Contents lists available at ScienceDirect Journal of Neurolinguistics journal homepage: www.elsevier.com/locate/jneuroling Review Understanding bilingual brain function and structure changes? U bet! A unified bilingual experience trajectory model Vincent DeLuca * , Katrien Segaert, Ali Mazaheri, Andrea Krott ** Center for Human Brain Health, School of Psychology, University of Birmingham, United Kingdom ARTICLE INFO Keywords: Bilingualism Neuroplasticity Individual differences Executive functions ABSTRACT A growing body of research shows that the brain adapts functionally and structurally to specific bilingual experiences. These brain adaptations seem related to modulations in cognitive pro- cesses (specifically the executive functions). However, the trajectory of these adaptations is varied and seems at least partially dependent on different aspects of language exposure and use. Here we provide a review of the existing theoretical models covering bilingualism-induced neuroplasticity. Moreover, we propose a unifying framework (Unifying the Bilingual Experience Trajectories, UBET) to more comprehensively map the relationship between the various neuro- cognitive adaptations and different aspects of bilingual experience trajectories, focusing on in- tensity and diversity of language use, language switching, relative proficiency, and duration of bilingual experience. Crucially, we also outline predictions regarding both relationships between different bilingual experience factors and relationships between the measurable neurocognitive adaptations. Our framework offers a theoretical backdrop and clear testable predictions for future large-scale empirical studies on individual differences in bilingual trajectories and their effects on neurocognitive adaptations. 1. Introduction In the past decade, there has been a stark increase in research examining the effects of bilingualism on the mind and brain. This research suggests that bilingualism modulates domain-general cognitive processes, specifically executive control (Bialystok, 2017; Bialystok, Craik, & Luk, 2012), likely through changes in brain structure and function (Li, Legault, & Litcofsky, 2014; Pliatsikas & Luk, 2016). However, inconsistencies exist between studies as to whether and how these effects manifest both at behavioral (Antoniou, 2019; Valian, 2015) and neural levels (García-Pentón, Fernández García, Costello, Duñabeitia, & Carreiras, 2016). The arguments for no bilingual effects on neurocognition are predicated on the notion that if any adaptations (or ‘advantages’) do stem from bilingualism, then they should be robust across all groups and types of bilinguals when compared to monolingual controls (see e.g. Nichols, Wild, Stojanoski, Battista, & Owen, 2020). However, this line of argument is flawed. Given the complexity of the bilingual experience, it is likely that a range of cognitive and neural adaptations are variably required to adequately handle the demands associated with engaging with more than one language (Bialystok, 2017; Li et al., 2014). Examining bilinguals as a homogeneous group would, then, potentially conflate these experiences and modulate the measurable neurocognitive outcomes (see e.g. Bak, 2016; Bialystok, 2016; Poarch & Krott, 2019). Indeed, several models have been proposed, which attempt to both explain the existing variance within the data and offer predictions for future work examining individual differences in bilingualism and their https://doi.org/10.1016/j.jneuroling.2020.100930 Received 14 January 2020; Received in revised form 28 April 2020; Accepted 26 May 2020 * Corresponding author. 52 Pritchatts Road, Birmingham, B15 2TT, UK. ** Corresponding author. E-mail addresses: v.deluca@bham.ac.uk (V. DeLuca), a.krott@bham.ac.uk (A. Krott). Journal of Neurolinguistics 56 (2020) 100930 0911-6044/ © 2020 Elsevier Ltd. All rights reserved. T