Abstract of Chelsea Rohrer-Dann's's 2007 undergraduate honors thesis
The caudate nucleus has been shown to be a critical site of pathology in language and speech
impairments. The objective of this study was to provide a comparative analysis of the areal fraction
(AF) of Nissl-stained cell profiles in the left caudate nucleus of 26 individuals, representing 19 species
of anthropoids, grouped into humans, nonhuman apes, Old World monkeys, and New World monkeys.
AF is a method of measuring the gray matter density by providing a contrast between neuronal, glial
and endothelial cells and neuropil space. For many brain regions, larger-brained primates are known
to have decreased cell-packing density when compared to primates with smaller brains. Therefore,
it was hypothesized that larger-brained species would show reduced AF in the caudate nucleus.
However, no correlation was found between species mean caudate nucleus AF and brain weight, a
finding that does not support the hypothesis. Independent contrasts (IC) analysis was also performed
to account for the fact that these data points cannot be viewed as independent of each other since
individuals of sister taxa will show more similarity due to having a recent common ancestor. However,
the data did not fit the assumptions of the IC method under most branch length transformations
applied and there was no correlation observed between brain weight and caudate nucleus AF when
phylogeny was taken into account.
The FOXP2 gene, when mutated, has been shown to affect the development of the caudate nucleus,
causing severe language and speech impairments. This gene is highly conservative throughout
primates; there is, however, a two-amino acid change between the protein product of humans and that
of chimpanzees, gorillas, and rhesus monkeys, at least one of which has been suggested to have
functional consequences for language and speech. Lower gray matter density provides more space
for interconnections and because the caudate nucleus has been shown to be crucial in language and
speech, another hypothesis examined in the current study was that the caudate nucleus would show
decreased AF in humans compared to other primates. Kruskal-Wallis tests of AF among the four
taxonomic groups showed a significant decrease of the AF values of humans compared to Old World
monkeys. Subsequent Mann Whitney U tests to examine the variation between humans and the rest
of the anthropoids grouped together also revealed a significant decrease in the mean AF of humans.