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

Background: Lateralised representation of language in monolinguals is a well-established finding, but the situation is much less clear when there is more than one language. Studies to date have identified a number of factors that might influence the brain organisation of language in bilinguals. These include proficiency, age of acquisition and exposure to the second language. The question as to whether the cerebral lateralisation of first and second languages are the same or different is as yet unresolved. Methods: We used functional transcranial Doppler sonography (FTCD) to measure cerebral lateralisation in the first and second languages in 26 high proficiency bilinguals with German or French as their first language (L1) and English as their second language (L2). FTCD was used to measure task-dependent blood flow velocity changes in the left and right middle cerebral arteries during word generation cued by single letters. Language history measures and handedness were assessed through self-report questionnaires. Results:The majority of participants were significantly left lateralised for both L1 and L2, with no significant difference in the size of asymmetry indices between L1 and L2. Asymmetry indices for L1 and L2 were not related to language history, such as proficiency of the L2. Conclusion:  In highly proficient bilinguals, there is strong concordance for cerebral lateralisation of first and second languages.