The Source of Enhanced Cognitive Control in Bilinguals: Evidence From Bimodal-Bilinguals

Abstract

Bilinguals often outperform monolinguals on nonverbal tasks that require resolving conflict from competing alternatives. The regular need to select a target language is argued to enhance executive control. We investigated whether this enhancement stems from a general effect of bilingualism (the representation of two languages) or from a modality constraint that forces language selection. Bimodal bilinguals can, but do not always, sign and speak at the same time. Their two languages involve distinct motor and perceptual systems, leading to weaker demands on language control. We compared the performance of 15 monolinguals, 15 bimodal bilinguals, and 15 unimodal bilinguals on a set of flanker tasks. There were no group differences in accuracy, but unimodal bilinguals were faster than the other groups; bimodal bilinguals did not differ from monolinguals. These results trace the bilingual advantage in cognitive control to the unimodal bilingual's experience controlling two languages in the same modality.A growing number of studies have reported advantages in nonverbal executive control tasks for bilingual children (Bialystok, 2001;Carlson & Meltzoff, 2008;Mezzacappa, 2004) and adults (Bialystok, Craik, Klein, & Viswanathan, 2004;Bialystok, Craik, & Ryan, 2006;Costa, Hernandez, & Sebastián-Gallés, 2008). One explanation for this enhancement is that the regular use of two languages requires a mechanism to control attention and select the target language-an experience that may enhance a general control mechanism. Evidence from neuroimaging and patient studies suggests that the same neural regions (e.g., dorsolateral prefrontal and anterior cingulate cortices) are engaged during both languageswitching tasks and nonverbal control tasks, supporting the interpretation that the mechanism for language control and selection is domain general (Fabbro, Skrap, & Aglioti, 2000;Fan, Flombaum, McCandliss, Thomas, & Posner, 2003;Hernandez, Dapretto, Mazziotta, & Bookheimer, 2001;Rodriguez-Fornells et al., 2005).We investigate whether the bilingual advantage in executive control stems from the conflict that arises from the need to select only one language for production or from the bilingual's representation of two language systems. Bilinguals who know two spoken languages (unimodal bilinguals) cannot produce two words at the same time; that is, they cannot simultaneously say dog and perro. In contrast, bimodal bilinguals who know both a spoken and a signed language can produce lexical items from both languages at the same time (Emmorey, Borinstein, Thompson, & Gollan, 2008 ; Casey & Emmorey, in press;Pyers & Emmorey, 2008). Like unimodal bilinguals, bimodal bilinguals have two available languages, but they are not necessarily constrained to select only one language for production, even in monolingual interactions. Here, we examine whether the bilingual advantage in executive control stems from a modalityspecific articulatory constraint that forces language selection. If so, bimodal bilinguals should not show the same advanta...