Abstract:

Lexical tones are a key component of tonal language. The accurate perception of different Mandarin lexical tones is essential for Mandarin-speaking children to process spoken Chinese. Previous studies based on the speech perception model of nontonal language proposed perceptual narrowing theory, while later studies indicated that the perception of lexical tones might be more complicated. Event-related potentials (ERPs) are an effective tool that can investigate Mandarin-speaking children's implicit perception of lexical tones. Some studies targeting children's speech perception have demonstrated that mismatch responses (MMRs, including MMN and p-MMR) indicate the development of phonetic representation. Moreover, these studies have provided empirical evidence in this field. However, ages between 2 and 4 years old needs to be explored empirically. The current study used MMN and p-MMR as the neural correlates of lexical tone perception of Mandarin in the pre-attention stage and investigated the development of lexical tone processing mechanisms in 2- to 4-year-old children. In addition, we paid attention to the influence of category information and the size of the deviance at the acoustic level.

Sixteen 2- to 4-year-old Mandarin-speaking children (7 boys and 9 girls; mean age: 3.4 years old; range: 2.6 - 3.10 years old) were recruited in this study. The study used the oddball paradigm and designed two experiments: Experiment 1 investigated the mismatch responses in across-category lexical tone perception in the participants. The stimuli consisted of three syllables: /yi1/ (T1), /yi2/ (T2), and /yi3/ (T3). T3 was assigned as the standard; T1 was assigned as the large deviant; T2 was the small deviant. This experiment investigated how the deviance size affects children's perception of different lexical tones in the across-category condition. Experiment 2 investigated the mismatch responses of the participants to the perception of tones in the same category and were designated yi3a (T3a) and yi3b (T3b) within the category. These tones have the same phonological information but differ from the standard T3 stimulus in the acoustic information (based on the frequency and contour). The distances between T3a & T3 and T3b & T3 were modulated in Praat to control the distances of T1 & T3 and T2 & T3, respectively. This experiment detected the influence of the degree of similarity on the acoustic information on lexical tone perception without changing the category information.

The results of the EEG data showed that (1) only the across-category large-deviance pair (T1/T3) elicited an obvious MMN response, indicating that 2- to 4-year-old Mandarin-speaking children can distinguish tones with obvious category boundaries in the pre-attention stage; (2) the across-category small-deviance pair (T2/T3) and the two kinds of within-category deviations (T3a/T3; T3b/T3) achieved the same data performance: no significant MMN or p-MMR was elicited. However, the internal mechanisms of these two conditions are different. The former may indicate that 2- to 4-year-old Mandarin-speaking children are in the period of transition from p-MMR to MMN. The latter may reflect that children have established a lexical tone category, and due to their immature perception ability, it is impossible for them to distinguish the differences among within-category lexical tones.

In summary, the current study has filled the age gap in the relevant lexical tone perception neural mechanism research and has revealed what is crucial to children's lexical tone neural mechanism of perception development. Moreover, this research has identified the size difference between isometric across-category stimulation with within-category stimulation, which innovatively provides a reference for future research. The conclusion indicates that 2- to 4-year-old Mandarin-speaking children are in the period of lexical tone category formation. In the pre-attention stage, the obvious category boundary of lexical tones can be distinguished accurately. For the inconspicuous category boundaries of lexical tones, the neural mechanism is transforming from p-MMR to MMN. The magnitude of the deviance does not affect lexical tone perception within the same category.

Key words: Mandarin-speaking children, lexical tone perception, the categories of lexical tones, size of deviant