The purpose of this study was to examine the relationships among mentalizing abilities, self-reported autism traits, and two white matter tracts, uncinate fasciculus (UF) and inferior longitudinal fasciculus (ILF), in neurotypical adults. UF and ILF were hypothesized to connect brain regions implicated in a neuroanatomical model of mentalizing. Data were available for 24 neurotypical adults (mean age = 21.92 (4.72) years; 15 women). Tract-based spatial statistics (TBSS) was used to conduct voxelwise cross-participant comparisons of fractional anisotropy (FA) values in UF and ILF as predicted by mentalizing abilities and self-reported autism traits. Self-reported autism traits were positively related to FA values in left ILF. Results suggest that microstructural differences in left ILF are specifically involved in the expression of subclinical autism traits in neurotypical individuals.

Diffusion-weighted magnetic resonance imaging (dMRI) has been used to non-invasively investigate the integrity of white matter and the connectivity of the brain. In this work, high angular resolution diffusion imaging (HARDI), an advanced dMRI methodology was developed and employed in bonnet macaque monkeys to study the connectivity of the orbitofrontal cortex (OFC) and amygdala, two gray matter regions involved in making reward-guided decisions. With age, it is believed that there is a decline in the white matter connectivity between these two regions, also known as uncinate fasciculus (UF), and that this affects reward-value assignment and feedback learning in older adults. The analysis pipeline involved correction for distortions due to eddy currents and field inhomogeneity, noise reduction using a local principal component analysis based technique and subsequent registration to the high-resolution T1-weighted images. Gray matter regions corresponding to OFC and amygdala were identified on the T1-weighted images and probabilistic tractography was carried out to delineate the tracts belonging to UF. The output connectivity map from tractography was used to extract imaging parameters of interest such as fractional anisotropy, axial and radial diffusivity along the UF. A significant reduction in the fractional anisotropy index and the axial diffusivity index along the UF tract was observed with increased age of monkeys. Compared to the left hemisphere, stronger trends were observed in the right hemisphere of the monkeys, indicating possible laterality.

Il est de plus en plus reconnu que les connexions du cerveau jouent un rôle important sur la fonction cérébrale, en particulier les fonctions cognitives supérieures comme le langage. Cependant l’imperfection des techniques traitant les connexions macroscopiques humaines a empêché l’avancement de nos connaissances sur l'anatomie des faisceaux. Nous nous appuyons ainsi aujourd’hui essentiellement sur la littérature du XIXème siècle. Les définitions des trajets et des connexions anatomiques de nombreux faisceaux sont constamment débattues. En utilisant l'imagerie de diffusion, nous réévaluons les anatomies des faisceaux clés dans une grande cohorte saine. Nous utilisons une nouvelle approche de segmentation des faisceaux qui vise à reproduire la méthode introduite par les dissectionistes. Celle-ci définit un tract comme l’ensemble des fibres passant par une même tige, minimisant ainsi l’a priori sur leurs terminaisons. Nous nous concentrons sur les faisceaux d'association des capsules externe et extrême, notamment les faisceaux occipito-frontal inférieur (FOFI) et unciné (FU) impliqués dans le circuit du langage ventral. Nous passons en revue la littérature sur ces tracts, fournissons des descriptions détaillées de leurs connectivités anatomiques et donnons un nouvel éclairage sur leur asymétrie et organisation interne. Dans une première étude, nous confirmons que les deux faisceaux ont de plus vastes projections dans le cortex qu'on ne le pensait, et nous présentons de nouveaux résultats concernant les branches asymétriques des faisceaux. Dans une deuxième étude, nous étudions en profondeur le FU et ses sous composantes. Nous résolvons un débat d’un siècle en exhibant clairement sa frontière avec le FOFI et nous identifions pour chaque sous composante des caractéristiques anatomiques distinctives y compris des asymétries. Ces résultats apportent un éclairage nouveau sur le FOFI et le FU qui sera crucial pour démêler leurs rôles multifonctionnels. / The importance of the brain’s connections for cerebral function is increasingly emphasized especially for higher cognitive functions like language. But the imperfection of the techniques used to address the human macroscopic connections has prevented the advancement of our knowledge on the anatomy of fibre pathways. Thus we rely heavily on XIXth century literature. Controversy surrounding the anatomical course and connections of many fibre pathways persists. Using diffusion imaging, we reevaluate the anatomies of key pathways in a large healthy cohort. We use a novel tract segmentation approach that aims to reproduce the method introduced by dissectionists – defining a tract as all fibers passing through a stem, thus minimizing a priori on their terminations. We focus on the association pathways of the external and extreme capsules, namely the inferior fronto-occipital (IFOF) and uncinate fasciculi (UF), implicated in the ventral language circuitry. We review the literature on these tracts, provide detailed descriptions of their connectional anatomies and present new insights regarding their asymmetry and internal organization. In a first study, we confirm that both tracts have more extensive projections within the cortex than previously thought and present new results regarding asymmetrical tract branches. In a second study we further investigate the UF including its subcomponents. We resolve a century old debate by clarifying its elusive boundary with the IFOF and reveal the distinctive anatomical features including asymmetry patterns of each subcomponent. These results shed new light on the IFOF and UF and will be crucial for disentangling their multifunctional roles.