Brain homology and function: an uneasy alliance.

Current efforts to homologize brain regions across species are often motivated by the expectation that "functional homology" can be deduced from structural homology. Research guided by this "extrapolationist" assumption has been quite successful in molecular biology and, to a lesser extent, in comparative neurobiology. For example, several studies have shown that the hippocampal formation performs similar behavioral functions in birds and mammals, despite significant differences in both anatomy and physiology. However, the extrapolationist assumption can also impede progress because it disregards the possibility that brain regions may change their function during the course of evolution. For example, data gathered at the end of the 19th century on the behavioral effects of large telencephalic lesions were quite confusing until Ferrier recognized that the lesion effects simply differ between species. This realization gave rise to the concept of "functional encephalization," according to which behavioral functions generally shift from "lower" to "higher" brain regions as one ascends the so-called phylogenetic scale. This idea is now discredited, but there is still no adequate theory to explain the species differences in lesion effects. The present paper outlines how one might begin to construct a theory of evolutionary changes in brain function.