K-shell decomposition reveals hierarchical cortical organization of the human brain

In recent years numerous attempts to understand the human brainwere undertaken from anetwork point of view. A network framework takes into account the relationships between the different parts of the system and enables to examine how global and complex functionsmight emerge fromnetwork topology. Previouswork revealed that the human brain features ‘small world’ characteristics and that cortical hubs tend to interconnect among themselves. However, in order to fully understand the topological structure of hubs, and how their profile reflect the brain’s global functional organization, one needs to go beyond the properties of a specific hub and examine the various structural layers that make up the network. To address this topic further, we applied an analysis known in statistical physics and network theory as k-shell decomposition analysis.The analysis was applied on a human cortical network, derived fromMRIDSI data of six participants. Such analysis enables us to portray a detailed account of cortical connectivity focusing on different neighborhoods of inter-connected layers across the cortex. Ourfindings reveal that the human cortex is highly connected and efficient, and unlike the internet network contains no isolated nodes. The cortical network is comprised of a nucleus alongside shells of increasing connectivity that formed one connected giant component, revealing the human brain’s global functional organization. All these components were further categorized into three hierarchies in accordancewith their connectivity profile, with each hierarchy reflecting different functional roles. Such amodelmay explain an efficient flowof information from the lowest hierarchy to the highest one, with each step enabling increased data integration. At the top, the highest hierarchy (the nucleus) serves as a global interconnected collective and demonstrates high correlationwith consciousness related regions, suggesting that the nucleusmight serve as a platform for consciousness to emerge. ‘..And you ask yourself, where ismymind?’The pixies (Where ismymind)