Cortical Hierarchy and The Dual Counterstream Architecture.

Hierarchy is one of the major organizational principals of the cortex and underscores modern computational theories of cortical function. Based on a review of the literature on the local microcircuit and its role in the amplification of inputs, we develop the argument that distance dependent changes in connectivity and oscillatory coherency of the inputs to an area represent the structural and functional signatures of hierarchy. Statistical modeling of these structural signatures demonstrates that inputs from multiple levels to target areas across different levels show remarkable consistency. Close examination of inter-areal connectivity reveals a dual counterstream architecture with well-defined distance-dependent feedback and feedforward pathways in the supra- and infragranular layers, suggesting a multiplicity of feedback pathways with well defined functional properties. These findings are coherent with feedback connections providing a generative network involved in a wide range of cognitive functions. Exploring the laminar basis of inter-areal interactions, we suggest, can be overcome with concerted experiments capitalizing on recent technical advances in tract-tracing, high-resolution fMRI and optogenetics thereby allowing a much improved understanding of the computational properties of the cortex.