The emergence and evolution of mammalian neocortex.

Cortical variation in mammals and other terrestrial vertebrates, re-examined by current comparative methodology (out-group analysis), indicates that separate lateral (olfactory), dorsal and medial (hippocampal) pallial or cortical formations arose with the origin of vertebrates. Although the exact origin of mammalian isocortex (so-called neocortex) is still disputed, it appears that the earliest mammals already had a six-layered isocortex with ten to 20 functional subdivisions. Among placental mammals, at least, isocortex has expanded numerous times, producing additional cortical subdivisions. Because these expansions were independent transformations of a simpler cortex, they produced subdivisions that are not homologous.