An enduring question in the study of second-language acquisition concerns the relative contributions of age of acquisition (AOA) and ultimate linguistic proficiency to neural organization for second-language processing. Several ERP and neuroimaging studies of second-language learners have found that neural organization for syntactic processing is sensitive to delays in second-language acquisition. However, such delays in second-language acquisition are typically associated with lower language proficiency, rendering it difficult to assess whether differences in AOA or proficiency lead to these effects. Here we examined the effects of delayed second-language acquisition while controlling for proficiency differences by examining participants who differ in AOA but who were matched for proficiency in the same language. We compared the ERP response to auditory English phrase structure violations in a group of late learners of English matched for grammatical proficiency with a group of English native speakers. In the native speaker group, violations elicited a bilateral and prolonged anterior negativity, with onset at 100 msec, followed by a posterior positivity (P600). In contrast, in the nonnative speaker group, violations did not elicit the early anterior negativity, but did elicit a P600 which was more widespread spatially and temporally than that of the native speaker group. These results suggest that neural organization for syntactic processing is sensitive to delays in language acquisition independently of proficiency level. More specifically, they suggest that both early and later syntactic processes are sensitive to maturational constraints. These results also suggest that late learners who reach a high level of second-language proficiency rely on different neural mechanisms than native speakers of that language.

Although anecdotally there appear to be differences in the way native speakers use and comprehend their native language, most empirical investigations of language processing study university students and none have studied differences in language proficiency, which may be independent of resource limitations such as working memory span. We examined differences in language proficiency in adult monolingual native speakers of English using an ERP paradigm. ERPs were recorded to insertion phrase structure violations in naturally spoken English sentences. Participants recruited from a wide spectrum of society were given standardized measures of English language proficiency, and two complementary ERP analyses were performed. In between-groups analyses, participants were divided on the basis of standardized proficiency scores into lower proficiency and higher proficiency groups. Compared with lower proficiency participants, higher proficiency participants showed an early anterior negativity that was more focal, both spatially and temporally, and a larger and more widely distributed positivity (P600) to violations. In correlational analyses, we used a wide spectrum of proficiency scores to examine the degree to which individual proficiency scores correlated with individual neural responses to syntactic violations in regions and time windows identified in the between-groups analyses. This approach also used partial correlation analyses to control for possible confounding variables. These analyses provided evidence for the effects of proficiency that converged with the between-groups analyses. These results suggest that adult monolingual native speakers of English who vary in language proficiency differ in the recruitment of syntactic processes that are hypothesized to be at least in part automatic as well as of those thought to be more controlled. These results also suggest that to fully characterize neural organization for language in native speakers it is necessary to include participants of varying proficiency.