Brain-structural differences underlying dialect competence in the bilingual network.

Research in multilingualism has provided evidence for brain structural differences between monolinguals and bilinguals. Less is known about speakers who, apart from a standard language, also show competence in a variety (dialect) of this language ("bidialectals"). In these populations, cultural differences are minimized and language competence can be compared against a common ground. We hypothesize that bidialectals show brain-structural differences to non-bidialectals, and capitalize on one of the world's largest dialect corpus available for German. A competent dialect group (N = 26) and a group without dialect competence (N = 23) were compared by using brain structural measures, including gray matter volume (GMV) and cortical thickness (CT). Results demonstrate a whole-brain group difference, seen for CT in right orbitofrontal cortex, and for GMV in bilateral middle temporal gyrus and bilateral insula. Notably, CT in right fusiform cortex as well as GMV in bilateral middle temporal gyrus and right insula co-varied with dialect competence differently for the two groups. The structural differences and covariations with dialect competence are discussed on the background of code switching and language control. The findings suggest that dialect competence may shape brain structure in ways similar to bilingualism.