Dynamic Neural Network Reorganization Associated with Second Language Vocabulary Acquisition: A Multimodal Imaging Study

Abstract

It remains unsettled whether human language relies exclusively on innately privileged brain structure in the left hemisphere or is more flexibly shaped through experiences, which induce neuroplastic changes in potentially relevant neural circuits. Here we show that learning of second language (L2) vocabulary and its cessation can induce bidirectional changes in the mirror-reverse of the traditional language areas. A cross-sectional study identified that gray matter volume in the inferior frontal gyrus pars opercularis (IFGop) and connectivity of the IFGop with the caudate nucleus and the superior temporal gyrus/supramarginal (STG/SMG), predominantly in the right hemisphere, were positively correlated with L2 vocabulary competence. We then implemented a cohort study involving 16 weeks of L2 training in university students. Brain structure before training did not predict the later gain in L2 ability. However, training intervention did increase IFGop volume and reorganization of white matter including the IFGop-caudate and IFGop-STG/SMG pathways in the right hemisphere. These "positive" plastic changes were correlated with the gain in L2 ability in the trained group but were not observed in the control group. We propose that the right hemispheric network can be reorganized into language-related areas through usedependent plasticity in young adults, reflecting a repertoire of flexible reorganization of the neural substrates responding to linguistic experiences.

The neuroanatomy of bilingualism: how to turn a hazy view into the full picture

“…However, these last two studies used a completely different measure and methodology to those of the former studies. In line with results in cross-sectional studies, there are also two longitudinal studies: one showed increased FA in CC for learners vs. non-learners (Schlegel et al, 2012) and another one showed increased WM volume inside the right IFG (Hosoda et al, 2013).…”

“…Hypothesis (Abutalebi & Green, 2007;Green & Abutalebi, 2013), there is just one region predicted by the model that consistently shows up across studies as a structural difference due to bilingualism: the left/right IFG (Hosoda et al, 2013;Klein et al, 2014;Grogan et al, 2012;Luk et al, 2011;García-Pentón et al, 2014). Some of the studies used alternatives to the traditional methods of VBM and ROI-based analysis and analyzed the whole brain to reveal effects of bilingualism in the IFG and in the connections between there and other regions (Luk et al, 2011;Klein et al, 2014;García-Pentón et al, 2014).…”

“…Some of the studies used alternatives to the traditional methods of VBM and ROI-based analysis and analyzed the whole brain to reveal effects of bilingualism in the IFG and in the connections between there and other regions (Luk et al, 2011;Klein et al, 2014;García-Pentón et al, 2014). Furthermore, others studies support these results showing that this region is also sensitive to L2 AoA and proficiency (Grogan et al, 2012;Hosoda et al, 2013;Klein et al, 2014). Additionally, several WM structural studies also confirmed differences between bilinguals and monolinguals in the tracts connecting IFG with many other regions in the frontal lobe (including the contralateral side) and the temporal, parietal and occipital regions in the back of the brain, specifically the CC (Luk et al, 2011;Pliatsikas et al, 2015;Gold et al, 2013;Mohades et al, 2012) and the IFOF (Luk et al, 2011;Pliatsikas et al, 2015;Mohades et al, 2012;Gold et al, 2013;Cummine & Boliek, 2013).…”

The effects of bilingualism on the white matter structure of the brain

“…Further evidence for the effects of late L2 learning was presented by Hosoda and colleagues (23), who reported a crosssectional and a longitudinal study on Japanese learners of English. Participants in the cross-sectional study demonstrated a positive correlation between the size of their L2 vocabulary and the FA values in the right hemisphere, including the subcortical WM beneath IFG, the arcuate fasciculus, and the IFGcaudate nucleus and IFG-superior temporal gyrus pathways.…”

“…Although it appears that WM changes are to be expected in early bilinguals, the evidence on late bilinguals is still inconclusive. There is evidence that linguistic training can indeed affect WM structure (22), but it also appears that these effects might be temporary if the training is terminated (23). In this study, we scanned a group of highly proficient and highly immersed late L2 speakers of English who did not receive any linguistic training at the time of testing and compared them with a group of monolinguals with a TBSS analysis.…”