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 Time-locked Cortical Processing of Speech in Complex Environments(2021) Kulasingham, Joshua Pranjeevan; Simon, Jonathan Z; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Our ability to communicate using speech depends on complex, rapid processing mechanisms in the human brain. These cortical processes make it possible for us to easily understand one another even in noisy environments. Measurements of neural activity have found that cortical responses time-lock to the acoustic and linguistic features of speech. Investigating the neural mechanisms that underlie this ability could lead to a better understanding of human cognition, language comprehension, and hearing and speech impairments. We use Magnetoencephalography (MEG), which non-invasively measures the magnetic fields that arise from neural activity, to further explore these time-locked cortical processes. One method for detecting this activity is the Temporal Response Function (TRF), which models the impulse response of the neural system to continuous stimuli. Prior work has found that TRFs reflect several stages of speech processing in the cortex. Accordingly, we use TRFs to investigate cortical processing of both low-level acoustic and high-level linguistic features of continuous speech. First, we find that cortical responses time-lock at high gamma frequencies (~100 Hz) to the acoustic envelope modulations of the low pitch segments of speech. Older and younger listeners show similar high gamma responses, even though slow envelope TRFs show age-related differences. Next, we utilize frequency domain analysis, TRFs and linear decoders to investigate cortical processing of high-level structures such as sentences and equations. We find that the cortical networks involved in arithmetic processing dissociate from those underlying language processing, although bothinvolve several overlapping areas. These processes are more separable when subjects selectively attend to one speaker over another distracting speaker. Finally, we compare both conventional and novel TRF algorithms in terms of their ability to estimate TRF components, which may provide robust measures for analyzing group and task differences in auditory and speech processing. Overall, this work provides insights into several stages of time-locked cortical processing of speech and highlights the use of TRFs for investigating neural responses to continuous speech in complex environments.Item Development of the translaminar circuits in the mouse cortex(2020) Deng, Rongkang; Kanold, Patrick O; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The elaborated connections among cortical neurons form the cortical circuits, which are essential mechanisms underlying various cortical functions such as sensory perception, motor control, and other cognitive functions. The cortical circuits are composed of excitatory neurons and GABAergic interneurons. Excitatory neurons send excitatory connections to cortical neurons, while inhibitory neurons send inhibitory connections. Building the neural circuits is no easy task involving complex genetic programs and the influence of the environment through sensation. Malformation of the cortical circuits during development is implicated in causing neurological disorders, but our knowledge about the developmental process is scarce. The work in this dissertation uses in vitro electrophysiology in brain slices from transgenic mice to investigate how the excitatory connections onto GABAergic interneurons in the primary auditory cortex develop during the first two postnatal weeks. Furthermore, this dissertation explores the mechanisms that could regulate the early development of the cortical circuits by testing the requirement of sensory epithelium and N-methyl-D-aspartate receptors (NMDARs) in the early postnatal development of the neural circuits in the primary sensory cortex and temporal association cortex (TeA), respectively. Results from these studies fill crucial gaps in our understanding of how GABAergic interneurons are integrated into the cortical circuits and highlight the importance of sensory epithelium in the normal development of excitatory connections onto cortical GABAergic interneurons. My results also showed impaired development of GABAergic connections onto excitatory neurons lacking functional NMDARs in the TeA, suggesting an essential role of NMDARs for the early development of inhibitory circuits in the cortex.Item EFFECTS OF AGING ON MIDBRAIN AND CORTICAL SPEECH-IN-NOISE PROCESSING(2016) Presacco, Alessandro; Andreson, Samira; Simon, Jonathan Z.; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Older adults frequently report that they can hear what they have been told but cannot understand the meaning. This is particularly true in noisy conditions, where the additional challenge of suppressing irrelevant noise (i.e. a competing talker) adds another layer of difficulty to their speech understanding. Hearing aids improve speech perception in quiet, but their success in noisy environments has been modest, suggesting that peripheral hearing loss may not be the only factor in the older adult’s perceptual difficulties. Recent animal studies have shown that auditory synapses and cells undergo significant age-related changes that could impact the integrity of temporal processing in the central auditory system. Psychoacoustic studies carried out in humans have also shown that hearing loss can explain the decline in older adults’ performance in quiet compared to younger adults, but these psychoacoustic measurements are not accurate in describing auditory deficits in noisy conditions. These results would suggest that temporal auditory processing deficits could play an important role in explaining the reduced ability of older adults to process speech in noisy environments. The goals of this dissertation were to understand how age affects neural auditory mechanisms and at which level in the auditory system these changes are particularly relevant for explaining speech-in-noise problems. Specifically, we used non-invasive neuroimaging techniques to tap into the midbrain and the cortex in order to analyze how auditory stimuli are processed in younger (our standard) and older adults. We will also attempt to investigate a possible interaction between processing carried out in the midbrain and cortex.