Language experience and the brain: variability, neuroplasticity, and bilingualism

In the last two decades, we have learned that there is far greater neuroplasticity in adulthood than previously understood. Experiences as varied as driving a taxi cab, playing a musical instrument, physical exercise, or playing video games change the brain both functionally and structurally (e.g. Bavelier, Achtman, Mani, & Föcker, 2012; Erickson et al., 2011; Herholz & Zatorre, 2012; Maguire et al., 2000). In this context, it should come as no surprise that a life in two languages would do the same. The specific consequences of bilingualism may be unique, but the fact of using two languages actively has implications for cognition and the brain networks that support it (e.g. Abutalebi et al., 2012; Bialystok, Craik, & Luk, 2012; Kroll, Dussias, Bice, & Perrotti, 2015). The article by García-Pentón, García, Costello, Duñabeita, and Carreiras (2015) asks what evidence exists to suggest that bilingualism imposes structural changes in the brain and what that evidence implies about whether the use of two or more languages confers cognitive advantages to bilingual speakers. Other recent reviews, notably Li, Legault, and Litckofsky (2014), have also evaluated the available structural data on the consequences of bilingualism. The word “hazy” in the title of the García-Pentón et al. (2015) paper suggests that there is a problem with the existing research on this topic and the paper is indeed oriented towards two problems. One is that the authors claim that the quality of the data on the consequences of bilingualism for neuroanatomical change is variable: studies are underpowered, use a range of analytic methods that may not be comparable, and anatomical results from similar studies lack convergence. The other concerns the implications that this evidence holds for whether bilinguals are advantaged relative to monolinguals. In this commentary, we argue that neither of these problems creates haze. Rather, the approach taken to the interpretation of structural imaging data requires additional complexity that considers the relationship between neural function and structure, particularly in identifying the mappings between function to structure and the role of individual differences. We suggest that even if themappings between neuroanatomical structure and brain function can be understood in detail, the implications for potential cognitive advantages in behaviour will require a theoretical model that maps behaviour to brain structure and function, and that also takes into account differences in the way that bilingualism is manifest for different types of bilinguals across the lifespan (e.g. Bialystok, Craik, Green, & Gollan, 2009; Luk & Bialystok, 2013). Finding changes in neuroanatomical structure as a function of bilingualism need not imply that therewill be cognitive advantages to bilinguals that are observed in behavioural performance, nor vice versa. These behavioural advantages need to be understood as well and each of these investigations, at multiple levels of analysis, requires a more sophisticated theoretical model. Here, we consider what sort of framework is needed to begin to address the apparent inconsistencies in the evidence reviewed by García-Pentón et al. (2015). We then consider the relationship of this work to the extensive body of research on the consequences of bilingualism for behaviour and ask what assumptions might need to be made to begin to understand the causal mechanisms that underlie these consequences.