Introduction to the NeuroImage Special Issue: “In vivo Brodmann mapping of the human brain”

To achieve the important goal of developing well-grounded mechanistic models of the function of neural circuits, localized changes in brain activity and the end-points of axonal pathways need to be associated with specific well-characterized neural substrates. The reawakening of scientific interest inmyeloarchitecture, as implemented using high resolution structural MRI, affords deeper insights into principles of cortical organization which can be integrated with appropriate crossing-fiber dMRI tractography. Once the location of changes in brain activity in a given human brain has been identified, via the individual subject's own native myelin-based in-vivo cortical atlas, the corresponding cytoarchitecture could be looked up in a concordance atlas. The papers of this Special Issue offer analysis tools and examples of in-vivo native cortical atlases of individual human subjects, in which the boundaries of several cortical areas can be clearly identified. With a native cortical map for each subject, spatial smoothing can be viewed as no longer required. As described elsewhere by Turner (2013), mechanistic modeling of the relationship between structure, function and connectivity in anatomically distinct brain areas without spatial smoothing may gain greatly in plausibility. Despite two centuries of neuroanatomy and the genius of such pioneers as Ramon y Cajal, we are still very unsure of the nature and function of the component parts of the human brain. The situation is worst when it comes to cortical gray matter. The 0.23 m area of gray matter in human brain has been known for more than a century to show many compact subregions (e.g. Brodmann areas) defined by their distinctive cytoarchitecture and myeloarchitecture. Ideally, a mechanistic explanation that enables valid prediction requires clear definition of the given mechanism's components, their specific functional roles, and how these sub-functions are integrated into the operation of the mechanism as a whole. Some would argue that reasonably accurate predictions may only be achievable when we can specify components at nanometer scale across the entire brain. However, given the comparatively uniform structure of cortical areas and the anatomical discriminability of subcortical nuclei, it may be more pragmatic to start with these as the units of analysis andmechanistic modeling. This would limit the number of components to nomore than 200,which togetherwithmore than 20 different neurotransmitters, neuropeptides and corticosteroids should already provide a requisite level of complexity. Consensus remains to be built, however, regarding how many such cortical regions can and should be distinguished, and there is an urgent need for a useful concordance atlas between myeloarchitecture and cytoarchitecture in the same cadaver human brains. Research in the human myeloarchitecture has made little progress in a century. Details of myeloarchitecture, such as the heavily myelinated stria of Gennari in the primary visual cortex, are often far more easily visible than in cytoarchitecture. However, there has been little speculation or research regarding the functional role of specific myeloarchitecture in each