A B S T R A C T Evidence shows that second language (L2) processing depends on the Age of Acquisition (AoA), proficiency and differences between L1 and L2 grammar. Here we focus on the influence of the latter factor on L2 processing. To this end, we tested early (AoA = 3 years) and highly proficient Spanish-Basque and Basque-Spanish bilinguals by means of Event-Related Potentials (ERPs) while processing noun morphology in Basque (Experiments 1 and 2) and Spanish (Experiments 3 and 4). Both

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... ioral and electrophysiological results revealed significant differences between L1 and L2 speakers: non-natives made more errors and elicited a smaller P600 for violations than natives when processing ergative and allative morphology in Basque and accusative, dative and allative in Spanish. These findings reveal that, even for early and highly proficient bilinguals, (a) L2 processing is modulated by L1 grammar and (b) native vs. non-native differences obtain only when L1 and L2 morphological categories differ but not otherwise.