Cortical specification of mice and men.

The developmental basis for the localization of function in the mammalian cortex remains a controversial issue. The newly formed rodent cortex displays a considerable uniformity in terms of its connectivity. This contrasts with the primate, where even the first formed connections can show a high degree of areal specificity. An important clue to understanding these species differences can be obtained by examining how and when the sensory periphery exerts its organizing influence on the developing cortex. In rodents the developmental timetable ensures that the organizational control of the periphery persists late in development, when neurons are forming their first connections. By contrast, in primates the late onset and prolonged duration of corticogenesis result in the periphery being able to exert its influence much earlier, during the phase of precursor proliferation. Differences and similarities between primate and rodent corticogenesis are highly informative. In rodents, recent results with molecular markers show that regional differences in developmental potential exist in the cerebral cortex before innervation from the periphery. Similar findings are predicted in primates. It is to be expected that a more complete understanding of the rules governing the emergence of distinct cortical areas will come from resolving how afferent specification acts within the confines of such regional specializations.