Elsevier

Neuroscience Letters

Volume 585, 12 January 2015, Pages 48-53
Neuroscience Letters

Neural processing of speech in children is influenced by extent of bilingual experience

https://doi.org/10.1016/j.neulet.2014.11.011Get rights and content

Highlights

  • Auditory neural function reflects a child’s bilingual experience.

  • Neural consistency and spectral encoding track with amount of bilingual experience.

  • Bilingualism enhances auditory processing of select acoustic aspects of speech.

Abstract

Language experience fine-tunes how the auditory system processes sound. Bilinguals, relative to monolinguals, have more robust evoked responses to speech that manifest as stronger neural encoding of the fundamental frequency (F0) and greater across-trial consistency. However, it is unknown whether such enhancements increase with increasing second language experience. We predict that F0 amplitude and neural consistency scale with dual-language experience during childhood, such that more years of bilingual experience leads to more robust F0 encoding and greater neural consistency. To test this hypothesis, we recorded auditory brainstem responses to the synthesized syllables ‘ba’ and ‘ga’ in two groups of bilingual children who were matched for age at test (8.4 ± 0.67 years) but differed in their age of second language acquisition. One group learned English and Spanish simultaneously from birth (n = 13), while the second group learned the two languages sequentially (n = 15), spending on average their first four years as monolingual Spanish speakers. We find that simultaneous bilinguals have a larger F0 response to ‘ba’ and ‘ga’ and a more consistent response to ‘ba’ compared to sequential bilinguals and we demonstrate that these neural enhancements track with years of bilingual experience. These findings support the notion that bilingualism enhances subcortical auditory processing.

Introduction

Acquisition of a second language enhances how sound is processed both cortically [19] and subcortically [14]. While bilingualism’s influence on cortical areas has been extensively evaluated [see [5] for a review] its effects on subcortical auditory processing is a recent topic. Assessments of subcortical processing have revealed that bilingual adolescents demonstrate greater across-trial neural consistency and encode the fundamental frequency (F0) of speech more robustly than monolinguals [14] ; however, whether the degree of these enhancements is dependent on the extent of second language experience is unknown. The subcortical auditory response of newborns is not biased to the native language of their parents [10], but young adults show enhanced subcortical processing of native language features [10], [13], implicating an emergence of spoken-language dependent tuning of the auditory brainstem during childhood. Therefore, we hypothesize that second language learning during childhood leads to additional structural and/or functional changes in the neural circuitry underlying auditory communication, with the amount of plasticity being commensurate with the amount of bilingual experience. This leads to the prediction that among age-matched bilinguals, the encoding of the F0 of speech and the consistency of the response will be greater in children who learned their second language earlier in life. To test this prediction, the current study compared F0 encoding strength and response consistency across two groups of bilingual children who differed in their age of second language acquisition.

Section snippets

Participants

Electrophysiological responses to the synthesized syllables ‘ba’ and ‘ga’ were collected in 27 school-aged children (8.4 ± 0.67 years, 17 female) recruited from Los Angeles, California. All children were Spanish–English bilinguals and all but four were from low socioeconomic backgrounds as measured by maternal education, which has previously been used as a reliable index of socioeconomic status in children [26], [28] (high-school or less: n = 23; some college or beyond: n = 4). Two sequential

Results

Simultaneous bilinguals had a larger evoked response to the F0 and tended to have greater across-trial consistency that was specific to the response to the vowel relative to the sequential bilinguals. Across groups, these neural measures related to amount of bilingual experience.

Discussion

Previously, we found that adolescent bilinguals had larger and more consistent auditory brainstem responses to speech relative to adolescent monolinguals [14], [16]. Now when comparing younger bilingual children who differ in their amount of bilingual experience, we find that children who have spoken both languages since birth have greater F0 encoding of ‘ba’ and ‘ga’ and more consistent responses to ‘ba’ than age-matched peers who spent half as many years using two languages. Moreover, amount

Conclusions

Bilingual experience during childhood can foster plasticity in the neural encoding of sound. Bilingual children who learned their two languages simultaneously from birth had enhanced F0 encoding and more consistent evoked responses to ‘ga’ compared to bilingual children who learned their two languages sequentially. These findings suggest that enhanced F0 encoding and neural consistency emerge with increasing experience communicating in two languages during childhood and support the notion that

Acknowledgements

The authors thank Samantha O’Connell, Dana L. Strait and Jason Thompson for help with data collection and Trent Nicol, Adam Tierney, and Elaine Thompson for helpful comments on earlier versions of the manuscript. This research is funded by NSF SMA1015614, NIH DC009399 and HD059858, NAMM, and the Hugh Knowles Center, Northwestern University.

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      However, our data did not support the hypothesis that more extensive language exposure in early childhood leads to more robust processing of that language in adulthood, as reflected in the brainstem evoked responses, as we did not find a difference between the SIM and L1-English groups, who differed primarily in terms of amount of exposure to English during the first five years of life (along with exposure to an L2, of course). Thus, analogous to Krizman et al. (2015), who investigated bilingual children, our findings demonstrate that earlier AoA leads to more robust neural responses in the auditory brainstem to sounds in the respective language, lasting until at least early adulthood. Similarly, neuroimaging studies using resting-state fMRI have shown that earlier L2-AoA leads to stronger functional connectivity within and between auditory-related networks involved in language processing (Liu et al., 2017).

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    1

    Present address: Department of Speech, Language, and Hearing Sciences, Department of Psychology Affiliate, Cognitive Sciences Program Affiliate, University of Connecticut Storrs, Connecticut, USA.

    2

    www.brainvolts.northwestern.edu.

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