The importance of developmental timing in cortical specification.

Environmental control of gene expression can occur early or late during development, and this is relevant to understanding species differences in cortical specification. Experiments in the developing visual system of the primate show that the areal limits of striate cortex are specified by the thalamic inputs, so that afferent specification of cortex appears as a general feature of mammalian development. Primates differ from nonprimates in that thalamic afferents control very early stages of corticogenesis when symmetrical cell division is forming the pool of striate neuron precursors. Other cortical features are specified much later in primates than in nonprimates. We speculate that the early specification of certain features and the late specification of others contribute to the sophistication of the cerebral cortex characteristic of primates.