In this chapter, we consider different definitions of bilingualism, underscoring the reality that there are various aspects which should be taken into account when investigating bilingualism, particularly when designing studies and choosing participants. Bilingualism is a complex construct and should be viewed on a continuum. Crucially, many key details about bilinguals’ backgrounds need to be reported in studies to make results comparable and clearly linkable to the specific study sample. Relative proficiency level seems to be the most influential factor, but it is by no means the only factor relevant for studying bilingualism. Rather, individual differences and their variability, dynamically related dimensions and their interaction over time, speech environment and their changes, language use habits, socioeconomic background, and so on have been reported to influence language processing and even brain function to some extent.

Because the majority of the world’s population speaks more than one language, in this chapter we consider that a “bilingual mental lexicon” can be viewed as the default, which may be a more accurate way of exploring lexical and conceptual memory. We transition to examine how concepts and words are represented in the mind, with a particular emphasis on the dynamic, developmental nature of word-to-concept mapping and the distributed, overlapping characteristics of semantic representations. Against this background, we review several theoretical models of word processing – including word recognition, production, and translation. The first set of models includes the Word Association Model and the Concept Mediation Model. We then discuss another set of models that offer a more detailed account of the conceptual system by emphasizing the degree of overlap that exists between conceptual representations in the two languages. In the last section of the chapter, we review two localist-connectionist models: the BIA/BIA+/BIA-d and the Multilink Models.

This chapter presents criteria characterizing the “bilingual experience” examined on three different levels: language processing, cognitive processing, and structural and functional changes in the brain. On all three levels, numerous studies have been conducted and have yielded inconclusive results. A bilingual experience is a change-inducing event leading to speedy adaptations on different levels of processing, with brain changes at its basis to accommodate for additional demands and specific requirements which are dependent on the length and intensity of the bilingual experience. A surge of proposals on how to measure the bilingual experience has recently appeared in the literature. The brain adapts from early on, even in infants, allowing for early indications of the effects of bilingual experience, in particular on perception and attentional aspects. The experience-dependent alterations in the brain at various locations, intensities, and timelines seem to align with our current understanding of the cognitive neuroscientific effects of bilingualism much more than previous views of separate brain areas involved in the processing and representation of each language.

In this chapter, we review work on a range of bilingual aphasias – the language impairments that occur due to a lesion or atrophy in the brain. We begin the chapter by discussing two theoretical approaches, namely the localizationist and dynamic accounts, which explain the extent to which one or both languages are affected. We see that the severity and type of aphasia that result from a lesion depends on its size and location. Additionally, a number of other nonlesion factors can affect the severity of the resulting aphasia. The premorbid variables that have been most studied are frequency of language use, AoA, and L2 proficiency. We then address how bilinguals are assessed for aphasia (e.g., the Bilingual Aphasia Test), and the possibilities for treatment and recovery. In this regard, the following important questions are discussed: In which language(s) should treatment be provided and what are the differential outcomes? If rehabilitation is given in only one language, is there cross-language generalization to the language not treated? We conclude that the most common pattern is parallel recovery in which both languages improve at a similar rate.

In this chapter, we reinforce the book’s aim to shed light on changes inflicted on language, cognition, and the brain rather than to focus on advantages and disadvantages of being bilingual. To obtain a more realistic picture of bilingualism, its assets (i.e., what is easier), and its difficulties (i.e., what is taxing and leads to high consumption of mental resources), we have drawn on research from various disciplines. We conclude the book by identifying complexity as the major issue for research on bilingualism. The complexity problem is fundamental to definitions of bilingualism and the characterization of bilingual participants in empirical studies, leading to discussions about its assessment as a dichotomous or continuous variable. Considering bilingualism as an experience and how such experience impacts overall language development, cognition, and the brain at different levels are related to usage-based approaches of examining bilingualism as well as a concern regarding confounding and moderating variables. The shift for designing research in the field of bilingualism seems to necessarily be more interdisciplinary in nature than in the past.

This chapter distinguishes studies on the mind from studies investigating the brain. By describing linguistic, psychological, and cognitive neuroscience approaches, some aspects which have been found to be relevant for bilingualism have only been studied in linguistic terms, leaving open whether certain findings are limited to the mind level only or whether there is any correspondence on the brain level. Other aspects, well researched in linguistic or psychological studies, have not yet been taken up in neuroscientific studies, leaving the question of whether certain variables would change the results or explain variance unanswered. We then look at details of learning in the brain and in particular at brain plasticity, a lifelong available characteristic of the brain. The chapter also addresses the notion that for linguists, acquisition and learning are not the same, since context in general and factors such as input quality and quantity must be taken into account. In brain terms, there is only learning. We then discuss different factors influencing language acquisition and learning and reveal that individual differences can be found among these factors to a great extent.

In this chapter, we explore neural representations and language processing in the bilingual brain. We begin by discussing key language areas and lateralization in bilinguals and look at some of the primary functions of the hemispheres, noting that, in fact, many brain functions – including bilingual language processing – are distributed across both hemispheres. We then consider the function of the four lobes of the brain and identify important regions for language, including Broca’s and Wernicke’s areas, which have been found to support language production and comprehension, respectively. The chapter then addresses how early or late exposure to a second language (L2) affects the cortical representation of the two languages. We then turn to specific processes of language use, namely how bilinguals comprehend and produce language. Overall, we have seen that the neural representations and processing of two languages are far from being fully understood. Nonetheless, we are gaining a clearer picture of the dynamic nature of two languages in one brain and the many individual differences that can affect this, such as age of acquisition (AoA) and proficiency.

For bilinguals, the use and knowledge of one language affects how they process the other. Various cross-linguistic influences (CLI) can be observed in both language production and comprehension across all domains of linguistics. We start by broadly exploring the concept of transfer, both negative and positive, and forward and reverse. In doing so, we identify various classifications of CLI. Following this, we review key studies on phonological, lexical, morphological, and syntactic transfer along with other types such as discursive, pragmatic, and sociolinguistic. We find that there are a number of factors that can determine the degree to which transfer emerges or whether it happens at all. We then review studies investigating the ability to switch between the two language systems. In doing so, we look at theoretical models that explain what facilitates language switching and what empirical studies tell us about the neural and electrophysiological activity that arises in language switching. Finally, we discuss dreaming and bilingualism and argue that dreaming in an L2, contrary to popular belief, does not necessarily imply fluency in that language.

In this chapter, we report on the effects of language learning in adults beyond the language system. We discuss the hypothesis that knowing and practicing multiple languages has an effect on cognition, in particular on executive functions, and the hypothesis that in the bilingual brain, both languages are always active. Current hypotheses on developed cognitive control abilities in bilingual speakers is presented. The final part of the chapter is dedicated to recent research on the protective effects of bilingualism for aging and its contribution to recovery from bilingual aphasia due to brain trauma.