Adults seem to have greater difficulties than children in acquiring a second language (L2) because of the alleged “window of opportunity” around puberty. Postpuberty Japanese participants learned a new English rule with simplex sentences during one month of instruction, and then they were tested on “uninstructed complex sentences” as well as “instructed simplex sentences.” The behavioral data show that they can acquire more knowledge than is instructed, suggesting the interweaving of nature (universal principles of grammar, UG) and nurture (instruction) in L2 acquisition. The comparison in the “uninstructed complex sentences” between post-instruction and pre-instruction using functional magnetic resonance imaging reveals a significant activation in Broca's area. Thus, this study provides new insight into Broca's area, where nature and nurture cooperate to produce L2 learners' rich linguistic knowledge. It also shows neural plasticity of adult L2 acquisition, arguing against a critical period hypothesis, at least in the domain of UG.

A foreign language (a language not spoken in one's community) is difficult to master completely. Early introduction of foreign-language (FL) education during childhood is becoming a standard in many countries. However, the neural process of child FL learning still remains largely unknown. We longitudinally followed 322 school-age children with diverse FL proficiency for three consecutive years, and acquired children's ERP responses to FL words that were semantically congruous or incongruous with the preceding picture context. As FL proficiency increased, various ERP components previously reported in mother-tongue (L1) acquisition (such as a broad negativity, an N400, and a late positive component) appeared sequentially, critically in an identical order to L1 acquisition. This finding was supported not only by cross-sectional analyses of children at different proficiency levels but also by longitudinal analyses of the same children over time. Our data are consistent with the hypothesis that FL learning in childhood reproduces identical developmental stages in an identical order to L1 acquisition, suggesting that the nature of the child's brain itself may determine the normal course of FL learning. Future research should test the generalizability of the results in other aspects of language such as syntax.

One of the most fundamental and universal properties of human language is a phenomenon called displacement. In the present study, we used multichannel event-related potentials (ERPs) to identify the nature of this phenomenon with Japanese, a subject-object-verb (SOV) language of relatively free word order. The ERPs of sentences of canonical word order (CC) were compared with those of non-canonical word order in two types of Japanese complex sentences; namely, in those which can be described as being in a middle-scrambled condition (MSC) and in those in a long-scrambled condition (LSC). The sustained anterior negativity (SAN) and the P600 in the pregap position were observed in the LSC, compared to the CC, and they are consistent with previous findings. The SAN, exhibiting a tripartite nature in morphology and scalp distribution, mainly reflected a storage cost of scrambled elements in sentence comprehension. The subsequent P600 had a left fronto-temporal maximum, distinguished from a posterior P600, taken as a reflector of the thematic role assignment in previous related studies. It is argued that the P600 in the present study reflects a cost of structural integration intensively depending on the case marker information. A compositional interpretation of sentence meanings was also observed, reflected in an anterior negativity at the postgap verbal position, which cannot be differentiated at the pregap verbal position in the languages of subject-verb-object (SVO) word order.