The present study investigated the role of proficiency in late second-language (L2) processing using comparable stimuli in German and Italian. Both sets of stimuli consisted of simple active sentences including a word category violation, a morphosyntactic agreement violation, or a combination of the two. Four experiments were conducted to study high- and low-proficiency L2 learners of German as well as high- and low-proficiency L2 learners of Italian. High-proficiency L2 learners in both languages showed the same event-related potential (ERP) components as native speakers for all syntactic violations. For the word category violation, they displayed an early anterior negativity (ELAN), an additional negativity reflecting reference-related processes, and a late P600 evidencing processes of reanalysis. For the processing of the morphosyntactic error, an anterior negativity (LAN) and a P600 were observed, whereas for the combined violation, the same ERP components were found as in the pure category violation. In high-proficiency L2 learners, the timing of the processing steps was equivalent to that of native speakers, although some amplitude differences were present. Low-proficiency L2 learners, however, showed qualitative differences in the agreement violation characterized by the absence of the LAN and quantitative differences reflected in a delayed P600 in every violation condition. These findings emphasize that with a high proficiency, late L2 learners can indeed show native-like neural responses with the timing approximating that of native speakers. This challenges the idea that there are fundamental differences in language processing in the brain between natives and late L2 learners.
