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| George F. Striedter |
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Dept. Neurobiology & Behavior, UC Irvine, 2205 McGaugh Hall, Irvine, CA 92697-4550, USA
e-mail: gstriedt@uci.edu |
Presentation: |
| 2002-10-05, 10:00-10:40 |
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| Evolutionary and comparative perspectives of the human brain. |
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| When discussing human brain evolution, anthropologists tend to focus on measures of overall brain size but often remain rather vague about how evolutionary changes in brain size could effect changes in behavior. Neuroscientists, on the other hand, like to focus on structure-function relationships, but often disregard species differences in brain anatomy and function (fearing, perhaps, that species differences would undermine the “model systems” approach). Partly because of these divergent attitudes, we still know relatively little about how human brains differ from those of other primates. In an effort to bridge this gap, I will review some published data on primate brains from my perspective as an evolutionary neurobiologist. Reasonably clear, though not uncontested, is that the prefrontal, parietal, and temporal cortices are larger in humans than one would expect (for a primate with a brain the size of ours) and contain at least some cortical areas that are not apparent in other primates. At subcortical levels, human brains are unusual in having (for example) a very large pulvinar nucleus, an enlarged parvocellular red nucleus, a poorly differentiated hypothalamus, and an anemic dorsal cochlear nucleus. Clearly, the human brain is a mosaic of both hyper- and hypotrophied features. In isolation, many of these species differences seem peculiar, but they begin to make sense when we consider the functional circuits to which the regions belong. The atrophy of the dorsal cochlear nucleus, for example, is probably related to the almost complete absence of ear movements in humans, and pulvinar hypertrophy is probably related to the enlargement of prefrontal, parietal and temporal cortices, with which the pulvinar is heavily interconnected. In addition, human brain evolution probably involved some major connectional and functional changes that are not apparent from comparative cytoarchitectural investigations. For example, the human brain probably exhibits considerably more functional asymmetry than is apparent cytoarchitecturally, and axons from several parts of the human frontal lobe probably “invaded” brainstem regions that they did not innervate ancestrally, leading to what one might call “functional encephalization”. Although it is generally difficult to determine the causal mechanisms (e.g. selective pressures) behind evolutionary changes in brain anatomy, it is increasingly possible to make educated guesses about their functional and/or behavioral consequences. Given the ongoing explosion of information about human brains, now is a good time to think seriously about what features of the human brain make us different from other animals. |
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