Age of acquisition impacts the brain differently depending on neuroanatomical metric

Abstract

Although researchers generally agree that a certain set of brain areas underlie bilingual language processing, there is discrepancy regarding what effect timing of language acquisition has on these regions. We aimed to investigate the neuroanatomical correlates of age of acquisition (AoA), which has been examined previously, but with inconsistent results, likely influenced by methodological differences across studies. We analyzed gray matter density, volume, and thickness using whole-brain linear models in 334 bilinguals and monolinguals. Neuroanatomical correlates of AoA differed depending on gray matter metric. Relative to early bilinguals, late bilinguals had thicker cortex in language processing and cognitive control regions, and greater density in multiple frontal areas and the right middle temporal and supramarginal gyri. Early bilinguals had greater volume than late bilinguals in the left middle temporal gyrus. Overall, volume was the least sensitive to AoA-related differences. Multiple regions not classically implicated in duallanguage processing were also found, which highlights the important role of whole-brain analyses in neuroscience. This is the first study to investigate AoA and gray matter thickness, volume, and density all in the same sample. We conclude that cognitive models of bilingualism should consider the roles of development and neuroanatomical metric in driving our understanding of bilingual and monolingual language organization. K E Y W O R D S age of acquisition, bilingualism, gray matter, language development, magnetic resonance imaging, neuroanatomy, replication crisis

Cortical Thickness in bilingual and monolingual children: Relationships to language use and language skill

“…Therefore, some of the bilingual children may have heard some English in the home, while other bilingual children may not have been exposed to English until ages 4–6, when they entered school. Research suggest that the age at which a second language was acquired impacts brain structure ( Claussenius-Kalman et al, 2020 ; Klein et al, 2014 ). Specifically, theories such as the sensorimotor hypothesis ( Hernandez and Li, 2007 ) suggest that when a language is learned in infancy, it builds from low-level sensory information to complex speech in the same way that any native language develops.…”

Highly proficient, balanced bilingualism is related to thinner cortex in two cognitive control regions

“…These findings may be caused by neurobiological predispositions, as a result of learning, or as an interaction between these two factors 4,8,17,31 . The debate over the influence of nature versus nurture is pertinent here: Are highly proficient, balanced bilinguals better at learning the L2 and maintaining the L1 because their neurocognitive characteristics facilitate it, or does conscious retention of the L1 and exceptional L2 learning cause changes in neurocognition?…”

“…One limitation of this study is that other neuroanatomical metrics beyond cortical thickness may capture other language history–related effects. We chose to examine cortical thickness on the basis of results from our previously published research (Claussenius‐Kalman et al 17 …”

“…This structural plasticity is thought to be the result of changes in dendritic growth, synaptic pruning, and the amount of glia, with the direction of growth/shrinkage depending on the mastery level, skill type, developmental stage, and particular brain region 16 . In our previously published research, 17 we examined the neuroanatomical effects of language history (specifically AoA) according to three different neuroanatomical metrics (gray matter volume, density, and cortical thickness) across two different neuroimaging analysis software packages (Statistical Parametric Mapping (SPM)‐12 and FreeSurfer). Before our study, eight previous studies had examined the relationship between individual measures of gray matter structure (for example, Klein et al 18 .…”

Clear Theories Are Needed to Interpret Differences: Perspectives on the Bilingual Advantage Debate

“…Indeed, there is a large amount of literature focusing on differences between bilinguals and monolinguals without making claims regarding behavioural consequences or "bilingual advantages". Hernandez, 2020;DeLuca et al, 2020;Del Maschio et al, 2019;Gullifer et al, 2018). These studies will be of great use to further develop theories on neural changes in response to language learning and/or bilingualism (e.g., Pliatsikas, 2019).…”