This thesis investigates whether there are differences in language comprehension and language production between highly advanced/near-native adult learners of a second language (late L2ers) and native speakers (L1ers), and if so, how they should be characterised. In previous literature (Sorace & Filiaci 2006, Sorace 2011 inter alia), nonconvergence of the near-native grammar with the native grammar has been identified as most likely to occur at the interface between syntax and another cognitive domain. This thesis focuses on grammatical and ungrammatical representations at the syntax-lexicon interface between very advanced/near-native Dutch learners of English and native speakers of English. We tested differences in syntactic knowledge representations and real-time processing through eight experiments. By syntactic knowledge representations we mean the explicit knowledge of grammar (specifically word order dependence on lexical-semantics) that a language user exhibits in their language comprehension and production, and by realtime processing we mean the language user’s ability to access implicit and explicit knowledge of grammar under time and/or memory constraints in their language comprehension and production. To test for systematic differences at the syntax-lexicon interface we examined linguistic structures in English that differ minimally in word order from Dutch depending on the presence or absence of certain lexical items and their characteristics; these were possessive structures with animate and inanimate possessors and possessums in either a prenominal or postnominal construction, preposed adverbials of location (locative inversions) followed by either unergative or unaccusative verbs, and preposed adverbials of manner containing a negative polarity item (negative inversions) or positive polarity item followed by either V2 or V3 word order. We used Magnitude Estimation Tasks and Speeded Grammaticality Judgement Tasks to test comprehension, and Syntactic Priming (with/without extra memory load) and Speeded Sentence Completion Tasks to test production. We found evidence for differences in comprehension and production between very advanced, near-native Dutch L2ers and native speakers of English, and that these differences appear to be associated with processing rather than with competence. Dutch L2ers differed from English L1ers with respect to preferences in word order of possessive structures and after preposed adverbials of manner. However, these groups did not differ in production and comprehension with respect to transitivity in locative inversions. We conclude that even among highly advanced to near-native late learners of a second language there may be non-convergence of the L2 grammar. Such non-convergence need not coincide with the L1 grammar but may rather be a result of over-applying linguistic L2 knowledge. Thus, very advanced to near-native L2ers still have access to limited (meta)linguistic resources that under time and memory constraints may result in ungrammatical language comprehension and/or production at the syntax-lexicon interface. In sum, in explaining interface phenomena, the results of this study provide evidence for a processing account over a representational account, i.e. Dutch L2ers showed they possess grammatical knowledge of the specific L2 linguistic structures in comprehension and production, but over-applied this knowledge in exceptional cases under time and/or memory pressure. We suggest that current bilingual production models focus more on working memory by including a separate memory component to such models and conducting empirical research to test its influence on L2 production and comprehension.

Language acquisition research frequently concerns itself with linguistic development and result of the acquisition process with respect to a first or subsequent language. For some, it seems tacitly assumed that a first language, once acquired, remains stable, regardless of exposure to and the acquisition of additional language(s) beyond the first one in childhood. Research on language attrition (language loss) questions the validity of this assumption and raises questions that will not only help in describing and explaining the nature of linguistic attrition, but also shed light on the mental (cognitive) representation of human language. The goal of this dissertation is to contribute to the general program of research that investigates possible domains of first language attrition and its cause(s). More specifically, I endeavor to test the predictions and theoretical tenability of the Interface Hypothesis (Sorace and Filiaci 2006) as applied to language attrition (e.g. Tsimpli et al 2004). The Interface Hypothesis claims that certain linguistic properties, namely those at external interfaces such as the syntax/discourse interface, are especially vulnerable to optionality in language acquisition (see Sorace and Serratrice 2009). For attrition, it predicts that, upon sufficient exposure, linguistic properties that are dependent on interfaces between the linguistic computational system and external domains of cognition (such as pragmatics and discourse structure) are more vulnerable to erosion than those that lie internally to the linguistic system (e.g. syntax/semantic interface) or those that are purely syntactic in nature. Within this framework, attrition is hypothesized to either be due to direct interference from the L2 or due to linguistic processing deficits that are a byproduct of being bilingual. The comprehensive nature of this case study, which tests the L1 grammar of an adult native speaker of Spanish after 25 years of uninterrupted naturalistic exposure to Brazilian Portuguese across the different property types, not only allows for an examination of possible domains of attrition (e.g. external interfaces, internal interfaces, syntax) but also allows for teasing apart of the cause of attrition by combining both untimed and timed methodologies. Although the main focus of this dissertation is to test the limits and explanatory value of the Interface Hypothesis, the data will also be examined in light of other theories such as Paradis' (2004) Activation Threshold Hypothesis and Jakobson's (1940) Regression Hypothesis to determine the extent to which various theories might best explain the data to be obtained.

Linguists studying language acquisition often assume that once a first language is fully acquired, its mental linguistic representation remains constant and stable. Observations of native language attrition due to the influence of a dominant second language have led researchers to rethink the nature of the first language and consider the possibility that the mental representation of our first language may not be completely stable. The purpose of this dissertation is to investigate specific areas of the first language that may be particularly vulnerable to L1 attrition if exposed to a dominant L2. I test Sorace's (2003) Vulnerable Interface Hypothesis, and propose and test the Structural Overlap Hypothesis. The Vulnerable Interface Hypothesis for first language attrition claims that linguistic properties located in the interfaces between the linguistic computational system and external domains (e.g. discourse or pragmatics) are particularly vulnerable to attrition, while internal interfaces (e.g. the syntax-semantics interface) are only somewhat vulnerable to attrition. The domain of narrow syntax is assumed to remain stable unless the L1 begins to attrite in childhood (Montrul, 2008). The Structural Overlap Hypothesis assumes that properties which exhibit structural overlap between the L1 and L2 are more vulnerable to L1 attrition. The predictions of both hypotheses are tested using 15 L1 German adult attriters whose dominant L2 is English, in order to observe the degree of stability of the linguistic system in adult onset bilinguals. Four linguistic properties of German are examined, which are grouped in two pairings of a purely syntactic property with a grammatically related interface property. 15 monolingual L1 German speakers and 15 monolingual L1 English speakers serve as controls. The data obtained also shed light on a frequently debated question of attrition research, viz. whether L1 attrition is due to transfer from the L2, or a decrease in the linguistic processing capacity due to competition of a dominant L2, or both.