Interneuron Heterogeneity in Neocortex

Complex behavior and cognitive functions, such as perception, attention and memory, are believed to be intimately associated with the ~1–2 mm thin neocortical sheet, that forms the outer envelope of mammalian brains1, 2. Neocortical function is believed to ultimately arise from the communication among the intricately interconnected constituent excitatory and inhibitory neurons3–9. In the late 1950’s and early 1960’s, experiments revealed small repeating functional units of several thousand cells, that are vertically arranged into cylinders/slabs (traversing all cortical layers) of about half a millimeter in diameter10,11. Functional modules, as exemplified by these cortical columns, have been found in many neocortical areas, establishing the notion that the neocortical sheet may be viewed as a mosaic of small repeating modules1,7,8,12–18. Understanding the structure and function of these modules, therefore, provides the foundation for understanding neocortical function.