Electronic Thesis and Dissertation Repository


Doctor of Philosophy




Marc Joanisse


In this thesis, I investigated the neural correlates of bilingualism, and how individual differences in both brain and behaviour affect second language processing. To date, theories of bilingualism have tended to treat bilinguals as a uniform group, while in practice they vary greatly in both experience and ability. By examining how individual differences in proficiency and age of acquisition contribute to second language learning and processing, I sought to address this issue. In chapter two, I used event-related potentials to investigate how age of acquisition and proficiency modulate processing of a novel versus a grammatical rule that is similar across languages. I provided evidence that both age of acquisition and proficiency, in addition to bilingual status, modulate processing of a novel grammatical rule. In contrast, only proficiency predicted processing of a similar grammatical rule. Thus, while the similarities between languages affect second language processing, the degree of their influence is modulated by individual differences in second language experience. In chapter three, I used functional magnetic resonance imaging to investigate how bilinguals represent their shared, integrated lexicons in the brain. Several areas showed differing patterns of representation, while univariate analyses in these areas showed no differences in levels of activation. The separate representation of first and second languages in these regions provides a possible basis for the neurocognitive realization of a shared, integrated lexicon proposed by many theories of bilingualism. In chapter four, I used diffusion tensor imaging to investigate how AoA modulates white matter microstructure, examining white matter tracts in the left and right hemispheres that underlie language processing. Group statistics suggested that second language speakers as a whole may have lower fractional anisotropy, while the within-group analysis revealed that white matter integrity is sensitive to individual experience. Chapter five discusses the relevant findings of the previous chapters, and considers how individual differences arise. Next, I make recommendations for theories of bilingual language processing, and close with a discussion of future research directions.