Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item CHARACTERIZATION OF CHRONIC MONOCULAR DEPRIVATION AND ESTROGEN ADMINISTRATION IN ADULT RODENTS(2018) Sengupta, Deepali Clare; Quinlan, Elizabeth M; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Reduced synaptic plasticity and excitatory synapse density contribute to age-related cognitive decline, and constrain recovery of function from injury in adults. A parallel reduction in circulating sex hormones in both sexes, particularly estrogens, exacerbates this decline in synaptic plasticity. Conversely, estrogen therapy in aged members of many species restores synapse density, promotes synaptic plasticity, and improves learning/memory. Importantly, acute estrogen administration can promote rapid synaptogenesis, and these new synapses can be stabilized by activity. Here I ask if estrogen treatment can promote synaptic plasticity in the primary visual cortex (V1) of aged rats. I demonstrate robust expression of estrogen receptors (ERs) in V1 of adult male and female rats, suggesting an opportunity to enhance plasticity with estrogens. I test this hypothesis following the induction of amblyopia by chronic monocular deprivation (cMD). I show that cMD reduces thalamic innervation from the deprived eye, and increases molecular markers which constrain plasticity, consistent with observations that the deficits induced by cMD are highly resistant to reversal. Surprisingly, cMD did not change markers for excitatory synapses, suggesting a homeostatic increase in synapses serving the non-deprived eye (NDE) to maintain synaptic density within an optimal range. Importantly, visually-evoked potentials (VEPs) induced by repetitive visual stimulation to the deprived eye depress more rapidly than those of the NDE, consistent with cMD inducing an increase in the probability of neurotransmitter release (Pr) at synapses in the cMD pathway. In contrast, treatment of cMD adults with a single dose of 17α estradiol significantly increased markers for excitatory synapses, and estradiol treatment followed by visual stimulation also increased markers for excitatory synapse activity. Repetitive estradiol treatments increased excitatory synapse markers, but not synaptic activity markers. Furthermore, one dose of estradiol enhanced VEP amplitude following repetitive visual stimulation, however this was observed only in response to stimulation of the NDE. As presynaptic ERs are known to increase Pr at glutamatergic synapses, this suggests that the effects of estradiol are specific to spared synapses where Pr has not been up-regulated by deprivation. Exploiting this selectivity may allow for receptive field remapping of spared inputs around a scotoma or cortical infarctItem Modeling the Influence of Phase Boundaries and Oxygen Interstitials on the Nucleation and Growth of Deformation Twins in the Alpha-Phase of Titanium Alloys(2015) Joost, William Joseph; Ankem, Sreeramamurthy; Kuklja, Maija M; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Twinning is an important deformation mechanism in many hexagonal close packed metals, including alpha-titanium alloys. However, the processes of twin nucleation, growth, and interaction with other defects are not well understood. Further, many aspects of deformation twinning are difficult to interrogate experimentally owing to the small time and length scales of the governing mechanisms. In this study we apply a combination of theoretical and computational materials science techniques, leveraged with experimental data, to quantify the effects of alpha-beta phase boundaries and oxygen interstitials on twin nucleation, twin growth, and ultimately mechanical behavior in titanium alloys. Combined results from finite element method and analytical dislocation modeling demonstrate that elastic and plastic interaction stresses across the interface between the alpha- and beta-phases are responsible for the experimentally observed anisotropy in the deformation behavior of dual-phase alloys. Interaction stresses also promote slip and twinning at up to 30% lower applied stress than predicted from Schmid's Law, significantly affecting performance in many applications. The complex interactions of phase boundaries, dislocations, and deformation twins modify the preferred deformation mechanism and promote twinning for some loading orientations. In order to quantify the interaction between oxygen interstitials and (10-12) twin boundaries, we employ atomistic simulations using a newly developed modified embedded atom method potential and density functional theory. Our investigation reveals that a twin boundary alters interstitial formation energy by as much as 0.5 eV while also stabilizing a tetrahedral interstitial, which is unstable in the bulk. Further, the activation barriers for diffusion in the region near a twin are uniformly lower than in the bulk; an atom diffusing across the twin boundary moves through several paths with peak activation barriers more than 0.3 eV lower than for comparable diffusion far from the twin. Despite accelerated kinetics, oxygen diffusion still occurs much more slowly than twin growth, suggesting that oxygen interstitials contribute to experimentally observed time-dependent twinning. Together, these results provide new insight while enabling predictive modeling and purposeful development of improved titanium alloys across a wide range of applications.Item RAPID ADAPTIVE PLASTICITY IN AUDITORY CORTEX(2010) Atiani, Serin; Shamma, Shihab A; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Navigating the acoustic environment entails actively listening for different sound sources, extracting signal from a background of noise, identifying the salient features of a signal and determining what parts of it are relevant. Humans and animals in natural environments perform such acoustic tasks routinely, and have to adapt to changes in the environment and features of the acoustic signals surrounding them in real time. Rapid plasticity has been reported to be a possible mechanism underling the ability to perform these tasks. Previous studies report that neurons in primary auditory cortex (A1) undergo changes in spectro-temporal tuning that enhance the discriminability between different sound classes, modulating their tuning to enhance the task relevant feature. This thesis investigates rapid task related plasticity in two distinct directions; first I investigate the effect of manipulating task difficulty on this type of plasticity. Second I expand the investigation of rapid plasticity into higher order auditory areas. With increasing task difficulty, A1 neurons' response is altered to increasingly suppress the representation of the noise while enhancing the representation of the signal. Comparing adaptive plasticity in secondary auditory cortex (PEG) to A1, PEG neurons further enhance the discriminability of the sound classes by an even greater enhancement of the target response. Taken together these results indicate that adaptive neural plasticity is a plausible mechanism that underlies the performance of novel auditory behaviors in real time, and provide insights into the development of behaviorally significant representation of sound in auditory cortex.