Transmodal association hubs of the cerebral cortex: maps, models, and mechanisms

Connectivity hubs of the human brain exert a major influence on function and are strongly concentrated within areas of transmodal association cortex. These areas play a central role in supporting abstract, multimodal cognition and also show dramatic expansion through both ontogeny and phylogeny, suggesting that their evolution may underlie our unique cognitive abilities. In this review, we reconcile different methods for defining and mapping transmodal cortical association hubs (TCAHs), assess various models of their evolution, and examine the developmental mechanisms implicated in their emergence. The available anatomical, comparative, and developmental data support a model in which the extension of postnatal brain development in larger-brained species interacts with conserved areal patterning mechanisms that distinguish primary sensorimotor from association cortex to accentuate regional differences in maturational rates. These regional developmental heterochronicities drive the relative expansion of TCAHs and give rise to their distinctive structural and functional properties. Extending the duration of human postnatal neurodevelopment thus offers a simple, scalable mechanism for adjusting the fraction of cortical resources devoted to unimodal or multimodal processing throughout cortical evolution.