Coordination of executive functions in monolingual and bilingual children

Abstract

Two groups of 8-year-old children who were monolingual or bilingual completed a complex classification task in which they made semantic judgments on stimuli that were presented either visually or auditorily. The task requires coordinating a variety of executive control components, specifically working memory, inhibition, and shifting. When each of the visual and auditory tasks was presented alone, performance was comparable for children in the two groups. Combining the two modalities into a dual-task paradigm made the task more difficult, and on this combined task bilingual children maintained better accuracy than monolingual children, especially on the visual task. The results are interpreted in terms of the enhanced ability of bilingual children to coordinate the executive control components required in performing this complex task.

Introduction

The development of executive control is one of the most significant cognitive achievements during childhood (Carlson, 2005, Jones et al., 2003, Zelazo et al., 2008), a process that continues into adolescence (Best, Miller, & Jones, 2009). The majority of the research investigating this issue examines development of the component processes that together make up the executive function. One framework commonly used for this purpose is that proposed by Miyake and colleagues (2000) consisting of three core components: selective attention and inhibition, shifting, and working memory. Although there continues to be debate about the precise core components, the approach has led to productive research showing the gradual emergence of executive control during the early years and the expanding repertoire of the cognitive behaviors it supports.

There are two limitations to this approach. The first is that the contributions of individual core components are difficult to isolate empirically and relate to complex performance that is characteristic of real-life tasks. Experimental tasks rarely produce pure measures of these components (Best & Miller, 2010), and in studies that attempt to isolate them performance is typically best described by interactions among multiple components (Garon, Bryson, & Smith, 2008). Because real-life tasks rarely recruit a single core component, the components need to be managed and coordinated for effective performance and the ability to manage them might be different from their individual development. Thus, some higher order management function might be another aspect of the development of executive control. Multitasking is an example of an everyday performance in which a variety of executive control components are required and is difficult to understand in terms of individual components.

The second limitation is that experience may modify the development of executive control, requiring more complex descriptions of development. Bilingualism has been shown to be one such experience (for a review, see Bialystok, Craik, Green, & Gollan, 2009). Executive control develops earlier in bilingual children than in comparable monolinguals (Adi-Japha et al., 2010, Bialystok, 2010, Carlson and Meltzoff, 2008, Yang et al., in press), and bilingual adults continue to outperform monolinguals on such tasks (Bialystok et al., 2004, Colzato et al., 2008, Costa et al., 2008, Prior and MacWhinney, 2010, Treccani et al., 2009). The studies used different tasks, but all involved some subset of the component processes normally considered to be part of executive control. However, it is not known whether bilingualism affects individual core components or is a coordinating function that allows the components to function together. For example, in tasks such as the Simon task (Simon & Wolf, 1963) and the flanker task (Eriksen & Eriksen, 1974) that include congruent and incongruent trials, all participants perform equivalently on single blocks of congruent trials but bilinguals outperform monolinguals on both congruent and incongruent trials when presented in mixed blocks (Bialystok et al., 2004, Costa et al., 2008). At a minimum, these results indicate that the bilingual performance cannot be attributed to a single component such as inhibition, even though inhibition is clearly affected by bilingualism (Blumenfeld & Marian, 2011). Thus, a more complete understanding of the development of executive control is obtained by examining groups with different relevant experiences.

Why would bilingualism modify executive control? There is now overwhelming evidence that both languages are always active to some degree, even in contexts that clearly support only one of the languages (e.g., Francis, 1999, Grainger, 1993, Kroll and de Groot, 1997, Rodriguez-Fornells et al., 2002, Thierry and Wu, 2007). For example, Marian, Spivey, and Hirsch (2003) conducted an eye movement study with Russian–English bilinguals using the visual world paradigm and showed that eye movements to pictures of words named in English were disrupted by pictures in the stimulus array whose Russian translation had overlapping phonology with the English target item. There was no reason to process the Russian name for the object, but participants did this automatically and that Russian name affected performance on the English task. Therefore, bilingualism places individuals in a “dual-task” situation for which executive control is constantly required. Speakers must construct speech plans according to the current context (i.e., working memory), selectively attend to linguistic structures in the target language while ignoring competition from the other language (i.e., inhibition), and monitor progress of the interaction (i.e., shifting). Thus, the impact of bilingualism may be found less in the individual core components of executive control than in their coordination and management to enable bilingual language use.

The possibility that bilingualism enhances performance in a dual-task situation was tested by Bialystok, Craik, and Ruocco (2006). Younger and older participants who were monolingual or bilingual classified stimuli into two semantic categories. The stimuli were presented either visually and required a manual key press or auditorily and required a spoken response. Tasks were performed separately and then combined into a dual-task presentation. The semantic categories were letters or numbers and animals or musical instruments. For the visual task, the letter–number stimuli were strings of digits or letters and the animal–music stimuli were colored photos presented in the center of the computer monitor. For the auditory task, the letter–number stimuli were the names of letters or numbers spoken by a female voice and the animal–music stimuli were WAV files of those sounds played through headphones. The dependent variable was the number of correctly classified items in each condition. Bilinguals of both age groups were able to correctly classify more items than monolinguals in the dual-task condition when the visual task consisted of items from the letter–number category, but no language group differences were found in the animal–music task.

The current study adapted this paradigm to investigate how bilingualism influences children’s performance in a complex task that requires coordination of several components of executive control. Multitasking is a common activity that involves all three core components and, therefore, requires their coordination. Thus, rather than focusing exclusively on individual processes, the task examines the ability to manage a complex set of executive control demands. Only stimuli from the animal–music classification were used. Unlike the previous study (Bialystok et al., 2006), the two stimuli in the dual-task conditions were presented simultaneously; participants could respond to either modality first and the next trial began after both responses had been registered. In contrast to previous research that focused on individual components of executive control, the current study investigated the coordination of these components in complex performance with less attention to their individual contribution. The components themselves may be at different levels as well in the two groups, but the assumption is that the need for coordination is above and beyond those individual levels. Therefore, a bilingual advantage in this task would contribute to understanding the differences previously reported and would have implications for understanding the role of executive control in a world where multitasking is increasingly common.