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 Reward modulation of inhibitory control during adolescence: An age related comparison of behavior and neural function(2010) Hardin, Michael George; Fox, Nathan A; Human Development; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The developmental period of adolescence is distinguished by a transition from the dependent, family-oriented state of childhood to the autonomous, peer-oriented state of adulthood. Related to this transition is a distinct behavioral profile that includes high rates of exploration, novelty-seeking, and sensation-seeking. While this adolescent behavioral profile generally aids in the transition to autonomy, it comes at a cost and is often related to excessive risk-taking behavior. Current models attribute the adolescent behavioral profile to a developmental discordance between highly sensitive reward-related processes and immature inhibitory control processes. Specifically, reward-related processes appear to develop in a curvilinear manner characterized by a heightened sensitivity to reward that peaks during adolescence. On the other hand, inhibitory processes show a protracted linear developmental trajectory that begins in childhood and continues gradually throughout adolescence. Thus, the unique developmental trajectories of these two sets of processes leave the adolescent with highly sensitive, reward-driven processes that can only be moderately regulated by gradually developing inhibitory processes. Despite the usefulness of these models of adolescent behavior, they remain incompletely supported by data, as few studies specifically examine the interaction between reward-related and inhibitory processing. The current study addresses this particular gap in the adolescent neural development literature by administering a reward-modified inhibitory control task to children, adolescents, and young adults during functional neuroimaging. Three key findings emerged from the current study. First, adolescents showed greater inhibition-related neural responses than both adults and children when potential monetary reward was available. Second, adolescents reliably showed greater striatal recruitment with reward than both adults and children. These differences in striatal response occurred as all three age groups showed significant reward-related behavioral improvements. Third, when reward was not present, adolescents and children showed deficient inhibitory behavior relative to adults. Findings from this study support models proposing interactive relationships between heightened adolescent sensitivity to reward and protracted development of inhibitory control. Additionally, the current findings expand these models by suggesting heightened adolescent sensitivity to reward may facilitate developmentally inefficient inhibitory control processes in a bottom-up manner.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 Noninvasive neural decoding of overt and covert hand movement(2010) Bradberry, Trent Jason; Contreras-Vidal, José L.; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)It is generally assumed that the signal-to-noise ratio and information content of neural data acquired noninvasively via magnetoencephalography (MEG) or scalp electroencephalography (EEG) are insufficient to extract detailed information about natural, multi-joint movements of the upper limb. If valid, this assumption could severely limit the practical usage of noninvasive signals in brain-computer interface (BCI) systems aimed at continuous complex control of arm-like prostheses for movement impaired persons. Fortunately this dissertation research casts doubt on the veracity of this assumption by extracting continuous hand kinematics from MEG signals collected during a 2D center-out drawing task (Bradberry et al. 2009, NeuroImage, 47:1691-700) and from EEG signals collected during a 3D center-out reaching task (Bradberry et al. 2010, Journal of Neuroscience, 30:3432-7). In both studies, multiple regression was performed to find a matrix that mapped past and current neural data from multiple sensors to current hand kinematic data (velocity). A novel method was subsequently devised that incorporated the weights of the mapping matrix and the standardized low resolution electromagnetic tomography (sLORETA) software to reveal that the brain sources that encoded hand kinematics in the MEG and EEG studies were corroborated by more traditional studies that required averaging across trials and/or subjects. Encouraged by the favorable results of these off-line decoding studies, a BCI system was developed for on-line decoding of covert movement intentions that provided users with real-time visual feedback of the decoder output. Users were asked to use only their thoughts to move a cursor to acquire one of four targets on a computer screen. With only one training session, subjects were able to accomplish this task. The promising results of this dissertation research significantly advance the state-of-the-art in noninvasive BCI systems.Item Relative Salience of Envelope and Fine Structure Cues in Zebra Finch Song(2010) Vernaleo, Beth A.; Dooling, Robert J.; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation examines the perceptual salience of several acoustic cues in zebra finch song. Birdsong has long served as an animal model of speech development. Both are learned during a sensitive period, and require auditory feedback for learning and maintenance. Zebra finch song is commonly studied due to its stereotyped nature. Song syllables are complex, containing multiple cues that are modulated over millisecond time scales. Using psychoacoustic methods, male zebra finches were tested on discrimination of changes to their own and conspecific songs. Females and budgerigars were also tested, since they have auditory experience with song, but do not sing. Three types of synthetic songs were created to determine which acoustic cues in song were most salient to birds. Same-seed noise songs were made of syllable envelopes filled with the same piece of random Gaussian noise. This removed spectral structure but kept song envelope cues intact. Random noise songs were made of each syllable envelope filled with a unique piece of noise. This provided more complex fine structure to the same song envelope. Lastly, Schroeder songs were made of Schroeder harmonic waveforms with the same duration as song syllables. In Schroeder waveforms, spectrum and envelope are constant, but phase changes occur across frequencies. Two types of song changes were tested: single interval duration doublings and single syllable reversals. All birds were much more sensitive to syllable changes than to interval changes. For natural song, there was a duration effect on performance for male zebra finches only. Performance on syllable reversals shorter than 100 milliseconds was positively correlated with syllable duration. In Schroeder song, where only fine temporal structure changes with reversal, all three groups showed a duration effect. Thus, females and budgerigars may focus less on fine structure in natural song than males. In the absence of song spectral structure, birds relied on syllable envelope cues for reversal discrimination. Thus, removal of a single cue from song did not greatly affect reversal discrimination. However, birds performed best when all cues were present. This is reminiscent of human speech, in which multiple redundant cues are used for speech recognition.Item spatial and temporal characteristics of electromagnetic activity in the brain prior to reaches to visual targets(2010) Bonin, Claudia; simon, jonathan z; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The electromagnetic activity in the brain associated with the preparation of reaching movements has been studied extensively in monkeys using direct cell recordings from neurons and in humans using electroencephalography (EEG) and functional magnetic resonance imaging (fMRI). The research presented here extends those lines of investigation into human reaching movements using magnetoencephalography in order to include higher frequencies of activation not available through EEG and temporal resolution not available through fMRI. supplemental materials included in a separate file.Item On The Way To Linguistic Representation: Neuromagnetic Evidence of Early Auditory Abstraction in the Perception of Speech and Pitch(2009) Monahan, Philip Joseph; Idsardi, William J; Poeppel, David E; Linguistics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The goal of this dissertation is to show that even at the earliest (non-invasive) recordable stages of auditory cortical processing, we find evidence that cortex is calculating abstract representations from the acoustic signal. Looking across two distinct domains (inferential pitch perception and vowel normalization), I present evidence demonstrating that the M100, an automatic evoked neuromagnetic component that localizes to primary auditory cortex is sensitive to abstract computations. The M100 typically responds to physical properties of the stimulus in auditory and speech perception and integrates only over the first 25 to 40 ms of stimulus onset, providing a reliable dependent measure that allows us to tap into early stages of auditory cortical processing. In Chapter 2, I briefly present the episodicist position on speech perception and discuss research indicating that the strongest episodicist position is untenable. I then review findings from the mismatch negativity literature, where proposals have been made that the MMN allows access into linguistic representations supported by auditory cortex. Finally, I conclude the Chapter with a discussion of the previous findings on the M100/N1. In Chapter 3, I present neuromagnetic data showing that the re-sponse properties of the M100 are sensitive to the missing fundamental component using well-controlled stimuli. These findings suggest that listeners are reconstructing the inferred pitch by 100 ms after stimulus onset. In Chapter 4, I propose a novel formant ratio algorithm in which the third formant (F3) is the normalizing factor. The goal of formant ratio proposals is to provide an explicit algorithm that successfully "eliminates" speaker-dependent acoustic variation of auditory vowel tokens. Results from two MEG experiments suggest that auditory cortex is sensitive to formant ratios and that the perceptual system shows heightened sensitivity to tokens located in more densely populated regions of the vowel space. In Chapter 5, I report MEG results that suggest early auditory cortical processing is sensitive to violations of a phonological constraint on sound sequencing, suggesting that listeners make highly specific, knowledge-based predictions about rather abstract anticipated properties of the upcoming speech signal and violations of these predictions are evident in early cortical processing.Item Probing Postural Stability Mechanisms in Locomotion(2009) Logan, David Michael; Jeka, John J; Kinesiology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)It is not currently known if those upright stability mechanisms utilized in standing posture are present in locomotion. In this investigation, subjects walked or stood on a treadmill in three speed conditions (posture, 1 km/h, 5 km/h) in front of a visual scene consisting of randomly oriented triangles. The triangles translated in the Anterior-Posterior (A/P) direction in either a low or high amplitude condition. Frequency response functions (FRFs) of both the A/P displacement of bilateral kinematic markers and their corresponding segment angles in response to the visual scene translations were computed. Gain and phase of these FRFs had consistent responses in high amplitude visual conditions in the trunk (hip and shoulder displacements, trunk angle), which motivated further comparisons within the trunk during posture and locomotion. In doing so, the postural processes of orientation and equilibrium control were teased apart during locomotion.Item Molecular Mechanisms of Neuronal Development(2009) Wang, Philip Yung-cheng; Quinlan, Elizabeth M; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Neuronal development relies on the coordination of various biological mechanisms, including the trafficking and function of neurotransmitter receptors and synaptic cell adhesion molecules (CAMs). In this dissertation, I investigated various distinct, yet related, mechanisms of neuronal development: the roles of synaptic adhesion-like molecules (SALMs) in neurite outgrowth and cell adhesion, and the transient expression of N-methyl D-aspartate receptors (NMDARs) at growth cones of young hippocampal neurons. First, I showed that the SALMs, a newly discovered family of CAMs, regulate changes in neurite outgrowth with distinct morphological characteristics. Through transfections of primary hippocampal neurons, I investigated the roles of each SALM in neurite outgrowth. In addition to neurite outgrowth, SALMs are involved in synapse formation. In a parallel study, I further investigated SALM function in development by examining the formation of SALM-mediated cell-cell contacts, and their implications on synaptogenesis. In my final study, I investigated the transient expression of NMDARs at axonal growth cones of young hippocampal neurons. While NMDAR function at synapses is well known, their roles earlier in development are less characterized. The data indicate that NMDARs are present and functional at axonal growth cones of young hippocampal neurons. Somatic whole-cell recordings of young neurons reveal NMDAR-mediated currents in response to local application of NMDA at axonal growth cones, while calcium imaging experiments show that these NMDARs elicit localized calcium influx. Together, the studies in this dissertation give insights into the recurring phenomena of proteins and mechanisms that have dual/multiple roles throughout neuronal development. While a considerable amount of information is known about various biological events that occur at opposite ends of the developmental spectra, the mechanisms connecting them are often enigmatic, but can be elucidated through examining the proteins that they share in common.Item COMPARATIVE STUDIES ON THE STRUCTURE OF THE EARS OF DEEP-SEA FISHES(2009) Deng, Xiaohong; Popper, Arthur N; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Many deep-sea fishes have sensory adaptations for living at great depths with very limited light. While such adaptations are best known in the visual system, it is likely that there are also adaptations in the auditory system that enable deep-sea fishes to use the "auditory scene." However, there are few data on the inner ear of deep-sea fishes. The purpose of this study was to add to those data. Since deep-sea fishes are rarely taken alive, this study was done through comparative anatomical investigations. Three families were chosen from two major deep-sea fish fauna: benthopelagic and mesopelagic. In Antimora rostrata (family Moridae, deep-sea cods), the inner ear structure and its coupling to the swim bladder were analyzed and compared with similar systems found in shallow-water fishes. Part of the membrane labyrinth is thick and rigid. The elaborate structure of the saccular epithelium and the close contact between the ear and swim bladder suggests enhanced hearing sensitivity. In the family Melamphaidae (bigscales and ridgeheads), five species from three genera show broad interspecific variation in the saccular otolith shapes, including having a long otolithic "stalk" in two genera. The presence of this "stalk" corresponds with a gradual change in the saccular maculae. A special type of ciliary bundle on the saccule may have enhanced sensitivity to bundle displacements. Ears were compared between six species of Macrouridae (grenadiers and rattails) that live at different depths. The saccule/lagena size ratio seems to increase with depth, especially between a mesopelagic and a benthopelagic species in the genus Nezumia, in which the benthopelagic species has an enlarged saccule associated with sound production. These findings support the hypothesis that some deep-sea fishes have evolved specializations for inner ear function. While it is not possible to test hearing in deep-sea fishes, the various adaptations found suggest that at least some such species have evolved specialized structures to enable them to use sound in the deep-sea. Some features in the ears of deep-sea fishes that have never been seen in the ears of other vertebrates, which further reveals the structural diversity of fish inner ears in general.Item Retention of a novel visuomotor gain in patients with Parkinson's disease is context-specific(2009) Venkatakrishnan, Anusha; Contreras-Vidal, José L; Kinesiology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hypometria or reduced movement amplitude is a major concern in Parkinson's disease (PD) since it impairs multiple functional activities of daily living, including fine motor control tasks, such as handwriting. Recent research using virtual or computer-based environments, wherein visual information about hand movement is altered and dissociated from perception (e.g., position sense or kinesthesia) of hand movement itself, has shown increases in handwriting size in patients with PD. In fact, preliminary findings in our laboratory have shown that gradual alterations in visual feedback of movement facilitate adaptation of handwriting size in patients with PD, plausibly by recruiting neural networks other than the basal ganglia, such as those in cerebellum. The purpose of this study was to determine whether these adaptive effects persist after a week following visuomotor training in patients with PD and can favorably transfer to other functional writing and drawing tasks. Thirteen patients with Parkinson's disease and twelve healthy, age-matched subjects practiced handwriting either under gradually manipulated (intervention) or intact (placebo) visual display of handwriting size. The results from this study show for the first time, that these adaptive effects may persist for at least up to a week in PD; however, a single training session seemed inadequate to transfer these acquired changes to paper-pen writing and drawing. Additionally, experimental manipulation of task demands during training also helped maintain movement quality in patients with PD as against the placebo group. These findings have important implications in designing rehabilitative interventions to enhance functional sensorimotor performance in patients with PD.