Cognition presumably emerges from neural activity in the network of association connections between cortical regions that is modulated by inputs from sensory and state systems and directs voluntary behavior by outputs to the motor system. To reveal global architectural features of the cortical association connectome, network analysis was performed on >16,000 reports of histologically defined axonal connections between cortical regions in rat. The network analysis reveals an organization into four asymmetrically interconnected modules involving the entire cortex in a topographic and topologic core-shell arrangement. There is also a topographically continuous U-shaped band of cortical areas that are highly connected with each other as well as with the rest of the cortex extending through all four modules, with the temporal pole of this band (entorhinal area) having the most cortical association connections of all. These results provide a starting point for compiling a mammalian nervous system connectome that could ultimately reveal novel correlations between genome-wide association studies and connectome-wide association studies, leading to new insights into the cellular architecture supporting cognition.T he cerebral cortex is the core of the brain's cognitive system (1, 2). Emerging evidence suggests that misdirected and/or dysfunctional cortical connections established during neurodevelopment, or degenerative events later in life, are fundamental to cognitive alterations associated with brain disorders like Alzheimer's disease, autism spectrum disorder, and schizophrenia (3). Presumably, an understanding of biological mechanisms underlying cognition and the control of voluntary behavior rests at least partly on the structure-function wiring diagram of the cortex. Design principles of this neural circuitry are based on a network of interactions between distributed nervous system regions, and on the underlying function of their constituent neuron populations, and individual neurons.Unfortunately, a global structure-function wiring diagram of the cortex has not yet been elaborated (4). A necessary, but not sufficient, prerequisite for establishing this basic plan is a comprehensive structural model of cortical connectivity (5-7). Such a "roadmap" could then be used as a database scaffolding for molecular, cellular, physiological, behavioral, and cognitive data and for modeling (8)-analogous to a Google Maps for the brain. The research strategy described here provides the starting point for such a model, as well as a framework, benchmark, and infrastructure for developing a global account of nervous system structural network organization as a whole.The conceptual framework underlying our strategy to analyze global nervous system connection architecture is twofold. First, because of considerable complexity-for example, human isocortex on one side has 6-9 billion neurons (9-11) interconnected by orders-of-magnitude-more synapses-three hierarchical (nested) levels analysis are considered (12, 13). A macroconnection be...