In sum, we did not find any evidence that subjects with right (or even bilateral) hemispheric language organization also have rightward insular surface asymmetry.
Examining the spatial location of this trajectory on the insular surface revealed that these extremes were located in the anterior dorsal and ventral insula and in the dorsal and posterior part of the long insular gyri, respectively.
Structural asymmetries, expressed as laterality indices (LIs), were calculated according to the formula: LI = [(L − R)/(L + R)] × 100 where L (left) and R (right) reflect the cortical volume, insular surface area or cortical thickness in the left or right hemisphere.
Having the possibility to investigate these relationships further, we examined whether these effects were driven by hemispheric differences in the extent of insular surface rather than the thickness of its cortex.
Finally, insular surface asymmetries showed significant correlations with the functional asymmetries of activity for gesture and language in the insula itself (r13 = 0.49, P = 0.033; r13 = 0.47, P = 0.038; respectively, one-tailed).
Here, we also corroborate that the left-sided asymmetry of the insular surface, as well as the asymmetry of its volume, correlates well with the lateralization of language (i.e. verbal fluency).
Although it is not clear to what extent such structural differences are inherent or experience-driven, there is evidence that some inter-hemispheric insular asymmetries can be observed as early as at birth, with greater insular surface in the left hemisphere (Li et al., 2014).
As far as insular surface is considered, the correlations with functional activity in the SMG ROI (r25 = 0.29, P = 0.07, one-tailed) as well as in the functional insular ROI (r25 = 0.32, P = 0.06, one-tailed) showed only clear trends towards significance.
All in all, only the insular surface exhibited significant leftward asymmetry in both analyses (mean LI for structural ROI = 5.91, SD = 3.02; a significant difference in cluster size = 0.05 > P > 0.01 for vertex-wise analysis in favour of the left side).
Although we observed significant correlations between insular volume or surface asymmetries and functional lateralization of gesture and language, correlations never imply causation.
