The basic non-uniformity of the cerebral cortex
The mammalian cerebral cortex varies in size over 100.000x across species. Because of its layout as a sheet, of its columnar organization and of the view of the cortex as being entirely originated from radial migration of newborn neurons from the proliferating ventricular zone (VZ), the evolution of such diversity in cortical size is usually considered to involve variation in the number of precursor cells in the VZ, each of which would give rise to a column of neurons in the cortex (Rakic, 1988). This scenario would be compatible with the widespread view that the number of neurons in a cortical column is constant across mammalian species (Rockel et al., 1981). Cortical expansion, therefore, would occur through the addition of more columnar modules, each with the same number of neurons.
In this paper, we examined whether there is indeed such a basic uniformity in the number of neurons beneath a unit surface area of the cerebral cortex across nine primate species and the closely related Tupaia. By determining the total number of neurons N in one cerebral cortical hemisphere, the total surface area A of that cortical hemisphere, and the average ratio N/A for each species, we find that, contrary to the expected, this ratio N/A is not constant. Rather, this ratio varies 3x across species in a way that is not related to cortical mass, thickness or area, but correlates with variations in neuronal density.
Our findings challenge the notion of uniformity in the number of neurons contained in a cortical column, a notion that is widely used or implied in models of cerebral cortical evolution, development, and connectivity. Although a variable number of neurons beneath one square mm of cerebral cortex does not invalidate the essence of models that view cortical expansion as a result of an increase in the number of neuronal precursors, it does call for an examination of how connectivity models are numerically impacted it the cortical unit is no longer considered constant in number of neurons across species.