The article by Marner et al. in this issue of the Journal of Comparative Neurology reports that there is an extensive loss of myelinated fibers from the white matter of the human cerebral hemispheres during normal aging. It raises the possibility that changes in gray matter, and especially any loss of its component neurons, might not be the principal reason for the mild cognitive decline that accompanies normal aging. To put this in perspective, we will first briefly examine some of the

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... for neuronal loss from human and nonhuman primate cerebral cortex in normal aging and then consider the changes that occur in white matter.