Specializations of the Cortical Microstructure of Humans

A fundamental and long-standing question in neuroscience is what is special about the neocortex of humans and how does it differ from that of other species. It is generally thought that during evolution, the neocortex expanded in larger brains due to the addition of microcircuits with the same basic structure. Hence, it is considered unlikely that new types of neurons or other fundamental differences exist. Here, we focus on certain features that have been highlighted by comparative microanatomical studies of the neocortex that emphasize the differences between the human neocortex and that of other mammals. The notion of uniformity in the neocortex with respect to the density and the types of neurons found in columns has been recently challenged. We present evidence that the uniformity of cortex has been overstated and that there are actually wide variations in cortical organization. The proportion of GABAergic interneurons is higher in primates than in rats and, moreover, certain subtypes of interneurons are lacking or greatly modified in some species. Together with the distinct synaptology in the neuropil and in the number of synapses per neuron, these differences clearly highlight the variability in the design of microcircuits between cortical areas and species. Given our ignorance regarding many aspects of the organization of the cerebral cortex and the great number of mammalian species whose cortical structure remains unexplored, what we currently conceive as general diagrams of basic neocortical circuits should be considered an oversimplification. Based on our studies and those of a number of other laboratories, we support the idea that the human neocortex shows unique specialization that is likely to be crucial for human cortical function.