Constructing the mammalian neocortex: the role of intrinsic factors

The mammalian neocortex is subdivided into regions that are specialised for the processing of particular forms of information. These regions are distinct in terms of their cytoarchitecture, electrophysiology, and connectivity. How this regional diversity is generated through development is currently a topic of considerable interest and has centered upon two main issues. First, to what extent are these regions prespecified by intrinsic genetic mechanisms? Second, what is the influence of extrinsic activity in transmitting signals that ultimately shape functional regions? Historically, experimental evidence has tended to emphasise the role of extrinsic influences, but the identification and analysis of several genes that are expressed asymmetrically in the developing neocortex have tempered this viewpoint. We review current literature from the standpoint that intrinsic influences act early in neocortical development to generate molecular patterning whose main role is the guidance of long-range projections from the dorsal thalamus. Extrinsic influences appear to generate receptive fields for peripheral input, the summation of which determines the areal extent of particular neocortical region.