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date: 01 October 2022

Bilingualism: A Cognitive and Neural View of Dual Language Experiencefree

Bilingualism: A Cognitive and Neural View of Dual Language Experiencefree

  • Judith F. KrollJudith F. KrollDepartment of Psychology, University of California, Irvine
  •  and Guadalupe A. MendozaGuadalupe A. MendozaUniversity of California, Irvine


There has been an upsurge of research on the bilingual mind and brain. In an increasingly multilingual world, cognitive and language scientists have come to see that the use of two or more languages provides a unique lens to examine the neural plasticity engaged by language experience. But how? It is now uncontroversial to claim that the bilingual’s two languages are continually active, creating a dynamic interplay across the two languages. But there continues to be controversy about the consequences of that cross-language exchange for how cognitive and neural resources are recruited when a second language is learned and used actively and whether native speakers of a language retain privilege in their first acquired language. In the earliest months of life, minds and brains are tuned differently when exposed to more than one language from birth. That tuning has been hypothesized to open the speech system to new learning. But when initial exposure is to a home language that is not the majority language of the community—the experience common to heritage speakers—the value of bilingualism has been challenged, in part because there is not an adequate account of the variation in language experience. Research on the minds and brains of bilinguals reveals inherently complex and social accommodations to the use of multiple languages. The variation in the contexts in which the two languages are learned and used come to shape the dynamics of cross-language exchange across the lifespan.


  • Cognitive Psychology/Neuroscience

What happens to language processes when individuals learn and use more than one language? In most places in the world, speaking two or more languages is common, suggesting that bilingualism is not a special condition but one that reflects the ordinary adaptations that individuals make as they are exposed to language. But the consequences of that exposure over the lifespan vary as a function of the age at which the two languages were acquired, the context in which they were learned and are used, and the cognitive resources available to the speaker (e.g., Kroll, Bice, et al., 2021; Kroll, Takahesu Tabori, et al., 2021). In the past three decades, there has been increasing interest and excitement about understanding how bilingualism might reveal the plasticity of the mind and the brain (see Bialystok, this volume; Bialystok, 2017; Pliatsikas, 2020). This article explores how language processes themselves change when two or more languages are in play. These two lines of research are related to one another, but it is only recently that studies have asked directly how language processing might engage cognitive resources and change as a result (e.g., Kroll & Rossi, in press; Salig et al., 2021). Here, bilingual language processing is examined as well as how these processes emerge in development and how the language processes and their consequences together impact how cognitive and neural mechanisms are recruited in ways that ultimately shape the bilingual mind and brain.

The last 30 years of research have provided robust evidence for the claim that the bilingual’s two languages are highly interactive, changing dynamically when they are in contact with one another, and sustaining the influence of the other language even when that language is not being used. The history of this work has been reviewed extensively (e.g., Dijkstra & Van Heuven, 2018; Kroll & Navarro-Torres, 2018; Kroll et al., 2015). For present purposes the focus is on those findings that have created the foundation for the ideas presented in this article.

Reading, Listening, and Speaking Two Languages

When bilinguals read or listen to words in one language, the other language comes online in parallel, producing joint activation even when individuals intend to use one language alone (e.g., Dijkstra, 2005; Shook & Marian, 2013). The activation of the language not in use has been observed regardless of the similarity of the two languages (e.g., Morford et al., 2011; Thierry & Wu, 2007), across all levels of proficiency (e.g., Kroll et al., 2002), and regardless of whether bilinguals are processing words in sentence context or in isolation (e.g., Schwartz & Kroll, 2006; Titone et al., 2011). The persistent finding of cross-language activation across conditions that might have otherwise been expected to reduce or eliminate the influence of the language not in use suggests that it is a basic feature of bilingualism. Even more surprising is that when bilinguals plan speech in one language alone, the other language is engaged, at times to the point of having the phonology of the unintended language on the tip of a speaker’s tongue (e.g.,Kroll & Gollan, 2014; Kroll et al., 2006). Because the initiation of spoken production is under the control of the speaker, the observation that the nontarget language is briefly available is counterintuitive, again suggesting that the co-activation of the two languages is not optional but a stable feature of the bilingual’s language system.

The logic of the many studies that have demonstrated the presence of cross-language activation has been to manipulate features of the relationship between the target language and the language not in use and ask whether that relationship holds any consequence for performance. For a person who is monolingual, the relationship to the other language should have no bearing on performance. Therefore, demonstrating that bilinguals do not behave like monolinguals would serve to show that bilinguals are taking the cross-language information into account. At the lexical level, this has been shown by asking bilinguals to recognize or produce cognates (translations that share similar word form as well as meaning) and interlingual homographs or false friends (words that share similar form across language but that have distinct meanings). Whether these cross-language pairs confer benefits or costs for bilinguals depends on the particular properties of the experimental design, but cognates generally facilitate recognition and production whereas interlingual homographs produce interference; these effects can be documented in measures of behavior and in measures of brain activity (e.g., Dijkstra et al., 1998; Midgley et al., 2011; Van Heuven et al., 2008). Likewise, there appears to be an implicit activation of the translations of words in the language not in use in measures of both behavior and brain activity (e.g., Meade et al., 2017; Morford et al., 2011; Thierry & Wu, 2007).

The initial research on cross-language activation was focused largely at the lexical level to examine the processes of word recognition and word production. Studies of grammar and phonology reveal a similar dynamic exchange across the bilingual’s two languages (e.g., Chang, 2012; Dussias & Sagarra, 2007; Goldrick et al., 2014; Hartsuiker et al., 2004). Although there are constraints on cross-language interactions that are shaped by linguistic features specific to each of the two languages (e.g., whether one of the languages is a tone language or whether one language marks grammatical gender and the other does not), the evidence is compelling in demonstrating that the bilingual’s two languages are dynamic—changing in response to one another and in response to the contexts in which they are used.

Cross-Language Transfer and Bidirectional Influence

Traditional accounts of second language learning have focused on the transfer from the first or more dominant language, the L1, to the second or less dominant language, the L2. Although a diverse set of mechanisms has been proposed to account for how knowledge in the L1, particularly at the level of the grammar, might enable or constrain L2 learning (MacWhinney, 2005; Pienemann et al., 2005), the underlying principle is that existing linguistic knowledge will be exploited to guide the acquisition of new information. There is also recognition that not only is there transfer from L1 to L2 but that the dynamic nature of the interactions across the two languages comes to produce transfer from the L2 to the L1 and that bidirectional transfer occurs as soon as L2 learning begins.

The evidence for the influence of the L2 on the L1 has been reviewed in a number of recent papers and chapters (e.g., Kroll et al., 2021). Briefly, it has shown that the processing of the native or dominant L1 is affected by knowledge of the L2, even when proficiency in the L2 is just beginning to develop. Bice and Kroll (2015) tested a group of young adult native English speaking students learning Spanish as an L2. Like many lexical studies on cross-language activation, they asked whether cognate status would influence performance in a word recognition task. Critically, the word recognition task, in this a case lexical decision (i.e., is the string of letters a real word?), was performed in English, the learners’ native language. In behavioral measures of reaction time and accuracy, there were no differences for either learners or monolingual English speakers for words that were cognates in English and Spanish and for words that were not. But in electrophysiological measures, there was a modulation of the N400 component of the Event Related Potentials (ERPs) that showed that the learners were differentially sensitive to words in English that were cognate translations of words in Spanish. The result suggests that even before learners become aware of the relationship between the two languages, there are changes in brain activity in the L1 that reflect the exposure to the L2. (See Brice et al., 2021 for a recent longitudinal study on L2 immersion that tracked brain activity over time using functional magnetic resource imaging [fMRI] and showed that the largest changes associated with increasing L2 proficiency were found for the L1.) Similar findings have been reported for L2 learning when assessing the L2 directly, with evidence that brain activity shows almost immediate changes in the context of L2 learning even when behavior for L2 is at chance and exposure to the L2 has been minimal (e.g., McLaughlin et al., 2004).

Demonstrating that the L1 is sensitive to the L2 is a first step in revealing the openness of the native language to L2 influence. But recent studies have gone beyond demonstrating sensitivity to show that the L1 changes dynamically as bilinguals move from one context to the next and as the demands of the environment impinge on language processing (e.g., Green & Abutalebi, 2013). In the final section of this article, this issue is revisited to consider what those contexts might look like and how bilinguals adapt to the demands of their environment. The implications of finding that the L2 affects the L1 are profound because they challenge assumptions about the native language and require that bilingualism be understood as a far more fluid process than how it has been previously conceptualized. Most traditional accounts of language processing and language acquisition attribute a privileged role to the native language, assuming that the first learned language early in life becomes a stable system that may influence the acquisition of new language knowledge but that itself is not necessarily changed by that process. The assumption has been that the goal of the L2 learning is to achieve a stable native-speaker level of proficiency. Recent studies have suggested that not only is the L1 changed, with the bilingual speaker different than monolingual speakers of either of their two languages (e.g., as per Grojean’s 1989 admonition that the bilingual is not two monolingual speakers in one), but that the L1 comes to be regulated to enable bilinguals to use the two languages appropriately in the face of varying contextual demands.

Much of the research that has contributed to an understanding of bilingual language processes and their neural basis has been conducted with young adults, many of whom acquired the L2 after early childhood, typically in school contexts. But much of the world is bilingual and, in the United States, the most common bilinguals are heritage speakers of a home language other than English (American Academy of Arts and Sciences, 2017). It is important to recognize that for bilingual speakers who have a home language that differs from the community language, there is often a switch of language dominance, from the home language in early childhood to the community language later in schooling and in adulthood. Early language exposure is quite varied and the long-term consequences that result, regardless of whether the dominant language changes or not, are not well understood. The experience of emerging bilingualism for individuals first exposed to two or more languages early in life is considered next.

Bilingualism During Infancy and Early Childhood

People become bilingual or multilingual in a multitude of ways, with experiences in early life proving to be significant determinants of language acquisition. The typical trajectory for young children who learn a language consists of being exposed to their mother tongue as infants, babbling at six months of age and speaking full sentences by three years of age (e.g., Kuhl, 2004). But how does variation in early life experiences affect the typical language development trajectory? Even in the earliest stages of life there is evidence for differences between monolingual exposed infants and multilingual exposed infants. Studies show that bilingual infants between four and a half and six months old acquire a sensitivity for the languages of their environment such that they can distinguish between their two languages despite these being rhythmically similar (e.g., Bosch & Sebastián-Gallés, 1997). However, even in the earliest stages of life, learners may become bilingual differently based on their experience.

Some individuals achieve bilingualism by acquiring their first and second languages simultaneously. In contrast, sequential bilingualism is the process by which bilingualism is achieved by learning the L2 sometime after learning the L1 earlier in life. Differences between simultaneous and sequential bilingualism have been demonstrated in the past literature. Kaltsa and colleagues (2020) found differences between simultaneous and late sequential bilinguals aged between 8–10 years of age, with vocabulary and syntactic skills closely related for simultaneous bilinguals but not for sequential bilinguals. Beyond simultaneous versus sequential bilingualism, the experience of bilinguals at later stages of life varies greatly based on their early language experience.

Research on bilingual infants has shown evidence of significant effects of bilingual exposure even in the earliest stages of life. To gain a better understanding of the effects of early exposure to two languages, Ferjan Ramírez et al. (2017) examined brain activity in 11-month-old English monolingual and Spanish–English bilingual infants. They used magnetoencephalography (MEG) to track brain activity as the babies listened to speech sounds in English and Spanish. The results revealed that both monolingual and bilingual infants showed a stronger response to an English than Spanish contrast, however the bilingual infants showed greater flexibility in phoneme discrimination suggesting a broader tuning to speech linked to exposure to more than one language in infancy. The bilingual babies also showed activation of brain areas involved in executive function, suggesting that exposure to two languages is beginning to have consequences beyond language itself. In an earlier study, Werker and Tees (1984) showed that by the end of the first year of life, babies have tuned to the language or languages to which they have been exposed. Babies exposed to more than one language appear to be more open to speech sounds in other languages than their monolingual counterparts, something that has been called the “perceptual wedge” in recent studies tracking brain activity in young bilingually exposed infants (Petitto et al., 2012).

Additional research has shown that bilingual infants, despite being nonverbal, can discriminate between their two languages even before they speak their first word, demonstrating the importance of language input during the earliest stages of life (Werker & Byers-Heinlein, 2008). Similarly, Weikum and colleagues (2007) found a bilingual advantage such that when bilingual infants saw a silent video clip, they—unlike monolingual infants—were able to regain interest when the silent speaker on the screen switched languages. The same advanced perceptual attentiveness was found in a study testing Spanish–Catalan bilingual infants (Sebastián-Gallés et al., 2012). What is remarkable about the latter study is that it took place in Barcelona, Spain, but using the same English–French videos that Weikum et al. created for their study in Vancouver, Canada. Babies in Barcelona hear Spanish and Catalan, not English and French, and yet they are advantaged relative to monolingual babies in a way that suggests that it is bilingualism itself rather than specific language exposure that may be critical to development. These studies on infants demonstrate the importance of early language exposure, and suggest further inquiry into how this might extend into early childhood.

Bilingualism in early childhood has been linked to enhanced linguistic and cognitive performance in past research. Preschool aged bilingual children were faster and more accurate at a vocabulary assessment and executive attention tests in comparison to their monolingual counterparts (Yang et al., 2011). A similar advantage was reported in kindergarten aged bilingual children who performed significantly better on an executive function task battery in comparison to monolingual children, suggesting an advanced use of cognitive operations involved in language switching may benefit children (Carlson & Meltzoff, 2008). Pelham and Abrams (2014) found that bilinguals generally suffer from a lexical access deficit in comparison to monolinguals, such that they have a smaller vocabulary size in that particular language. However, the habitual use of two languages has been linked to executive function benefits, possibly as a means of compensating for reduced lexical access (see Bialystok et al., 2010). In a provocative study, Haranto et al. (2019) reported data from a longitudinal study on over 18,000 children that showed that bilingualism appears to reduce the deleterious consequences of poverty on executive function in low socioeconomic status (SES) children. Early life language experiences have been linked to these long-term consequences for both language and cognition, but how does variation or diversity in language experience itself play a role in shaping these outcomes?

The Effects of Variation in Early Language Experience

Focusing on diversity in early language experience offers an illuminating lens for studying bilingualism more broadly. Past research has taken the first few years of life, generally between birth and five years of age, to be critical for language development. Historically, this has been discussed and debated around the idea of a critical or biologically sensitive period for language learning (e.g., Birdsong, 2018; Johnson & Newport, 1989; Lenneberg, 1967). If language development is constrained by maturational mechanisms that unfold as a function of age, then learning an L2 following that critical period would be expected to be a limited endeavor. Indeed, a great deal of research, particularly on the acquisition of L2 syntax, has taken the approach of asking whether L2 learners past the hypothesized critical period are restricted in their ability to acquire particular features of the L2 (e.g., Clahsen & Felser, 2006). While there is robust evidence for the effects of age of acquisition (see Hartshorne et al., 2018 for a big data approach to bolster the claims about critical periods), other findings, in part fueled by developments using the methods of cognitive neuroscience, suggest far more plasticity in new language learning throughout the lifespan than initially assumed (e.g., Pliatsikas, 2020; Steinhauer, 2014). The shift of focus has also opened new lines of research on how language learning beyond the first few years of life might be influenced by diversity in language experience.

Individuals come to learn and use two or more languages in different ways. Speakers vary in the contexts in which they may use their languages, the frequency with which each language is spoken, and whether they code-switch with other speakers. Relatively few studies have examined the enduring consequences of early life language experience. If the first years of life are the most important for learning a new language, how do differences in early life experience come to affect later language acquisition?

As noted in the section Cross-language Transfer and Bidirectional Influence, the most common bilingual language experience in the United States is that of being a heritage speaker, with a home language other than English. Many heritage learners become bilingual with the language of the community, learning the community language along with the home language or later as an L2. The politics surrounding bilingual education have the consequence that few heritage speakers are schooled in the home language but instead switch to the community language as young children when they enter school. Ultimately, many heritage speakers become dominant in the language of the community with a high degree of variability in spoken proficiency and literacy in the home language. The profile of language development for heritage speakers has produced a range of claims about how heritage language development differs from native language development in a language that is learned and maintained as the language of the community or from L2 development (e.g., López et al., 2019; Polinsky & Scontras, 2020).

Many children who speak a heritage language are children of immigrants or individuals who do not speak the dominant language of the environment. This creates a complex linguistic environment for children who may serve as language brokers, translating and interpreting for their parents or family members and for other adults such as teachers and neighbors who cannot understand their heritage language (e.g., McQuillan & Tse, 1995). The experience of language brokering engages cognitive resources and has been hypothesized to have a number of significant linguistic and cognitive consequences (e.g., López & Vaid, 2018).

Another form of heritage language use is that of children who use dialectal variations of the dominant community language. An example are children in the United States who are bidialectal and speak African American Vernacular English (AAVE). AAVE is the variety of English natively spoken by African Americans (Labov, 2010). Past research has found a relationship between the usage of AAVE and spelling patterns, such that Black college students who speak AAVE devoice single final consonants when spelling in contrast to their White counterparts who do not (Treiman, 2004). Washington (2001) outlined the poor reading achievement of children who regularly use AAVE; however, she pointed out that other factors such as low socioeconomic status may contribute to African American children’s reading performance in comparison to nonminority children. Like other heritage speakers of a home language that is not supported in school, studies have documented a decrease in the use of African American English by children as early as the first grade (e.g., Craig & Washington, 2004).

Contemporary research on language development and bilingualism is just beginning to fully consider the consequences of variation in early language experience; however, two sources of evidence provide provocative support for the idea that features of early language experience may be available many years later even when they have not been actively used. Perhaps the most dramatic example comes from studies of international adoptees. Unlike heritage speakers who switch from a home language to a community language but where the home language is active and available to varying degrees, for international adoptees, there is typically an abrupt switch of language environment, with the result that young infants and children are cut off entirely from the language to which they were exposed from birth. Studies of infant speech perception demonstrate very early learning, with the speech system tuned to the language to which babies are exposed by the end of the first year of life (e.g., Kuhl, 2004; Kuhl et al., 1992; Werker & Tees, 1984). But what happens when children are cut off from the input to the native language? The evidence suggests that although adoptees may have little conscious awareness of the native language, there is residual information that facilitates relearning (e.g., Choi et al., 2017; Oh et al., 2010) and that their brain activity as young adults reveals their exposure as infants (e.g., Pierce et al., 2015).

Pierce et al. (2015) compared three groups of adolescents and young adults who were living in Montreal. The international adoptees were born in China and exposed to Mandarin as infants but then adopted by French-speaking Canadians with no subsequent Mandarin input. A second group were native French speakers matched in age to the adoptees, and a third group were age-matched bilinguals who had grown up in homes with both Mandarin and French. Using fMRI, Pierce et al. showed that the brain activity of the adoptees was somewhere between the native French speakers and the fully bilingual speakers, suggesting that some features of the initial exposure were still evident many years later. This demonstration does not establish how much of a foundation that residual brain activity might provide towards relearning but suggests that despite the absence of any conscious knowledge of Mandarin, the exposure during infancy has enduring consequences.

A second source of information about the enduring consequences of early language experience comes from studies of childhood overhearers, adults who as children were exposed to a language other than the dominant language of the community, typically in the context of home and family, but unlike heritage bilinguals, never learned to speak and read that language. Au et al. (2002) reported a study of young adults who were childhood overhearers of Spanish. Their performance was compared to native speakers of Spanish and late learners of Spanish as an L2 on features of Spanish phonology and grammar. Although the childhood overhearers were not native-like, on measures of spoken production (e.g., voice onset times, accentedness), they were much closer to native speakers than the L2 learners of Spanish. On measures of Spanish grammar and morphology, they were more similar to the L2 learners than to the native speakers. Like the studies of international adoptees, the evidence on childhood overhearers suggests that there are extended consequences of early exposure, even when the language is not used actively. Studies that have examined relearning of the lost or overheard languages have produced mixed results but with a suggestion that at least under some circumstances, there are benefits in relearning (e.g., Oh et al., 2019).

The studies on early bilingual exposure converge in demonstrating dramatic consequences for the tuning of the speech system in the earliest months of life, with evidence for enduring traces of initial language experience well into early adulthood. In the next section of the article, the Contexts and Condition for Bilingualism are addressed to understand how beyond exposure itself, there are distinct contexts that shape the many forms that bilingualism takes.

Contexts and Conditions for Bilingualism

The course of initial acquisition is only one factor that determines how bilinguals manage two languages in one mind. Over the course of their lives, bilinguals who speak the very same languages may come to live and work in different environments that may support or fail to support the use of each language. In what follows, the studies reviewed attempt to identify the features of the environment that play a unique role in accounting for the experience of being bilingual. A question is frequently asked about the languages themselves. Does the form of bilingualism depend on which two languages are paired? At one level, the answer to that question is obviously yes because languages differ in many ways, and those differences influence the opportunities for cross-language transfer and interaction. Yet, at another level, there is remarkable convergence across bilingual language pairings for the principles that have been described. Speakers of languages that are quite distinct, like Chinese and English (e.g., Thierry & Wu, 2007) and individuals who are bimodal bilinguals, using one spoken language and another signed language, reveal cross-language interactions that are largely similar to those seen for bilinguals whose languages are more similar. (See Morford & Kroll, 2021 for a review of research on bimodal bilingualism.)

Interactional Contexts and Social Networks

Individuals may achieve a high level of proficiency in each of their two languages but still use them in quite different ways. In some contexts, one language may be used as the language of the home and community and another language may be the language of work and school. In locations that are truly bilingual, there may be opportunities to use both languages almost interchangeably. For bilinguals who find themselves immersed in an environment in which the dominant language of the community is their L2, there may be few opportunities to speak their L1 with others. In the past, the characterization of the linguistic and cultural contexts for language learning and language use was primarily the goal of sociolinguistics. More recently, there has been an attempt to ask how the mechanisms that enable language learning and language processing are affected by the contexts in which they occur.

In the first section of this article, Reading, Listening, and Speaking Two Languages, the idea that both languages are always active and always interacting, even when bilinguals find themselves using one language alone, was discussed. That observation, on its own, might be hypothesized to create a serious problem for bilingual speakers that challenges their ability to select the intended language in the appropriate context. The fact that it does not create a problem suggests that speakers adapt to the dynamics of language co-activation by developing mechanisms to regulate the two languages and to engage domain-general cognitive control in ways that are sensitive to the demands of the environment (e.g., Green, 1998; Green & Abutalebi, 2013). Those mechanisms affect language learning and language processing themselves (e.g., Bogulski et al., 2019; Zirnstein et al., 2018) and the domain-general cognitive resources that support them (e.g., Adler et al., 2020; Hsu et al., 2021). Although there is a great deal that is unknown about the relationship between language regulation and cognitive control (e.g., Declerck & Philipp, 2015; Hervais-Adelman & Babcock, 2020) what is clear in many studies is that adapting to the competing demands of the environment in which the two languages are used is crucial. In the present discussion, language regulation refers to the coordination and engagement of a range of executive functions that serve to modify the availability of each language in response to the demands placed on bilingual speakers. While the modulation of each of the bilingual’s two languages will draw on domain-general cognitive control mechanisms, there is not necessarily a one-to-one correspondence between language and cognitive control processes.

Beatty-Martínez et al. (Beatty-Martinez, Navarro-Torres, & Dussias, 2020; Beatty-Martinez, Navarro-Torres, Dussias, Bajo, et al., 2020) investigated the consequence of different interactional contexts for language processing for three groups of Spanish–English bilinguals who were all native speakers of Spanish and highly proficient in English as the L2, but who lived in different environments. One group lived in Granada, Spain, where Spanish is the language of the home and community but where English is used at school and work. The two languages are used separately and there is little intra-sentential code-switching. A second group lived in San Juan, Puerto Rico, where the two languages are used more cooperatively and there is frequent code-switching. The third group lived in the United States in State College, Pennsylvania, where the community is largely monolingual speakers of English. These bilinguals code-switch with each other but have limited opportunities in the larger community to speak Spanish. The bilinguals in each location performed a speeded picture naming task in both languages and the AX-Continuous Performance Test (AX-CPT), a cognitive measure that identifies proactive and reactive control (e.g., Braver et al., 2001). Bilinguals in all groups were able to name pictures at a high level of accuracy. The critical finding that Beatty-Martínez et al. (Beatty-Martinez, Navarro-Torres, Dussias, Bajo, et al., 2020) reported was that the bilinguals immersed in the United States, in a context where they could not reliably expect others to speak Spanish, produced a pattern of picture naming that revealed the engagement of proactive cognitive control. Bilinguals with high proactive control were better able to maintain their L1, Spanish, in the face of an environment that itself offered little support. Although the other two contexts are quite distinct from one another, in each there are cues to language use that do not require the same level of monitoring to determine with whom each language can be spoken.

The results of the Beatty-Martínez et al. (Beatty-Martinez, Navarro-Torres, Dussias, Bajo, et al., 2020) study suggest that it is not simply whether or not bilinguals engage in code-switching, nor whether they are proficient in both languages that determines language performance. In line with the adaptive control hypothesis (Green & Abutalebi, 2013), bilinguals adapt to the demands of the environment in selectively drawing on cognitive mechanisms to enable the active maintenance of the two languages. It is not only about being able to speak each language proficiently, but also about the decisions that must be made about which language you can speak in the presence of which interlocutors. Although the goal of this study was not to address the cognitive consequences of bilingualism directly, the results also suggest that characterizing language experience more fully may be necessary to determine the conditions under which bilingualism confers benefits, costs, or little if any consequences for cognition (see Navarro-Torres et al., 2021 for additional discussion of the characterization problem).

Interactional contexts are of course not only restricted to geography. In different environments, bilingual speakers are members of social networks that may differ linguistically and culturally. A set of studies has begun to identify the way that social networks may influence language processing for bilinguals and monolinguals alike (e.g.,Gullifer & Titone, 2020; Gullifer et al., 2018; Lev-Ari, 2018). Gullifer and Titone developed a measure of language entropy to identify the number of different languages that a speaker uses in different social contexts. The measure is an index of the uncertainty that bilinguals face in using each of their two languages across speakers and contexts (see Gullifer & Titone, 2021 for a discussion of uncertainty more generally). Using both behavioral and neurocognitive measures, they showed that bilingual speakers with high language entropy, using different languages across different environments, appeared to engage greater proactive cognitive control than bilingual speakers without this requirement (see Pot et al., 2018 for a similar finding with older adult bilinguals). Like the Beatty-Martínez et al. (Beatty-Martinez, Navarro-Torres, Dussias, Bajo, et al., 2020) findings, the research on language entropy demonstrates that variation in the context of language use may be a critical factor for understanding differences across otherwise similar groups of bilinguals.

Other recent studies have shown that language variation itself may be important. Bice and Kroll (2019) used a training paradigm to teach Spanish–English bilinguals and monolingual English speakers words in Finnish that obeyed a vowel harmony rule. Neither Spanish nor English has this linguistic feature, making the word learning task more difficult than a simple paired associated learning task with novel words. The study recorded behavioral measures and electroencephalography (EEG). For pragmatic reasons having to do with the move of the laboratory in which the study was being conducted, half of the data were collected in Pennsylvania, in a dominant monolingual English environment, and the other half in Southern California, in a linguistically diverse environment. The most interesting result was for monolingual speakers, all of whom identified as English-only speakers with minimal knowledge of another language. Those in Southern California showed brain activity at the end of the experiment that revealed sensitivity to generalization of the rules in Finnish even when presented with a novel exemplar. The monolinguals in Pennsylvania did not. Although both groups revealed initial learning of the rule and neither group revealed behavioral evidence for generalization, the pattern of brain activity suggests, at least in a preliminary way, that the linguistic diversity of the environment itself may create a greater openness to new language learning. This was a serendipitous finding as a result of the two locations of data collection, so needs to be taken cautiously, but the results contribute to an emerging area of research that is showing that not only bilinguals, but also monolinguals vary as a function of the context in which language processing occurs, and that far from the idea that variation is simply noise, that it may enhance both language and cognition.

How the Cognitive and Neural Consequences of Bilingualism are Shaped by Language

The research reviewed points to a complex dynamic in which the bilingual speaker’s two languages are in a perpetual state of interaction that is influenced by their knowledge of the two languages and by the requirements placed on them to adapt to the environments in which they speak each language. It is beyond the scope of the present article to review the extensive evidence on how bilingual minds and brains change as a function of their experience (see Bialystok, 2017 and also this volume; Pliatsikas, 2020). Instead, the next question asked is on what causal role these language processes might play in shaping the cognitive and neural outcomes of bilingualism.

As language learners acquire and use an L2, there is not only transfer from the native or more dominant L1 to the L2 but marked changes in the L1 itself. It has been hypothesized that these changes to the L1 are not simply passive effects on language processes but contribute to the bilingual speaker’s ability to regulate the more dominant language (e.g., Bogulski et al., 2019; Zirnstein et al., 2018).

Although there are many observations of native language change, including an extensive body of research on language attrition (e.g., Schmid, 2010), the studies that most brought this issue to the attention of the field were the asymmetric processing costs observed in language switching (e.g., Meuter & Allport, 1999; and see Bobb & Wodniecka, 2013 and Declerck & Philipp, 2015 for reviews). Unlike code-switching which occurs in normal discourse contexts between bilingual speakers (e.g., Beatty-Martínez, Navarro-Torres, & Dussias, 2020), language switching is a laboratory paradigm in which bilingual speakers are cued to name pictures or digits in one of their two languages. The finding that Meuter and Allport reported, and that has since been replicated many times, is that there are greater switch costs to the L1 than to the L2. At first, that finding seemed counterintuitive because the native or dominant language might be thought to be more active and resilient than the second or less dominant language. The switch cost asymmetry came to be understood as a reflection of the control processes that are engaged to navigate the use of the more dominant L1 when the less dominant L2 was also required (e.g., Green, 1998; Kroll & Gollan, 2014). If both languages are active, then the more dominant language would have to be suppressed to enable speech planning in the less dominant language. When a switch is required back into the dominant language, the hypothesis is that additional resources would have to be engaged to overcome the momentary inhibition. Indeed, in some instances, not only are there asymmetric costs but production in the more dominant language may temporarily become slower than production in the less dominant language (e.g., Declerck et al., 2020).

Subsequent research has debated the nature of the mechanisms that affect the L1 in this way and the scope of the observed inhibition of the L1. The point for the present discussion is that it is typically the L1 that is affected, changing rapidly in different task environments. Other studies have demonstrated similar phenomena when the switch occurred over a longer period of time, e.g., after naming a series of pictures in one language and then switching to the other language (e.g., Misra et al., 2012; Van Assche et al., 2013). It is not only that the larger social context of bilingualism matters, but also that the immediate demands on bilingual speakers require continual adjustments, with even momentary exposures creating these changes (e.g., Degani et al., 2020). These adjustments appear to be a natural feature of what it means to be bilingual.

Where else are these dynamics seen? When individuals are immersed in an L2 environment, there is a shift in the availability of the native or dominant language, with the L1 less available (e.g., Baus et al., 2013; Linck et al., 2009). As we saw in the Beatty-Martínez et al. (Beatty-Martinez, Navarro-Torres, Dussias, Bajo, et al., 2020) study of interactional context, when bilingual speakers are immersed in an L2 with relatively few L1 speakers available, they seem to rely on proactive cognitive control to maintain the L1. The presence of control mechanisms that are tuned by bilingual experience may function not only to maintain the L1, but to coordinate across competing language and cognitive demands. A number of studies have shown that this coordination may be key to enabling bilinguals to use the L2 proficiently (e.g., see Zirnstein et al., 2018 for an illustration of how the interplay of language regulation and cognitive control enables immersed bilinguals to engage in prediction processing while comprehending sentences, and Morales et al., 2015 for an example of how bilingualism modulates the coordination across the components of control). These consequences of bilingualism are complex, both for language processes and for the cognitive and neural processes that support language. What is key is that the complexity that has been captured in recent research is systematic, pointing to the underlying causal mechanisms that adapt as individuals become bilingual and use two languages actively.

Concluding Remarks

There is much that has not been discussed in this brief article. Notably, there is a substantial body of research on bilingual language processes at the sentence level that has addressed the issues of variation and context which have been discussed here (e.g., Dussias et al., 2019). Research on code-switching, where some bilinguals switch languages in the middle of an utterance, has been a topic of particular interest in this regard both because it is highly regular and appears to reflect the social interface between language processing, cognition, and the brain (e.g., Beatty-Martínez & Dussias, 2017; Fricke et al., 2016; Kaan et al., 2020). Likewise, although the consequence of early exposure to bilingualism during infancy has been discussed, the extensive research on the perceived accentedness of speech (which, like code-switching, provides an opportunity to examine the interplay between language processes and the sociocultural environment in which it is embedded; e.g., Kutlu, 2020) has not been addressed.

Across the lifespan, the consequences of bilingualism for aging have not been considered (e.g., Zhang et al., 2020). Much of the research on bilingualism and aging addresses the cognitive and neural consequences (e.g., Abutalebi et al., 2015; and see Bialystok, this volume), without reference to language processes, although evidence has suggested that the control mechanisms for young adult bilinguals may also be involved (e.g., Mendez, 2019). Finally, only the studies that track the trajectory of new language learning for young and old bilingual and monolingual adults as they acquire a new language were discussed. That research has suggested that bilinguals may be advantaged with respect to new language learning (e.g., Kaushanskaya & Marian, 2009) that language learning itself may counter some aspects of cognitive decline in old age (e.g., Antoniou et al., 2013), and that some of the cross-language processing and control dynamics that have been discussed here may be important (e.g., Bogulski et al., 2019; Hirosh & Degani, 2018).

Since 2000, there have been profound discoveries about the ways in which bilingual language experience comes to impact our minds and brains. The story that is emerging is complex because language experience itself is both complex and varied. But the lesson learned in this period is that bilingualism is not only of interest in its own right, but also as powerful lens to reveal the relations between language, the mind, the brain, and the social world. Much of what has been reviewed in this article would not have been known had monolingual speakers alone been examined.

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