Brain networks may be examined at any level of their hierarchy, and over the past 150 years, an enormous literature has developed that addresses various structural and functional properties of brain and neural networks. For example, genetic and biochemical mechanisms of cortical patterning and circuit development have been described (Cowan et al., 1984; O’Leary et al., 2007; Sur and Rubenstein, 2005), and neuroanatomical tracing studies have refined our conceptions of local and distributed connectivity patterns (Carmichael and Price, 1996; Felleman and Van Essen, 1991). These are remarkable advances, but they have largely taken place in non-human systems. As a result, much of our knowledge about human brain function and organization rests upon extrapolations from such model systems. The advent of methods for human neuroimaging changed this situation by enabling the comprehensive examination of macroscopic brain activity, and more recently, connectivity, in living subjects. These techniques have facilitated the exploration of human brain networks, and this article reviews recent progress in understanding the development of functional brain networks in humans.
