ABSTRACT
The neocortex is a sheet of tissue that varies greatly in surface area, although only somewhat in thickness, across mammalian species (Kaas, 2000). Neurones throughout this variable sheet are similarly arranged in layers, and much of the processing is done within local circuits of neurones that are highly interconnected in vertical arrays across the thickness of cortex, wherever they are located. Thus, some aspects of neocortical structure and function are relatively uniform across this sheet. Nevertheless, we have known from the 1782 report of Gennari (see Gross, 1997) that this sheet is not completely uniform in structure, and from the time of Broca (1861) at least, that it is not completely uniform in function. Different regions of the neocortex have long been recognized as mediating different functions, and early neuroanatomists such as Brodmann (1909) laboured to identify and delimit such
bodies and used regional differences in the detailed appearance of various sections of cortex to reveal the locations of functional subdivisions of the brain. Brodmann in particular studied the brains of many different species of mammals and developed elaborate and complete theories or proposals of how their cortex is subdivided into areas. In brief, Brodmann proposed that humans, with a large sheet of neocortex, have many areas, nearly 50, while small-brained mammals with little neocortex, such as hedgehogs, have fewer than 15. Even so, hedgehogs and other small-brained mammals shared some areas with humans, such as area 17 (or primary visual cortex), and other areas in such mammals were “composites”, such as 5 plus 7, that had differentiated into separate areas in humans and other large-brained mammals. Thus, Brodmann presented a broad, comprehensive theory of brain organization and evolution. According to this theory, the neocortex is divided into a number of areas that varies across mammalian taxa, thereby accounting at least partly for variations in behaviour and ability. Humans and other large-brain mammals have more areas, some areas are shared across species from a common ancestor, and some are new. New areas emerged by differentiating from old areas.
