Enhanced music sensitivity in 9-month-old bilingual infants

 This study explores the influence of bilingualism on the cognitive processing of language and music. Specifically, they investigate how infants learning a non-tone language (a language which does not use tones such as English) perceive linguistic and musical pitch and how bilingualism affects cross-domain pitch perception. Dutch monolingual and bilingual infants of 8–9 months participated in the study. All infants had Dutch as one of the first languages. The other first languages, varying among bilingual families, were not tone or pitch accent languages. To do this, they carried out 2 experiments.

The perception of human speech and music is shaped by initial sensitivities at birth and later learning from the environment. Language-wise, infants are born with the ability to discriminate a wide range of native and non-native sound contrasts at birth. In the first year of life, infant sensitivity shifts towards the native language. Just as language, infants show initial sensitivity to music. After hearing repetitions of the original musical tone notes, infants of 5–10 months after birth are able to detect changes of single notes and of internal reordering of multiple notes.

The main questions of this research were :

(1) what are non-tone language learning infants’ specific perceptual mechanisms for linguistic and musical pitch?

(2) Does growing up in a bilingual environment alter infants’ perception of language and music?

Experiment 1

A total of 42 Dutch monolingual and bilingual infants aged 9 months participated in the experiment. All bilingual infants were acquiring Dutch as one of their native languages, and the other language varied across participants. The degree of exposure to the non-dominant language was no less than 20% via a Multilingual Infant Language Questionnaire. The mean (standard deviation, SD) degree of exposure to Dutch was 55% (17%) for bilingual infants. All parents reported normal hearing, no exposure to a tone language, and no excessive (more than 2 h per day) music exposure at home for their children. No parent worked as a musician as her/his profession. Data from six participants were excluded from analyses for the following reasons: fussiness (3), program error during the experiment (1), and looking time difference exceeding 2 SD from the mean (2).

This experiment tested the children in lexical pitch contrasts. Four lexical tones exist in Mandarin Chinese (Fig. 1a): high-level (T1), middle-rising (T2), low-dipping (T3), and high-falling (T4). The tone-bearing syllable was /ta/. Both /ta1/ “build” and /ta4/ “big” are legal words in Mandarin Chinese. The vocalisations of a Chinese female speaker were recorded. Four natural T1–T4 pairs were recorded. 

Infants sat on their caretakers’ lap in the test booth, facing the screen and the hidden camera during the experiment. No visual or auditory distractions were present in the booth. An experimenter observed infants through a closed-circuit TV in a room adjacent to the test booth. The infants went through three phases during the experiment: habituation, test, and post-test.  Repeated tokens of one tone were provided in the habituation phase. The test phase began when the mean looking time of the last three trials in the habituation phase fell below 65% of the mean looking time of the first three trials. Two trials of tokens of the other tone were presented in the test phase. In the post-test phase, a novel stimulus was presented to verify infants’ general attention, followed by a children’s song at the end to boost infants’ pleasure in participating the experiment. During the experiment, the dependent variable was infant looking time. The length of each trial was controlled by infant gazing: one trial ended when the infant looked away for more than 2 s, and then the next trial began.  Discrimination was indicated by looking time rebound upon hearing the new stimulus during the phase change (from habituation to test phase).  The visual stimuli were static bull’s eye in the habituation, test, and post-test phases and random toy pictures appearing on a 3*3 grid when the children’s song was played. Caregivers were blind to the purpose of the test as well as the acoustic stimuli presented to infants throughout the experiment.

The main effect of the phase change (the difference between the two last trials in the habituation phase and the two trials in the test phase) was not significant. Neither was the interaction between language background and phase change. Infants in both language backgrounds failed to discriminate the contrast.

Experiment 2

To understand infants’ pitch processing across domains, a musical pitch contrast was examined in Experiment 2. Forty-eight Dutch monolingual and bilingual infants aged 9 months participated in Experiment 2. To prevent potential perceptual biases, all infants were different from those in Experiment 1. The same infant selection criteria as in Experiment 1 were adopted.  The mean (SD) degree of exposure to Dutch was 53% (16%) among bilingual infants. Eventually, data from 36 participants were included in analysis. Data of 12 infants from the initial sample pool were excluded from analyses for the following reasons: tone or pitch accent language exposure after birth (3), fussiness (3), crying (3), unable to habituate (1), and inattentiveness (2).

To ensure the cross-domain comparison, the musical (violin) tonal stimuli were generated from the same contrast used in Experiment 1. The violin contrast shared the exact same pitch contour as the tonal contrast in Experiment 1, but differed in timbre. 

The same procedure as in Experiment 1 was adopted.

Infants’ mean looking times between the last two habituation trials and the two test trials were compared using an RM ANOVA.  The main effect of the phase change was significant, the interaction between language background and the phase change was also significant. Splitting the data by language background, paired samples t-test shows that the phase change was not significant for the monolingual group. Bilingual but not monolingual infants discriminated the contrast. 9-month-old monolingual infants did not discriminate the musical pitch contrast, whereas their bilingual peers succeeded.

Discussion

Infants are predisposed to attending to musical melodies upon birth.  Nevertheless, 9-month-old Dutch monolingual infants failed to show discrimination in the current experiment. This is likely to be due to the acoustic properties of the stimuli, as single notes (less than 500 ms per token) were used with the sole difference lying in the final drop in pitch (50 Hz difference in the final part) between the two tokens. The most interesting finding of the current study is that unlike their monolingual peers, bilingual infants discriminated the musical contrast. This is unlikely to be due to attentional or memory factors since (1) bilinguals did not perform better when perceiving a similar contrast in Experiment 1; (2) all infants included in the analysis passed the attention criterion; and (3) the cognitive load of the specific habituation paradigm is relatively low. Facing a more complicated learning environment, bilinguals may be more sensitive to the subtle acoustic differences in the incoming stimuli, and this sensitivity is not restricted to speech contrasts but extends to the music domain. It is surprising that bilingual infants discriminated the violin tonal contrast more than the linguistic tonal contrast. In the perception of lexical pitch, no discrimination was found in Dutch monolingual and bilingual infants. A tentative explanation might be that the contrast may be too difficult for infants at this age. This interpretation, however, is not in line with previous findings reporting initial sensitivity to pitch in neonates. Alternatively, non-tone language learning infants may pay little attention to non-native pitch at the end of the perceptual attunement period, since lexical pitch does not contrast word meaning in their native language inventory. From the results of experiment 2 however we hypothesize that bilingual infants present another cognitive advantage, heightened acoustic sensitivity, compared to their monolingual peers.