Hemispheric asymmetries in dorsal language pathway white-matter tracts: A magnetic resonance imaging tractography and functional magnetic resonance imaging study

Abstract

Introduction Previous studies have shown that the arcuate fasciculus has a leftward asymmetry in right-handers that could be correlated with the language lateralisation defined by functional magnetic resonance imaging. Nonetheless, information about the asymmetry of the other fibres that constitute the dorsal language pathway is scarce. Objectives This study investigated the asymmetry of the white-matter tracts involved in the dorsal language pathway through the diffusion tensor imaging (DTI) technique, in relation to language hemispheric dominance determined by task-dependent functional magnetic resonance imaging (fMRI). Methods We selected 11 patients (10 right-handed) who had been studied with task-dependent fMRI for language areas and DTI and who had no language impairment or structural abnormalities that could compromise magnetic resonance tractography of the fibres involved in the dorsal language pathway. Laterality indices (LI) for fMRI and for the volumes of each tract were calculated. Results In fMRI, all the right-handers had left hemispheric lateralisation, and the ambidextrous subject presented right hemispheric dominance. The arcuate fasciculus LI was strongly correlated with fMRI LI ( r = 0.739, p = 0.009), presenting the same lateralisation of fMRI in seven subjects (including the right hemispheric dominant). It was not asymmetric in three cases and had opposite lateralisation in one case. The other tracts presented predominance for rightward lateralisation, especially superior longitudinal fasciculus (SLF) II/III (nine subjects), but their LI did not correlate (directly or inversely) with fMRI LI. Conclusion The fibres that constitute the dorsal language pathway have an asymmetric distribution in the cerebral hemispheres. Only the asymmetry of the arcuate fasciculus is correlated with fMRI language lateralisation.

Keywords: Arcuate fasciculus; Broca; Geshwind; Wernicke; laterality index; superior longitudinal fasciculus.

Figure 1.

Example of tridimensional reconstructions of the dorsal language pathway tracts in the left hemisphere (circles: arcuate fasciculus [AF], arrowheads: superior longitudinal fasciculus [SLF] II; arrow: SLF III; crosses: temporo-parietal inferior parietal lobe connection of SLF [SLF TP-IPL]).

Figure 2.

Example of bilateral AF tridimensional reconstructions in a single subject (L: left; R: right).

Figure 3.

Correlation between the laterality indices of the functional magnetic resonance imaging and of the AF, SLF II/III and SF TP-IPL.