Recent studies have shown that when bilinguals or multilinguals read written words, listen to spoken words, or plan words that they intend to speak in one language alone, information in all of the languages that they know is momentarily active. That activation produces cross-language competition that sometimes converges to facilitate performance and sometimes diverges to create costs to performance. The presence of parallel activation across languages has been documented in comprehension, in studies of word recognition, and also in production, in studies of lexical speech planning. The observation that one of the two or more languages cannot be switched off at will is particularly surprising in production, where the intention to express a thought should be guided by conceptually driven processes. Likewise, in comprehension, recent studies show that placing words in sentence context in one language alone is insufficient to restrict processing to that language. The focus of current research on the multilingual lexicon is therefore to understand the basis of language nonselectivity, to consider how the language in use is ultimately selected, and to identify the cognitive consequences of having a lexical system that is open to influence by the languages not in use. In this article, we review the recent cognitive and neural evidence on each of these issues, with special consideration to the question of how the nature of the evidence itself shapes the conclusions drawn about the organization and access to the lexicon in individuals who speak more than one language.

Research treats divergences between monolingual and heritage grammars in terms of performance — ‘L1 attrition’, e.g. lexical retrieval — or competence — ‘incomplete acquisition’, e.g. lack of overt tense markers (e.g. Montrul 2008, Polinsky 1995, Schmid 2010, Sorace 2004). One classic difference between monolingual and Heritage German is reduction in morphological case in the latter, especially loss of dative marking. Our evidence from several Heritage German varieties suggests that speakers have not lost case, but rather developed innovative structures to mark it. More specifically, Heritage German speakers produce dative forms in line with established patterns of Differential Object Marking (Bossong 1985, 1991; Aissen 2003), suggesting a reallocated mapping of case. We take this as evidence for innovative reanalysis in heritage grammars (Putnam & Sánchez 2013). Following Kamp and Reyle (1993) and Wechsler (2011, 2014), the dative adopts a more indexical discourse function, forging a tighter connection between morphosyntax and semantic properties. Moribund grammars deploy linguistic resources in novel ways, beyond narratives of ‘attrition’ and ‘incomplete acquisition’.

Morphological brain changes as a consequence of new learning have been widely established. Learning a second language (L2) is one such experience that can lead to rapid structural neural changes. However, still relatively little is known about how levels of proficiency in the L2 and the age at which the L2 is learned influence brain neuroplasticity. The goal of this study is to provide novel evidence for the effect of bilingualism on white matter structure in relatively proficient but late L2 learners who acquired the second language after early childhood. Overall, the results demonstrate a significant effect on white matter fractional anisotropy (FA) as a function of L2 learning. Higher FA values were found in a broad white matter network including the anterior thalamic radiation (ATR), the inferior fronto-occipital fasciculus (IFOF), the Uncinate Fasciculus (UF), and the inferior longitudinal fasciculus (ILF). Moreover, FA values were correlated with age of L2 acquisition, suggesting that learning an L2, even past childhood, induces neural changes. Finally, these results provide some initial evidence that variability in the age of L2 acquisition has important consequences for neural plasticity.

Healthy non-pathological aging is characterized by cognitive and neural decline, and although language is one of the more stable areas of cognition, older adults often show deficits in language production, showing word finding failures, increased slips of the tongue, and increased pauses in speech. Overall, research on language comprehension in older healthy adults show that it is more preserved than language production. Bilingualism has been shown to confer a great deal of neuroplasticity across the life span, including a number of cognitive benefits especially in executive functions such as cognitive control. Many models of bilingual language processing have been proposed to explain bilingual language processing. However, the question remains open of how such models might be modulated by age-related changes in language. Here, we discuss how current models of language processing in non-pathological aging, and models of bilingual language processing can be integrated to provide new research directions.