Abstract
The middle longitudinal fascicle (MdLF) is a long, associative white matter tract connecting the superior temporal gyrus (STG) with the parietal and occipital lobe. Previous studies show different cortical terminations, and a possible segmentation pattern of the tract. In this study, we performed a post-mortem white matter dissection of 12 human hemispheres and an in vivo deterministic fiber tracking of 24 subjects acquired from the Human Connectome Project to establish whether a constant organization of fibers exists among the MdLF subcomponents and to acquire anatomical information on each subcomponent. Moreover, two clinical cases of brain tumors impinged on MdLF territories are reported to further discuss the anatomical results in light of previously published data on the functional involvement of this bundle. The main finding is that the MdLF is consistently organized into two layers: an antero-ventral segment (aMdLF) connecting the anterior STG (including temporal pole and planum polare) and the extrastriate lateral occipital cortex, and a posterior-dorsal segment (pMdLF) connecting the posterior STG, anterior transverse temporal gyrus and planum temporale with the superior parietal lobule and lateral occipital cortex. The anatomical connectivity pattern and quantitative differences between the MdLF subcomponents along with the clinical cases reported in this paper support the role of MdLF in high-order functions related to acoustic information. We suggest that pMdLF may contribute to the learning process associated with verbal-auditory stimuli, especially on left side, while aMdLF may play a role in processing/retrieving auditory information already consolidated within the temporal lobe.
Highlights
In the last decade, a renewed interest in white matter tracts studied both in vivo [“virtual” dissection by diffusion tensor magnetic resonance imaging (MRI) -diffusion tensor imaging (DTI)- and intraoperative electrical stimulation] and post-mortem [different modifications of the Klinger technique (Agrawal et al, 2011) in most studies] has led to an improved understanding of the morphology and function of the complex system of human associative fibers involved in several higher neurocognitive functions.The middle longitudinal fascicle (MdLF) is classically defined as an associative longitudinal fiber bundle connecting temporal, parietal, and occipital lobes
Its fibers run in an antero-posterior direction in the deep white matter of the superior temporal gyrus (STG) directed toward the parieto-occipital junction
Delicate dissection of the fibers of these two subcomponents showed that the anterior, deeper, branch of the MdLF runs from the planum polare (PP), lateral anterior STG (aSTG), and temporal pole (TP) to the LOC, and to lesser extent to the lateral extension of the Cu
Summary
The middle longitudinal fascicle (MdLF) is classically defined as an associative longitudinal fiber bundle connecting temporal, parietal, and occipital lobes This white matter pathway has received less attention and is one of the most puzzling bundles because its morphology and function are still poorly understood. The parietal cortical terminations have been the object of debate and some controversial results between post-mortem and in vivo studies are reported This may be partially due to the fact that MdLF was first recognized in monkeys, where it interconnects the superior temporal gyrus (STG) with the inferior parietal lobule (IPL) (Seltzer and Pandya, 1984). DTI studies in humans have shown cortical terminations at the level of the IPL, namely at both the angular (AG) and supramarginal (SMG) gyri (Menjot de Champfleur et al, 2013; Makris et al, 2017)
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