The mammalian neocortex is highly dynamic, demonstrating incredible variability in size and complexity across species. This phenotypic diversity, however, evolved from an extinct common ancestor. By illuminating organization of this mammalian ancestor, we can better understand the common features and the constraints of the mammalian neocortex across all species. Brain organization is not preserved in fossilized tissue. Therefore, examinating the brain of extant species, such as the short-tailed opossum, and comparing it to the brains of other mammals, provides the best available data for understanding the brain organization of early mammals. We investigated both corticocortical and thalamocortical connections of parietal cortical areas as well as the visual acuity in the short-tailed opossum (Monodelphis domestica). This species was chosen because it is thought to share many features with early mammaliforms (including body morphology, ecological niche, and gross brain morphology). We also discuss the neocortical organization of the somatosensory and motor systems across small-brained mammals. This provides a more comprehensive understanding of similar features of organization, inherited from the common ancestor (homologous) as well as features of organization that are unique to this particular species. For studies of cortical connections of parietal cortex in Monodelphis domestica, injections of anatomical tracers were placed in four different cortical areas and both injection sites and retrogradely labeled cells were related to myeloarchitectonic boundaries of cortical fields. Using these techniques, we identified rostral and caudal somatosensory fields (SR and SC, respectively) on either side of primary somatosensory cortex (S1), as well as a multimodal region caudal to SC (termed MM). Together with the second somatosensory area (S2), these five areas compose an interconnected somatosensory/multisensory network in Monodelphis. Next, we investigated the thalamocortical connections of SR, S1, SC, and MM. In contrast with thalamocortical connections described in previous studies in the closely related Virginia opossum (Didelphis virginiana), Monodelphis domestica does not have strong projections from ventral lateral/ventral anterior nucleus to S1, suggesting a different pattern of motor organization in Monodelphis neocortex, and further complicating the hypothesized organization of the common mammalian ancestor. Finally, we provided the first behavioral measure of visual acuity in any American opossum. We discuss the significance of this finding, both in the context of future research on the visual system of Monodelphis as well as in the context of visual system organization across mammals.
|Advisor:||Krubitzer, Leah A.|
|Commitee:||Bales, Karen, Britten, Kenneth, Recanzone, Gregg, Trainor, Brian|
|School:||University of California, Davis|
|School Location:||United States -- California|
|Source:||DAI-B 77/04(E), Dissertation Abstracts International|
|Keywords:||Comparative, Evolution, Marsupial, Somatosensory, Visual acuity|
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