Herein, we contextualize, problematize, and offer some insights for moving beyond the problem of monolingual comparative normativity in (psycho) linguistic research on bilingualism. We argue that, in the vast majority of cases, juxtaposing (functional) monolinguals to bilinguals fails to offer what the comparison is supposedly intended to do: meet the standards of empirical control in line with the scientific method. Instead, the default nature of monolingual comparative normativity has historically contributed to inequalities in many facets of bilingualism research and continues to impede progress on multiple levels. Beyond framing our views on the matter, we offer some epistemological considerations and methodological alternatives to this standard practice that improve empirical rigor while fostering increased diversity, inclusivity, and equity in our field.

Recent years have seen an expansion in the research related to structural brain adaptations related to the acquisition and processing of additional languages. However, the accumulating evidence remains to a great extent inconsistent, with a large variety of cortical, subcortical, and cerebellar effects reported in various studies. Here we propose that the variability in the data can be explained by the differences in the language background and experiences of the tested samples. We also propose that the field should move away from monolithic bilingual versus monolingual comparisons; instead, it should focus on the experiences of the bilingual groups as predictors of structural changes in the brain, and also employ longitudinal designs to test the dynamic effects of active bilingualism. The implications of the proposed approaches for the suggested benefits of bilingualism on ageing and patient populations are also discussed.

Long-term management of more than one language has been argued to contribute to changes in brain and cognition. This has been particularly well documented in older age, where bilingualism has been linked to protective effects against neurocognitive decline. Since memory difficulties are key aspects of this decline, herein we examine potential effects of bilingualism on the hippocampus, a brain structure related to memory that is particularly vulnerable to cognitive ageing. Hippocampal volume has been shown to increase as a result of second language learning and use in younger adults. However, it is unknown if this is maintained throughout the lifespan. We examine hippocampal volume and episodic memory performance in a participant sample consisting of healthy older individuals with a wide range of experiences in exposure and using a second language. Results reveal greater hippocampal volume calibrated to degree of quantified dual language use. Our results mirror those of immersive active bilingualism in younger populations, suggesting that long-term active bilingualism leads to neuroprotective effects in the hippocampus. We discuss this in the context of literature proposing bilingualism-induced brain reserve in the older age.

Cognitively demanding experiences, including complex skill acquisition and processing, have been shown to induce brain adaptations, at least at the macroscopic level, e.g. on brain volume and/or functional connectivity. However, the neurobiological bases of these adaptations, including at the cellular level, are unclear and understudied. Here we use bilingualism as a case study to investigate the metabolic correlates of experience-based brain adaptations. We employ Magnetic Resonance Spectroscopy to measure metabolite concentrations in the basal ganglia, a region critical to language control which is reshaped by bilingualism. Our results show increased myo-Inositol and decreasedN-acetyl aspartate concentrations in bilinguals compared to monolinguals. Both metabolites are linked to synaptic pruning, a process underlying experience-based brain restructuring. Interestingly, both concentrations correlate with relative amount of bilingual engagement. This suggests that degree of long-term cognitive experiences matters at the level of metabolic concentrations, which might accompany, if not drive, macroscopic brain adaptations.

Bilingualism has been associated with increases in compensatory mechanisms to age-related neurocognitive decline thus delaying dementia symptom onset and leading to a more favorable trajectory of neurocognitive aging. However, most research to date has examined bilingualism-induced effects on neurocognition within older age ranges or young adults - with middle-aged individuals typically not being a population of interest. Furthermore, bilingualism is often treated as a dichotomous variable, despite it being a heterogeneous experience on an individual level. In the present study, we employed diffusion tensor imaging (DTI) to examine whether bilingualism, and the degree of engagement in bilingual experience, modulates the nature or rate of white matter decline associated with aging. DTI data and language history data were collected from a cohort of monolingual and bilingual individuals spanning a wide age range. Two separate analyses were run. First, generalized additive models were run on matched monolingual and bilingual samples, examining effects of age on the trajectory of white matter integrity and how bilingualism modulates this effect. This analysis revealed a significant effect of age within the monolingual group for fractional anisotropy values in the right superior longitudinal fasciculus. However, the age effect within the bilingual group was not significant, indicating a faster decline in white matter integrity within the monolingual cohort. Second, general linear models were run on the entire participant sample, examining an interaction between age and degree of bilingual engagement on white matter integrity. Results from this analysis indicate that increased engagement in bilingual language use across the lifespan correlates with a slower decline in white matter integrity with age. Together these results indicate bilingualism, and specifically degree of bilingual engagement, impacts the trajectory of age-related decline in white matter integrity across the lifespan.