Confusing similar words: ERP correlates of lexical-semantic processing in first language attrition and late second language acquisition.

First language (L1) attrition is a socio-linguistic circumstance where second language (L2) learning coincides with changes in exposure and use of the native-L1. Attriters often report experiencing a decline in automaticity or proficiency in their L1 after a prolonged period in the L2 environment, while their L2 proficiency continues to strengthen. Investigating the neurocognitive correlates of attrition alongside those of late L2 acquisition addresses the question of whether the brain mechanisms underlying both L1 and L2 processing are strongly determined by proficiency, irrespective of whether the language was acquired from birth or in adulthood. Using event-related-potentials (ERPs), we examined lexical-semantic processing in Italian L1 attriters, compared to adult Italian L2 learners and to Italian monolingual native speakers. We contrasted the processing of classical lexical-semantic violations (Mismatch condition) with sentences that were equally semantically implausible but arguably trickier, as the target-noun was "swapped" with an orthographic neighbor that differed only in its final vowel and gender-marking morpheme (e.g., cappello (hat) vs. cappella (chapel)). Our aim was to determine whether sentences with such "confusable nouns" (Swap condition) would be processed as semantically correct by late L2 learners and L1 attriters, especially for those individuals with lower Italian proficiency scores. We found that lower-proficiency Italian speakers did not show significant N400 effects for Swap violations relative to correct sentences, regardless of whether Italian was the L1 or the L2. Crucially, N400 response profiles followed a continuum of "nativelikeness" predicted by Italian proficiency scores - high-proficiency attriters and high-proficiency Italian learners were indistinguishable from native controls, whereas attriters and L2 learners in the lower-proficiency range showed significantly reduced N400 effects for "Swap" errors. Importantly, attriters and late L2 learners did not differ in their N400 responses when they belonged to the same proficiency subgroup. Attriters also showed an enhanced P600 response to both kinds of lexical-semantic anomalies, which we discuss as reflecting increased conflict-monitoring and conscious "second thought" processes specifically in attriters. Our findings provide some of the first ERP evidence of attrition effects, and are compatible with accounts of ongoing neuroplasticity for language in adulthood. Proficiency, rather than age-of-acquisition, seems to be the key factor in modulating certain neurocognitive responses, not only within L2 learners but also in L1 attriters.