Neural Plasticity of Language Systemsevidence from fMRI experiments with adult language learners

Résumé

Functional specialisation and plasticity are fundamental organising principles of the brain. Language is a uniquely human phenomenon that requires a delicate balance between neural specialisation and plasticity, and language learning offers the perfect window to study these principles in the human brain. Though the human brain exhibits a remarkable ability to support a variety of languages that may be acquired at different points in the life span, the capacity for neural reorganisation decreases with age. Further, language is a complex construct involving linguistic as well as visual, auditory, and motor processes. The current doctoral thesis asked two main questions: (1) Do large-scale functional changes accompany language learning in adulthood? and (2) Are these neural changes similar across different language systems such as reading, speech comprehension, and verbal production? These questions were investigated in three fMRI experiments with adult language learners. In Experiments I and II, comprehension and production were examined in 30-to-60-year-old intermediate and advanced language learners and functional learning-related changes in each modality were comprehensively characterised. In Experiment III, hemispheric lateralisation of reading, speech comprehension, and verbal production were compared and contrasted, and the analyses were extended to a second longitudinal study with a contrasting participant sample. Robust evidence was found for significant functional plasticity well into adulthood, and results showed that different language systems exhibited different patterns of hemispheric specialisation and plasticity. The results have theoretical and practical implications for our understanding of fundamental principles of neural organisation of language, language learning in healthy populations, and language testing and recovery in patients.