First- and Second-language Phonological Representations in the Mental Lexicon Nu ´ria Sebastian-Galle ´s 1,2 , Antoni Rodrı ´guez-Fornells 2,3 , Ruth de Diego-Balaguer 4 , and Begon ˜a Dı ´az 1,2 Abstract & Performance-based studies on the psychological nature of linguistic competence can conceal significant differences in the brain processes that underlie native versus nonnative knowl- edge of language. Here we report results from the brain activity of very proficient early bilinguals making a lexical decision task that illustrates this point. Two groups of Spanish–Catalan early bilinguals (Spanish-dominant and Catalan-dominant) were asked to decide whether a given form was a Catalan word or not. The nonwords were based on real words, with one vowel changed. In the experimental stimuli, the vowel change in- volved a Catalan-specific contrast that previous research had shown to be difficult for Spanish natives to perceive. In the control stimuli, the vowel switch involved contrasts common to Spanish and Catalan. The results indicated that the groups of bilinguals did not differ in their behavioral and event-related brain potential measurements for the control stimuli; both groups made very few errors and showed a larger N400 com- ponent for control nonwords than for control words. However, significant differences were observed for the experimental stimuli across groups: Specifically, Spanish-dominant bilinguals showed great difficulty in rejecting experimental nonwords. Indeed, these participants not only showed very high error rates for these stimuli, but also did not show an error-related negativity effect in their erroneous nonword decisions. However, both groups of bilinguals showed a larger correct- related negativity when making correct decisions about the experimental nonwords. The results suggest that although some aspects of a second language system may show a re- markable lack of plasticity (like the acquisition of some foreign contrasts), first-language representations seem to be more dynamic in their capacity of adapting and incorporating new information. & INTRODUCTION Learning a second language as an adult is a difficult task. In fact, it is so hard that most human beings, in spite of their efforts, fail to attain native performance levels. Among the most difficult aspects of a second language to be mastered is its sound system. It is very difficult (maybe impossible) to both perceive and produce a foreign language with a native accent. Common experi- ence indicates the prevalence of these difficulties, in par- ticular when the second language is learned late in life. However, training studies with adult participants have shown that significant improvements are possible with very low proficiency, or even monolingual, participants (see, for reviews, Sebastia ´n-Galle´s, 2005; Sebastia ´n- Galle´s & Kroll, 2003; Strange, 1995). The existence of brain plasticity for speech sounds has been attested in a range of studies addressing the question of learning difficult L2 phonemic contrasts. In most of these studies, along with behavioral measures, the mismatch negativity (MMN) component has been used as an index of phonological discrimination. This measure shows a high sensitivity to the physical properties of the stimuli and, interestingly, it is observed in the absence of conscious realization of the contrast. Several studies have revealed significant differences (an increase in the amplitude) when the MMN is elicited in the presence of a linguistic (phonological) contrast, when compared with a foreign linguistic phonetic contrast (Sharma & Dorman, 2000; Na ¨a ¨ta ¨nen et al., 1997; Phillips et al., 1995). In addition, several training studies have shown that electrophysio- logical changes can be induced through short but in- tensive programs (McClelland, Fiez, & McCandliss, 2002; Tremblay, Kraus, & McGee, 1998; Tremblay, Kraus, Carrell, & McGee, 1997). One aspect of the Tremblay, Kraus, and McGee (1998) study was that significant changes in the electrophysiological signatures were observed, before any behavioral improvement could be detected. This result could be taken as an indication that although no behavioral discriminations may be observed for nonnative contrasts, they may actually be detected at a subconscious level, and so behavioral measures may 1 GRNC, Parc Cientı ´fic Universitat de Barcelona & Hospital Sant Joan de De ´u, Spain, 2 Universitat de Barcelona, Spain, 3 Institucio ´ Catalana de Recerca i Estudis Avanc ¸ats (ICREA), Spain, 4 E ´ cole Normale Supe´rieure, France D 2006 Massachusetts Institute of Technology Journal of Cognitive Neuroscience 18:8, pp. 1277–1291