Early perception of lexical tones by non-tone-learning infants
Infants start with a general sensitivity to speech sounds. Initially they can discriminate a wide range of phonetic contrasts that do not exist in their native language. Between 6 and 10 months of age, infants’ discrimination of non-native contrasts declines, while their discrimination of native contrasts increases. This process is called perceptual attunement (PA). Most previous work on PA has focused on consonants or vowels, but a rapidly growing body of studies considers PA for lexical tone.
Languages may contrast tones at different levels. For intonation, tone is contrastive at the utterance level. For lexical tone, tones contrast at the word level (only in tone languages). Crucially, intonation and lexical tone are both marked by variation in pitch (fundamental frequency, F0), which raises the issue how infants learn to disentangle pitch contrasts at the word level from pitch at the utterance level.
Neonates are universally sensitive to pitch contours. Yet as early as 4–6 months, differences in tone perception are observed between infants acquiring a tone language and infants who are not. Tone-learning infants retain their initial ability to discriminate tone, whereas infants learning a non-tone language lose their discrimination around 6-9 months, their use of pitch for word recognition by 10 months and their ability to learn tone-to-word associations by 18 months.
Research at the BabyLab Utrecht in collaboration with Liquan Liu and Ao Chen addresses PA for lexical tone by tracking the development of tone perception in Dutch non-tone-learning (NTL) infants during the two years of life. We are considering questions such as: How does sensitivity to tone develop after 9 months? Do NTL infants permanently lose their sensitivity to tone, while not losing their sensitivity to intonation? How irreversible are the effects of tonal PA? How is tonal PA related to developing knowledge of intonation? How does phonetic salience of pitch contrasts affect tonal PA? How does tonal PA affect word learning ability? How does bilingualism affect NTL infants’ tone perception ability? How are pitch perception and musical perception related in NTL infants? Is pitch perception in NTL infants affected by cognitive maturation as well as exposure? In this colloquium I will present an overview the results of recent studies which addressed these questions.
Our results converge to show that the acoustic sensitivity and phonological sensitivity to non-native tones slowly dissociate from the second year of life onward. Infants’ discrimination of a non-native tone contrast shows a U-shaped pattern, with an increase of tone discrimination early in the second year of life. Bilingual NTL infants show a pattern of facilitation: their sensitivity to tones is enhanced and re-emerges earlier as compared to monolinguals. We also found evidence that tone perception is subject to cognitive maturation, independently of exposure. Regarding word learning, infants’ ability to use the same tone contrast shows a declining pattern, in monolinguals and bilingual infants alike. In sum, halfway their second year of life, NTL infants’ acoustic sensitivity to pitch variation increases, yet the linguistic (phonological) function lost.