fMRI-guided tractography of language processing streams in the healthy brain

Introduction: Recovery of language function after aphasic stroke appears to be related not only to the intactness of cortical language areas but also to the integrity of the interconnecting temporo-frontal fibre tracks. Our study was set out to compile a neuroanatomical atlas of the language network using a probabilistic fibre tracking tool based on Diffusion Tensor Imaging (DTI) for mapping intra- and interhemispheric connections of fMRI-defined cortical activation knots.

Methods: We examined 19 healthy subjects with fMRI and DTI. The main knots of the language network were identified by a language comprehension task with auditory presentation of (i) intelligible sentences and (ii) non-intelligible sentences of reversed speech. The peak voxels of the group activation were equally expanded and served as seed points for the DTI-based probabilistic fibre tracking.

Results: Comparing the intelligible and reversed sentences, the group analysis revealed bilateral, but left lateralized activation with two peaks in the inferior frontal gyrus [IFG, pars orbitalis (“orbIFG“) and pars triangularis (“triIFG“)] and three activation peaks in the temporal lobe [temporal pole (“TP“), anterior (“aMTG“) and posterior (“pMTG“) middle temporal gyrus). The fibre tracking revealed the following interconnecting pathways:

(i) Left intrahemispheric temporo-frontal: (a) high probability was found in all subjects for a ventral connection via the uncinate fasciculus and the external capsule for all temporal areas to both the orbIFG and triIFG; (b) additional high probability for a dorsal connection via the superior longitudinal fasciculus was found for the posterior temporal seed area and the dorsal frontal target area [pMTG to orbIFG (in 4 of 19 subjects), TP to triIFG (4/19), aMTG to triIFG (6/19) and pMTG to triIFG (19/19)].

(ii) Interhemispheric temporal: (a) TP with highest probability via the anterior commissure (AC, 7/19), splenium of the corpus callosum (sCC, 6/19) and the genu/rostum of the CC (gCC 6/19); (b) aMTG via the sCC (11/19), the AC (5/19) and the gCC (3/19); (c) pMTG via the sCC (19/19).

(iii) Interhemispheric frontal: (a) orbIFG via the gCC (19/19) and additionally via the AC (4/19); (b) triIFG via the anterior truncus of the CC (19/19).

Conclusions: These results may serve as a neuroanatomical atlas of the language network in patients with aphasia after a focal lesion for better interpretation of altered language activation during recovery after stroke.