UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
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 given thesis/dissertation in DRUM.
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Item Subtask Control in Human Locomotion(2014) Logan, David Michael; Jeka, John J; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Maintenance of upright posture during walking is one the most important tasks to ensure flexible and stable mobility, along with speed adjustment, wayfinding and obstacle avoidance. These underlying functions, or subtasks, are simultaneously coordinated by the nervous system, which relies heavily on sensory feedback to obtain continual estimates of self-motion. This dissertation reports the findings of four experiments which made use of visual and mechanical perturbations to probe the interplay of these subtasks during treadmill walking. To confront the inherent nonlinearity of human gait, novel frequency domain analyses and impulse response functions that take into account phase of the gait cycle were used to characterize perturbation-response relationships. In the first experiment, transient visual scene motion was used to probe how visual input simultaneously influenced multiple subtasks, but at different phases of the gait cycle. In the second experiment, kinematics and muscle activity response variables showed an amplitude dependency on visual scene motion during walking that indicates vision is reweighted in a manner similar to standing posture. The third experiment used a metronome to constrain walking, revealing two time scales of locomotive control. The final experiment made use of both visual and mechanical perturbations simultaneously to probe the subtasks of postural orientation upright and positional maintenance on the treadmill. Doing so revealed that the nervous system prioritizes control of postural orientation over positional maintenance. In sum, this dissertation shows that sensory and mechanical perturbations provide insight as to how the nervous system controls coexisting, underlying functions during walking.Item The Effects of Age-related Differences in State Estimation on Sensorimotor Control of the Arm in School-age Children(2011) King, Bradley Ross; Clark, Jane E; Kinesiology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Previous research examining sensorimotor control of arm movements in school-age children has demonstrated age-related improvements in performance. A unifying, mechanistic explanation of these improvements is currently lacking. This dissertation systematically examined the processes involved in sensorimotor control of the arm to investigate the hypothesis that improvements in performance can be attributed, in part, to developmental changes in state estimation, defined as estimates computed by the central nervous system (CNS) that specify current and future hand positions and velocities (i.e., hand `state'). A series of behavioral experiments were employed in which 5- to 12-year-old children and adults executed goal-directed arm movements. Experiment 1 demonstrated that improvements in proprioceptive functioning resulted in an increased contribution of proprioception to the multisensory estimate of hand position, suggesting that the CNS of children flexibly integrates redundant sensorimotor feedback based on the accuracy of the individual inputs. Experiment 2 demonstrated that improvements in proprioceptive functioning for localizing initial hand position reduced the directional variability of goal-directed reaching, suggesting that improvements in static state estimation contribute to the age-related improvements in performance. Relying on sensory feedback to provide estimates of hand state during movement execution can result in erroneous movement trajectories due to delays in sensory processing. Research in adults has suggested that the CNS circumvents these delays by integrating sensory feedback with predictions of future hand states (i.e., dynamic state estimation), a finding that has not been investigated in children. Experiment 3 demonstrated that young children utilized delayed and unreliable state estimates to make on-line trajectory modifications, resulting in poor sensorimotor performance. Last, Experiment 4 hypothesized that if improvements in state estimation drive improvements in sensorimotor performance, then exposure to a perturbation that simulated the delayed and unreliable dynamic state estimation in young children would cause the adults to perform similarly to the young children (i.e., eliminating age-related improvements in performance). Results from this study were equivocal. Collectively, the results from these experiments: 1) characterized a developmental trajectory of state estimation across 5- to 12-year-old children; and, 2) demonstrated that the development of state estimation is one mechanism underlying the age-related improvements in sensorimotor performance.Item Temporal Structure in Zebra Finch Song: Implications for the Motor Code and Learning Process(2008-07-07) Glaze, Christopher Mulholland; Carr, Catherine; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)One of the touchstone questions in neuroscience is how the nervous system encodes complex behavioral sequences such as speech. With experience-dependent learning, well-defined anatomy and complex temporal organization, zebra finch song has served as an excellent model system for these questions. Male songs are learned from older males during a sensitive period that includes song memorization and vocal learning guided by auditory feedback. Once learned, song acoustics are hierarchically organized into syllables, continuous stretches of vocalization separated by silent gaps, which are arranged into stereotyped sequences termed motifs; on a finer scale, syllables are composed of one or more notes, vocalizations with a homogenous spectral profile. Although much is known about the song system, progress has been limited by conflicting data on the neural basis of the acoustic hierarchy and the role this organization plays during learning: While behavioral and electrophysiological studies have suggested separate circuits and learning stages for individual syllables and syllable sequence, these models have been challenged by physiological evidence that songs are actually driven by a clock-like mechanism that does not segment songs into different units. We have analyzed and modeled trial-to-trial timing variability in zebra finch song acoustics to investigate whether the hierarchy is in fact represented in the song system and learning process. Using automated template matching and dynamic time warping, we made millisecond-precise timing measurements in tens of thousands of recordings of both adult and juvenile song. In each adult song, we find rendition-to-rendition tempo variability that is spread across syllables and gaps; however syllable lengths stretch and compress with tempo changes proportionally less than gaps, \ie\ they are less ``elastic." Such non-uniformity is at odds with the simplest clock-based model in which songs are driven by a timing mechanism that paces song evenly across syllable-gap sequences. On the other hand, in a subsequent analysis we factored out tempo changes and used the remaining variability to investigate subsyllabic timescales that contradict the hierarchical model as well. Here, we find length variability that is specific to 10-msec song slices and independent of neighboring vocalization, yet correlated across motifs, providing the first behavioral evidence for a 5-10 msec timescale of song representation and an interaction with a neuromodulatory source operating on a much slower timescale. We have developed a model of song production constrained by the timing data; modeling suggests that adult song may be produced by an underlying chain of activity on a single 5-10 msec timescale, but with properties such as synaptic strength that do correspond to the acoustic hierarchy. Finally, we analyzed juvenile song within the same framework and investigated how the timing properties we modeled may develop during sensorimotor learning. The behavioral data indicate a period towards the end of learning in which syllable sequences become more stereotyped, tempo increases selectively among gaps, and independent timing variability falls two- to threefold across syllables and gaps. In remarkable contrast, over this same period we find no changes in patterns of global tempo variability or the fine timescale patterns indicative of chaining mechanisms. Overall, the developmental data suggest a final phase of song learning in which syllable-based representations are consolidated into the longer sequence-based chaining mechanisms proposed for the adult system. A similar process of linking simpler chains to form more functional activity patterns has been proposed for neocortex and other models of sequence learning in mammalian systems. In this respect, adult zebra finch song representations may be most analogous with procedural memory and overlearned sequences such as repetitive speech patterns.