This data collection entails a range of data collected from monolingual and bi/multilingual adult participants living in and around Birmingham, United Kingdom. This includes questionnaire data (mainly from the Language and Social Background questionnaire), EEG data collected during the execution of cognitive control tasks (Flanker task, Simon task, Colour-Shape switching task, Number-letter task) and structural MRI brain scans. For the majority of participants all type of data is available.The number of children learning a second language at school in the UK has dramatically decreased in the past 10 years, even though learning a second language has many benefits. It broadens understanding of different cultures, improves literacy and employability, and importantly, may also be good for the brain. Being bilingual appears to have a positive effect on a collection of high level cognitive abilities, often referred to as executive functions such as response inhibition and task switching. Speaking another language might also slow down cognitive decline by delaying the onset of dementia. The reason for these effects is thought to be the constant inhibition of one language when speaking the other and frequent language switches. Moreover, it is known that poor executive functions are linked to social problems including antisocial behaviour or addiction. Improving executive functions via language learning might potentially lessen such problems. Despite a growing and large body of evidence in favour of executive advantages, some studies have not been able to replicate the findings. One often suggested explanation for these inconsistencies is the heterogeneity of the bilingual population. Bilingual speakers differ with respect to when they started to learn their second language, how fluently they speak it and how often they use each language. All these factors have been found to be independently related to executive function abilities, but no study to date has systematically investigated these factors. Identifying the precise characteristics that bring out a bilingual advantage is crucial in identifying the conditions under which improved executive control behaviour can be achieved. The proposed study will use a multimodal approach to provide a systematic and thorough investigation into the effects of bilingual key characteristics (second language proficiency, age of second language acquisition, and language use) on executive control behaviour as well as the underlying brain differences. We will obtain from bilingual and monolingual speakers measures of individual differences in language proficiency and use, measures of behaviour in executive function tasks, and of brain differences. For the latter, we will measure differences in brain structure (grey and white matter) and communication between brain areas (functional brain connectivity). Participants will be tested on the two executive functions that have most strongly been associated with bilingual advantage: inhibition and switching ability. Participants' brain structure will be measured using voxel-based morphometry (VBM), their structural connectivity using diffusion imaging (DTI). Bilingual functional brain connectivity will for the first time be measured using electro-encephalography (EEG), a particularly good real-time measure of brain communication. Furthermore, we will determine the neural sources of the EEG functional connectivity measures by testing a subset of both bilingual and monolingual participants additionally on one inhibition task and one switching task while simultaneously measuring their EEG and blood-oxygen-level dependent (BOLD) signals via functional magnetic resonance imaging (fMRI). In contrast to EEG, fMRI is an excellent measure of where in the brain processing differences occur. Combined EEG-fMRI is a novel approach to the bilingual advantage question. Critically, all these measures will be acquired in the same participants. This will enable us to describe the complex interactions between behaviour, brain structure, and neural functions in executive function tasks and how those interactions are modulated by the individual participant characteristics. It will be the first time that such a multidisciplinary and comprehensive approach is used to investigate the brain networks underlying bilingualism and the benefits in executive control they might support.

Participants were young adults (age 18-30). They were recruited via the Research Participation Scheme of the School of Psychology at the University of Birmingham as well as through social media outlets. Some were paid course credits, others received money for participation. Participants filled in the Language and Social Background Questionnaire (LSBQ), the Oxford Quick Placement Test (OQPT), a phonemic and a semantic verbal fluency task in both English and their other language, the Switching Experience & Environments Questionnaire (SEEQ), the Standardised Progressive Matrices assessment, and the National Adult Reading Test (NART). They also took part in a Flanker task, Simon task, Colour-Shape switching task, and the Number-Letter switching task. Finally, their brain structure was scanned (DTI and T1 VBM scans) using a Siemens MAGNETOM Prisma 3T MRI system.