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UBC Theses and Dissertations

Dissociable roles for nucleus accumbens core and shell in regulating risk-based decision making Stopper, Colin Michael


Decision making under conditions of risk and uncertainty constitutes a fundamental aspect of society. Few routine cost/benefit decisions are independent of any consideration of risk and uncertainty, from investing and financial matters to simple assessments of time management and resource allocation. Neuropsychological studies with brain-damaged patients gave initial insights into the cortical contributions to risk-based decision making. Subsequent imaging work has allowed for an understanding of the neural functioning of patients afflicted by disorders which impair risk-based decision making and has also implicated various subcortical structures, including the nucleus accumbens, in these types of decisions. Recent research in humans has shown that nucleus accumbens activation precedes risk-taking or risk-seeking on a financial decision making task. Additionally, animal research has determined that lesions of the nucleus accumbens bias choice away from larger but riskier rewards. The current experiments expand upon these findings by demonstrating that inactivation of a subregion of the accumbens, the shell, is responsible for this bias while the other subregion, the core, mediates the speed at which these decision are made. The effects of accumbens inactivation on risky choice appeared to be due to a reduced tendency to choose the riskier option following trials where rats chose risky and received reward (i.e., reduced win-stay performance), suggesting reduced reward sensitivity. Additionally, this set of experiments demonstrates that instead of inducing risk-aversive tendencies, inactivation of the nucleus accumbens interferes with general value judgments. Specifically, accumbens inactivation induces a slight reduction in preference for the larger reward when the risk/uncertainty component is eliminated. Additionally, inactivation only shifts choice preference away from the more valuable option when it is larger and probabilistic. These data suggest that in addition to effort- and delay-based decision making, the nucleus accumbens also mediates risk-based decision making. In addition to decisions under risk, the nucleus accumbens also seems to play a smaller, yet significant role in judgments of overall value and utility.

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