Consistent with our hypothesis of better maintenance of white matter integrity in bilinguals, higher FA values in the CC (p < 0.05, corrected, ) were seen in the bilingual group, extending posteriorly to the bilateral superior longitudinal fasciculi, and anteriorly to the right inferior frontal – occipital fasciculus and uncinate fasciculus. Monolinguals did not show higher FA values than bilinguals in any voxel. TBSS was also used to assess between-group differences in axial and radial diffusivity. There was no group difference in AD, but monolinguals had higher RD in the body of the CC, overlapping with some of the areas where bilinguals had higher FA (p < 0.05, corrected, ). Note that the FA values in the genu of CC for both groups (bilinguals: M = 0.47, SD = 0.04; monolinguals: M = 0.44, SD = 0.03) were lower than those usually reported for younger adults (M = 0.55, SD = 0.04 from Davis et al., 2009
), as would be expected.
Figure 1 Group differences in FA and RD. a, Increased FA in bilinguals compared to monolinguals. Colored voxels (orange-red) are significant clusters (p < 0.05, corrected) in corpus callosum, extending to bilateral superior longitudinal fasciculi and right (more ...)
To test the hypothesis that greater WM integrity in the bilinguals would be associated with a more distributed pattern of functional activity, we assessed functional connectivity of two seed voxels. We chose seeds in bilateral inferior frontal gyri because these regions are important for bilingual language switching (Garbin et al., 2010
; Luk et al., 2011
), and we found greater FA in the right inferior frontal gyrus, as well as in the CC connecting the frontal lobes, in bilinguals (see ). The right inferior frontal seed (MNI: x = 38, y = 31, z = −14) was located in gray matter adjacent to where we observed a group difference in FA (see ). The left seed was located in the homologous region of the inferior frontal gyrus (x = −38, y = 31, z = −14; ). Although no FA difference was observed here, this seed was very similar to a region identified in a meta-analysis of brain regions active during bilingual switching (Luk et al., 2011
, x = −33, y = 21, z = −14).
Figure 2 Resting-state functional connectivity for monolinguals and bilinguals. a, Locations of the two seed voxels in LIFG and RIFG. b, c, Correlations between activity in LIFG and RIFG seed voxel and activity in the positive and negative brain networks across (more ...)
For the left inferior frontal seed, we found a statistically reliable pattern that differentiated bilinguals from monolinguals (accounting for 6% of the variance in the data, p < 0.001). Positive correlation values in indicate a positive correlation between seed activity and activity in the warm-colored regions seen in ; a negative correlation indicates the opposite pattern, i.e., that seed activity was positively correlated with activity in the cool-colored regions. These correlation distributions were significantly different between the groups (Wilcoxon normal approximation Z = −4.2, p < 0.0001). Thus, for the left inferior frontal gyrus, bilinguals had stronger functional connectivity with bilateral middle temporal gyri, right inferior parietal lobule, precuneus, bilateral middle occipital gyri, and left caudate. This increase in connectivity with posterior regions suggests more long-range frontal – occipital and frontal – parietal functional connections in the bilinguals. In contrast, the monolinguals showed stronger functional connectivity between the left inferior frontal region and other anterior areas, including bilateral precentral gyri, right superior frontal gyrus, right middle frontal gyrus, and left medial frontal gyrus, suggesting stronger functional connections within the frontal lobes and closer neighbors (see Notes for more information). This pattern of stronger functional connectivity between anterior – posterior regions in bilinguals and stronger intrafrontal connectivity in monolinguals is consistent with the higher FA values seen in long-range WM connections in the bilinguals, particularly in the longitudinal fasciculi.
The right inferior frontal gyrus seed also showed a statistically significant functional connectivity pattern that differentiated the two language groups (p < 0.001, accounting for 4.4% of the data variance). The correlations in the groups across the resting state run were less distinct than seen for the left inferior frontal region (see ), but were nevertheless significantly different (Wilcoxon normal approximation Z = 3.4, p < 0.0008). Interestingly, the pattern of group differences was similar to that seen for the left inferior frontal region (see Notes for more information). Monolinguals had stronger functional connectivity between the right inferior frontal seed and other frontal regions, whereas bilinguals had stronger functional interactions between the seed and occipitoparietal regions ().