Age-related comparative study of interhemispheric communication in monolinguals and bilinguals

The corpus callosum is the largest white matter structure of the human brain, and it enables interhemispheric transfer of information. Research has shown that bilinguals have larger anterior regions of the corpus callosum when compared to monolinguals. Besides, as a potential consequence of age-related thinning of the corpus callosum, older adults seem to experience less efficient interhemispheric transfer in certain tasks. Through three different behavioural studies, the work described in this thesis analysed the impact of bilingualism and age on the communication between the hemispheres. To that aim, I recruited several groups of younger (18-25 years) and older (65-85 years) monolinguals and bilinguals to take part in one lab-based and two web-based studies which used the divided-visual field paradigm to measure interhemispheric transfer through reaction times and accuracy. Apart from one single experiment, no significant difference between monolinguals’ and bilinguals’ interhemispheric transfer was found. However, old age of participants was generally responsible for slower reaction times and had an impact on the bilateral processing advantage, a well-researched phenomenon where interhemispheric transfer is more efficient than intrahemispheric communication for more demanding tasks. These results seem to suggest that although lifelong bilingualism can increase white matter integrity, it does not have an impact on the speed of interhemispheric transfer. Ageing, on the contrary, remains an important factor to consider when studying the corpus callosum and its function.