The importance of being hierarchical

Our understanding of cortical electrophysiology and anatomy at the single-cell level has led to the present day insight in to the function of connections linking cortical areas. This made it possible to elaborate the cortical hierarchy in the early 1990s and was a prerequisite for the development of present day generative models of perception. These computational hierarchical models make strong predictions concerning the roles of feedforward (FF) and feedback (FB) pathways, including their segregation and topographical precision in both directions. This shows that instead of a single stream in the upper and lower compartments of the cortex there is in fact a bi-directional counter-stream in each compartment of the cortex. A significant advance in this field will require more detailed anatomy hand in hand with a network analysis of the directed and weighted cortical matrix.

► Counter-streams are a major organizational feature of cortical pathways. ► Going beyond driving and modulation as a characterization of cortical pathways. ► Models of cortical hierarchy require single-cell resolution. ► Hierarchy is at the heart of modern theories of brain function. ► There is a need to derive weighted models of cortical hierarchy.