The brain, and most networks, contain a rich club–the group of high degree nodes that are strongly interconnected to each other. However, I wanted to know if the group of nodes with diverse connectivity across a network’s sub-systems–connector hubs–are even more strongly connected. I analyzed at the c elegans, macaque, and human brains, air traffic, and the US power grid. My intuition was correct. In every network, connector hubs form a club that is even more tightly interconnected than the rich club. Given that connector hubs have diverse connectivity across a network, I coined this club: the diverse club. Importantly, different nodes are members of the diverse club and the rich club. We also found that most of the shortest paths throughout the network pass through the diverse club, not the rich club.
I think that connector hubs evolved to allow for modularity and efficient communication between nodes. In what I think was one of the cooler aspects of the paper, I ran an evolutionary simulation, where networks are selected based on increased modularity and decreased shortest paths. Basically, we remove edges that decrease modularity the most and do not increase the sum of shortest paths. In these networks, a diverse club, but not a rich club, evolves.
The interpretation of many previous brain network analyses could be dramatically altered in light of this paper, particularly because the rich club and the diverse club have different cognitive functions. While the diverse club is likely responsible for integration and coordination across the brain and is involved in so-called task positive function, I think that the rich club maintains the stability of the network’s dynamics and is active during introspective thought. The diverse club is highly represented in the fronto-parietal control network, while the rich club is highly represented in the default mode network. Moreover, the rich club is situated the farthest—connectivity wise—from primary sensory and motor functions. In the macaque structural network, rich club nodes exhibit very high in-degree—many white matter connections terminate on these nodes. Thus, the dynamics of the rich club are largely constrained by the summary of strong rhythmic outputs from the entire network. An analogy can be made in social networks where members that exhibit a high in-degree, like politicians, are “slaves to their own power”, as they are only able to act in limited, and often slow, ways that mostly reflect the entire social network. That is the rich club. In contrast, the diverse club is not as constrained and sits evenly between the rich club and primary sensory and motor functions, allowing it to modify and interact with the entire brain.
The diverse club, M. A. Bertolero, B. T. T. Yeo & M. D’Esposito, Nature Communications volume 8, Article number: 1277 (2017)