International Journal of Bilingualism

In the domain of language and audition, studies have shown large individual differences, within the normal range (i.e. in healthy, non-expert individuals), in performance on tasks involving speech sound processing, vocabulary knowledge, and reading, these in both monolingual and bilingual participants and in native and non-native language contexts. These individual differences have often been related to individual differences in brain structure. Evidence for structural differences is especially striking since brain structure can be assumed to be more stable, or less malleable, than brain function. Brain function, on the other hand, can be expected to change, or be plastic, after only very short periods of training/learning. The present paper provides a review of studies that have investigated the brain structural correlates of normative individual differences in aspects of language-related performance, these spanning a hierarchy in terms of the underlying complexity of processing and brain networks involved. Specifically, the review is structured so as to describe work examining the following domains, which involve progressively increasing levels of complexity in terms of the posited perceptual/cognitive sub-functions involved: 1) lower-level acoustic processing; 2) phonetic processing, including non-native speech sound learning, learning to use pitch information linguistically, non-native speech sound articulation, and phonetic expertise; 3) working memory for verbal and for pitch information; 4) semantics, in the context of lexical knowledge and of semantic memory; 5) reading; 6) syntax, both natural and artificial; 7) bilingualism; and finally 8) executive control of language in the contexts of fluency and of speech-in-noise processing. Results are discussed and synthesized in the context of lower to higher-level brain regions thought to be functionally involved in these respective domains, which are very often, if not always, the very ones that structurally partly predict domain-specific performance.

Over the past several years, the field of bilingual speech perception has seen a substantial increase in both the number of publications and in the amount of interest directed at its findings. Consequently, the time is ripe to assess the state of the field, what we have accomplished and where we have yet to go. Although we cannot capture the full state of the field in the space of this paper, we hope to summarize the major trends that have led to the current state and take stock of its future directions. To that end, we focus our review on the relative merits of single phonemes versus whole words and phrases when investigating bilingual speech, the efficacy of the different training paradigms that have been attempted and we focus, in particular, on the role of individual differences in predicting learning outcomes. We conclude our review by highlighting recent developments demonstrating that identifying individual differences in ability pre-training can result in more efficacious training paradigms. Goals for future research are also discussed.

Early acquisition of a second language influences the development of language abilities and cognitive functions. In the present study, we used functional Magnetic Resonance Imaging (fMRI) to investigate the impact of early bilingualism on the organization of the cortical language network during sentence production. Two groups of adult multilinguals, proficient in three languages, were tested on a narrative task; early multilinguals acquired the second language before the age of three years, late multilinguals after the age of nine. All participants learned a third language after nine years of age. Comparison of the two groups revealed substantial differences in language-related brain activity for early as well as late acquired languages. Most importantly, early multilinguals preferentially activated a fronto-striatal network in the left hemisphere, whereas the left posterior superior temporal gyrus (pSTG) was activated to a lesser degree than in late multilinguals. The same brain regions were highlighted in previous studies when a non-target language had to be controlled. Hence the engagement of language control in adult early multilinguals appears to be influenced by the specific learning and acquisition conditions during early childhood. Remarkably, our results reveal that the functional control of early and subsequently later acquired languages is similarly affected, suggesting that language experience has a pervasive influence into adulthood. As such, our findings extend the current understanding of control functions in multilinguals.

Individuals who develop bilingually typically outperform monolinguals on tests of executive functions. This advantage likely reflects enhanced prefrontal function, but the mechanisms that underlie this improvement are still poorly understood. This article describes a theory on the nature of the neural underpinnings of improved executive function in bilinguals. Specifically, we propose that growing up in a bilingual environment trains a gating system in the striatum that flexibly routes information to the prefrontal cortex. This article is divided into three sections. Firstly, literature establishing a three-way connection between bilingualism, executive function, and fronto-striatal loops is summarized. Secondly, a computational model of information processing in the basal ganglia is described, illustrating how the striatal nuclei function to transfer information between cortical regions under prerequisite conditions. Finally, this model is extended to describe how bilingualism may “train the brain,” enabling improved performance under conditions of competitive information selection during information transfer. Theoretical implications and predictions of this theory are discussed.