Abstract
Large mammalian brains show “encephalization”: that is, in larger brains, the neocortex claims a disproportionately high percentage of their volume (Jerison, 1973; Jerison, this volume). The magnitude of encephalization is impressive, and its manner is strikingly regular across multiple mammalian radiations (Hofman, 1989). I would like to take advantage of this forum to speculate on the relationship of encephalization to current research on the ontogenetic regulation of neuron number and type in the cortex. The discussion is organized around three questions:
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1.
Can any persistent feature of development account for the disproportionately large volume of the cortex in large brains?
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2.
Why do developmental stabilizing mechanisms permit cortical hypertrophy? For example, in the spinal cord there is some trophic relationship between the number of neurons and peripheral muscle and sensory mass, regulated by normally-occurring cell death (Hamburger and Levi-Montalcini, 1949; reviewed in Hamburger and Oppenheim 1982; Oppenheim, 1981). Why is relative cortical volume allowed to grow so large with respect to the volume of its input and terminal zones?
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3.
Can development give us any clue as to how local areas of cortex get wired for their particular functions? Two views of the brain have competed for decades, whether the brain is best understood as a generalized computing device, or as an accretion of specialized capacities. While some of the functional change in larger brains might be described as faster, more powerful, and more general computing, the most prominent functional changes are the addition of particular, specialized skills. These skills, like echolocation, language, predictive prey tracking and the like, are apparently represented in a modular way in the brain. How can a generalized and regular neocortical hypertrophy be reconciled with the development of modularly-organized special functions?
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Finlay, B.L. (1991). Control of Cell Number and Type in the Developing and Evolving Neocortex. In: Finlay, B.L., Innocenti, G., Scheich, H. (eds) The Neocortex. NATO ASI Series, vol 200. Springer, Boston, MA. https://doi.org/10.1007/978-1-4899-0652-6_4
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