Converging Evidence for Differential Specialization and Plasticity of Language Systems

Abstract

Functional specialization and plasticity are fundamental organizing principles of the brain. Since the mid-1800s, certain cognitive functions have been known to be lateralized, but the provenance and flexibility of hemispheric specialization remain open questions. Language is a uniquely human phenomenon that requires a delicate balance between neural specialization and plasticity, and language learning offers the perfect window to study these principles in the human brain. In the current study, we conducted two separate functional MRI experiments with language learners (male and female), one cross-sectional and one longitudinal, involving distinct populations and languages, and examined hemispheric lateralization and learning-dependent plasticity of the following three language systems: reading, speech comprehension, and verbal production. A multipronged analytic approach revealed a highly consistent pattern of results across the two experiments, showing (1) that in both native and non-native languages, while language production was left lateralized, lateralization for language comprehension was highly variable across individuals; and (2) that with increasing non-native language proficiency, reading and speech comprehension displayed substantial changes in hemispheric dominance, with languages tending to lateralize to opposite hemispheres, while production showed negligible change and remained left lateralized. These convergent results shed light on the long-standing debate of neural organization of language by establishing robust principles of lateralization and plasticity of the main language systems. Findings further suggest involvement of the sensorimotor systems in language lateralization and its plasticity.SIGNIFICANCE STATEMENTThe human brain exhibits a remarkable ability to support a vast variety of languages that may be acquired at different points in the life span. Language is a complex construct involving linguistic as well as visual, auditory, and motor processes. Using functional MRI, we examined hemispheric specialization and learning-dependent plasticity of three language systems—reading, speech comprehension, and verbal production—in cross-sectional and longitudinal experiments in language learners. A multipronged analytic approach revealed converging evidence for striking differences in hemispheric specialization and plasticity among the language systems. The results have major theoretical and practical implications for our understanding of fundamental principles of neural organization of language, language testing and recovery in patients, and language learning in healthy populations.

What Can Glioma Patients Teach Us about Language (Re)Organization in the Bilingual Brain: Evidence from fMRI and MEG

“…A large number of neurocognitive studies have since revealed how the dynamics of the language network change after the acquisition of a second language. These empirical findings have suggested that this neuroplastic process permeates both brain structure and function [ 2 , 3 , 6 , 67 ]. This process has also been explored using longitudinal approaches.…”

“…A remarkable feature of the central nervous system is its inherent capacity to dynamically reorganize its structure and function depending on the environment [ 1 , 2 , 3 , 4 , 5 , 6 ]. This capacity for neuroplasticity can give rise to changes in both healthful (e.g., acquisition of a new language; [ 7 , 8 ]) and pathological (e.g., presence of brain lesions; [ 4 , 5 ]) contexts.…”

Cerebral lateralisation of first and second languages in bilinguals assessed using functional transcranial Doppler ultrasound

“…Study 2 included participants with proficiency levels varying from basic to advanced as measured by the standardised Quick Placement Test. Gurunandan et al (2020) reported that increasing proficiency of L2 accompanied more divergent lateralization patterns between L1 and L2. This result was not replicated in our study, with participants of all proficiencies showing similar LI strengths across all tasks.…”

“…It is worth highlighting that our studies only used expressive language tasks, which typically produce strong lateralisation. Where discrepancies in laterality have previously been reported, this has been for receptive language tasks - both in behavioural contexts (dichotic listening), and in neuroimaging (comprehension or lexical decision tasks) ( Gurunandan et al , 2020 ; Hull & Vaid, 2007 ; Wartenburger et al , 2003 ). It is possible that the processes that drive this effect seen in the literature are not recruited during expressive language production.…”

“…In contrast, Dehaene et al , 1997 , found, consistent with other studies, that when listening to L1, there was consistency between participants in the locus of activation in the left hemisphere, but when listening to L2, there was substantial variability from person to person, not just within a hemisphere, but also in terms of which hemisphere was most activated. A recent study of basic and advanced L2 learners by Gurunandan et al (2020) reported that, whereas language production tended to be left-lateralised in both languages, in receptive tasks, the two languages tended to lateralise to opposite hemispheres, with this effect increasing with language proficiency. For language production, the size of the laterality index showed only weak agreement between L1 and L2, regardless of proficiency.…”