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
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item TRANSFORMATIONS OF TASK-DEPENDENT PLASTICITY FROM A1 TO HIGHER-ORDER AUDITORY CORTEX(2016) Elgueda Gonzalez, Diego Enrique; Shamma, Shihab A; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Everyday, humans and animals navigate complex acoustic environments, where multiple sound sources overlap. Somehow, they effortlessly perform an acoustic scene analysis and extract relevant signals from background noise. Constant updating of the behavioral relevance of ambient sounds requires the representation and integration of incoming acoustical information with internal representations such as behavioral goals, expectations and memories of previous sound-meaning associations. Rapid plasticity of auditory representations may contribute to our ability to attend and focus on relevant sounds. In order to better understand how auditory representations are transformed in the brain to incorporate behavioral contextual information, we explored task-dependent plasticity in neural responses recorded at four levels of the auditory cortical processing hierarchy of ferrets: the primary auditory cortex (A1), two higher-order auditory areas (dorsal PEG and ventral-anterior PEG) and dorso-lateral frontal cortex. In one study we explored the laminar profile of rapid-task related plasticity in A1 and found that plasticity occurred at all depths, but was greatest in supragranular layers. This result suggests that rapid task-related plasticity in A1 derives primarily from intracortical modulation of neural selectivity. In two other studies we explored task-dependent plasticity in two higher-order areas of the ferret auditory cortex that may correspond to belt (secondary) and parabelt (tertiary) auditory areas. We found that representations of behaviorally-relevant sounds are progressively enhanced during performance of auditory tasks. These selective enhancement effects became progressively larger as you ascend the auditory cortical hierarchy. We also observed neuronal responses to non-auditory, task-related information (reward timing, expectations) in the parabelt area that were very similar to responses previously described in frontal cortex. These results suggests that auditory representations in the brain are transformed from the more veridical spectrotemporal information encoded in earlier auditory stages to a more abstract representation encoding sound behavioral meaning in higher-order auditory areas and dorso-lateral frontal cortex.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.Item Auditory edge detection: the dynamics of the construction of auditory perceptual representations(2006-04-27) Chait, Maria; Poeppel, David; Simon, Jonathan Z; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation investigates aspects of auditory scene analysis such as the detection of a new object in the environment. Specifically I try to learn about these processes by studying the temporal dynamics of magnetic signals recorded from outside the scalp of human listeners, and comparing these dynamics with psychophysical measures. In total nine behavioral and Magneto-encephalography (MEG) brain-imaging experiments are reported. These studies relate to the extraction of tonal targets from background noise and the detection of change within ongoing sounds. The MEG deflections we observe between 50-200 ms post transition reflect the first stages of perceptual organization. I interpret the temporal dynamics of these responses in terms of activation of cortical systems that participate in the detection of acoustic events and the discrimination of targets from backgrounds. The data shed light on the statistical heuristics with which our brains sample, represent, and detect changes in the world, including changes that are not the immediate focus of attention. In particular, the asymmetry of responses to transitions between 'order' and 'disorder' within a stimulus can be interpreted in terms of different requirements for temporal integration. The similarity of these transition-responses with commonly observed onset M50 and M100 auditory-evoked fields allows us to suggest a hypothesis as to their underlying functional significance, which so far has remained unclear. The comparison of MEG and psychophysics demonstrates a striking dissociation between higher level mechanisms related to conscious detection and the lower-level, pre-attentive cortical mechanisms that sub-serve the early organization of auditory information. The implications of these data for the processes that underlie the creation of perceptual representations are discussed. A comparison of the behavior of normal and dyslexic subjects in a tone-in-noise detection task revealed a general difficulty in extracting tonal objects from background noise, manifested by a globally delayed detection speed, associated with dyslexia. This finding may enable us to tease apart the physiological and behavioral corollaries of these early, pre-attentive processes. In conclusion, the sum of these results suggests that the combination of behavioral and MEG investigative tools can provide new insights into the processes by which perceptual representations emerge from sensory input.