Cortical folding and reduced connection distances in the human brain

Why do sulci occur in the human brain in particular parts of the cortical sheet? To obtain quantitative evidence on how folding may help to reduce the amount of white matter involved in connecting cortical regions, we measured the geodesic distance (GD) across the cortical white-matter surface, and the length of the connections and the number of streamlines (SN), between 180 cortical regions in the Human Connectome Project (HCP) Multimodal Parcellation atlas using diffusion tractography at 7 T in 178 HCP participants. For relatively long connections (GD >90 mm), we quantify how in many cases the connection distance is much shorter than the geodesic distance, with the presence of folding making this possible. We show how sulci can help to reduce the wiring cost defined as the length of the streamlines multiplied by the number of streamlines between all cortical regions. The wiring cost function may be minimized in evolution by utilizing factors such as the "tension" between cortical regions caused by the number of streamlines and the distance that could be saved arising from having sulci in particular locations in the cortical sheet. The cortical folding may also bring key hierarchically organized cortical processing streams closer together to minimize conduction time.