Yamamoto Kayako, Sakai Kuniyoshi L
Department of Basic Science, Graduate School of Arts and Sciences, The University of TokyoTokyo, Japan.
Japan Society for the Promotion of ScienceSaitama, Japan.
Front Psychol. 2017 May 23;8:829. doi: 10.3389/fpsyg.2017.00829. eCollection 2017.
In adult second language (L2) acquisition, individual differences are considerably large even among people with similar experiences. The neural mechanisms underlying this variability would include structural plasticity of language-related pathways. To elucidate such neuroplasticity, we focused on the transitional period of adolescence, which is associated with certain plasticity toward maturation following the sensitive period of language acquisition (≤12 years old). The adolescent brain would thus be influenced by age-dependent factors, as well as performances in L2. Here, we examined individual differences in L2 performances controlling the duration of experience to reveal the differential signatures of performances and age on the plasticity of structural properties in major language-related pathways. We recruited Japanese students at two ages, i.e., junior (age: 13-14) and senior (age: 16-17) high-school students, all of whom started to expose to English at age 12 or 13. We divided them into subgroups, so that either L2 performance [Junior (High)/Senior (Low)] or age [Senior (Low)/Senior (High)] was matched in group comparisons; the duration of L2 experience was also controlled between the Senior (Low) and Senior (High) groups. We then examined the thickness and fractional anisotropy (FA) of the dorsal and ventral pathways, i.e., the arcuate fasciculus (Arcuate) and inferior fronto-occipital fasciculus (IFOF), respectively, using semi-automatic methods for selecting regions without branches. Regarding FA in the left Arcuate, the Senior (High) group showed significantly higher FA than the other two groups, indicating performance-related group differences. Further, FA in the left Arcuate was selectively correlated with the accuracy of a syntactic task. Regarding the thickness of the left Arcuate, the Senior (High) and Senior (Low) groups showed significantly larger thickness than the Junior (High) group, indicating age-related group differences. These differential performance-related and age-related signatures were evident on the left Arcuate alone, in contrast to the right Arcuate that showed only mild differences in thickness, and to the bilateral IFOF that lacked either signature. Our results suggest that the left dorsal pathway continued to develop to adolescence, and that performance differences in a syntactic task can be predicted by its FA, independent of age and the duration of experience.
在成人第二语言（L2）习得中，即使是经历相似的人群，个体差异也相当大。这种变异性背后的神经机制可能包括与语言相关通路的结构可塑性。为了阐明这种神经可塑性，我们聚焦于青春期这个过渡阶段，它与语言习得敏感期（≤12岁）后的成熟过程中的特定可塑性相关。因此，青少年大脑会受到年龄相关因素以及第二语言表现的影响。在这里，我们通过控制学习时长来研究第二语言表现的个体差异，以揭示表现和年龄对主要语言相关通路结构特性可塑性的不同特征。我们招募了两个年龄段的日本学生，即初中生（年龄：13 - 14岁）和高中生（年龄：16 - 17岁），他们均在12岁或13岁开始接触英语。我们将他们分成亚组，以便在组间比较中使第二语言表现[初中（高）/高中（低）]或年龄[高中（低）/高中（高）]相匹配；高中（低）组和高中（高）组之间的第二语言学习时长也得到了控制。然后，我们使用半自动方法选择无分支区域，分别检测背侧和腹侧通路，即弓状束（Arcuate）和额颞枕下束（IFOF）的厚度和分数各向异性（FA）。关于左侧弓状束的FA，高中（高）组的FA显著高于其他两组，表明存在与表现相关的组间差异。此外，左侧弓状束的FA与句法任务的准确性存在选择性关联。关于左侧弓状束的厚度，高中（高）组和高中（低）组的厚度显著大于初中（高）组，表明存在与年龄相关的组间差异。这些与表现和年龄相关的不同特征仅在左侧弓状束上明显，与之形成对比的是，右侧弓状束仅在厚度上有轻微差异，双侧额颞枕下束则缺乏任何一种特征。我们的研究结果表明，左侧背侧通路在青春期仍持续发育，并且句法任务中的表现差异可以通过其FA来预测，而与年龄和学习时长无关。
