The role of pitch characteristics on the development of lexical tone perception in the first year of life : evidence from Cantonese-learning infants

Abstract

Cumulative research on infant speech perception has demonstrated that infants begin life with a remarkable ability to discriminate a wide range of phonetic contrasts, both native and non-native. Within the first year of life, infants’ phonetic discrimination ability undergoes a significant reorganization from being language-general to being language-specific. However, growing evidence has revealed that the developmental pattern is more complex than previously thought. Not all contrasts are easily discriminated, and certain contrasts can only be discriminated in the second half of the first year. This “exceptional” pattern raises a question as to what underlies the emergence of different developmental trajectories. In particular, why are certain contrasts initially difficult for infants to discriminate?

The present dissertation examines Cantonese-learning infants’ ability to discriminate lexical tones in their native language. Lexical tones are considered an expansion of the phoneme inventory in a tone language, where pitch variations also signal lexical meaning differences. While numerous infant speech perception research has focused on phonetic segments (i.e., vowels and consonants), the perceptual development of lexical tones is relatively less well-understood. The aims of the present dissertation are two-fold. First, this dissertation expands previous findings from Mandarin and examines whether some Cantonese tone contrasts are difficult to discriminate for Cantonese-learning infants. Second, this dissertation explores the role of pitch characteristics as a possible factor influencing the development of infant lexical tone discrimination.

Cantonese contrasts its lexical tones through changes in two specific dimensions: pitch height and pitch contour. To test the possible role of pitch characteristics in determining infants’ development of lexical tone perception, Study 1 employed a visual habituation-dishabituation paradigm to test a cross-age and cross-contrast design. Two age groups (4-6 and 8-10 months) of Cantonese-learning infants were tested on three pairs of Cantonese lexical tone contrasts varying in the extent to which pitch contour and pitch height differed: (1) high-rising T25 vs. mid-level T33, (2) high-rising T25 vs. low-falling T21, and (3) high-rising T25 vs. high-rising T23. Study 2 examined Cantonese-speaking adults’ perception of the same lexical tone contrasts to provide further evidence for an overall better discrimination accuracy in adults than in infants.

The results show that Cantonese-learning infants across both age groups discriminated the T25–T33 and T25–T21, but not the T25–T23, contrasts. Moreover, the effect of age was not observed, suggesting a lack of developmental changes in infants’ discrimination across ages. Nonetheless, the successful discrimination in Cantonese-speaking adults in Study 2 implies a possible improvement in discrimination after infants’ first year of life.

Our findings underscore the role of pitch characteristics on the development of lexical tone perception. In particular, infants treat distinct magnitude of pitch contour changes as more relevant or significant than subtle pitch height changes in the perception of contour tone distinctions. Moreover, a different development pattern is revealed, where certain lexical tones take longer to develop—probably beyond infancy. Thus, the present findings suggest that, not only in Mandarin but also in Cantonese, certain pairs of lexical tones are difficult to discriminate even for young infants.