The effects of bilingualism on toddlers’ executive functioning

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Abstract

Bilingual children have been shown to outperform monolingual children on tasks measuring executive functioning skills. This advantage is usually attributed to bilinguals’ extensive practice in exercising selective attention and cognitive flexibility during language use because both languages are active when one of them is being used. We examined whether this advantage is observed in 24-month-olds who have had much less experience in language production. A battery of executive functioning tasks and the cognitive scale of the Bayley test were administered to 63 monolingual and bilingual children. Native bilingual children performed significantly better than monolingual children on the Stroop task, with no difference between groups on the other tasks, confirming the specificity of bilingual effects to conflict tasks reported in older children. These results demonstrate that bilingual advantages in executive control emerge at an age not previously shown.

Introduction

There has long been interest in determining whether bilingualism leads to linguistic or cognitive differences in both children and adults. Research over the past two decades has revealed a number of differences that emerge from growing up with at least two languages (Bialystok, 2009a, Grosjean, 1989). In the most general terms, bilingualism leads to the development of strategies that are adaptive to the unique problem space with which bilingual infants are faced. During the early stages of language acquisition, for example, recent research on speech perception in bilingual and monolingual infants has shown that bilingual infants learn similar sounding words in a word learning task a few months later than monolinguals (Fennell, Byers-Heinlein, & Werker, 2007). However, bilingual infants of the same age can outperform monolinguals in learning word–object associations when the phonetic conditions favor their input (Mattock, Polka, Rvachew, & Krehm, 2010).

Research on vocabulary development in bilingual first language acquisition has shown that bilingual children produce their first words at around the same time as monolingual children (Genesee, 2003, Patterson and Pearson, 2004, Petitto et al., 2001). However, the evidence for differences in vocabulary development in bilingual and monolingual children is mixed, depending on the ages of the children and whether receptive or productive vocabulary is assessed. A smaller receptive vocabulary in each language compared with monolinguals has been reported in samples of preschool- and school-aged children (Bialystok et al., 2010, Bialystok et al., 2010, Mahon and Crutchley, 2006), but other studies have shown that the receptive vocabulary of school-aged children is close to that of monolinguals (Cromdal, 1999, Yan and Nicoladis, 2009). With respect to measuring expressive language, school-aged bilinguals tend to have a smaller vocabulary even when both languages are combined (Yan & Nicoladis, 2009). In younger bilinguals (<3 years of age), receptive and expressive vocabulary have been reported to be comparable to that of monolinguals when total or conceptual (total minus translation equivalents) vocabularies are compared even if very young bilinguals tend to have fewer words in each of their productive languages (Junker and Stockman, 2002, Oller and Eilers, 2002, Pearson et al., 1993, Pearson et al., 1995, Petitto and Kovelman, 2003).

Bilingualism also brings linguistic and cognitive advantages. Early studies showed that bilingual children performed better than monolingual children on a variety of tests of metalinguistic awareness (Ben-Zeev, 1977, Bialystok, 1987, Bialystok, 1988, Edwards and Christophersen, 1988, Galambos and Goldin-Meadow, 1990, Yelland et al., 1993). Although impressive, it is not completely surprising that a linguistic experience such as bilingualism would lead to an enhanced understanding of the structure and properties of language. More surprising is the evidence showing that bilingualism also leads to the precocious development of cognitive processes not confined to linguistic tasks. In a comprehensive review of the research on cognitive differences between bilingual and monolingual children, Bialystok (2001) concluded that there is growing evidence that bilingual children outperform monolingual children on a variety of tasks that require selective attention and cognitive flexibility tasks. Inhibitory processes are instrumental in such tasks because attention to misleading aspects of a stimulus must be suppressed to attend to the relevant ones. The inhibitory control recruited in such conflict situations is a key element of the executive function, a set of processes that are responsible for the conscious control of thought and action (Miyake et al., 2000, Posner and Rothbart, 2000). Other components of executive function include shifting of mental sets, updating information in working memory, and planning ability. Executive functioning shows age-related improvements well into adolescence, but the most significant enhancements happen during the preschool period (Carlson, 2005, Zelazo and Müller, 2002).

Research with children (Bialystok, 2005) and adults (Bialystok, Craik, Klein, & Viswanathan, 2004) has shown that bilinguals show better control over these executive processes than their monolingual counterparts. In children as young as 4 years of age, this advantage has been demonstrated with a range of tasks typically used to assess executive functioning. For example, Bialystok and her colleagues demonstrated a bilingual advantage in 4- and 5-year-olds using the Dimensional Change Card Sort (DCCS), a task in which children are given a series of cards to sort by one of two dimensions (color or shape) and then are asked to switch and sort by the other dimension (Bialystok, 1999, Bialystok and Martin, 2004). Thus, children need to ignore the color of the stimulus and attend to its shape to classify the cards correctly. This bilingual advantage in selectively attending to one cue in the context of a conflicting cue has also been reported for the Simon task (spatial conflict between stimulus and response) (Martin-Rhee & Bialystok, 2008), the ambiguous figure task (conflict between competing interpretations of a line drawing) (Bialystok & Shapero, 2005), and the global–local task (spatial competition between overall and constituent shapes) (Bialystok, 2010). A recent study by Carlson and Meltzoff (2008) comparing English–Spanish bilinguals with English 6-year-old monolinguals tested the generality of a bilingual advantage to a wide range of executive function measures by administering a battery of tasks. The main findings revealed a significant bilingual advantage on tasks that call for managing conflicting attentional demands (conflict tasks) but no such advantage on impulse control (delay tasks). It is noteworthy that the effect was robust even after controlling for socioeconomic factors such as parent education level. This pattern of findings suggests that conflict inhibition plays a role in the link between bilingualism and executive function and that precocious effects of bilingualism in executive functioning should be found in conflict tasks but not necessarily in delay tasks.

The prevailing interpretation of the bilingual advantage in executive control is that bilinguals have extensive practice in exercising selective attention and cognitive flexibility. This practice effect is assumed to derive from the fact that both languages are active when one of them is being used (Beauvillain and Grainger, 1987, Colomé, 2001, Costa, 2005, De Groot et al., 2000, Green, 1998, Rodriguez-Fornells et al., 2005). The management of attention to the target language may take place either by lowering the activation of the nontarget language (Finkbeiner, Gollan, & Caramazza, 2006) or by using domain-general suppression mechanisms to inhibit the nontarget language (Green, 1998). Empirical evidence for the effect created by the activation of two competing language systems comes from a number of sources. First, bilingual adults tend to name pictures more quickly and with fewer tips of the tongue when they know the translation equivalents (Finkbeiner et al., 2006, Gollan and Acenas, 2004, Gollan et al., 2005). This facilitory translation effect has recently been replicated with bilingual toddlers (Poulin-Dubois, Bialystok, Blaye, Polonia, & Yott, 2010). In a study on bilingual toddlers using event-related potentials (ERPs), Conboy and Mills (2006) reported differences in ERP latencies, amplitudes, and scalp distributions across mixed-language versus single-language conditions even after controlling for age and vocabulary size. Second, a recent study on bimodal bilinguals, a special population of bilinguals for whom there is less conflict for selection, supports the conflict hypothesis with behavioral evidence. Bimodal bilinguals are speech–sign bilinguals who frequently replace code blending (signs produced simultaneously with spoken words) with code switching (changing from one language to another) because both languages can be produced simultaneously (Emmorey, Borinstein, Thompson, & Gollan, 2008). Using a flanker task in which inhibition of attention to an irrelevant stimulus was required for efficient performance, speech bilinguals performed more rapidly than monolinguals, but bimodal bilinguals performed the same as monolinguals (Emmorey, Luk, Pyers, & Bialystok, 2008). Presumably, the opportunity for code blending reduces the conflict and decreases the need for executive control in managing language production.

Across a range of studies investigating a variety of abilities, it is clear that bilingualism is an experience that has significant consequences for cognitive performance. However, until recently, research on the cognitive performance of bilingual children had been tested only in children above 4 years of age. At what point do the inhibition and selective attention abilities of bilingual children deviate from the developmental trajectory of monolingual children? In one study, Bialystok and colleagues (in press) reported that 3-year-old bilinguals outperformed monolinguals on three tests of executive functioning, and new evidence has been reported suggesting that even 7-month-old bilinguals show a cognitive benefit in a switch task that requires inhibitory control (Kovacs & Mehler, 2009). However, these intriguing findings are based on a single task, and the percentage of exposure to the second language was not specified. More important, a recent study based on a similar paradigm with 8-month-old Spanish–Catalan bilinguals reported that both bilinguals and monolinguals were able to inhibit looking at the wrong location, although bilinguals showed a slight tendency to show inhibition earlier (Ibanez-Lillo, Pons, Costa, & Sebastian-Galles, 2010). These new results raise the exciting possibility that cognitive modifications from two environmental languages can be detected during the first 2 years of life. The current study contributes to this new direction by comparing 24-month-old bilinguals and monolinguals on a large battery of executive functioning tasks adapted for that age (Carlson, 2005) and documenting more complete information about the language history and cognitive level of the children than is usually undertaken in such research.

For the management of attention to two languages to lead to modifications in executive functioning, it would be necessary for children to differentiate between the two languages. There is evidence supporting the claim that bilingual children develop differentiated grammatical systems from the very beginning (Meisel, 2001). Prior to grammatical differentiation, research on early lexical development has shown that young bilingual children acquire translation equivalents or doublets (words in each language that have the same referent) from the time they first begin to speak or by the middle of the second year (Genesee & Nicoladis, 2007). The presence of a sizable set of doublets in very young bilinguals supports the hypothesis that bilinguals have two distinct lexical systems, making it necessary to switch across the two systems (Patterson & Pearson, 2004).

Further evidence for the distinct lexical systems during the early stages of language development in bilingual children comes from research on code mixing. Bilingual children in the one- and early two-word stages of language development are able to adjust their language use appropriately with parents who speak only their native or dominant language with them as well as with strangers (Comeau et al., 2003, Genesee et al., 1996, Nicoladis and Genesee, 1996, Petitto et al., 2001). The ability to make on-line adjustments to accommodate interlocutors’ language preferences or abilities confirms that language switching experience starts early in bilingual development. Finally, in the area of speech perception, discrimination and separation of bilingual infants’ two languages have recently been demonstrated, suggesting that separate representations for each language might begin within the first year of life (Bosch and Sebastián-Gallés, 1997, Byers-Heinlein et al., 2010, Werker and Byers-Heinlein, 2008).

Taken together, these findings on the language abilities of very young bilingual children suggest that language organization differs in monolingual and bilingual infants and that separation of the two language systems is evident very early in language acquisition. Whether these nascent differences in perception and production involve practicing inhibition in the same way as required by language production in older children and adults remains unknown. Nevertheless, the combined evidence that by 24 months of age bilingual children have already separated their two languages and have already gained some experience in switching between their two languages leads to the prediction that cognitive benefits of bilingualism should be observed much earlier than previously reported.

Section snippets

Participants

A total of 75 children were tested. Of these, 6 were excluded due to fussiness, 1 was excluded due to language delay, and 5 were excluded because their dominant language (L1) was neither English nor French. The remaining 63 children were divided into monolinguals and bilinguals based on exposure to their L1. Children whose exposure to their L1 was equal to or greater than 80% were included in the monolingual group (n = 30, 12 French speaking and 18 English speaking, mean age = 24.4 months, SD = 0.8,

Results

We first analyzed the vocabulary and Bayley scores of the two language groups to compare their basic language and cognitive skills. The MCDI was available in both L1 and L2 for 27 of the 33 bilingual children. As expected, the groups differed significantly on verbal ability as measured by vocabulary in L1. Monolingual children produced an average of 338.6 words (SD = 162.7), whereas bilingual children produced an average of 193.4 words (SD = 142.9), t(55) = 3.56, p < .00, d = 0.95. The two groups had a

Discussion

The current results explored the emergence of the bilingual advantage in executive functioning by investigating a group of 2-year-olds. Previous research using a comprehensive battery of tasks found such effects in 3-year-olds (Bialystok et al., in press), and another study using only a single task reported greater flexibility in bilinguals at 7 months of age (Kovacs & Mehler, 2009). The current study bridges these earlier results by administering a variety of tasks assessing different executive

Acknowledgments

This research was partially supported by a grant awarded by the Canadian Language and Literacy Research Network to Diane Poulin-Dubois, Ellen Bialystok, and Agnes Blaye, by a Natural Sciences and Engineering Research Council grant (#2003-07) to Diane Poulin-Dubois and by grant R01HD052523 from the US National Institutes of Health to Ellen Bialystok. We thank Amanda Guay, Alexandra Polonia and Jessica Yott for their assistance with testing and coding.

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