Elsevier

Brain Research Bulletin

Volume 57, Issues 3–4, February–March 2002, Pages 239-242
Brain Research Bulletin

Brain homology and function: An uneasy alliance

https://doi.org/10.1016/S0361-9230(01)00692-XGet rights and content

Abstract

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.

Section snippets

The hippocampal formation of mammals and birds

The topographical position of the hippocampal formation ranges from ventrolateral in humans to dorsomedial in marsupials, but Elliot Smith [3] recognized long ago that a dorsomedial hippocampus is probably primitive for mammals. Therefore, he reasoned, the dorsomedial telencephalon of reptiles and birds (i.e., sauropsids) is probably homologous to the mammalian hippocampal formation (HF).

Since then, several connectional similarities between the mammalian HF and the sauropsid dorsomedial

The theory of functional encephalization

Toward the end of the 19th century, the question of whether specific behavioral functions could be localized to restricted portions of the telencephalon was hotly debated [45]. Proponents of the “doctrine of cortical localization” pointed to data showing that restricted cortical lesions caused specific behavioral deficits, but their opponents argued just as forcefully that even large telencephalic lesions produced only minor and temporary impairments. The cortical localizationists ultimately

A skeptic’s take on extrapolationism

Even someone skeptical of the extrapolationist paradigm must acknowledge that homologous structures, be they molecules or brain regions, often do have similar functions. But is the functional similarity due to common ancestry per se, or simply to structural similarity, which also tends to go along with homology? In my opinion, functional similarity is far more closely associated with structural similarity than with homology. Convergent structural similarities generally point to functional

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