Depicting the anatomy of the gyral white matter: ubi sumus? quo vadimus?

Abstract

A cerebral gyrus is made up of an external layer of folded cortex and an inner core of white matter. The architecture of the core has specific features that make it distinct from the white matter of the deep brain regions. Limited externally by the gray matter that covers the top of the gyrus and the neighboring sulci, this gyral white matter is made up of a mix of fiber populations with multiple directions and destinations. The presence of densely packed fibers with multiple crossings, the proximity to the cortex, and the existence of inter-regional and inter-individual variations make the task of depicting this microanatomy extremely challenging. The topic is, however, of paramount relevance for both fundamental and applied neurosciences. This fiber colocalization is crucial for the functional role of each cerebral region and is key to clinical manifestations in cases of parenchymal damage. As track tracing, imaging, and dissection are based on different biological or physical principles, it is natural for their results to sometimes be different, but they are often complementary. As the amount of available information increases, it becomes fragmented due to the multiplicity of methods, target phenomena, and studied species. In this scoping review, we present the key concepts and map the primary sources of evidence regarding identifying the fiber pathways that compose the gyral white matter, enabling the discussion of avenues for future research. The general pattern in which these pathways are distributed in the gyral white matter was detailed, and the main variations as a function of brain topography were explained and illustrated with typical examples.

Unique longitudinal contributions of sulcal interruptions to reading acquisition in children

“…In order to compare the structural connectivity around the left pOTS proper and gaps in children with a continuous vs. an interrupted sulcus, we performed whole-brain probabilistic tractography from a superficial white-matter layer located under the left pOTS and gap cortex (see Figure 4c for a seed region in an example participant). Mean structural connectivity from these seeds from all children showed strong local connectivity to fusiform and inferior temporal gyri adjacent to the superficial white-matter seeds above the left pOTS, and extending to the middle temporal gyrus, parahippocampal cortex, and anterior OTS (Figure 4c middle panel), consistent with the presence of short, neighborhood association fibers traveling under the sulcal cortex (Dannhoff et al, 2023;Reveley et al, 2015). When comparing structural connectivity between children with interrupted and continuous left pOTS through permutation testing, children with sulcal interruption showed higher local connectivity, as indexed by density maps reflecting the number of reconstructed streamlines, in a cluster located around the posterior section of the left pOTS, after controlling for demographic variables (age, gender and handedness, p<0.01 threshold-free cluster enhancement -TFCE-uncorrected, Figure 4c bottom panel, leftmost map), suggesting a higher development of short-distance connections, in agreement with hypothesis (ii).…”

“…For instance, previous work has highlighted that junctions in gyral ridges, known as 3-hinge gyri, serve as hubs in cortico-cortical connections with higher connection degrees, stronger connections, and high betweenness centrality in graphs of structural connectivity (Zhang et al, 2020). The rich connectivity of such regions has also been documented through fine-grained postmortem dissections (Shinohara et al, 2020; as also reviewed in Dannhoff et al, 2023), revealing that 'pyramid shape crossings' are characterized by, and seem to result from, the convergence of both intraand inter-gyral U fibers. Crucially, we showed that such increased local U-shaped fibers also relate to reading skills.…”

“…In the case of sulcal interruptions, two explanations stand out as potential mechanisms underlying the behavioral benefits for reading of left pOTS interruptions. First, given that white-matter fibers in general, and especially long-distance projection fibers, tend to terminate more in gyri than sulci (Dannhoff et al, 2023;Ge et al, 2018), having an extra gyral crown, or superficial pli de passage (Mangin et al, 2019), in the left pOTS may allow to pack more connections, especially long-distance connections through e.g., potentially a combination of fibers from the (posterior) arcuate fasciculus, vertical occipital fasciculus and inferior longitudinal fasciculus (Bouhali et al, 2014;Grotheer et al, 2021;Moulton et al, 2019;Weiner et al, 2017;Yeatman et al, 2012). Such increased long-distance connections may support a more efficient reading network.…”