Correlates and Possible Mechanisms of Neocortical Enlargement and Diversification in Mammals

The mammalian neocortex varies greatly in size and internal organization across species. However it is often difficult to attribute specific cognitive abilities to corresponding cortical specializations. Here mammals with different sensory specializations are compared with their less, or differently, specialized relatives in order to identify trends in mammalian cortical evolution associated with increased behavioral abilities and sensory processing. In addition, some of the features of small versus large brains are considered in the context of evolution. The enlargement of cortex, changes to the organization of cortical areas, and the subdivision of cortex into additional areas, are seen as important trends correlated with the ability to process greater volumes of complex sensory information. Recent advances in the ability to manipulate gene expression during development suggest some of the mechanisms that have produced these changes. These mechanisms include alterations to a sensory surface (retina, cochlea, and skin) that affect neocortical maps through a cascade of inductive influences during development and more dramatic changes in brain organization that may result from duplication and subsequent specialization of cortical areas