Bifurcation in space: Emergence of functional modularity in the neocortex.

How does functional modularity emerge in a cortex composed of repeats of a canonical local circuit? Focusing on distributed working memory, we show that a rigorous description of bifurcation in space describes the emergence of modularity. A connectome-based model of monkey cortex displays bifurcation in space during decision-making and working memory, demonstrating this new concept's generality. In a generative model and multi-regional cortex models of both macaque monkey and mouse, we found an inverted-V-shaped profile of neuronal timescales across the cortical hierarchy during working memory, providing an experimentally testable prediction of modularity. The cortex displays simultaneously many bifurcations in space, so that the corresponding modules could potentially subserve distinct internal mental processes. Therefore, a distributed process subserves the brain's functional specificity. We propose that bifurcation in space, resulting from connectivity and macroscopic gradients of neurobiological properties across the cortex, represents a fundamental principle for understanding the brain's modular organization.