Regimented language training. Changes to brain and behaviour following intensive non-native language learning
Abstract: The influence of foreign-language acquisition on adult brain structure remains mostly unexplored. Whilst several cross-sectional studies have led to further understanding of how language can affect brain structure, only a few longitudinal studies have been conducted. We measured the effects of language acquisition on behaviour and brain structure at the Swedish Armed Forces Language School in two separate cohorts of army interpreters. Our hypothesis was that intensive language studies would lead to increases in associative memory performance, hippocampal volume, relevant cortical areas and connectivity between areas of the brain. In Study 1 we compared performance on 8 cognitive tasks before and after 3 months of language learning (n = 15) to a control group of university students (n = 19). Behavioural measures revealed increases in associative memory performance; remembering names and faces, a paired associates task, which is known to involve hippocampal processing. In Study 2 semiautomatic tracing of the hippocampus using FreeSurfer was compared to manual tracing and deemed equivalent to manual tracing in young adults (n = 44) but not in older adults (n = 47). Evaluation was based on data from earlier studies. Study 3 used FreeSurfer to measure grey matter volume in subcortical structures as well as cortical thickness in the next cohort of interpreters and university students (n = 14 and 17, respectively). Results revealed increases in hippocampal volume and cortical thickness of the left middle frontal gyrus, inferior frontal gyrus, and superior temporal gyrus. The right hippocampus and the left superior temporal gyrus, both key regions in language acquisition, increased relatively more in interpreters with higher language proficiency following training. Interpreters struggling to stay at the academy showed relatively higher increases in the middle frontal gyrus, a region that is known to be involved in processing difficult speech. In Study 4 the same group of interpreters as study 3 was measured using Diffusion Tensor Imaging. No changes in white matter microstructure were observed. A possible explanation is that interpreters had well developed tracts before starting their training at the academy. Effects in this direction could be observed as lower values of radial diffusivity in interpreters relative to controls. The combined results showed that grey matter volume in relevant language areas as well as the right hippocampus increased for interpreters compared to controls. Furthermore, increases were related to separate behavioural measures and ratings. The results indicate that grey matter plasticity is not only possible during language acquisition, but the extent and region of plastic changes varies between individuals.
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