Previous research has shown L1 attrition to be selective (Gürel 2004) and often restricted to structures at the interfaces between syntax and context/pragmatics, but not to occur with syntactic properties that do not involve such interfaces (Interface Hypothesis, Sorace & Filiaci 2006). This is supported by many studies exploring cross-linguistic influence effects in interface structures, such as the production and/or interpretation of null versus overt pronominal subjects, not only in L1 attriters (Tsimpli et al. 2004, Montrul 2004) but also in other bilingual groups with different language combinations, such as early bilinguals (Paradis & Navarro 2003, Sorace et al. 2009), and advanced late bilinguals (Belletti et al. 2007, Rothman 2009). The current hypothesis is that individual L1 attrition affects only the ability to process interface structures but not knowledge representations themselves (Sorace 2011). In this thesis, we first compared a well-studied syntax-pragmatics interface phenomenon (pronominal subjects in Spanish) with a non-interface structure (the Spanish personal preposition a, also known as Differential Object Marking, DOM). In Spanish, the distribution of null and overt subject pronouns is pragmatically constrained, whereas the presence of the preposition just depends on the animacy and specificity of the direct object. Participants included a group of attrited speakers of L1 Spanish who had been living in the UK for a minimum of 5 years, and a group of Spanish monolinguals. Using a naturalness judgment task and eye tracking while reading, participants were presented with anaphoric sentences in which number cues matched or mismatched predicted antecedent preferences (i.e. null pronoun: subject preference; overt pronoun: object preference). The DOM study also used a mismatch paradigm, crossing preposition presence (al vs. el) with animacy, where an animate object requires the prepositional form al and an inanimate object requires the article el. Offline ratings revealed equal mismatch sensitivity for both groups of participants with both structures. However, eye-tracking measures showed that monolinguals were reliably more sensitive than attriters to the pronoun mismatch, while both groups showed equal on-line sensitivity to the DOM mismatch, which reveals that attrition affects interface structures, but not non-interface structures. Second, we investigated the effects of recent (re)exposure to L1 input on attrition. A second group of attriters carried out the same experiment after having been exposed exclusively to Spanish in a monolingual Spanish-speaking environment for a minimum of a week. Their eye-tracking results patterned with the monolingual group. This novel manipulation shows that attrition effects decrease as a result of L1 exposure, which reveals that bilinguals are sensitive to input changes and that attrition affects online sensitivity rather than causing a permanent change in speakers’ L1 grammatical representations.

This thesis is based upon a longitudinal study of L1 attrition in two bilingual teenage siblings, J and her older brother S, with Korean as their L1 and English as their L2. The two teenagers' initial exposure to English occurred at the age of 9. When the two siblings began to have sustained exposure to and immersion in an English L2 environment, they were at the postpubertal age. They had been attending high school in a Mid-Western city in the U.S for about two years when the study was conducted. The longitudinal study addressed three main questions. The first question concerned the siblings' language (L1 and L2) development, language dominance and cross-lingusitic influence from a bidrectional perspective. The second question examined the evidence for L1 attrition in relation to lexis, morphology and syntax, and their relative levels of vulnerability. The third question examined the role of extra-linguistic factors in the process of L1 attrition. In order to address the above questions, the two siblings were observed in their home over a period of 8 months. The data for the longitudinal study included the siblings' (i) spontaneous speech interactions in the home (ii) oral narratives in the L1 and L2 and (iii) their responses to a language background questionnaire. The results showed that the two siblings' English L2 developed and improved over time, but that they continued to maintain their L1 (Korean) as their dominant language. The evidence from both siblings' L1 and L2 use supported bidirectional cross-linguistic influence (i.e. from the L1 onto the L2 and from the L2 onto the L1). L1 attrition occurred only minimally in relation to morphology (e.g. honorification, case particles, classifiers, and plural marking) and lexical choice, while the siblings' L1 syntax remained relatively stable over time. Overall, the siblings were largely successful in maintaining their L1. In part, this was because their L1 was already firmly established or entrenched, as their immersion in an L2 environment occurred only when they were older (around puberty). Furthermore, their frequent use of their L1 for social networking, along with their positive attitudes toward their heritage language, also played a crucial role in maintaining and stabilizing their L1. Finally, the theoretical and practical implications of the findings of the present study, as well as recommendations for the future research are discussed.

L1 attrition - which in the context of this study is defined as the decline of any native language skill (or portion thereof) in a healthy bilingual speaker (Ecke, 2004) - has been studied extensively for several decades. However, only few studies have examined the native speech of immigrants who use their L1 for professional purposes, such as language instructors (Isurin, 2007; Major, 1992; Porte, 1999, 2003). Furthermore, no research has been conducted comparing the L1 speech of such individuals with that of individuals who do not use their L1 for professional purposes. This study analyzed and compared L1 speech samples from two populations of German immigrants in the U.S., German Instructors and Other Professionals, and from a monolingual control group in Germany. It was hypothesized that German instructors may be less vulnerable to L1 attrition due to more frequent L1 use, a higher motivation to maintain the L1, and greater identification with the native language and culture. Data elicited through verbal fluency tasks, a film retelling task, a semi-structured interview, and a sociolinguistic questionnaire revealed significant differences between the control group and the two bilingual groups which point to L1 attrition (primarily as an access problem) in the bilingual speakers. The data also revealed significant differences between the German Instructors and the Other Professionals, suggesting that the severity of L1 attrition is not the same for all populations. In addition, a comparison of the two bilingual groups with regard to sociolinguistic variables, and correlations between linguistic measures and sociolinguistic variables also yielded interesting findings which have implications for L1 attrition research as well as L1 maintenance.

This thesis examines the L2 acquisition and L1 attrition of aspectual properties in Spanish. Specifically, it investigates preterite versus imperfect distinctions and the ongoing value of the Spanish present tense among English-speaking L2 learners and long-term Spanish immigrants. In contrast to previous research which explains L2 learners’ difficulties as a result of either maturational constraints or morphosyntactic development, this study provides a supplementary explanation focusing on L1 transfer of the semantic patterns of tense morphemes. The study proceeds by comparing data from L2 learners with that of long-term Spanish immigrants. Unlike what is argued for L2 acquisition difficulties, the difficulties immigrants may have with tense and aspect cannot stem solely from impairment reasons. These immigrants acquired the L2 as adults. Therefore, to the extent that L2 learners share similar patterns of errors with adult immigrants, L2 speakers’ difficulties cannot be unequivocally linked to causes related to impairment. Instead, following a selectional approach to aspectual variation (De Swart, 1998), it is argued that transfer of the selectional patterns of tense morphemes offers a more encompassing explanation of the difficulties L2 speakers face with tense and aspect. Data collection involved two truth-value judgment tasks, two acceptability judgment tasks and two elicited production tasks. Twenty long-term immigrants, twenty English-speaking L2 learners of Spanish, and twenty native speakers of Spanish serving as control participants took part in the study. Results show incorrect activation of aspectual patterns by both experimental groups and similar patterns of difficulties in some of the conditions under investigation. I conclude that transfer from the other language offers a more adequate explanation of the difficulties L2 learners face

This thesis examines the L2 acquisition and L1 attrition of aspectual properties in Spanish. Specifically, it investigates preterite versus imperfect distinctions and the ongoing value of the Spanish present tense among English-speaking L2 learners and long-term Spanish immigrants. In contrast to previous research which explains L2 learners’ difficulties as a result of either maturational constraints or morphosyntactic development, this study provides a supplementary explanation focusing on L1 transfer of the semantic patterns of tense morphemes. The study proceeds by comparing data from L2 learners with that of long-term Spanish immigrants. Unlike what is argued for L2 acquisition difficulties, the difficulties immigrants may have with tense and aspect cannot stem solely from impairment reasons. These immigrants acquired the L2 as adults. Therefore, to the extent that L2 learners share similar patterns of errors with adult immigrants, L2 speakers’ difficulties cannot be unequivocally linked to causes related to impairment. Instead, following a selectional approach to aspectual variation (De Swart, 1998), it is argued that transfer of the selectional patterns of tense morphemes offers a more encompassing explanation of the difficulties L2 speakers face with tense and aspect. Data collection involved two truth-value judgment tasks, two acceptability judgment tasks and two elicited production tasks. Twenty long-term immigrants, twenty English-speaking L2 learners of Spanish, and twenty native speakers of Spanish serving as control participants took part in the study. Results show incorrect activation of aspectual patterns by both experimental groups and similar patterns of difficulties in some of the conditions under investigation. I conclude that transfer from the other language offers a more adequate explanation of the difficulties L2 learners face

The present study aims to capture linguistic variation in subject distribution of two typologically similar languages, Greek and Chilean Spanish, considering adult monolingual and bilingual speakers of Greek as a heritage/minority language in Chile. The focus is on null and overt third-person subjects in topic-continuity and topic-shift contexts. Such structures involve the interface between syntax and discourse/pragmatics, a vulnerable domain in bilingualism. Previous research has shown overextension of the scope of the overt subject pronoun in contexts where null subjects are discursively expected (e.g. Tsimpli, Sorace, Heycock & Filiaci 2004). The Interface Hypothesis (IH) (Sorace 2011) was formulated to account for such findings, which obtain even in pairs of two null subject languages (Sorace, Serratrice, Filiaci & Baldo 2009). The key question as to the language-contact effects on subject distribution in pairs of two null subject languages requires further exploration while the combination of Greek and Spanish has been so far understudied. The IH is evaluated with new empirical data from a bilingual situation not studied before. Data from oral narratives and aural pronominal anaphora resolution were elicited from monolinguals and three types of bilinguals, namely first-generation immigrants, heritage speakers and L2 speakers of Greek residing in Chile. The monolingual data revealed differences in the use and interpretation of overt subject pronouns between Greek and Chilean Spanish. The crosslinguistic difference lies in the strong deictic properties of the Greek pronoun compared to its Spanish counterpart; hence differences obtain because of the relative strength of the two pronominal forms. No overextension of the scope of overt pronouns was found in bilinguals, against predictions stemming from the Interface Hypothesis. This may relate to the typological similarity between Greek and Spanish as well as to the nature of the Greek pronoun, which makes its use relatively categorical. Such findings lend support to the Representational account (Tsimpli et al. 2004). On the contrary, null subjects gave rise to optionality presumably due to their complexity, which demands higher degrees of computational efficiency. The Vulnerability Hypothesis (Prada Pérez 2018) may also account for the findings.