Common characteristics of vowels

This section reviews universal and language-specific characteristics in vowel production from cross-linguistic studies and those on the acquisition of Mandarin vowels. In particular, major findings are discussed on vocalic development in the first year of life and from the first year to the appearance of early meaningful speech. Due to similar courses of development in articulatory proficiency for children learning English and other languages, their data often show common characteristics in vowel development. Indeed, early vowels have been shown to be mainly central and front vowels. For example, in an early study of vowel development based on transcriptions of the vocalizations from 95 English-learning infants (Irwin, Reference Irwin1948), mid-front [ɛ, I] and mid-central (ə) vowels accounted for over 70% of vowel production during the first year. Among these vowels, [ɛ] occurred most often. The major trend in vowel development, according to Irwin, is an increasing production of back vowels from age 0 ; 11 to 2 ; 6.

Similarly, analysis of vocalizations by five infants aged 1 ; 1 showed that central and front vowels [ʌ (ə), ɛ, æ] are produced most often in V (monophthongs), VV (diphthongs) and CV (vowels with an initial consonant) syllables (Kent & Bauer, Reference Kent and Bauer1985). In addition to these three vowels, [ɑ] is often produced in CV syllables. These authors concluded that central and front vowels are produced more often than back vowels, and low vowels are produced more often than high vowels.

This developmental trend has been supported by most studies analyzing formant patterns. For example, Buhr's (Reference Buhr1980) longitudinal study found that the vowel spaces (defined by F1 and F2 values) of non-high front and central vowels [I, ɛ, æ, ʌ] become stabilized by the end of the first year, earlier than the stabilization of any back or high vowels. Furthermore, Kent & Murray's (Reference Kent and Murray1982) analysis of 21 infants aged 0 ; 3, 0 ; 6 and 0 ; 9 indicated that the vowel formant frequencies of younger children are more upward and to the right in the standard F1–F2 graph than those of older children and adults. That is, vowels produced from ages 0 ; 3 to 0 ; 9 are mainly mid-front or central vowels. This trend is supported by Matyear's (Reference Matyear1998) study of vowel formants in three English-learning infants (ages 0 ; 7 to 1 ; 0) who produced mostly non-high, non-back vowels.

Similar results are found in Mandarin-learning infants. Mid-front and mid-central vowels [ɛ, ə], often used in babbling before age 0 ; 8 (Chen, Reference Chen2005), have also been reported in first words by age 1 ; 8 (e.g. Jeng, Reference Jeng1979; Yue-Hashimoto, Reference Yue-Hashimoto1980). The emergence of the contrast of three corner vowels [i, ɑ, u] before age 2 ; 0 (e.g. Jeng, Reference Jeng1979; Hsu, Reference Hsu2003) must be the major development during the second year, since few utterances of both [i] and [u] have been observed before age 1 ; 0 (Chen, Reference Chen2005).

Language-specific characteristics of vowels

In addition to these common characteristics, language-specific vowel development is found in cross-linguistic studies and in studies of Mandarin-learning children.

Language-specific vowel production occurs by age 0 ; 10 (de Boysson-Bardies, Hallé, Sagart & Durand, Reference de Boysson-Bardies, Hallé, Sagart and Durand1989; de Boysson-Bardies, Reference de Boysson-Bardies, de Boysson-Bardies, de Schonen, Jusczyk, MacNeilage and Morton1993). The F2/F1 ratio of vowel production of 20 infants (aged 0 ; 10) from English, French, Algerian and Cantonese environments reflected the F2/F1 ratio in their corresponding adults' models (de Boysson-Bardies et al., Reference de Boysson-Bardies, Hallé, Sagart and Durand1989). For example, among these four language groups, the vowel production of both English-learning infants and their corresponding adult language group exhibited the highest F2/F1 ratio, reflecting the high frequency of mid- to high-front vowels [I, e] in the adult model. In contrast, the vowel production of infants in the Cantonese environment showed the lowest F2/F1 ratio, correlating with the lowest F2/F1 ratio of their adult model and reflecting the high frequency of low-back vowels [ɑ:, ɔ:, ɐ]. Similarly, in Stokes & Wong's study (Reference Stokes and Wong2002) of 40 Cantonese-speaking children from age 0 ; 10 to 2 ; 3, the three vowels [a], [ɛ] and [ɔ] were the first to emerge, achieving better than 50% accuracy by age 1 ; 3 to 1 ; 6. These were followed by [i] at age 1 ; 8 to 1 ; 11.

In addition to spectral characteristics and the distribution of vowels, language-specific influences in infants' vowel production have been examined in terms of height and place relations of V1 and V2 in CVCV sequences (de Boysson-Bardies, Reference de Boysson-Bardies, de Boysson-Bardies, de Schonen, Jusczyk, MacNeilage and Morton1993). In this second cross-linguistic study, the V1 and V2 elements in disyllabic utterances of infants and adults from four language environments (French, English, Swedish and Yoruban) tended to agree in height and frontedness. Utterances with vowels that vary with height from French-learning and English-learning infants showed a tendency toward higher V2, which is related to their adult models. Disyllabic productions of infants from Swedish and Yoruban environments displayed a tendency for a higher V1 to occur at high frequency, reflecting their corresponding adult models. However, no such systematic correlation was found in the variation of frontedness of V1 and V2. In conclusion, language-specific vowel production begins in babbling by age 0 ; 10, shortly before first words are produced.

Several language-specific characteristics in Mandarin-speaking children have also been reported but before these studies are reviewed, the Mandarin vowel system is introduced. Although analyses of the Mandarin vowel system remain unsettled on the number of surface vowels and distribution of allophones in the phonemic system, most studies propose 12 or 13 surface vowels and 4 to 6 vowel phonemes (Cheng, Reference Cheng1966; Cheng, Reference Cheng1973; Lin, Reference Lin1989). These analyses differ primarily with respect to the low vowels /a/ and /ɑ/ and the high vowel /ɨ/. All analyses regard [a] and [ɑ] as variants of the same vowel phoneme, the only low vowel in Mandarin. The low-back vowel /ɑ/ is considered a basic form by Cheng (Reference Cheng1973), whereas others propose /a/ as a phoneme, with [a, ɑ, ɛ] or [æ] as allophonic variations. The high vowel /ɨ/ has been regarded as phonemic and having two variants (Cheng, Reference Cheng1966; Cheng, Reference Cheng1973), or as an allophone of /i/. All 12 surface vowels of Cheng (Reference Cheng1966) and Cheng (Reference Cheng1973) (i, e, ɛ, y, ɨ [with 2 variants], ə, a, u, ɤ, o, ɑ) and an extra [ɔ] are included in Lin's (Reference Lin1989) analysis of Mandarin surface vowels (Table 1). The same set of vowels occurs in Mandarin spoken in Taiwan, except that the two allophonic variations of the high vowel /ɨ/ are not distinguished.

TABLE 1.

Single vowels in Mandarin

Lin's (Reference Lin1989) analysis of Mandarin surface vowels.

([ɛ] and [ɔ] are only used in forming diphthongs).

To analyze the frequency of Mandarin syllables, this study used a corpus of 1 177 984 Mandarin characters from written materials (e.g. journals, newspapers, textbooks; Liu, Chuang & Wang, Reference Liu, Chuang and Wang1975; Cheng, Reference Cheng1982), since no spoken corpus is available. Our analysis of this database, including over 900 000 syllables with both consonant and vowel components, shows that front vowels occur most frequently (44·65%), followed by central (31·48%) (including [a], if the low-central vowel [a] is considered the basic form) and back vowels (23·86%). As to vowel height, high vowels occur most frequently (42·09%), followed by mid vowels (35·21%) and low vowels (22·69%). The low-central vowel [a] (22·69%) and the high-front vowels [i, y] (25·80%) occur most frequently among all vowels.

Chen's (Reference Chen2005) finding that [ɛ] is produced early and that high and back vowels [i, I, u, Ʊ, o, ɔ] are rarely produced (less than 7%) has also been documented in English-learning infants (Irwin, Reference Irwin1948; Buhr, Reference Buhr1980; Kent & Bauer, Reference Kent and Bauer1985). The early emergence and increasing production of the low vowel [a] or [ɑ] by a Mandarin-learning infant (0 ; 3–0 ; 8) reflects the phonetic system of the ambient language (Chen, Reference Chen2005). This increasing production of [ɑ] is like that of Cantonese-learning infants (age 0 ; 10) (de Boysson-Bardies et al., Reference de Boysson-Bardies, Hallé, Sagart and Durand1989). Moreover, [a] or [ɑ] is already present in Mandarin infants' vowel inventories by the second year (e.g. Li, Reference Li, Cheng, Li and Tang1978; Yue-Hashimoto, Reference Yue-Hashimoto1980; Wang, Huang, Chen & Phillips, Reference Wang, Huang, Chen and Phillips1986).

Cross-linguistic differences in vowel production in babbling are found as early as age 0 ; 10, several months before the first words are produced. For example, in addition to the general trend for mid-front and mid-central vowels to occur with high frequency in Mandarin-learning infants, production of [a] or [ɑ] increases before age 1 ; 0, corresponding to the prominence of low vowels in Mandarin syllables mentioned above.

Common characteristics of consonants

Common developmental patterns of consonants are found in studies of children in English and other language environments. In terms of place of articulation, Irwin's (Reference Irwin1947a) longitudinal study of English-learning children reported that the frequency of velar and glottal consonants dramatically decreases by the end of the first year, when dental and labial stops [d, b, m] appear to be the second-most frequent consonants after [h]. Kent & Bauer (Reference Kent and Bauer1985) also found that bilabial and apical consonants occurred most often in the consonants produced by infants aged 1 ; 1 in various utterance patterns (e.g. CV, VCV and CVCV).

Regarding manner of articulation, Irwin (Reference Irwin1947b) indicated that after age 0 ; 6 the frequencies of [h] and [ʔ] decrease gradually, when various stops and nasals emerge around the time that the velum and epiglottis separate (Kent & Vorperian, Reference Kent and Vorperian1995). Similarly, stops, nasals and fricatives dominate in consonants produced by infants aged 1 ; 1 (Kent & Bauer, Reference Kent and Bauer1985). Among these, stops represent almost three-fourths of the total consonant production in the major syllabic pattern (CV). Although prevocalic consonants (in CV) tend to be frontal stops, postvocalic consonants (or consonantal endings) and intervocalic consonants tend to be continuants (fricatives, nasals in VC, and fricatives, glides and liquids in VCV) (Kent & Bauer, Reference Kent and Bauer1985; Stoel-Gammon, Reference Stoel-Gammon2002). Furthermore, the majority of prevocalic stops in CV syllables are voiced (voiced/voiceless ratio=3:1; Kent & Bauer, Reference Kent and Bauer1985). The distribution of consonants in terms of place, manner and voicing suggests that the voiced bilabial and apical stops [b, d] are the most frequent prevocalic consonants in English-learning infants' vocalizations (Kent & Bauer, Reference Kent and Bauer1985).

The developmental trends just reviewed for English-learning children can also be seen in children from other language environments. These apparent universal trends are: (1) labials and dentals occur more often than velars; and (2) stops and nasals occur more often than fricatives, liquids or glides.

In Mandarin-learning children, the developmental order of front and back consonants remains unclear. Similar to data on the production of early words in English-learning infants, labial and dental stops are generally acquired before velar stops (Zhu & Dodd, Reference Zhu and Dodd2000; Hsu, Reference Hsu2003), but back consonants (e.g. [k, kʰ, x] before [f, l, tsʰ]) have also been reported to emerge around age 1 ; 6 (Jeng, Reference Jeng1979). This discrepancy in findings was not attributed to consonant distribution patterns in adult language, in which velars occur as often as labials and less often than alveolars (Cheng, Reference Cheng1982). Further research is needed on both babbling and early speech to clarify these contradictory findings. For various manners of articulation, a universal developmental order was also found in Mandarin data. That is, nasals are acquired (age 0 ; 11 to 1 ; 8) before fricatives (1 ; 8 to 2 ; 0) and affricates (after 2 ; 2) (Yue-Hashimoto, Reference Yue-Hashimoto1980; Zhu & Dodd, Reference Zhu and Dodd2000).

Language-specific characteristics of consonants

In addition to universal trends in consonant development, consonant production by children in different linguistic environments varies significantly by individual place and manner category. Language-specific characteristics of consonant production can be found as early as age 0 ; 9 to 0 ; 10. That is, language-specific consonant production is found at an age similar to that for language-specific vowel production (de Boysson-Bardies & Vihman, Reference de Boysson-Bardies and Vihman1991; de Boysson-Bardies, Vihman, Roug-Hellichius, Durand, Landberg & Arao, Reference de Boysson-Bardies, Vihman, Roug-Hellichius, Durand, Landberg, Arao, Ferguson, Menn and Stoel-Gammon1992).

The consonant production of infants from four different linguistic environments (French, English, Swedish and Japanese) consistently showed language-specific distribution patterns throughout the 0- to 25-word stages (roughly age 0 ; 9 to 1 ; 5) in both babbling and early words (de Boysson-Bardies et al., Reference de Boysson-Bardies, Vihman, Roug-Hellichius, Durand, Landberg, Arao, Ferguson, Menn and Stoel-Gammon1992). French-learning infants produced significantly more labials than Swedish- and Japanese-learning infants, and French-learning infants produced velars the least frequently of all four groups. Furthermore, Swedish-learning infants produced stops significantly more often and nasals significantly less often than French-learning infants.

Language-specific characteristics are also found in Mandarin-learning children. Before reviewing consonant development in Mandarin-learning infants, the Mandarin consonant system is reviewed to provide essential background information. Mandarin has 21 consonants (initials) plus [ŋ], which occurs only in coda position (Luo, Reference Luo1992; Table 2). Aspiration, a distinctive feature in Mandarin, is used to differentiate six minimal pairs: [p/pʰ, t/tʰ, k/kʰ, tʂ/tʂʰ, ts/tsʰ, tɕ/tɕʰ]. The three series of affricates and fricatives, [tʂ, tʂʰ, ʂ], [ts, tsʰ, s] and [tɕ, tɕʰ, ɕ], are analyzed in different ways. Some researchers regard them as allophonic variations with complementary distributions, but others treat them as an independent series. The series [tʂ, tʂʰ, ʂ, ʐ] has lost its retroflex quality and is realized as [ts, tsʰ, s, z] in daily Mandarin conversations spoken in Taiwan. Another sound change in Mandarin spoken in Taiwan is that postvocalic [ŋ] is frequently replaced by [n] (Tse, Reference Tse1992).

TABLE 2.

Consonants in Mandarin

Reanalysis of consonants in a database with over 900 000 syllables (Liu et al., Reference Liu, Chuang and Wang1975; Cheng, Reference Cheng1982) shows that alveolars (including alveolars, retroflexes, alveo-palatals) occur significantly more often (73·3%) than either labials (13·62%) or velars (13·08%). In the current study, comparison of infants' vocalization and child-directed speech was facilitated by grouping consonants into three major categories: labials, alveolars and velars. Alveolars are used as a general category that includes dentals, post-alveolar retroflexes and palatals. Since infants lack mature dentition, the three pairs of unaspirated/aspirated affricates and fricatives in the adult system are grouped as one category (alveolar) because they share one property – raising the tongue above its presumed rest or neutral position. Moreover, alveolar is used for ease of comparison with previous studies. As in English and most other languages, stops occur most frequently in Mandarin (34·32%) among the five manners of articulation. One major characteristic of Mandarin is the high frequency of affricates (26·89%), which is slightly higher than that of fricatives (24·97%). Nasals (7·6%) and laterals (6·22%) are the least frequently used consonants in Mandarin.

In contrast to the universal acquisition order of front to back consonants, velar fricatives [x] are acquired earlier than dental [s, ʂ] and palatal fricatives [ɕ] (Jeng, Reference Jeng1979; Zhu & Dodd, Reference Zhu and Dodd2000), although velar fricatives seem to occur much less often than dental and palatal fricatives in adult language (Cheng, Reference Cheng1982). Among all three nasals in Mandarin, the labial nasal [m] is acquired first. Of the other two nasals, alveolar nasal [n] seems to be less stable than velar nasal [ŋ] in the developmental process. It was reported that [ŋ] is stabilized (1 ; 6–2 ; 0) before [n] (2 ; 1–2 ; 6), and syllable-final [n] is replaced by [ŋ] (Zhu & Dodd, Reference Zhu and Dodd2000). The late stabilization of [n] in the consonant production of Mandarin-learning children is different from that in English-learning infants and merits further study. Late acquisition of [n] is also found in Cantonese-learning children (Wong & Stokes, Reference Wong and Stokes2001).

The acquisition of affricates in Mandarin argues against Jakobson's implicational rule. Mandarin-learning children acquire affricates [ts, tɕ] earlier than [s, ɕ] (Hsu, Reference Hsu2003). Before affricates are completely acquired, they can be replaced either by dental stops [t] or by other affricates (e.g. [ts] substitutes for [tsʰ, tɕʰ]), but they have never been found to be replaced by fricatives (Zhu & Dodd, Reference Zhu and Dodd2000; Zhu, Reference Zhu2002). On the other hand, fricatives are sometimes replaced by affricates. For example, the affricate [tɕ] has been substituted for fricatives [s, ʂ, ɕ] (Zhu & Dodd, Reference Zhu and Dodd2000; Zhu, Reference Zhu2002; Hsu, Reference Hsu2003). This error pattern is also found in Cantonese-speaking children (So & Dodd, Reference So and Dodd1995). The production of more affricates than fricatives by Mandarin-learning children may relate to the high frequency of affricates in Mandarin.

Developmental continuity of vowels

The relationship between the vowel production systems in babbling and early speech was systematically examined in a longitudinal study of a single child from age 1 ; 2 to 1 ; 8 (Davis & MacNeilage, Reference Davis and MacNeilage1990). However, this comparison of vocalic production in babbling and early speech may have been biased by the sampling of vocalization types in Davis and MacNeilage's data, which reflected four times as much word production as babbling between 1 ; 2 and 1 ; 8, perhaps failing to capture the transition in vowel production. Studies on younger age groups, covering the transition from mostly babbling to increasing production of lexical items, may provide a more complete picture of the continuity of vowel development. The current study addressed this issue by examining vocalic production between 0 ; 7 and 1 ; 6, from babbling to early speech.

Developmental continuity of consonants

The phonetic systems of babbling and early speech in English-learning infants resemble each other in various aspects of consonant development. First, the frequently occurring consonants – stops and nasals [p, b, d, k, g, m, n, ŋ] – in early words (produced from 0 ; 10 to 1 ; 2; Waterson, Reference Waterson, Waterson and Snow1978) are a direct outgrowth of the most frequent sounds [b, t, d, g, m, ʔ] in babbling (Irwin, Reference Irwin1947a, Reference Irwin1947b), except for [h], which is produced frequently only in babbling (Mowrer, Reference Mowrer and Lass1980). Second, besides this general similarity, the consonant production patterns for each child are consistent in both babbling and early words (Vihman, Macken, Miller, Simmons & Miller, Reference Vihman, Macken, Miller, Simmons and Miller1985).

Third, initial stops and final fricatives occur with high frequency in both early words and babbling. This unequal distribution of initial stops and fricative endings in early words is shown by the substitution of stops for initial fricatives (e.g. [top] for [sop]) and of fricatives or affricates for final stops (as in [bæs] for [bæk]; Mowrer, Reference Mowrer and Lass1980). This unequal distribution also reflects sound patterns in babbling, in which stops occur more often than fricatives in initial position, and conversely fricatives (or continuants) occur more often than stops in final position (Kent & Bauer, Reference Kent and Bauer1985). Fourth, developmental continuity has also been found in aspiration, voicing and gliding. For example, the deaspiration of initial stops in early words (e.g. [bɛt] for [pɛt]) is an outgrowth of the high frequency of unaspirated initial stops in babbling (Mowrer, Reference Mowrer and Lass1980).

Fifth, in contrast to the prominence of voiced stops in initial position, consonants in final position tend to be voiceless stops, fricatives or nasals in both early words and babbling, as seen in the devoicing of final consonants in early words (e.g. [bʌk] for [bʌg]; [fʌs] for [fʌz]) and the high frequency of voiceless final consonants in babbling (Mowrer, Reference Mowrer and Lass1980; Kent & Bauer, Reference Kent and Bauer1985). Sixth, substitution of glides [w, j] for prevocalic liquids [l, r] in early words correlates with a higher frequency of prevocalic glides than liquids in babbling (Mowrer, Reference Mowrer and Lass1980). Seventh, children tend to reduce consonant clusters into single consonants (e.g. [ti] for [tri]) and to delete final consonants in keeping CV syllable structures in early words (Mowrer, Reference Mowrer and Lass1980). This developmental trend likely results from the higher frequency of CV syllables than of VC syllables and the scarcity of consonant clusters in babbling (Kent & Bauer, Reference Kent and Bauer1985). The parallel patterns of consonant production in babbling and early words reviewed above strongly support developmental continuity. However, phonetic development comprises both continuous and discontinuous processes.

As to the relative frequency of labials and alveolars, ‘labial regression’ is considered counter-evidence of continuity in consonant production between babbling and early words (MacNeilage, Davis & Matyear, Reference MacNeilage, Davis and Matyear1997). Three of the four children in MacNeilage et al.'s longitudinal study produced more alveolars in babbling, whereas three of them produced more labials in early words. That is, the ratio of labials to alveolars in early words (range=2·04–2·86) was greater than that in babbling. This high alveolar frequency in babbling was interpreted as reflecting bias from the ambient language. Also, the regression to labials, which are easier to produce in early words, was thought to facilitate the variegation of vowel production in early words. However, Vihman et al. (Reference Vihman, Kay, de Boysson-Bardies, Durand and Sundberg1994) found that labials are produced more often than alveolars in child-directed English speech. Thus, ‘labial regression’ may reflect the phonetic patterns of infants' ambient language and the argument of ‘labial regression’ should be examined with data from other language groups and more subjects.

In sum, since most studies on vowel development have focused either on babbling in the first year or word production in the second year and later, developmental continuity can be best approached by empirical studies that cover a broader age range, from late babbling to the second year. Moreover, verification of early evidence for language-specific vowel production (de Boysson-Bardies et al., Reference de Boysson-Bardies, Hallé, Sagart and Durand1989; de Boysson-Bardies, Reference de Boysson-Bardies, de Boysson-Bardies, de Schonen, Jusczyk, MacNeilage and Morton1993) requires additional studies of infants learning other languages and younger than 0 ; 10. These two issues are addressed by the current study.

Various aspects of consonant development in Mandarin-learning children differ from the general pattern of consonant development in other languages. These discrepancies can be attributed partly to consonant distribution patterns in the ambient language and dialectal influence, but more studies are needed to clarify these discrepancies and to trace the earliest time of language-specific consonant production. In particular, using larger samples would control for bias from individual differences and including younger age groups would allow comparison of the frequencies for various consonant categories in babbling and in early words.

Further studies on vowel and consonant development from babbling to early word production from less-studied languages might confirm the general patterns found so far and resolve contradictory findings on developmental order in babbling and early speech. Therefore, this cross-sectional study with Mandarin-learning infants investigated language-specific patterns and developmental continuity between babbling and the production of first words by asking two questions:

1. (1) Do vocalic and consonantal developmental patterns in Mandarin-learning infants resemble those of other language groups (reflecting underlying universal constraints) or do these patterns reflect the pattern of their major linguistic input?

2. (2) Do the patterns of vowel and consonant distribution observed at the early stages persist through later stages of infant vocalizations?