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 Response inhibition and the cortico-striatal circuit(2015) Bryden, Daniel William; Roesch, Matthew R; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The ability to flexibly control or inhibit unwanted actions is critical for everyday behavior. Lack of this capacity is characteristic of numerous psychiatric diseases including attention deficit hyperactivity disorder (ADHD). My project is designed to study the neural underpinnings of response inhibition and to what extent these mechanisms are disrupted in animals with impaired impulse control. I therefore recorded single neurons from dorsal striatum, orbitofrontal cortex, and medial prefrontal cortex from rats performing a novel rodent variant of the classic "stop signal" task used in clinical settings. This task asks motivated rats to repeatedly produce simple actions to obtain rewards while needing to semi-occasionally inhibit an already initiated response. To take this a step further, I compared normal rats to rats prenatally exposed to nicotine in order to better understand the mechanism underlying inhibitory control. Rats exposed to nicotine before birth show abnormal attention, poor inhibitory control, and brain deficits consistent with impairments seen in humans prenatally exposed to nicotine and those with ADHD. I found that dorsal striatum neurons tend to encode the direction of a response and the motor refinement necessary to guide behaviors within the task rather than playing a causal role in response inhibition. However the orbitofrontal cortex, a direct afferent of dorsal striatum, possesses the capacity to inform the striatum of the correct action during response inhibition within the critical time window required to flexibly alter an initiated movement. On the other hand, medial prefrontal cortex functions as a conflict “monitor” to broadly increase preparedness for flexible response inhibition by aggregating current and past conflict history. Lastly, rat pups exposed to nicotine during gestation exhibit faster movement speeds and reduced capacity for inhibitory behavior. Physiologically, prenatal nicotine exposure manifests in a hypoactive prefrontal cortex, diminished encoding of task parameters, and reduced capacity to maintain conflict information.Item An Investigation of Inhibitory Control in Bilingual Aphasia(2015) Sampson, Monica; Faroqi-Shah, Yasmeen; Hearing and Speech Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Speaking involves selecting words and syntactic structures from among numerous competing options. It has been suggested that constant practice in using inhibitory control (IC) to limit within and cross-language competition may be associated with better lexical-semantic IC in proficient bilingual speakers relative to monolingual speakers. This advantage is also theorized to generalize to IC advantages in non-linguistic tasks (bilingual advantage hypothesis; BAH; Bialystok, 2001). However, conflicting evidence with regard to bilingual IC advantages abound, and the nature of relationship between linguistic and domain-general inhibitory control abilities is poorly understood. Since IC is proposed to be critical for lexical retrieval, it is important to understand the nature of IC engaged in individuals with lexical retrieval deficits (aphasia). Bilingual speakers with aphasia provide an ideal platform to examine the relationship between language processing and IC because there are seemingly contradictory effects at play: while bilingualism may render an IC advantage, acquired brain injury may be associated with less efficient IC. These contrasting effects allow one to tease apart the effects of bilingualism on IC, the domain generality of the bilingual IC advantage, and relationship between bilingualism, IC and lexical selection. It is important to examine these effects relative to matched monolingual controls to understand (i) if there is a bilingual advantage in lexically based IC and, (ii) the domain generality of any bilingual IC advantage. To address these aims, IC engaged in (i) lexical retrieval (semantically blocked cyclic naming task), (ii) linguistic processing (Stroop task), and (iii) non-linguistic processing (flanker task) was compared in ten each of bilingual (Tamil-English) and monolingual (English) neurologically healthy speakers and participants with aphasia. Results from neurologically healthy participants revealed a bilingual advantage in the blocked cyclic naming task (lexical IC) but no advantages in the non-lexical Stroop and flanker tasks. Results from participants with aphasia revealed no support for the proposed bilingual advantages in all three experiments. Furthermore, there was no significant association between inhibitory control measures in the three experimental tasks for all participants. Contrary to the predictions of the BAH, the collective results of this study indicate that there is insufficient evidence for the role of bilingualism in modulating non-lexical IC advantages. This lack of consistent support for BAH questions the influence of bilingual experience in modulating non-linguistic inhibitory control. These findings also reveal that the relationship between inhibitory control and lexical retrieval is not influenced by language background (monolingual versus bilingual) in persons with aphasia.Item REGULATION OF THE INHIBITORY DRIVE IN THE OLFACTORY BULB(2013) Nunez-Parra, Alexia Francisca; Araneda, Ricardo C; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Animals are exposed to a variety of odor cues that serve as environmental guides for their exploratory and social behaviors. Two distinct but complementing pathways process chemosensory cues: the Main and the Accessory olfactory System (AOS). Sensory neurons send their axons to the olfactory bulb (OB), specifically to the main and the accessory olfactory bulb (MOB and AOB, respectively) where they synapse onto principal neurons, the mitral (MCs). The OB is the only relay center between sensory neurons and cortical and limbic structures and therefore important aspects of odor processing occur in this region. Specifically, a distinctive mechanism used for olfactory processing is a strict regulation of MCs output by inhibitory neurons called granule cells (GCs). Importantely, inhibition of MCs is a dynamic process; it is regulated by the constant addition of new GCs to the OB circuit throughout life, in a process known as adult neurogenesis. Little is known, however, about the contribution of adult born neurons to the processing of olfactory cues, known as pheromones. Detection of pheromones by the AOS is critical for proper display of social behaviors such as hierarchical dominance and mate recognition. Here, we studied how the integration of new-born neurons could be regulated. We found that the arrival of new neurons into the adult AOB increases after animals are exposed to aggression and mate cues, suggesting that these newly arrived neurons can add important plasticity to the AOB circuitry and modify olfactory processing under different behavioral contexts. In addition, GCs mediated inhibition in the OB is precisely controlled by an extensive centrifugal innervation. For example, cortical feedback projections and neuromodulatory afferents originating in the midbrain and basal forebrain excite GC, inhibiting MCs' and decreasing their output. Regulation of of GCs by inhibition has also been reported, however, the source of this inhibition and its relevance to olfactory processing is not known. Here we characterized inhibitory inputs onto GCs and show that GCs receive extensive inhibition from GABAergic neurons in the HDB/MCPO and from neighboring GCs. Moreover, we show, for the first time, that inhibition onto GCs is required for proper olfactory discrimination.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 ENZYME INHIBITION IN MICROFLUIDICS FOR RE-ENGINEERING BACTERIAL SYNTHESIS PATHWAYS(2009) LARIOS BERLIN, DEAN EDWARD; RUBLOFF, GARY W; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Enzyme-functionalized biological microfluidic (EF-BioMEMS) systems are an emerging class of lab-on-chip devices that manipulate enzymatic pathways by localizing reaction sites in a microfluidic network. An EF-BioMEM system was fabricated to demonstrate biochemical enzyme inhibition. Further, design optimizations to the EF-BioMEM system have been proposed which improve system sensitivity and performance. The pfs enzyme is part of the quorum-sensing pathway that ultimately produces the bacterial signaling molecule AI-2. An EF-BioMEM system was fabricated to investigate the pfs conversion activity in the presence of a transition state analogue inhibitor. A reduction in enzyme conversion was measured in microfluidics for increasing inhibitor concentration that was comparable to the response expected on a larger scale. This EF-BioMEMS testbed is capable of investigating other compounds that inhibit quorum sensing. Design improvements were demonstrates that improve overall system responsiveness by minimizing unintended reactions from non-specifically bound enzyme. EF-BioMEMS signal-to-background performance increased from 0.72 to 2.43.