Characteristics of hub regions in the superficial white matter.

The superficial white matter (SWM) comprises short-range fibers that connect adjacent cortical regions; however, its structural connectivity remains poorly understood. In a previous dissection study, we identified anatomical "crossings" where superficial white matter fibers converge from multiple directions. Building on this, the present study investigated superficial white matter characteristics associated with these crossings using diffusion-weighted imaging data from 10 individuals in the Human Connectome Project. A total of 605 regions of interest (ROIs) were placed at presumed crossing locations in the dominant hemisphere. Superficial white matter fibers were categorized by tract length (short, medium, and long), and streamline distribution was analyzed. Short- and medium-length fibers showed predominantly intragyral connections, while long fibers exhibited more diverse intragyral connectivity. Graph theoretical analysis identified high-efficiency regions of interest, putative hub regions, which were mapped onto cytoarchitectonic structures from the Economo-Koskinas atlas and compared with myelin concentration. High-efficiency regions for short- and middle-length fibers were mainly in primary motor and sensory areas, particularly visual regions, corresponding to high myelin concentration and cytoarchitectonic areas 1 and 5. In contrast, high-efficiency regions for long fibers were distributed in the association cortex (area 3). These findings provide new insights into the structural organization of superficial white matter and its relationship to cortical architecture.