*Do first language neural processes for morphosyntax transfer to the second language? * *Using event-related potentials to expand the evidence of cross-linguistic influence* Irene Finestrat, David Abugaber, Alicia Luque & Kara Morgan-Short (University of Illinois at Chicago) Ifines2@uic.edu This study examines whether individual differences in first language (L1) processing of morphosyntax transfer to second language (L2) using event-related potentials (ERPs) as a measure of language processing. Transfer of L1 skills to the L2 is well attested behaviorally (Sparks, 2012). However, whether L1 processing transfers to an L2 is relatively unknown. The present study brings together two areas of research to examine this question. First, the field of second language acquisition aims to account for variability in L2 performance and processing. Individual differences in ERP responses elicited during L2 grammatical processing have been explained by a variety of factors (Morgan-Short, 2014). For example, whether L2 learners elicit a N400 or P600 dominant response to L2 grammar has been accounted for by motivation and age of arrival to the L2 context (e.g., Tanner, Inoue, & Osterhout, 2014). Second, recent research in L1 processing has evidenced quantitative and qualitative differences among native speakers (e.g., Pakulak & Neville, 2010). For example, L1 speakers have also been found to vary in whether they show a P600 or N400 dominant response to L1 grammatical processing (e.g., Tanner & van Hell, 2014). Given the evidence for individual differences in both L1 and L2 processing, we explore whether individuals transfer their dominant processing strategy for L1 morphosyntax, i.e., either a P600 or N400, to L2 morphosyntax at early stages of learning. Participants were 42 native speakers of English who were L2 learners of Spanish enrolled in third and fourth semester college classes. ERP data were collected while they judged the grammaticality of 122 experimental sentences, along with 122 of filler sentences, in English and Spanish in two counterbalanced sessions. Experimental sentences were designed to assess processing of morphosyntactic agreement (see Table 1). First, for a subject-verb agreement condition, 60 sentences consisted of third-person singular and plural correct and violation sentences. Similarly, for a determiner-noun agreement condition, 62 correct and violation sentences contained singular and plural noun phrases. The English stimuli were translations of the Spanish sentences, but no participant was exposed to the same sentence in both languages as the stimuli were divided across lists. Participants read the sentences, one word at a time, and were asked to judge the grammaticality of the sentences using a response box. EEG data was processed following standard lab procedures. Then mean amplitude averages were calculated for each individual in standard N400 and P600 time windows, 300-500ms and 600-900ms, respectively. At the group level, repeated ANOVAS were performed to examine grand average ERP responses. In the L1, results revealed a P600 effect to both agreement conditions. However, no language-related ERP effects were evidenced for L2 at the group level. At the individual level, each individual’s N400 and P600 effect magnitudes were calculated in a central-parietal region of interest (electrodes C4, Cz, C3, P4, Pz, P3) in the time windows indicated above (see Figure 1 caption). Results revealed variability in the distribution of brain response dominance for both L1 and L2 morphosyntactic processing (see Figure 1) with a continuum of responses ranging from negative-dominant (larger N400) to positive-dominant (larger P600). In order to address the research question, we calculated each participant’s response dominance index (RDI, Tanner, Inoue, & Osterhout, 2014; Tanner & van Hell, 2014; Tanner, 2019), a measure of their relative response dominance (either N400 or P600 dominant). Correlation analyses between English and Spanish RDIs revealed that participants’ dominant ERP responses to grammatical processing were significantly correlated across languages, both for the SV (*r* = .75, *p *= .00) and the NP condition (*r* = .36, *p *= .02, see Figure 2). Results suggest that ERP response dominance is a stable trait, transferrable across languages, thereby constituting a source of individual differences in morphosyntactic processing, regardless of the language. Based on this pattern of results, we conclude that cross-linguistic influence also occurs at the processing level. *Table 1.* Grammaticality Judgment Task stimuli Condition Correct Control Violation SV agreement La mujer *dibuja* en su habitación. The lady *draws* in her bedroom. *La mujer *dibujan* en su habitación. *The lady *draw* in her bedroom. NP agreement La profesora enseña un *curso* este semestre. The professor teaches a *course* this semester. *La profesora enseña unos *curso* este semestre. *The professor teaches a few *course* this semester. *Note*. Italics indicate the critical word in each sentence. Violation sentences are marked with an asterisk *English (L1) * *Spanish (L2)* *Figure 1.* Distribution of N400 (grammatical minus ungrammatical) and P600 (ungrammatical minus grammatical) effect magnitudes (following Tanner & van Hell, 2014) across participants in English and Spanish over both conditions. Each dot represents one participant. The line represents equal N400 and P600 effect magnitudes: Individuals to the left of the line primarily showed an N400 effect, while individuals to the right primarily showed a P600 effect. *Subject Verb Agreement* *Noun Phrase agreement* Spanish RDI English RDI Spanish RDI English RDI *Figure 2.* Correlation between RDIs in English and Spanish for the SV (left) and NP (right) conditions. *REFERENCES* Morgan-Short, K. (2014). Electrophysiological approaches to understanding second language acquisition: A field reaching its potential.* Annual Review of Applied Linguistics, 34*, 15-36. Pakulak, E., & Neville, H. J. (2010). Proficiency differences in syntactic processing of monolingual native speakers indexed by event-related potentials.* Journal of Cognitive Neuroscience, 22*(12), 2728-2744. doi:10.1162/jocn.2009.21393 Sparks, R. L. (2012a). 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