Default heterogeneity: from a singular construct to multiple default network(s)

In the last quarter of a century, the default network (DN) has become a major focus of scientific research. Researchers have sought to understand its functional properties and relationship to various psychological, clinical, and social variables. Work on the DN has occurred alongside a tension between considering the DN as a unitary system versus treating it as a heterogenous system comprised of subcomponents or networks. A potential difficulty researchers face when recognizing the heterogeneity of the DN is knowing how these different fractionations compare to one another. In this review, we provide an overview of the evidence for heterogeneity of organization within the DN. We begin by reviewing the first group-level fractionations of the DN into different subsystems. Next, we consider recent individual-level fractionations of the DN that reveal organizational heterogeneity missed by group-level approaches. Lastly, we discuss how different approaches to brain network estimation may influence the measurement of DN organization. We conclude with a discussion of what researchers in the field of network neuroscience can do to increase the adoption and appreciation of DN heterogeneity. • The default network is not a unitary system. • Recent individual-level analyses reveal two distinct default networks. • Group-level analyses have identified three default subsystems. • The "core" default group-level subsystem includes both individual-level networks. • Subcortical structures differ in their affiliation to individual-level default networks.