A small world of weak ties provides optimal global integration of self-similar modules in functional brain networks.
The human brain is organized in functional modules. Such an organization presents a basic conundrum: Modules ought to be sufficiently independent to guarantee functional specialization and sufficiently connected to bind multiple processors for efficient information transfer. It is commonly accepted that small-world architecture of short paths and large local clustering may solve this problem. However, there is intrinsic tension between shortcuts generating small worlds and the persistence of modularity, a global property unrelated to local clustering. Here, we present a possible solution to this puzzle. We first show that a modified percolation theory can define a set of hierarchically organized modules made of strong links in functional brain networks. These modules are "large-world" self-similar structures and, therefore, are far from being small-world. However, incorporating weaker ties to the network converts it into a small world preserving an underlying backbone of well-defined modules. Remarkably, weak ties are precisely organized as predicted by theory maximizing information transfer with minimal wiring cost. This trade-off architecture is reminiscent of the "strength of weak ties" crucial concept of social networks. Such a design suggests a natural solution to the paradox of efficient information flow in the highly modular structure of the brain.
Levich Institute and Physics Department, City College of New York, New York, NY 10031, USA.
Lazaros K
LK
Gallos
Hernán A
HA
Makse
Mariano
M
Sigman
D001921:Brain / Q000033:anatomy & histology / Q000502:physiology*; D017709:Fractals; D006801:Humans; D008959:Models, Neurological; D009415:Nerve Net / Q000502:physiology*