"Neural Substrates of Primate Communication: A Comparative Histometric Study of the Orofacial Motor System" by Chester Charles Sherwood

Chester Charles Sherwood

Deposited 2003

Abstract
Sociality is an integral feature of the primate adaptive complex. Navigation of social relationships requires the transmission and decoding of communication signals. Primate species display a great diversity of communication strategies, many of which entail skilled motor control of orofacial muscles for their production such as facial expressions, vocalizations, and speech in humans. This dissertation was designed to investigate possible neural substrates of species-specific modes of communication by using comparative quantitative histological methods to study brain regions centrally involved in orofacial motor control. The brain regions studied included the trigeminal motor nucleus, facial nucleus, and hypoglossal nucleus of the brainstem and the primary motor cortex. In specific, the following hypotheses were tested: Are changes in the volume, neuropil space, or neuron number of brainstem orofacial motor nuclei related to phylogenetic differences in the communication behavior of primates? Is the microstructural organization of orofacial representation of primary motor cortex uniform across catarrhines, or is there interspecific variation that may be linked to skilled voluntary control of the face, jaws, and tongue in great apes and humans? Analyses of allometric scaling and phylogenetic independent contrasts were used to test for anatomical specializations of orofacial motor nuclei. Overall, results showed that several structural features of these motor nuclei, including nucleus volume, neuron number, and neuropil space were highly correlated with medulla and brain volume. Furthermore, there was little evidence that interspecific variation in the cytoarchitectural organization of these cranial motor nuclei reflects specialization for facial expression or human speech. In contrast, the microstructure of primary motor cortex exhibited several phylogenetic differences. Compared to the Old World monkeys examined, primary motor cortex of great apes and humans was characterized by increased thickness of superficial cortical layers, decreased neuron packing density, and increased proportions of neurofilament protein-immunoreactive neurons and parvalbumin-immunoreactive transcolumnar inhibitory interneurons. These modifications of the network of connections within primary motor cortex may underlie enhanced mobility and voluntary control of orofacial muscles. The results of this study, furthermore, help to place the neural substrates of human speech in phylogenetic context.