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Adolescence is a transition period during which individuals have increasing autonomy in decision-making for themselves (Casey, Jones, and Hare, 2008), often choosing among options about which they have little knowledge and experience. This process of individuation and independence is reflected as real-world risk taking behaviors (Silveri et al., 2004), including higher motor accidents, unwanted pregnancies, sexually transmitted diseases, drug addictions, and death (Casey et al., 2008). The extent to which adolescents continue to display increased behaviors with negative consequences during this period of life depends critically on their ability to explore and learn potential consequences of actions within novel environments. This learning is not limited to the value of the outcome associated with making choices, but extends to the levels of risk taken in making those choices. While the existing adolescence literature has focused on neural substrates of risk preferences, how adolescents behaviorally and neurally learn about risks remain unknown. Success or failure to learn the potential variability of these consequences, or the risks involved, in ambiguous decisions is hypothesized to be a crucial process to allow the individuals to make decisions based on their risk preferences. The alternative in which adolescents fail to learn about the risks involved in their decisions leaves the adolescent in a state of continued exploration of the ambiguity, reflected as continued risk-taking behavior. This dissertation comprises 2 papers. The first paper is a perspective paper outlining a paradigm that risk taking behavior observed during adolescents may be a product of each adolescent's abilities to learn about risk. The second paper builds on the hypothesis of the perspective paper by first examining neural correlates of risk learning and quantifying individual risk learning abilities and then examining longitudinal risk learning developmental trajectories in relation to real-world risk-trajectories in adolescent individuals. / Doctor of Philosophy / Adolescence is a transition period during which individuals have increasing autonomy in decision-making for themselves, often choosing among options about which they have little knowledge and experience. This process of individuation and independence begins with the adolescent exploring their world and those options they are ignorant of. This is reflected as real-world risk-taking behaviors, including higher motor accidents, unwanted pregnancies, sexually transmitted diseases, drug addictions, and death. We hypothesized and tested the premise that whether adolescents who succeeded or fail to learn about the negative consequences of their actions while exploring will continue to partake in behaviors with negative consequences. This learning is not limited to the value of the outcome associated with making choices, but extends to the range of possible outcomes of the choices or the risks involved. Indeed, the failure to learn the risks involved in decisions with no known information show continued and greater risk-taking behavior, perhaps remaining in a state of continued exploration of the unknown.

Previous research presents a paradox in relation to the value of exerting personal control such that personal control is generally reinforcing, but its value may also be limited in some individuals and under certain circumstances. Across two studies, this dissertation takes a step towards exploring the limitations of perceived control at the process-level by manipulating perceived control via the provision of choice. Manuscript 1 examined limitations of perceived control in the context of effort costs and found that actual control, but not illusory control, may be necessary to enhance motivation in the context of physical effort, suggesting that perceived control may be limited in the context of effort. Manuscript 2 examined limitations of perceived control in relation to self-reported symptoms of anhedonia and found that responsivity to personal control was diminished in those with higher levels of anhedonia. Together these studies examined factors associated with limitations in appetitive personal control and suggest avenues for future research exploring perceived control processes and how they may interface with reward processes, which has potential implications for developing interventions to alleviate reward-related deficits found in anhedonia. / Doctor of Philosophy / Past research has shown that exerting personal control (actively influencing things in your life) is generally desired and motivating, but for some individuals and in some circumstances personal control may be less desirable or motivating (sometime people do not want to be in control). Across two studies, this dissertation explored why perceived control (the belief that one has influence over outcomes in one's life) might not be desired or motivating. In both studies, participants experienced perceived control during experiments when they were given choices within computerized games, believing themselves to have control over outcomes in the game. Manuscript 1 examined how perceived control may be less desirable when people must exert physical effort and found that people may be less inclined to believe they have control when their choice leads to a physical effort requirement. Manuscript 2 examined whether people want to be in control when they are experiencing anhedonia, a set of psychiatric symptoms that includes diminished motivation and reduced responses to reward (for example, paying less attention to rewards in the environment). This study found that people with anhedonia symptoms did not seem to want to be in control as much as psychologically healthy people. During the computerized game, people with anhedonia did not try to make their own choices when they had an opportunity to. Together these studies examined different factors associated with people not wanting to be in control or finding personal control less motivating. This research has implications for developing therapies for people with anhedonia, particularly symptoms related to not actively taking control.

Pile-supported geogrid-reinforced embankments are an exciting new foundation system that is utilized when sites are limited by a soft soil or clay. In this system, an embankment is supported by a bridging layer, consisting of granular fill and one or multiple layers of geogrid reinforcement. The bridging layer transfers the load to piles that have been driven into the soft soil or clay. The load from the embankment induces large deformations in the geogrid reinforcement, causing tensile forces in the ribs of the geogrid. Many of the current methods used to design geogrid reinforcement for this system simplify the approach by assuming that the reinforcement has a parabolic deformed shape. The purpose of this thesis is to thoroughly examine the behavior of the geogrid in a pile-supported embankment system, in an effort to determine the accuracy of the parabolic deformed shape, and identify the most important parameters that affect reinforcement design. The geogrid was analyzed using a three-dimensional model that included a cable net to represent the geogrid and linear springs to represent the soil underneath the geogrid. A larger pressure was applied to the geogrid regions that are directly above the pile caps so that arching effects could be considered, and the stiffness of the springs on top of the pile were stiffer to account for the thin layer of soil between the geogrid and the pile cap. A Mathematica algorithm was used to solve this model using the minimization of energy method. The results were compared to another model of this system that used a membrane to represent the geosynthetic reinforcement. Additionally, the maximum strain was compared to the strain obtained from a geosynthetic reinforcement design formula. A parametric study was performed using the Mathematica algorithm by varying the pile width, embankment pressure applied to the soil, embankment pressure applied to the pile, stiffness of the soil, stiffness of the soil on top of the pile, stiffness of the geogrid, geogrid orientation, rotational stiffness of the geogrid, and the layers of geogrid reinforcement. / Master of Science

Posttraumatic stress disorder (PTSD) is accompanied by marked alterations in cognition and behavior, particularly when negative, high-value information is present (Aupperle, Melrose, Stein, & Paulus, 2012; Hayes, Vanelzakker, & Shin, 2012) . However, the underlying processes are unclear; such alterations could result from differences in how this high value information is updated or in its effects on processing future information. To untangle the effects of different aspects of behavior, we used a computational psychiatry approach to disambiguate the roles of increased learning from previously surprising outcomes (i.e. associability; Li, Schiller, Schoenbaum, Phelps, & Daw, 2011) and from large value differences (i.e. prediction error; Montague, 1996; Schultz, Dayan, & Montague, 1997) in PTSD. Combat-deployed military veterans with varying levels of PTSD symptoms completed a learning task while undergoing fMRI; behavioral choices and neural activation were modeled using reinforcement learning. We found that associability-based loss learning at a neural and behavioral level increased with PTSD severity, particularly with hyperarousal symptoms, and that the interaction of PTSD severity and neural markers of associability based learning predicted behavior. In contrast, PTSD severity did not modulate prediction error neural signal or behavioral learning rate. These results suggest that increased associability-based learning underlies neurobehavioral alterations in PTSD. / Master of Science

Cocaine use has contributed to 5 million individuals falling into the cycle of addiction. Prior research in cocaine dependence mainly focused on rewards. Losses also play a critical role in cocaine dependence as dependent individuals fail to avoid social, health, and economic losses even when they acknowledge them. However, dependent individuals are extremely adept at escaping negative states like withdrawal. To further understand whether cocaine use may contribute to dysfunctions in aversive learning, this paper uses fMRI and an aversive learning task to examine cocaine dependent individuals abstinent from cocaine use (C-) and using as usual (C+). Specifically of interest is the neural signal representing actual loss compared to the expected loss, better known as prediction error (δ), which individuals use to update future expectations. When abstinent (C-), dependent individuals exhibited higher positive prediction error (δ+) signal in their striatum than when they were using as usual. Furthermore, their striatal δ+ signal enhancements from drug abstinence were predicted by higher positive learning rate (α+) enhancements. However, no relationships were found between drug abstinence enhancements to negative learning rates (α±-) and negative prediction error (δ-) striatal signals. Abstinent (C-) individuals' striatal δ+ signal was predicted by longer drug use history, signifying possible relief learning adaptations with time. Lastly, craving measures, especially the desire to use cocaine and positive effects of cocaine, also positively correlated with C- individuals' striatal δ+ signal. This suggests possible relief learning adaptations in response to higher craving and withdrawal symptoms. Taken together, enhanced striatal δ+ signal when abstinent and adaptations in relief learning provide evidence in supporting dependent individuals' lack of aversive learning ability while using as usual and enhanced relief learning ability for the purpose of avoiding negative situations such as withdrawal, suggesting a neurocomputational mechanism that pushes the dependent individual to maintains dependence. / Master of Science

Column-supported, geosynthetic-reinforced embankments provide effective geotechnical foundations for applications in areas of weak subgrade soils. The system consists of a soil bridging layer with one or more embedded layers of geosynthetic reinforcement supported by driven or deep mixed columnar piles. The geosynthetic promotes load transfer within the bridging layer to the columns, allowing for larger column spacings and varied alignments. This technique is generally used when differential settlements of the embankment or adjacent structures are a concern and to minimize construction time. Recent increase in the popularity of this composite system has generated the need to further investigate its behavior and soil-structure interaction. Current models of geosynthetics are oversimplified and do not represent the true three-dimensional nature of the material. Such simplifications include treating the geosynthetic as a one-dimensional cable as well as neglecting stress concentrations and pile orientations. In this thesis, a complete three-dimensional analysis of the geosynthetic is performed. The geosynthetic was modeled as a thin flexible plate in a single square unit cell of the embankment. The principle of minimum potential energy was then applied, utilizing central finite difference equations. Energy components from vertical loading, soil and column support, as well as bending and membrane stiffness of the geosynthetic are considered. Three pile orentation types were implemented: square piles, circular piles, and square piles rotated 45° to the edges of the unit cell. Each of the pile orientations was analyzed using two distinct parameter sets that are investigated in previously published and ongoing research. Vertical and in-plane deflections, stress resultants, and strains were determined and compared to other geosynthetic models and design guides. Results of each parameter set and pile orientation were also compared to provide design recommendations for geosynthetic-reinforced column-supported embankments. / Master of Science