In the 15 years since the first edition of The Cognitive Neurosciences we have witnessed immense advances in the understanding of the intricacies of human cognitive abilities. As evident from the many articles in this as well as previous editions of this popular reference book, the progress that has been made can to a large extent be linked with studies of the cerebral cortex using very sophisticated noninvasive imaging methods in human subjects. These methods allow examination of human-specific cognitive functions directly in living people as they develop, as they are perturbed, and as they decline (e.g., Gazzaniga, 2008). It is thus somewhat paradoxical that during this very same time period the major advances in our understanding of the cellular and molecular mechanisms of cortical development and the models of evolutionary elaborations have derived almost entirely from studies of rodent brains.

Although basic principles of cortical development are probably similar in all species, the modifications of developmental events during evolution produce not only quantitative changes (e.g., the number of neurons; expansion in surface, timing, and sequence of cellular events; increase in number of synapses, etc.), but also many qualitative changes (e.g., the elaboration of new types of neurons and glia, and most importantly additions of novel specialized cytoarchitectonic areas associated with corresponding new pathways and patterns of connectivity). It has become evident that even essential genes that are responsible for survival of the species give different phenotypes in the mouse and human (Liao & Zhang, 2008). Furthermore, the timing and duration of cell genesis, the composition of the ventricular zone, and the ratios of cell proliferation, versus programmed cell death occurring in the enlarged subventricular and marginal zones of the embryonic cerebral cortex, suggest not only a slower development, but also expanded, diversified, and novel roles of these transitional layers in primates including human (Bystron, Blakemore, & Rakic, 2008). It is for this reason that the first three chapters in this volume are dedicated to the development and evolution of the cerebral cortex with a particular emphasis on humans and nonhuman primates.

The first chapter, by Rakic, Arellano, and Breunig, is dedicated to the prenatal development of the primate neocortex. It emphasizes the developmental features that are prominent and essential for the formation of the large and convoluted cerebral cortex. For example, there are marked species-specific differences in the timing and sequence of divergence of neural stem and radial glial cell lines as well as in the levels of their differentiation and longevity. There are also subclasses of neural stem cells that produce interneurons for the neocortex as well as for the association thalamic nuclei not detectable in rodents (Letinic & Rakic, 2001; Letinic, Zonku, & Rakic, 2002). These neurons may be involved in human-specific language and cognitive functions that do not exist in nonprimate species.

The second chapter, by Kostovic¥ and Judaš, describes the early development of neuronal circuitry of the human prefrontal cortex, which is most elaborated and enlarged in humans, and arguably does not exist in nonprimate species. These studies are helped by the increase in resolution of the MRI to the degree that one can visualize normal and possible abnormal columns in the human fetal neocortex (e.g., McKinstry et al., 2002).

The third chapter, by Preuss, which deals with evolutionary aspects of cortical development in the hominoids, provides a compelling account of recent evidence documenting unique features in the organization of the human brain. These new insights have been derived by using new technologies, in combination with more established techniques, to assess different aspects of the relation between structure and function in the brain, ranging from neuronal morphology to connectional pathways.

The fourth chapter, by Chalupa and Huberman, is concerned with unraveling the role of neuronal activity in the formation of eye-specific connections in nonhuman primates. It deals with the development, competition, and plasticity of the projections of the two eyes to the lateral geniculate nucleus and the formation of the ocular dominance columns in the primary visual cortex. Their work challenges the widely held notion that neuronal activity, in particular the retinal waves of activity, plays an instructional role in the formation of eye-specific retinogeniculate projections.

The fifth chapter, by Bunge, Mackey, and Whitaker, deals with human brain changes underlying improved cognitive abilities during childhood and adolescence, with a particular emphasis on fluid reasoning. Focusing primarily on prefrontal and parietal cortices and using the most advanced neuroimaging methods and conceptual approaches, this chapter aptly demonstrates the power of the developmental approach in linking brain mechanisms with higher cognitive functions.

Collectively, these studies may help in understanding the biological bases of the high level of cognitive ability that is achieved during primate evolution culminating in humans. However, from a practical perspective, the findings obtained from studies on human and nonhuman primates may be essential for the design of psychiatric drug therapies, since, for example, the capacity for regeneration has diminished during vertebrate evolution, and the absence of neurogenesis in the primate cerebral cortex (Bhardwaj et al., 2006) indicates that overcoming the brain's resistance to the acquisition of functionally competent new neurons will require an understanding of why neurogenesis ceases at the end of specific developmental time windows and why there are regional variations in this phenomenon (Rakic, 2002, 2006). Another difference is the existence of distinct types of interneurons in the human brain that are not detectable in rodent species (e.g., DeFelipe, Alonso-Nanclares, & Arellano, 2002). In addition, a subclass of interneurons of the thalamic association nuclei that origi-nate in the ganglionic eminence are not detectable in rodents (Letinic & Rakic, 2001). Likewise, unlike in rodents, in which interneurons arise from the ganglionic eminences, in primates these originate in large numbers in the enlarged subventricular zone (Letinic et al., 2002; Petanjek, Dujmovic, Kostovic, & Esclapez, 2008). These neurons may be involved in human-specific disorders such as schizophrenia that do not occur spontaneously in nonprimate species. Thus modifications in the expression pattern of transcription factors in the human forebrain may underlie species-specific programs for the generation of specific classes of cortical neurons that may be differentially affected in genetic and acquired neurological disorders (Lewis, 2000). These novel evolutionary traits may be more vulnerable to genetic mutations and environmental insults, and could be implicated in disorders of higher brain functions, such as autism, developmental dyslexia, Alzheimer's disease, and schizophrenia. Designs of new drugs and replacement therapies need to take into consideration these species-specific distinctions.