Perceptual Reorganization of Lexical Tones: Effects of Age and Experimental Procedure

The development of tone discrimination in infancy: Evidence from a cross-linguistic, multi-lab report

We report the findings of a multi-language and multi-lab investigation of young infants' ability to discriminate lexical tones as a function of their native language, age and language experience, as well as of tone properties. Given the high prevalence of lexical tones across human languages, understanding lexical tone acquisition is fundamental for comprehensive theories of language learning. While there are some similarities between the developmental course of lexical tone perception and that of vowels and consonants, findings for lexical tones tend to vary greatly across different laboratories. To reconcile these differences and to assess the developmental trajectory of native and non-native perception of tone contrasts, this study employed a single experimental paradigm with the same two pairs of Cantonese tone contrasts (perceptually similar vs. distinct) across 13 laboratories in Asia-Pacific, Europe and North-America testing 5-, 10- and 17-month-old monolingual (tone, pitch-accent, non-tone) and bilingual (tone/non-tone, non-tone/non-tone) infants. Across the age range and language backgrounds, infants who were not exposed to Cantonese showed robust discrimination of the two non-native lexical tone contrasts. Contrary to this overall finding, the statistical model assessing native discrimination by Cantonese-learning infants failed to yield significant effects. These findings indicate that lexical tone sensitivity is maintained from 5 to 17 months in infants acquiring tone and non-tone languages, challenging the generalisability of the existing theoretical accounts of perceptual narrowing in the first months of life. RESEARCH HIGHLIGHTS: This is a multi-language and multi-lab investigation of young infants' ability to discriminate lexical tones. This study included data from 13 laboratories testing 5-, 10-, and 17-month-old monolingual (tone, pitch-accent, non-tone) and bilingual (tone/non-tone, non-tone/non-tone) infants. Overall, infants discriminated a perceptually similar and a distinct non-native tone contrast, although there was no evidence of a native tone-language advantage in discrimination. These results demonstrate maintenance of tone discrimination throughout development.

Neural correlates of lexical-tone and vowel-quality processing in 6- and 9-month-old German-learning infants and adults

We examined the neurophysiological underpinnings of lexical-tone and vowel-quality perception in learners of a non-tonal language. We tested 25 6- and 25 9-month-old German-learning infants, as well as 24 German adults and expected developmental differences for the two linguistic properties, as they are both carried by vowels, but have a different status in German. In adults, both lexical-tone and vowel-quality contrasts elicited mismatch negativities, with a stronger response to the vowel-quality contrast. Six-month-olds showed positive mismatch responses for lexical-tone and vowel-quality contrasts, with an emerging negative mismatch response for vowel-quality only. The negative mismatch responses became more pronounced for the vowel-quality contrast at 9 months, while the lexical-tone contrast elicited mainly positive mismatch responses. Our data reveal differential developmental changes in the processing of vowel properties that differ in their lexical relevance in the ambient language.

Asymmetries in Infants' Vowel Perception: Changes in Vowel Discrimination in German Learning 6- and 9-Month-Old Infants

Infants' speech perception is characterized by substantial changes during the first year of life that attune the processing mechanisms to the specific properties of the ambient language. This paper focuses on these developmental changes in vowel perception. More specifically, the emergence and potential cause of perceptual asymmetries in vowel perception are investigated by an experimental study on German 6- and 9-month-olds' discrimination of a vowel contrast that is not phonemic in German. Results show discrimination without any asymmetry in the 6-month-olds but an asymmetrical pattern with better performance when the vowel changes from the less focal to the more focal vowel than vice versa by the 9-month-olds. The results concerning the asymmetries are compatible with the Natural Referent Framework as well as with the Native Language Magnet model. Our results foster two main conclusions. First, bi-directional testing must be mandatory when testing vowel perception. Second, when testing non-native vowel perception, the relation of the stimuli to the native language vowel system has to be considered very carefully as this system impacts the perception of non-native vowels.

Why Aren't All Cantonese Tones Equally Confusing to English Listeners?

English listeners often struggle to perceive tones, but some are easier than others. This study examined these phenomena grounded in the feature weighing perspective (FWP) and the Perceptual Assimilation Model for Suprasegmentals (PAM-S). Forty-seven English and Cantonese listeners completed 4,212 trials of Cantonese tone discrimination and sequence recall tasks. The English listeners showed asymmetrical perceptual patterns of discrimination but not sequence recall. Specifically, these English listeners discriminated T1-T5, T3-T5, and T4-T5 more accurately than T1-T4, T3-T4, and T1-T3. However, they recalled the contour tone and level tone sequences with similar accuracies. Results of the discrimination task aligned with the predictions of PAM-S but not FWP. However, results of the sequence recall task did not support PAM-S. Together, these results suggest that PAM-S only applies to simple discrimination, not abstract phonological processing with a high memory load.

Understanding the neural mechanisms for infants' perception of native and non-native speech

Though perceptual narrowing has been widely recognized as a process guiding cognitive development and category learning in infancy and early childhood, its neural mechanisms and traits at a cortical level remain unclear. Using an electroencephalography (EEG) abstract mismatch negativity (MMN) paradigm, Australian infants' neural sensitivity to (native) English and (non-native) Nuu-Chah-Nulth speech contrasts was examined in a cross-sectional design at the onset (5-6 months) and offset (11-12 months) of perceptual narrowing. Immature mismatch responses (MMR) were observed among younger infants for both contrasts, while older infants showed MMR response to the non-native contrast, and both MMR and MMN to the native contrast. Sensitivity to the Nuu-Chah-Nulth contrast at perceptual narrowing offset was retained yet stayed immature. Findings conform to perceptual assimilation theories, reflecting plasticity in early speech perception and development. Compared to behavioural paradigms, neural examination effectively reveals experience-induced processing differences to subtle contrasts at the offset of perceptual narrowing.

How Tone, Intonation and Emotion Shape the Development of Infants’ Fundamental Frequency Perception

Fundamental frequency (ƒ₀), perceived as pitch, is the first and arguably most salient auditory component humans are exposed to since the beginning of life. It carries multiple linguistic (e.g., word meaning) and paralinguistic (e.g., speakers’ emotion) functions in speech and communication. The mappings between these functions and ƒ₀ features vary within a language and differ cross-linguistically. For instance, a rising pitch can be perceived as a question in English but a lexical tone in Mandarin. Such variations mean that infants must learn the specific mappings based on their respective linguistic and social environments. To date, canonical theoretical frameworks and most empirical studies do not view or consider the multi-functionality of ƒ₀, but typically focus on individual functions. More importantly, despite the eventual mastery of ƒ₀ in communication, it is unclear how infants learn to decompose and recognize these overlapping functions carried by ƒ₀. In this paper, we review the symbioses and synergies of the lexical, intonational, and emotional functions that can be carried by ƒ₀ and are being acquired throughout infancy. On the basis of our review, we put forward the Learnability Hypothesis that infants decompose and acquire multiple ƒ₀ functions through native/environmental experiences. Under this hypothesis, we propose representative cases such as the synergy scenario, where infants use visual cues to disambiguate and decompose the different ƒ₀ functions. Further, viable ways to test the scenarios derived from this hypothesis are suggested across auditory and visual modalities. Discovering how infants learn to master the diverse functions carried by ƒ₀ can increase our understanding of linguistic systems, auditory processing and communication functions.

The tone atlas of perceptual discriminability and perceptual distance: Four tone languages and five language groups

Some prior investigations suggest that tone perception is flexible, reasonably independent of native phonology, whereas others suggest it is constrained by native phonology. We address this issue in a systematic and comprehensive investigation of adult tone perception. Sampling from diverse tone and non-tone speaking communities, we tested discrimination of the three major tone systems (Cantonese, Thai, Mandarin) that dominate the tone perception literature, in relation to native language and language experience as well as stimulus variation (tone properties, presentation order, pitch cues) using linear mixed effect modelling and multidimensional scaling. There was an overall discrimination advantage for tone language speakers and for native tones. However, language- and tone-specific effects, and presentation order effects also emerged. Thus, over and above native phonology, stimulus variation exerts a powerful influence on tone discrimination. This study provides a tone atlas, a reference guide to inform empirical studies of tone sensitivity, both retrospectively and prospectively.

Perceptual narrowing in face- and speech-perception domains in infancy: A longitudinal approach

During the first year of life, infants undergo a process known as perceptual narrowing, which reduces their sensitivity to classes of stimuli which the infants do not encounter in their environment. It has been proposed that perceptual narrowing for faces and speech may be driven by shared domain-general processes. To investigate this theory, our study longitudinally tested 50 German Caucasian infants with respect to these domains first at 6 months of age followed by a second testing at 9 months of age. We used an infant-controlled habituation-dishabituation paradigm to test the infants' ability to discriminate among other-race Asian faces and non-native Cantonese speech tones, as well as same-race Caucasian faces as a control. We found that while at 6 months of age infants could discriminate among all stimuli, by 9 months of age they could no longer discriminate among other-race faces or non-native tones. However, infants could discriminate among same-race stimuli both at 6 and at 9 months of age. These results demonstrate that the same infants undergo perceptual narrowing for both other-race faces and non-native speech tones between the ages of 6 and 9 months. This parallel development of perceptual narrowing occurring in both the face and speech perception modalities over the same period of time lends support to the domain-general theory of perceptual narrowing in face and speech perception.

Asymmetric memory for birth language perception versus production in young international adoptees

Adults who as children were adopted into a different linguistic community retain knowledge of their birth language. The possession (without awareness) of such knowledge is known to facilitate the (re)learning of birth-language speech patterns; this perceptual learning predicts such adults' production success as well, indicating that the retained linguistic knowledge is abstract in nature. Adoptees' acquisition of their adopted language is fast and complete; birth-language mastery disappears rapidly, although this latter process has been little studied. Here, 46 international adoptees from China aged four to 10 years, with Dutch as their new language, plus 47 matched non-adopted Dutch-native controls and 40 matched non-adopted Chinese controls, undertook across a two-week period 10 blocks of training in perceptually identifying Chinese speech contrasts (one segmental, one tonal) which were unlike any Dutch contrasts. Chinese controls easily accomplished all these tasks. The same participants also provided speech production data in an imitation task. In perception, adoptees and Dutch controls scored equivalently poorly at the outset of training; with training, the adoptees significantly improved while the Dutch controls did not. In production, adoptees' imitations both before and after training could be better identified, and received higher goodness ratings, than those of Dutch controls. The perception results confirm that birth-language knowledge is stored and can facilitate re-learning in post-adoption childhood; the production results suggest that although processing of phonological category detail appears to depend on access to the stored knowledge, general articulatory dimensions can at this age also still be remembered, and may facilitate spoken imitation.

Protracted Development on Native Tone Interpretation: Evidence From Mandarin-Learning Infants' Novel Word Learning

Studies have shown that infants from cultures with tone languages develop categorical perception of their native lexical tone before their first birthday, but few studies have explored whether, and when, they interpret the phonemic function of lexical tone in word learning. Two habituation-switch experiments were conducted to explore whether Mandarin-learning infants could exploit tonal cues during their word learning, and detect a change when the association of two word-object pairs was switched. In Experiment 1, two words were solely differentiated by their lexical tones (/fāi/ vs. /făi/), and Mandarin-learning infants failed to detect the switch of tones at 14 months, but succeeded at 18 months. In Experiment 2, two words were markedly distinct (/fāi/ vs. /bǒu/), and infants could detect the change of words as early as 14 months. The results indicate that infants may not refer to the lexical function of tone during their novel word learning until 18 months, even though infants from birth are able to distinguish the Tone 1 vs. Tone 3 contrast. Given that lexical tone is expressed by variations of the pitch contours, which are also related to intonation, infants’ increasing knowledge of both tone and intonation may contribute to their misinterpretation of pitch contours in word learning at 14 months and, further, to their development of a sophisticated use of the phonemic function of lexical tone at 18 months of age.

How Do Infants Disaggregate Referential and Affective Pitch?

Infants are faced with a challenge of disaggregating functions of pitch in the ambient language into affective, pragmatic or referential (the latter in tone languages only). This mini review discusses several factors that might facilitate the disaggregation of referential and affective pitch in infancy: acoustic characteristics of infant-directed speech, recognition of vocal affect, facial cues accompanying affective prosody, and lateralization of affective and referential prosody in the brain. It proposes two hypotheses concerning the role of audiovisual cues and brain lateralization

Perceptual Narrowing in Speech and Face Recognition: Evidence for Intra-individual Cross-Domain Relations

During the first year of life, infants undergo perceptual narrowing in the domains of speech and face perception. This is typically characterized by improvements in infants’ abilities in discriminating among stimuli of familiar types, such as native speech tones and same-race faces. Simultaneously, infants begin to decline in their ability to discriminate among stimuli of types with which they have little experience, such as non-native tones and other-race faces. The similarity in time-frames during which perceptual narrowing seems to occur in the domains of speech and face perception has led some researchers to hypothesize that the perceptual narrowing in these domains could be driven by shared domain-general processes. To explore this hypothesis, we tested 53 Caucasian 9-month-old infants from monolingual German households on their ability to discriminate among non-native Cantonese speech tones, as well among same-race German faces and other-race Chinese faces. We tested the infants using an infant-controlled habituation-dishabituation paradigm, with infants’ preferences for looking at novel stimuli versus the habituated stimuli (dishabituation scores) acting as indicators of discrimination ability. As expected for their age, infants were able to discriminate between same-race faces, but not between other-race faces or non-native speech tones. Most interestingly, we found that infants’ dishabituation scores for the non-native speech tones and other-race faces showed significant positive correlations, while the dishabituation scores for non-native speech tones and same-race faces did not. These results therefore support the hypothesis that shared domain-general mechanisms may drive perceptual narrowing in the domains of speech and face perception.