Functional brain outcomes of L2 speech learning emerge during sensorimotor transformation

Daniel Carey,Marc E Miquel,Bronwen G Evans,Patti Adank,Carolyn Mcgettigan

doi:10.1016/j.neuroimage.2017.06.053

Abstract

Sensorimotor transformation (ST) may be a critical process in mapping perceived speech input onto non-native (L2) phonemes, in support of subsequent speech production. Yet, little is known concerning the role of ST with respect to L2 speech, particularly where learned L2 phones (e.g., vowels) must be produced in more complex lexical contexts (e.g., multi-syllabic words). Here, we charted the behavioral and neural outcomes of producing trained L2 vowels at word level, using a speech imitation paradigm and functional MRI. We asked whether participants would be able to faithfully imitate trained L2 vowels when they occurred in non-words of varying complexity (one or three syllables). Moreover, we related individual differences in imitation success during training to BOLD activation during ST (i.e., pre-imitation listening), and during later imitation. We predicted that superior temporal and peri-Sylvian speech regions would show increased activation as a function of item complexity and non-nativeness of vowels, during ST. We further anticipated that pre-scan acoustic learning performance would predict BOLD activation for non-native (vs. native) speech during ST and imitation. We found individual differences in imitation success for training on the non-native vowel tokens in isolation; these were preserved in a subsequent task, during imitation of mono- and trisyllabic words containing those vowels. fMRI data revealed a widespread network involved in ST, modulated by both vowel nativeness and utterance complexity: superior temporal activation increased monotonically with complexity, showing greater activation for non-native than native vowels when presented in isolation and in trisyllables, but not in monosyllables. Individual differences analyses showed that learning versus lack of improvement on the non-native vowel during pre-scan training predicted increased ST activation for non-native compared with native items, at insular cortex, pre-SMA/SMA, and cerebellum. Our results hold implications for the importance of ST as a process underlying successful imitation of non-native speech.

Highlights

Producing speech in a non-native language requires phonemes to be deployed flexibly in a variety of lexical contexts (e.g., Flege and Hillenbrand, 1984; Levy and Law, 2010)
We explored the effects of imitation training on the acoustic accuracy of vowel production, for non-native and native vowels in isolation; we later tested imitation performance within syllabic contexts
To explore the role of sensorimotor transformation (ST) and imitation with respect to learning success, we probed whether individual differences in pre-scan acoustic training performance could account for subsequent variation in ST or imitation activation for nonnative versus native speech

Summary

Introduction

Producing speech in a non-native language requires phonemes to be deployed flexibly in a variety of lexical contexts (e.g., Flege and Hillenbrand, 1984; Levy and Law, 2010). L2 learners are faced with the non-trivial demands of perceiving and parsing the incoming speech acoustic signal, matching perceived phonemes to targets within phonological memory, transforming from phonological to motor targets, selecting and executing appropriate speech articulatory plans, and relaying auditory and somatosensory feedback in order to correct speech errors online (Hickok, 2012; see Guenther, 2006; Bohland et al, 2010; Simmonds et al, 2014a; Cogan et al, 2014; Parker-Jones et al, 2013; Chang et al, 2013) In light of these challenges, studies have begun to explore speech articulation and imitation, in tandem with earlier neural processes that reflect transformation from the perceived speech signal to the phonemic and motor representations that support speech articulation (i.e., sensorimotor transformation, ST; Cogan et al, 2014; Leonard et al, 2016; Parker-Jones et al, 2014; Carey et al, 2017). Advances have been made in understanding ST in native speech, comparably less work has explored ST processes with respect to L2, or changes in ST processes in the context of L2 learning

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