Learning a second language (L2) at a young age is a driving factor of functional neuroplasticity in the auditory brainstem. To date, it remains unclear whether these effects remain stable until adulthood and to what degree the amount of exposure to the L2 in early childhood might affect their outcome. We compared three groups of adult English-French bilinguals in their ability to categorize English vowels in relation to their frequency following responses (FFR) evoked by the same vowels. At the time of testing, cognitive abilities as well as fluency in both languages were matched between the (1) simultaneous bilinguals (SIM, N = 18); (2) sequential bilinguals with L1-English (N = 14); and (3) sequential bilinguals with L1-French (N = 11). Our results show that the L1-English group show sharper category boundaries in identification of the vowels compared to the L1-French group. Furthermore, the same pattern was reflected in the FFRs (i.e., larger FFR responses in L1-English > SIM > L1-French), while again only the difference between the L1-English and the L1-French group was statistically significant; nonetheless, there was a trend towards larger FFR in SIM compared to L1-French. Our data extends previous literature showing that exposure to a language during the first years of life induces functional neuroplasticity in the auditory brainstem that remains stable until at least young adulthood. Furthermore, the findings suggest that amount of exposure (i.e., 100% vs. 50%) to that language does not differentially shape the robustness of the perceptual abilities or the auditory brainstem encoding of phonetic categories of the language. Statement of significance: Previous studies have indicated that early age of L2 acquisition induces functional neuroplasticity in the auditory brainstem during processing of the L2. This study compared three groups of adult bilinguals who differed in their age of L2 acquisition as well as the amount of exposure to the L2 during early childhood. We demonstrate for the first time that the neuroplastic effect in the brainstem remains stable until young adulthood and that the amount of L2 exposure does not influence behavioral or brainstem plasticity. Our study provides novel insights into low-level auditory plasticity as a function of varying bilingual experience.

中文翻译：

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较早的第二语言学习年龄会在成人的听觉脑干中诱导更强大的语音编码，而与儿童早期语言接触的量无关

在年轻时学习第二语言 (L2) 是听觉脑干功能性神经可塑性的驱动因素。迄今为止，尚不清楚这些影响是否会在成年前保持稳定，以及儿童早期接触 L2 的量会在多大程度上影响其结果。我们比较了三组成人英法双语者根据相同元音引起的频率跟随反应 (FFR) 对英语元音进行分类的能力。在测试时，两种语言的认知能力和流利程度在（1）同时双语者（SIM，N = 18）之间匹配；(2) L1-English 连续双语者 (N = 14)；(3) L1-French (N = 11) 连续双语。我们的结果表明，与 L1-法语组相比，L1-英语组在识别元音方面表现出更清晰的类别边界。此外，FFR 也反映了相同的模式（即 L1-英语 > SIM > L1-法语中更大的 FFR 反应），而同样只有 L1-英语和 L1-法语组之间的差异具有统计学意义；尽管如此，与 L1-French 相比，SIM 中存在更大 FFR 的趋势。我们的数据扩展了先前的文献，表明在生命的最初几年接触一种语言会诱导听觉脑干的功能性神经可塑性，这种可塑性至少在成年后保持稳定。此外，研究结果表明暴露量（即 100% 与 50%）对该语言的感知能力的稳健性或语言语音类别的听觉脑干编码没有差异。重要性声明：先前的研究表明，在 L2 加工过程中，L2 习得的早期会诱导听觉脑干的功能性神经可塑性。这项研究比较了三组成人双语者，他们的 L2 习得年龄以及儿童早期 L2 的接触量不同。我们首次证明脑干中的神经可塑性效应在成年前保持稳定，并且 L2 暴露量不会影响行为或脑干可塑性。我们的研究提供了对作为不同双语经验的函数的低级听觉可塑性的新见解。