Edited by: Leher Singh, National University of Singapore, Singapore
Reviewed by: Jae Yung Song, University of Wisconsin System, United States; Nan Xu Rattanasone, Macquarie University, Australia
This article was submitted to Language Sciences, a section of the journal Frontiers in Psychology
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Tonal information is essential to early word learning in tone languages. Although numerous studies have investigated the intonational and segmental properties of infant-directed speech (IDS), only a few studies have explored the properties of lexical tones in IDS. These studies mostly focused on the first year of life; thus little is known about how lexical tones in IDS change as children’s vocabulary acquisition accelerates in the second year (
In tone languages, pitch is employed to differentiate lexical meanings. Consequently, in order to recognize or learn a word, a tone-language-learning infant must develop sensitivity to lexical pitch contours in addition to consonants and vowels; conversely, infants who learn non-tone languages need to pay attention to consonants and vowels but ignore pitch contours at the lexical level. Though a number of studies have looked at infants’ discrimination, recognition, and acquisition of tones (see
Infant-directed speech is a speech register caregivers (typically mothers) use when addressing their infants, and as such it is an important type of input in early language acquisition (
IDS is often claimed to facilitate language acquisition, although conflicting views have been proposed (
The robust evidence that infants prefer listening to IDS, however, does not necessarily indicate that such speech carries a particular linguistic function in terms of language learning. Another line of research has been devoted to identifying the well-specified linguistic information encoded by IDS. A number of studies have explored two questions on this topic: First, are the segmental (mainly vocalic) and suprasegmental (tonal) properties of IDS hyperarticulated compared with those of ADS? It may seem, on first blush, that the exaggerated intonation IDS entails vowel hyperarticulation. However, it is also possible that exaggerated intonation provides more variable vowels, and thus poses a learning problem for vowel categorization. Similarly, exaggerated intonation need not naturally result in tone hyperarticulation; on the contrary, it may distort tonal cues at the syllabic level. Second, if the segmental and suprasegmental properties of IDS are indeed hyperarticulated, is this hyperarticulation expressed in a way that may support language acquisition? Previous investigations into this possibility have produced mixed results on the segmental level (vowels and consonants) and few results of any kind on the suprasegmental level (lexical tones).
An example of vowel hyperarticulation was identified by
The mixed results on vowel hyperarticulation in IDS are only magnified in studies investigating whether IDS supports language acquisition. On the one hand,
A similar debate may be extended to tone hyperarticulation. Hypothetically, the exaggerated intonation of IDS might affect tonal properties in two possible ways. Specifically, lexical tones in IDS may either be hyperarticulated or alternatively distorted (hypoarticulated) due to the exaggerated prosody. Two types of acoustic evidence may indicate tone hyperarticulation in IDS. First, tones’ acoustic cues may be more prominent in IDS as compared with ADS. For example, as fundamental frequency (F0) is the primary cue to tone in Mandarin Chinese (
Results from several studies support the distortion prediction.
While these studies suggest that tones may be distorted in IDS compared with ADS, there is also evidence that mothers hyperarticulate tones in IDS. Following the methods in
Two studies further tested tone hyperarticulation in Cantonese IDS with different measurements.
Taken together, these findings indicate that Cantonese tones are hyperarticulated in early IDS compared with ADS, but that the degree of hyperarticulation diminishes by the end of the first year. They also suggest that tone hyperarticulation may be restricted to certain tones or positions (as in Northern Mandarin, where tone hyperarticulation is only present in utterance-final positions). In other words, it has not been conclusively established that lexical tones in IDS are hyperarticulated across the board. To date, studies of IDS have been conducted on different languages, with different data collection methods and different measurements, and have yielded conflicting results. These methodological issues must be taken into consideration before we draw any conclusions about the hyperarticulation of tone in IDS.
Tone languages studied in the existing literature on IDS include Cantonese, Mandarin Chinese, Northern Mandarin, and Taiwanese Mandarin, all of which have different tonal systems and prosodic patterns (e.g.,
Second, speech elicitation methods used in previous studies range from reading tasks to spontaneous speech, and from home settings to laboratory settings.
Finally, previous studies have employed a wide range of analyses to compare the ADS and IDS conditions.
Besides the methodological issues discussed above, the different ages of the children in the various studies may also have contributed to the contradictory results. Studies on vowel and tone hyperarticulation to date have mostly focused on IDS directed at children in the first year of life, and these results have often been interpreted from the perspective of “perceptual reorganization” (
Since tonal information is crucial to word meaning in tone languages, it is important to examine whether tone hyperarticulation persists when children are becoming proficient word-learners in the second year. The general prosodic modifications in IDS are known to change based on the child’s stage of language development (
Two studies have investigated age-related changes in lexical tones in IDS, but both focused on the first year of life, prior to the lexical spurt.
If age-related changes in IDS are explicitly tied to perceptual reorganization, we should expect any differences between ADS and IDS to diminish and disappear altogether as children reach 12 months of age. However, in a longitudinal study,
The current study set out to investigate tone hyperarticulation in Mandarin Chinese IDS at two points in time, both of which occur during the second year of life (the period of the lexical spurt). Our main research questions are: (1) Are tones in Mandarin IDS addressed to 18- and 24-month-old children generally hyperarticulated compared to tones in ADS? If so, we should expect to observe a larger F0 range for Tone 2, Tone 3, and Tone 4, a higher F0 for Tone 1, and possibly longer duration for tones in IDS vs. ADS, as shown by
Thirty-nine Mandarin-Chinese-speaking mother–child dyads participated in this study. The participant sample comprised two age groups: 18-month-olds (
A picture book titled
Overview of stimuli.
Tone of the second syllables | Tone 1 | Tone 2 | Tone 3 | Tone 4 |
---|---|---|---|---|
Pinyin | nán guā | hé lí | chéng bǎo | mí lù |
IPA | [mi2 lu4] | |||
Translation | ‘Pumpkin’ | ‘Beaver’ | ‘Castle’ | ‘Moose’ |
Participants were tested in a quiet room. Before the experiment, mothers were given a few minutes to get familiar with the book. In the IDS condition, the child sat on his or her mother’s lap, and the mother was instructed to read the story to her child the way she usually did at home. The mothers were specifically told they could use any sentences; the only requirement was to include the words on each page. In the ADS condition, the mothers were instructed to tell the story to the experimenter (female, a native speaker of Mandarin Chinese), taking into account that she was a college student. This was done to control the speech context and content in both conditions. The order of the two conditions was counterbalanced across participants. A ZOOM H1 recorder (with 16-bit resolution and a sampling rate of 44.1 kHz) was used to make audio recordings, and all sessions were videotaped. Each experimental session took about 15–20 min. All families received a book as a gift after the session.
The beginnings and endings of the target syllables (the second syllable of each target word) were annotated and extracted from the recordings in PRAAT (
We chose to acquire the maximum and minimum F0 for each syllable by marking them manually, rather than limiting tone measures to any specific segment(s) within the syllables. This was done for two reasons. First, the domain of tones (or Tone Bearing Units (TBUs)) is phonologically determined, and what constitutes a TBU in Mandarin Chinese is debatable (see
Taking these issues into account, to get a more accurate picture of tonal information, the first author manually marked the maximum F0 and minimum F0 following the methods from
For all the F0 measures, we followed
To understand whether tones differed between (i) ADS and IDS and (ii) IDS directed at 18-month-olds and IDS directed at 24-month-olds, we used linear mixed-effects models for all analyses. In the models, we included fixed factors of Age (18-month-old/24-month-old), Condition (ADS/IDS) and Tone (Tones 1, 2, 3, and 4) on these dependent measures: Minimum F0 (in Hz and ERB), Maximum F0 (in Hz and ERB), F0 range (in Hz and ERB, for Tone 2, Tone 3 and Tone 4, excluding Tone 1), and Syllable duration (in seconds), with Participant Number as a random factor, and allowing for random slopes for Condition and Tone (
We used the lme4 package (
For Maximum F0 (Hz) (
Box plots of Maximum F0 (Hz) for ADS and IDS addressing 18- and 24-month-old children
Final model for Maximum F0 (Hz).
Parameters | Estimate | |||
---|---|---|---|---|
(Intercept) | 16.8043 | 0.401 | 41.942 | <0.001*** |
Condition (IDS) | 1.403 | 0.400 | 3.518 | 0.001** |
Tone2 | -0.483 | 0.259 | -1.866 | 0.063 |
Tone3 | -0.893 | 0.253 | -3.532 | <0.001*** |
Tone4 | 1.130 | 0.246 | 4.600 | <0.001*** |
Age (24 months) | 0.647 | 0.538 | 1.202 | 0.237 |
Condition (IDS): Age (24 months) | -1.480 | 0.582 | -2.545 | 0.015* |
Results for Minimum F0 (Hz) (
Box plots of Minimum F0 (Hz) for ADS and IDS addressing 18- and 24-month-old children.
Final model for Minimum F0 (Hz).
Parameters | Estimate | |||
---|---|---|---|---|
(Intercept) | 16.214 | 0.334 | 48.600 | <0.001*** |
Condition (IDS) | 0.552 | 0.248 | 2.225 | 0.030* |
Tone2 | -1.981 | 0.300 | -6.608 | <0.001*** |
Tone3 | -2.970 | 0.274 | -10.825 | <0.001*** |
Tone4 | -1.161 | 0.252 | -4.610 | <0.001*** |
Age (24 months) | 0.390 | 0.408 | 0.954 | 0.346 |
Condition (IDS): Age (24 months) | -0.870 | 0.352 | -2.548 | 0.014* |
For the measure of F0 range (
Box plots of F0 range (Hz) for ADS and IDS addressing 18- and 24-month-old children.
Final model for F0 range (Hz).
Parameters | Estimate | |||
---|---|---|---|---|
(Intercept) | 8.201 | 0.576 | 14.227 | <0.001*** |
Condition (IDS) | 0.969 | 0.352 | 2.751 | 0.006* |
Tone3 | 0.050 | 0.632 | 0.079 | 0.937 |
Tone4 | 0.480 | 0.600 | 0.800 | 0.424 |
Age (24m) | -1.288 | 0.764 | -1.686 | 0.094 |
Tone3: Age (24 months) | 1.848 | 0.884 | 2.091 | 0.037* |
Tone4: Age (24 months) | 2.467 | 0.855 | 2.884 | 0.004** |
The last measure was duration (
Box plots of syllable duration (s) for ADS and IDS addressing 18- and 24-month-old children.
Final model for syllable duration (s).
Parameters | Estimate | |||
---|---|---|---|---|
(Intercept) | 0.493 | 0.011 | 44.314 | <0.001*** |
Tone2 | 0.070 | 0.014 | 5.230 | <0.001*** |
Tone3 | -0.003 | 0.015 | -0.197 | 0.844 |
Tone4 | 0.028 | 0.013 | 2.137 | 0.034* |
Our main goal was to provide a global measure of tone hyperarticulation, however, tone hyperarticulation may also suggest that tonal contrasts are enhanced in IDS. In addition to comparing the tonal cues between ADS and IDS, we explored whether the contrast between Tone 1 and Tone 4 was enhanced in IDS
First, we took all occurrences of Tones 1 and 4 across the two age groups into analysis. A paired samples
Both Minimum F0 and Maximum F0 of lexical tones were higher (in both Hz and ERB) in IDS addressing 18-month-old children than in ADS, but no similar differences were observed between ADS and IDS addressing 24-month-children. This pattern suggests that mothers in the study raised the pitch level of tones when they addressed 18-month-old children, but maintained ADS-like pitch height when addressing 24-month-olds. F0 range (Hz and ERB), on the other hand, showed a difference between ADS and IDS across ages: F0 range was larger in IDS compared with ADS for both 18- and 24-month-olds. As for duration, our results showed that tones were not lengthened in either age group.
Our results showed that tone hyperarticulation was present in IDS addressing 18- and 24-month-old children, but the specific tonal cues differed between the two groups: for 18-month-olds, Tone 1 had a higher F0 in IDS, and Tones 2, 3, and 4 had higher F0 and a larger F0 range in IDS. For 24-month-olds, all four tones remained the same pitch level in the two speech registers, though Tones 2, 3, and 4 in IDS still had a larger pitch range in IDS. As a secondary cue to lexical tone (
This study examined lexical tones in Mandarin Chinese IDS addressing 18- and 24-month-old children, at the age of the vocabulary spurt. The study had two main goals: to test whether tones are hyperarticulated in IDS compared with ADS in Mandarin Chinese, and to explore how tones in IDS vary with the age of the addressee during the period of vocabulary spurt. To accomplish these goals, we measured the acoustic cues of lexical tones in ADS and IDS in a semi-spontaneous story-telling task. The results demonstrated that tone hyperarticulation and age-related changes are observed in Mandarin Chinese IDS addressing toddlers.
Our research questions were: (i) Are tones in Mandarin IDS addressed to 18- and 24-month-old children hyperarticulated compared to tones in ADS? (ii) Do lexical tones in Mandarin Chinese IDS change when the mother is addressing an 18-month-old child vs. a 24-month-old child? Our results build on past studies on lexical tones in IDS addressing preverbal children (
Taken in the context of previous studies exploring lexical tones in IDS addressing preverbal children and preschool children, our results contribute to the timeline of tonal changes in IDS by providing evidence for tone hyperarticulation in the second year.
A question that follows from these findings is: why do tonal cues in IDS change over time? It seems likely that the change in the pitch level (minimum and maximum F0) is related to the general prosodic exaggeration, as the degree of prosodic exaggeration in IDS may also decline from 18 to 24 months when children have become more verbal and their word learning accelerates. However, since studies on tone hyperarticulation (including the current study) usually focus on the syllabic level, little is known about whether tonal cues coincide with other prosodic features of IDS. Crucially, our results showed that the pitch range (of Tones 2, 3, and 4) remained enlarged in IDS even when the pitch height had declined to the ADS level at 24 months, suggesting that mothers may hyperaticulate lexical tones during the period of vocabulary spurt in support of word learning.
A relationship between the quality of IDS and children’s language development has often been assumed in research on IDS (e.g.,
Thus, we must be cautious in interpreting our results as direct evidence for the linguistic function of IDS in word learning. Indeed, although the current study demonstrates that tone hyperarticulation remains present in language input during the vocabulary spurt period, it does not necessarily indicate that children benefit from this linguistic phenomenon. Several studies have explored the correlation between the quality of IDS and children’s language outcomes. For instance,
The pitch measures of tones addressed to our 24-month-old group showed a different pattern from the findings in
A limitation of the current design is that we used only one target word for each tone. We also took steps to avoid generating contrasts between the target words by ensuring that the phonemes of the target syllables differed from each other. As vowels and tones may interact (
Another useful future direction for study would be to examine whether tone hyperarticulation is related to the prosodic marking of focused words. Previous research has shown that, in English IDS, mothers tend to put contextually new words (focused words) at utterance-final positions, and these focused words usually carry prosodic marking in the form of higher pitch and a larger pitch range (
This study investigated the tone hyperarticulation phenomenon in Mandarin Chinese IDS in the second year of life and revealed age-related changes of tonal cues in IDS addressed to 18-month-old vs. 24-month-old children. These findings may contribute to an understanding of the role of IDS in tone acquisition and word learning. Mothers may hyperarticulate lexical tones in order to provide more fine-grained information for language acquisition. However, it may be premature to interpret these findings as direct evidence for the linguistic function of IDS.
There was no ethical committee in Utrecht Institute of Linguistics (UiL OTS), Utrecht University when data was collected for this study. This study was approved by UiL OTS and was carried out in accordance with the research guidelines in UiL OTS. All participants gave written informed consent.
MH contributed to the design of the experiments, data collection, phonetic annotation, data analysis, and drafting the manuscript. NdJ and RK contributed to the experimental design, data analysis, and revision of the manuscript.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
We are grateful to Taohualing Kindergarten (Mrs. Aihua Zou and Mrs. Yuhong Li), Gezhouba Dongshan Kindergarten (Mrs. Huan He) and Gezhouba Early Education Center (Mrs. Hong Xie) in Yichang, China for their kind support and coordination in recruiting participants. We thank Run Chai for her help with part of data collection. We thank all families who participated in this study. We also acknowledge the members of Utrecht Babylab and Menghui Shi for their input on experimental design and data analysis. We thank the reviewers for their valuable comments and suggestions.
The Supplementary Material for this article can be found online at:
We use the term “Mandarin Chinese” in this paper in reference to “Putonghua” or “Standard Chinese,” the official language spoken in China. It should be distinguished from Taiwanese Mandarin, another variety of Mandarin Chinese spoken in Taiwan.
An example of the R codes for these models is: sqrt(max_hz) ∼ Condition ∗ Tone ∗ Age + (1 + Condition + Tone| Participant Number).
An example of the R codes is: sqrt(max_hz) ∼ Condition ∗ Tone + (1 + Condition + Tone| Participant Number).
An example of the R codes is: sqrt(max_hz) ∼ Condition ∗ Tone + (1 + Condition| Participant Number).
The box plots in our paper show the first and third quantiles, medians, and outliers (included in analysis). All
It should be noted that the results reported in Section “Exploring the Enhancement of Tone 1 – Tone 4 Contrast” are exploratory. Our tokens per tone were few resulting in low statistical power for this specific analysis. An alternative way of measuring tonal contrast enhancement would be to compare the tone space area of selected tones (Tones 1, 2, and 4) in ADS and IDS as in