sampling rate of 10 KHz with a 12-bit resolution . The beginning and ending boundaries of each word were set at the amplitude peak of the first or last periodic cycle that was visually distinct in the time waveform .

An estimate of f 0 averaged over a 25 . 6 ms window was obtained at the beginning and ending boundaries of the word and at each peak and valley of the intervening contour . Peaks and valleys were defined as points at which f 0 reversed direction and continued in the new direction until a glide of at least one semitone was completed .

The f 0 estimates were obtained by computerized or manual editing methods . First , samples were analyzed in ILS by algorithms based on linear predictive coding and cepstral analysis . Manual editing , which involved counting the pitch periods , was then used for voiced portions of the signal that the automatic routines could not analyze . This typically occurred when intensity was weak , f 0 was elevated or rapidly changing , or the voice quality was breathy or harsh , characteristics that are frequently present at the boundaries of utterances in infant speech (Oller & Smith , 1977) . The calculations were based on dif ferences between local peaks or zero- crossings , both of which were located automatically by the computer , with a resolution accuracy of 0 . 1 ms . For an f 0 of 313 Hz , therefore , the measurement accuracy was within about 2 Hz . Examples of the instrumental analysis shown in Fig . 1 were plotted by CSpeech (Milenkovic & Read , 1992) . The examples show falling tones in a monosyllable and a disyllable spoken by NBL . In spite of the dif ference in word length , the contours expressed in the two words were similar in terms of the falling direction and magnitude of the pitch movement (see Section 2 . 4 . 2) . They dif fer primarily in temporal aspects such as the onset of the steepest part of the fall .

For subsequent analysis , the magnitude of all f 0 changes in the contour was expressed in the octave scale (one octave 5 12 semitones , one semitone 5 100 cents) . The computations disregarded segments of the contour that were less than one semitone in magnitude . This criterion was used by Allen (1983) on the basis of evidence that a semitone roughly equals a just-noticeable-dif ference in pitch glide perception (cf . the 6% criterion described by Rossi , 1978) .

All measurements and judgements used in the analysis were made by the author . Reliability measurements for 72 accented syllables (equivalent to about 19% of the data sample) were carried out by a second judge , and judgements of the emphasis levels for 50 words were made by a third judge . The judges’ measurements of accented syllable duration were within 2 ms of one another , and their mean measurements of average f 0 at the boundaries were within 9 Hz . The judges’ assessments of emphasis levels were in agreement in 84% of the cases .


400

200

0

Am

plitu

def 0

(Hz)

Am

plitu

def 0

(Hz)

Time 413.5 ms

400

200

0

Figure 1 . Acoustic analysis of the single-word utterance bottle [baba] spoken by NBL at 20 months (top panel) and the nuclear-accented word sock [sak] in the phrase that ’ s her SOCK spoken by NBL at 23 months (bottom panel) . Time between cursors 5 duration of the disyllable bottle 5 413 . 5 ms . Voiced duration of the monosyllable 5 279 . 7 ms (To complete the 413 . 5 ms display window , the waveform in the bottom panel includes a 133 . 8 ms nonperiodic portion of the syllable corresponding to the closure and part of the burst for the final consonant) .

2 . 4 . Representing intonation

2 . 4 . 1 . Autosegmental representation

The analysis of intonation used in this study stems from the empirical work of Ashby (1978) , autosegmental models presented by Liberman (1979) and Pierrehumbert (1980) , and the nuclear tone approach of Cruttenden (1986) . In autosegmental

D . Snow 394

H*

Fun

dam

enta

l fre

quen

cy (

log

scal

e)

H*

L# L#

Accentedfinal

Monosyllables

e.g., SOCKDisyllablese.g., BOttle

Accentednonfinal

Unaccentedfinal

Figure 2 . Falling intonation in accented , phrase-final monosyllables (left) and trochaic disyllables (right) . (H* 5 high pitch accent and L 4 5 low boundary tone . Bold lines indicate accent range . The contours are based on Ashby , 1978 , and the target pitch levels are adapted from Liberman , 1979 , and Pierrehumbert , 1980 . )

models , tonal patterns are represented independently from segments and syllables by a sequence of high and low target pitch levels , two of which are discussed here and shown in Fig . 2 .

Fig . 2 gives a schematic representation of the falling contours that apply to single-word utterances and to the nuclear accented , phrase-final word of multiword utterances having a declarative meaning . In monosyllables such as CUP , JUICE , and DOG (left-hand side of Fig . 2) , the two pitch levels defining the contour are a high pitch accent (H*) and a low boundary tone , which is written L% in Pierrehumbert (1980) and L 4 in this study . The target pitch levels are linked to syllables via ‘‘tune-text association rules’’ (Liberman , 1979) which assign H* to the accented syllable and L 4 to the end of the phrase-final syllable .

The accent range (bold line in Fig . 2) corresponds to the prominent falling pitch movement that connects the two target pitch levels . The fall in pitch corresponding to the accent range is often preceded by a slight rise (light line) . As this secondary pitch movement becomes more prominent (adding to the ‘‘complexity’’ of the contour) , its starting point may be designated in some analyses by an additional low target pitch level . Nuclear tones with a complex rise-fall pattern are discussed in the next section as variants of the falling H*L 4 melody having ‘‘intensification’’ or a feature of ‘‘delay’’ .

The melody sequence for monosyllables also applies to two-syllable trochaic words , for example , BOttle , BAby , and PUppy (right-hand side of Fig . 2) . These words consist of a nonfinal accented syllable and a final unaccented syllable . The association rules again assign H* to the accented syllable and L 4 to the end of the phrase-final syllable . Thus , the theory schematically represented in Fig . 2 suggests


that an underlying tonal sequence is expressed in a similar way in one- and two-syllable words .

2 . 4 . 2 . Acoustic representation

Autosegmental representations are ideally suited to the goal of describing the abstract nature of tones and the consistent associations that are found between ‘‘tunes’’ and ‘‘texts’’ (Cruttenden , 1986) . To analyze the acoustic (or surface) characteristics , however , it is necessary to measure the contours resulting from the underlying pitch level targets . A contour model using tonal features (direction , accent range , and complexity) is useful for this purpose because such features are structurally well-defined and measurable .

Tonal features

In light of the regular correspondences between the tonal features and meaning , it is desirable to maintain a two-fold distinction between falls and rises and to treat both range and complexity as gradients within these directional classes (Cruttenden , 1986) . This is very similar to the approach taken by Brazil (1975) who treats rise-fall as an ‘‘intensified’’ variant of fall , where intensification conveys a heightened degree of speaker ‘‘involvement’’ . Similarly , Gussenhoven (1983a , 1983b) analyzes rise-fall as a variant of fall having the feature [delay] , which has the general meaning of ‘‘out of the ordinary’’ or ‘‘significant’’ .

To capture the relationships among direction , accent range , and complexity , the ‘‘intonation circle’’ shown in Figs 3 – 5 organizes systematically varying pitch contours in a continuum . 1 Simple unidirectional falls are represented at the top pole and simple unidirectional rises at the opposite pole . All of the infinitely varying intervening tones are complex , that is , there are two constituents 2 (cf . Fig . 2) : a major pitch glide (bold line) and a minor pitch glide (light line) .

Complexity is represented in two steps . First , the magnitude of the minor pitch glide is expressed as a proportion of the major glide . The proportion is then converted to an equivalent angular deviation away from the appropriate vertical axis (0 to 90 degrees) . Thus , as complexity increases , the contour’s plotted position swings towards the horizontal axis .

The schema shows that contour direction (the direction of the major pitch movement) is closely related to complexity . The gradient variations that change complexity also lead to categorical alternations in direction at the horizontal axis . As a result , falling and rising tones are represented in the upper and lower semicircles , 3

respectively . Finally , accent range (the magnitude of the major pitch glide) is represented by

distance from the center of the circle . Thus , as accent range increases , the contour’s plotted position moves towards the outer edge of the circle .

To compute the location of a child’s nuclear tones in ‘‘intonational space’’ , a

1 In the ‘‘Scandinavian Accent Orbit’’ , O ̈ hman (1967) represents accent patterns in a similar circular continuum .

2 The schema in Figs 3 – 5 does not allow for more than two glides . If a contour had three or more glides (all with a magnitude of 100 cents or more) , only the two largest were used in this analysis .

3 Traditional rise-fall and fall-rise contours correspond to the left and right halves of the circle , respectively . Within either complex category , location in the upper or lower quadrant specifies whether a given tone is basically falling or rising .

D . Snow 396

Child ChildGroup Group

Monosyllables DisyllablesC

ompl

exity

(de

gree

s)

0 1200 2400

Accent range (cents)

0

90

Figure 3 . Mean location of nuclear tone contours at relative session 2 1 by child and word-length type . (Letters 5 last initial of child’s name , e . g ., ‘‘D’’ is for AD . See text for explanation of dashed and dotted lines . )

vector was specified by two components—accent range (distance from the center) and complexity (angular deviation away from the zero-degree reference line) . As an example , the disyllabic contour shown in the upper panel of Fig . 1 had a simple falling glide (359 . 8 Hz to 225 . 6 Hz , or 808 cents) . Because there were no other glides , complexity would be represented at 90 degrees on the intonation circle . Accent range would be represented by plotting the contour a distance of 808 cents from the center . The corresponding data for the monosyllabic contour in the bottom panel of Fig . 1 were 374 . 5 to 205 . 3 Hz , or 1041 cents .

3 . Results

Durations of the monosyllabic and disyllabic speech samples are listed in Table 3 . Measures of the children’s falling tones are given in Table 4 .

The mean location of tonal contours on the intonation circle is plotted by child and word-length type for sessions 2 1 to 1 in Figs 3 – 5 , respectively . For each word- length type , the relationship of the contour components is summarized by a curvi- linear regression line in polar coordinates . An additional straight line indicates the


Child ChildGroup Group


ompl

exity

(de

gree

s)

0 1200 2400

Accent range (cents)

0

90

Figure 4 . Mean location of nuclear tone contours at relative session 0 by child and word-length type . (Symbols are as in Fig . 3 . )

mean complexity value (the mean contour location corresponds to the intersection of the straight line and the regression curve) .

Inspection of the paired data points plotted for each child shows that children tended to use the same general region of ‘‘intonational space’’ regardless of dif ferences in the length of words actually spoken . This consistency of tone production is most striking at relative session 1 (three months after the onset of syntax ; Fig . 5) , but it is a clearly emerging property of the children’s intonation in the single-word period as well (relative session 2 1 ; Fig . 3) . The regression curves also indicate that contours for monosyllabic and disyllabic words were very close to one another at session 2 1 , and to an even greater extent , at session 1 . At session 0 (Fig . 4) , the contours were similar in accent range but varied in complexity .

3 . 1 . Statistical e y aluation

The repeated measures model assumes homogeneity of correlations across the repeated measures . Because one of the repeated measures in this study is time (i . e ., sessions spaced three months apart) , the assumption of stability of correlations

D . Snow 398

Child Child Group


ompl

exity

(de

gree

s)

0 1200 2400Accent range (cents)

0

90

Group

Figure 5 . Mean location of nuclear tone contours at relative session 1 by child and word-length type . (Symbols are as in Fig . 3 . )

T ABLE III . Mean duration and standard deviation (ms) for monosyllabic and disyllabic words by relative session

Session Number of

syllables Mean

duration Standard deviation

2 1

0

1

all

1 2 1 2 1 2 1 2

415 667 287 626 357 670 353 654

103 155 58

129 54 73 90

121

might be questionable . Accordingly , the statistical procedure that was used is Hotelling’s T 2 test , an alternative procedure that is applicable when the conditions required for repeated measures cannot be assumed (e . g ., Winer , 1971) .

Separate multivariate tests were used to analyze ef fects for accent range ,


T ABLE IV . Means and standard deviations (SD) of contour components by word-length type (1 5 monosyllables , 2 5 disyllables) and relative session . High 5 maximum f 0 , Low 5 minimum f 0

High (Hz) Low (Hz) Accent

range (cents) Complexity (degrees)

Minor glide (cents)

Session Type Mean SD Mean SD Mean SD Mean SD Mean SD

2 1 1 2

432 467

92 167

214 216

46 54

1222 1243

504 632

100 106

14 34

380 629

261 550

0 1 2

379 430

60 120

205 209

34 38

1128 1221

322 553

83 105

13 23

290 551

179 320

1 1 2

398 419

84 77

207 211

35 30

1123 1138

303 288

91 97

21 19

366 591

161 219

all 1 2

403 439

80 124

209 212

38 40

1158 1201

375 495

92 102

18 25

345 590

201 375

complexity , and the magnitude of the minor glide . In each case , the analysis assessed the dif ference between one- and two-syllable words in the three sessions (ef fect of word length) . This analysis used the children’s mean within-session monosyllabic / disyllabic dif ference scores (dif ference 5 disyllables 2 monosyllables) and tested these dif ference scores against zero .

The tests also evaluated changes in the disyllabic / monosyllabic dif ferences over time (ef fect of development) by using between-session dif ference scores . A between-session score was defined as the dif ference between a child’s within-session dif ference score at session X and that child’s within-session dif ference score at session X 1 1 . Between-session dif ferences were evaluated for session 2 1 to 0 (the first half of the study) and for session 0 to 1 (the second half of the study) .

The multivariate analyses indicated that there was no overall ef fect of word length on accent range ( F (3 , 6) 5 0 . 31 , p 5 0 . 82) , contour complexity ( F (3 , 6) 5 3 . 58 , p 5 0 . 09) , or the minor glide ( F (3 , 6) 5 3 . 05 , p 5 0 . 11) . The analyses also showed no ef fect of development (i . e ., between-session changes) on accent range ( F (2 , 7) 5 0 . 23 , p 5 0 . 80) , contour complexity ( F (2 , 7) 5 0 . 65 , p 5 0 . 55) , or the minor glide ( F (2 , 7) 5 0 . 19 , p 5 0 . 84) .

4 . Discussion

4 . 1 . Accent range

This investigation showed that 18- to 24-month-old children used approximately the same accent range in monosyllabic and disyllabic falling tones . Inasmuch as the disyllables were longer than the monosyllables by ratios in the neighborhood of 2 : 1 , the findings indicate that accent range was relatively unaf fected by substantial dif ferences in the length of the tone-bearing strings .

The observed dissociation between intonation and utterance length is consistent with previously reported research . For example , comparable dif ferences in utterance length did not af fect f 0 variability in the speech of children (Robb et al . , 1989) , and

D . Snow 400

dif ferences between short- and medium-length sentences did not af fect f 0 downdrift in adults (Pierrehumbert , 1979) .

Comparative data even more directly related to the present study were reported by Ashby (1978) , who studied adults’ expression of normal emphatic nuclear tones in words or phrases that were multisyllabic , for example , LIKEly (nuclear accented syllables are written in capital italics) and monosyllabic , for example , ILL . In the accented syllable of the multisyllabic falls , Ashby reported that ‘‘the peak and gradient of the fall are similar to those observed for the monosyllabic falls’’ (p . 335) . Although mean f 0 values at the right edge of the analyzed strings were not compared explicitly in Ashby’s study , the pre-boundary landmarks of the multisyllabic and monosyllabic contours were similar (see the first four ‘‘knots’’ in Fig . 2) , implying that accent range tends towards a constant in individual adult speakers . Inspection of monosyllabic and multisyllabic Swedish data presented by Lyberg (1979) suggests similar conclusions .

The relative stability of nuclear tones that Ashby observed in adults and the present study found in children is of special interest in light of recent contributions of autosegmental phonology to the study of intonation . Autosegmental theory was originally developed to account for tonal patterns in words of dif ferent length in African tone languages (Goldsmith , 1976) . A similar framework has been used to analyze intonation (e . g ., Liberman , 1979) . In an autosegmental approach , the key idea is that tonal structures are represented in the grammar independently from other linguistic levels such as those devoted to lexical or morphological information , a property of intonation that Liberman refers to as ‘‘abstractness’’ . The notion of spreading the underlying tones across the appropriate domains parsimoniously accounts for the formal regularity of contours produced in utterances of dif ferent length .

In the present study , children in the presyntactic stage of linguistic development demonstrated the kind of tonal stability that autosegmental theory is intended to capture . One interpretation of this finding is that children begin to use tones as abstract structures of the grammar before they expressively control other features of syntax such as word combinations .

4 . 1 . 1 . Phonetic ef fects

Data from adult studies , however , indicate that accent range is not entirely unaf fected by nontonal characteristics such as syllable length (e . g ., Beckman & Ayers , 1993) . In Ashby’s study , for example , accent range in the adults’ monosyllabic productions varied by one or two semitones depending on whether the syllable rime was long or short (the duration dif ference , which was not reported by Ashby , was 61 ms in the present study) . However , the much larger duration increment that separates monosyllabic from multisyllabic sequences (301 ms in the present study) did not appear to add appreciably to the accent range used by Ashby’s adult subjects nor that used by children in this study , suggesting that f 0 is controlled dif ferently in short versus long syllable rimes than in monosyllabic versus multisyllabic words . Both child and adult speakers appear to limit at least certain types of context- induced fluctuations in f 0 , especially across syllable boundaries , perhaps in order to maintain the distinctiveness of the underlying melodic form across tone-bearing strings of dif ferent length .


4 . 2 . Contour complexity

Unlike accent range , the complexity of children’s nuclear tones appeared to be af fected in some instances by the number of syllables in which the tones were conveyed . When complexity was measured by the prominence of the minor rising glide with respect to the major falling one (angular deviation in the intonation circle) , the data at relative session 0 (Fig . 4) suggested that disyllabic contours were more complex than monosyllabic contours . However , the statistical analysis indicated there were no consistent ef fects of word length over the six-month period of study . This fluctuating pattern of results suggests that the children’s use of contour complexity is variable during the period of early meaningful speech .

At relative session 0 , when the apparent context-related dif ference in complexity was greatest , the children were making the transition to combinatorial speech (mean age of 21 months) . At that time , the tones they produced in disyllables like BAby tended to have a moderate initial rise , whereas the corresponding tones they produced in monosyllables like HAT had little or no initial rise (in fact , these tones often had a short rising of fglide instead 4 ) .

Because the more prominent initial rise in the disyllables could also be described as a larger pitch obtrusion , the vocal characteristics of 21-month-old children reported in the present study may represent a precursor of the development observed by Allen & Hawkins (1980) in slightly older children . In their study , the reader may recall , three-year-olds used a larger pitch obtrusion in nonfinal accented syllables , for example , the accented syllable of PIrate dog , than in similarly accented final syllables such as DOG . To account for the dif ference , Allen & Hawkins suggested that the children may extend or exaggerate the dif ferential accent patterns that adult speakers also use to distinguish between phrase-final and -nonfinal syllables .

Adult data support their hypothesis that accentual dif ferences among syllables may be related to phrase position . For example , listening experiments described by Pierrehumbert (1981) indicated that the timing of the peak accent may dif fer according to the syllable’s length and phrase position . Perceptually , for example , the ideal synthesized accent peak in a long , nonfinal syllable , such as the initial syllable of RI y al , was further from the left edge than in a short , final syllable like BIT . Pierrehumbert’s analysis suggested that the timing dif ference was due to the fact that the nonfinal accented syllable ( RI y al , cf . BAby , PIrate dog ) may have only one target pitch-level , whereas the final syllable ( BIT , cf . HAT , DOG ) has multiple targets (two in the framework used in this paper) . To adjust for this final / nonfinal asymmetry in number of pitch levels per unit of duration , adults may accelerate the timing of the peak accent in final syllables and delay it elsewhere . To resolve the same mismatch in what might be called the tonal density of syllables , evidence from the present study and from Allen & Hawkins’ research suggests that young children

4 The rising of fglides were typically very low in amplitude and did not perceptually modify the declarative meaning of the falling tone . For these reasons , the ‘‘fall plus of fglide’’ contours seemed to be variants of the falling pattern rather than examples of the fall-rise patterns described by Pierrehumbert (1980) and Ward & Hirschberg (1985) . It is possible that these of fglides reflect compensatory phonatory mechanisms of the type discussed by Baken & Orlikof f (1987) , which might serve to prevent inappropriate cessation of voicing in the vicinity of utterance boundaries .

D . Snow 402

may modulate the complexity of the nuclear tone (Gussenhoven’s feature [delay]) in addition to adjusting the timing . 5

From the present data , however , it cannot be determined whether children chose to use the [ 2 delay] category in final syllables and [ 1 delay] in nonfinal syllables , or whether a given category , e . g ., [ 2 delay] , was realized dif ferently in final versus nonfinal syllables . The influence of position-in-utterance on the timing of target pitch-levels is a complex prosodic phenomenon that has been identified as a priority for further study in adult speech (Pierrehumbert , 1981) . The present findings relating to tonal complexity characteristics in child speech , and the uncertainties surrounding their interpretation , suggest that this is an equally important target for future research in developmental intonation as well . 6

4 . 3 . Related issues

This study focused on children’s expressive development of falling contours . Two important issues that remain to be addressed in future research are : 1) Do rising contours reflect the same kind of formal regularity as falling contours? and 2) Do children show evidence of comprehending tonal features before they control them productively? Some perspectives on each of these questions are given next .

4 . 3 . 1 . Rising contours

Structurally , rising contours could be viewed as the simple inverse of their falling counterparts . Developmentally , however , they do not appear to be as similar to falling tones as the structural parallels might imply . Generally , rising tones are later-developing acquisitions than the corresponding falling tones (Crystal , 1986 ; cf . Clumeck , 1980) . To account for the dif ferent rates of acquisition , some researchers have speculated that the voice production mechanism controlling rising pitch requires greater physiological ef fort than the mechanism controlling falling pitch (Li & Thompson , 1977) . Others have argued that rising tones are associated with a more complex linguistic system than falling tones , at least in English (Cruttenden , 1981) . For either or both of these reasons , it is likely that rising contours in early child speech will be more variable than falling contours , as some limited data from English-speaking children suggest (Snow & Stoel-Gammon , 1994) . Thus , the stability of accent range that was observed in this study as early as the single-word period might not be reflected in rising contours until a later stage of linguistic development .

4 . 3 . 2 . Comprehension of intonation

Studies of acquisition of tone languages suggest that children understand at least some linguistic uses of tone before 12 months (Tse , 1978 ; Clumeck , 1980) . However , little is known about the child’s early comprehension of intonational

5 The duration of phrase-final and -nonfinal accented syllables was nearly equal at relative session 0 (Snow , 1994) . Thus , the tone density asymmetries at that time approached the conditions exemplified in Pierrehumbert’s research by the nonfinal but intrinsically long syllable in RI y al compared to the phrase-final but intrinsically short syllable in BIT .

6 Phrase position is also associated with alternations of pitch patterns that have been observed in two- and three-year-old children acquiring tone languages (Clumeck , 1980) .


contours , partly because the pragmatic and attitudinal meanings of intonation , unlike the referential meaning of early words , are dif ficult to represent and confirm in the nonverbal context . For the same reasons , the child’s understanding of particular tonal features like accent range is even more dif ficult to infer .

Perhaps the most tangible meaning conveyed by nuclear tones (via the nuclear accent) is semantic focus . Some investigators have concluded that children under six years do not yet comprehend the linguistic significance of the nuclear accent , because they do not seem to demonstrate the preferential attention to nuclear versus non-nuclear words that an understanding of focus might predict (Cutler & Swinney , 1989) . Other investigators have arrived at dif ferent conclusions . Du Preez (1974) , for example , found that young children most often imitated the nuclear word and , to a lesser extent , postnuclear words in sentences modelled by an adult . His findings suggest that the portion of utterances marked by the nuclear tone claims a special priority—what might be called a preferential attention—in children’s sentence perception and recall .

One possible reason for these conflicting conclusions is that the prosodic skills addressed by the two studies were at dif ferent developmental levels . The type of accentuation used in the Cutler & Swinney study would probably be referred to as ‘‘narrow focus’’ , a type of focussing in which the interpretation depends on the listener’s attention to the verbal context . An additional characteristic of narrow focus is that the nucleus often falls on a non-lexical and / or nonfinal word (Cruttenden , 1986) .

In contrast , the child-directed speech described by Du Preez probably involved ‘‘broad focus’’ (sometimes called ‘‘normal stress’’) , in which the interpretation is more closely tied to the nonverbal context , and the nucleus , in a more prototypical manner , tends to fall on the last content word of the phrase . Perhaps the canonical type of focussing was understood by the one-year-olds in Du Preez’ study , whereas more complex patterns were still being acquired by the older children in Cutler & Swinney’s investigation .

The children in Du Preez’ study were the same age as the children in the present study at session 2 1 (18 months) . Comparisons between the receptive implications of the former study and the expressive findings of the latter suggest that children comprehend at least some basic features of intonation by the time they begin to control those or closely related tonal features expressively .

In summary , the present study and previously reported research support the viewpoint that the child’s knowledge of intonation develops over time . As expressed cogently by Cruttenden (1986) , some core features of intonation are controlled receptively and expressively by children at very early stages of development—some features may even be innate—while other subtle aspects of intonation are not understood in adultlike ways until age 10 or later (Cruttenden , 1974 ; 1985) .

4 . 4 . Conclusions

The accent range that children use in falling tone contours is independent of the timing dif ference between monosyllabic and disyllabic strings in which the tones are expressed . This suggests that children’s intonation reflects some of the formal regularities that are observed in adult speech and accounted for by autosegmental theory . Although adultlike properties of intonation were most apparent at the end

D . Snow 404

of the two-word stage (age 24 months) , a similar stability of accent range was present to some extent 6 months earlier when the children used only single-word utterances . These findings suggest that children begin to express features of the grammar that are represented tonally before they express features represented by word combinations or morphology . At the same time , variability in the children’s use of contour complexity suggests that other aspects of intonation such as tone-timing coordination skills continue to develop during the period of early meaningful speech and probably throughout childhood .

This research is based on the author’s doctoral dissertation study at the University of Washington . The work was supported in part by a grant from the National Institute on Deafness and other Communication Disorders (Grant No . DC00520) awarded to Carol Stoel-Gammon , and by an Institutional National Research Service Award , NIH Grant No . 1 T32 DC00033-01 , ‘‘Research Training in Speech and Hearing Sciences’’ (University of Washington , Fred D . Minifie , Project Director) , supported by the National Institute on Deafness and other Communication Disorders . Portions of this study were reported at the Biennial Meeting of the Society for Research in Child Development , Indianapolis , 30th March – 2nd April 1995 . I particularly wish to thank Carol Stoel-Gammon for generous assistance throughout this study . I am also grateful to Linda Swisher and the editor and reviewers of this journal for valuable suggestions . Thanks to Suzanne Quigley and Patrick Feeney for their help with the audiometric screenings . Special thanks also to Margaret Kehoe and Kresent Gurtler for their contributions to the reliability portion of the study .

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