Taking the measure of diversity: comparative alternatives to the model-animal paradigm in cortical neuroscience

Cortical neuroscience is founded on studies of a very few model organisms, mainly rats, cats, and macaque monkeys. The concentration of effort on such a few species would be defensible if cortical organization were basically uniform across mammals, as is commonly believed. Although there is little reason to doubt that some features of cortical organization are indeed widespread among mammals, phyletic variation in cortical organization is far more extensive than has generally been appreciated or acknowledged. Rats, for example, differ from other mammals in the genetics and chemistry of their cortical neurons, in connectivity and areal organization, and in the functions of specific cortical regions. Likewise, macaque monkeys, although widely used as models of the human visual system, lack a number of features found in human visual cortex. Given the variability of cortical organization, how should neuroscientists approach the study of nonhuman species, and what can we reasonably expect to learn from them? First, by examining a wider range of species than are currently employed, and by using modern techniques of phyletic analysis, neuroscientists can more rigorously identify those features of cortical organization that are, in fact, widely shared among mammals or among particular mammalian subgroups. Second, by taking account of variations, neuroscientists can abstract more reliable and general principles of structure-function relationships in the nervous system. Finally, freed from the doctrine of basic uniformity, neuroscientists can pursue the study of human cortical specializations, and so advance our understanding of what distinguishes humans as a biological species.