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Tool-use and the Chimpanzee Brain: An Investigation of Gray and White Matter, and a Focused Study of Inferior Parietal Microstructure Open Access

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The goal of this dissertation is to investigate neural correlates of tool-use in chimpanzees and provide the groundwork for comparisons with humans and other primates. The dissertation contains three studies that integrate different techniques to investigate brain structures associated with tool-use in chimpanzees. The first two studies use neuroimaging techniques to investigate gray and white matter differences associated with tool-use performance in a sample of captive chimpanzees. The first study uses tensor-based morphometry to study gray matter, while the second study uses diffusion tensor imaging and tract-based spatial statistics to study white matter tracts. The third study uses histological methods to focus on the microstructure of the inferior parietal lobe in the chimpanzee. This type of integrative approach allows for a comprehensive study of tool-use and how it is linked with specific aspects of chimpanzee cortical anatomy, and can inform our understanding of how the brain evolved and adapted to tool-use and tool-making in hominins. The results from this dissertation show that the chimpanzee brain has specific tracts and areas that are associated with tool-use with distinct patterns of connectivity, similar to what is observed in humans. Chimpanzees, however, appear to rely more on areas involved in sensorimotor processing for tool-use than do humans. Human tool-use may therefore differ from that of chimpanzees in the recruitment of more areas associated with cognitive control and increased connectivity between areas involved in higher order cognitive functions. The additional cortical areas that have been identified in the human IPL that appear to lack chimpanzee homologues also may be essential for tool-related cognition in humans, especially cognition linked with complex processes such as stone tool-making. Investigations of both gray and white matter showed that chimpanzees also have notable sex differences in their neural correlates of tool-use. Male and female chimpanzees may process tool-use differently, and these differences may be linked with previously reported sex differences in tool-use behaviors. To fully understand evolutionary changes in the brain that occurred in the human lineage since its split from the Pan-Homo last common ancestor, comparative work must be performed between humans and chimpanzees, and especially work that can reveal the structure and function of the chimpanzee brain to ensure the validity of any comparisons. This dissertation has provided a characterization of tool-use in the chimpanzee brain based on multiple modalities, and these results can form the basis of direct comparisons between chimpanzees, humans, and other primate species in the future.

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