氏名： 大村 優
People tend to discount the value of reward as the time of receipt delays (delay discounting), and as the probability of receipt decreases (probability discounting). Humans and animals often must make decisions on choosing between a high-risk-high-return option and a low-risk-low-return option, and between an immediate small reward and a delayed large reward. Since these types of decision making are often associated with critical outcomes (e.g., drug dependence), determining psychological mechanism underlying the choice and finding the property of its mechanism are important issues for psychologist. This thesis consists of two studies.
In Study 1, I and collaborators examined whether delay discounting and probability discounting are stable tendencies within each individual, so as to explore the possibility for measuring the degrees of these tendencies to be used for clinical and personality psychological purposes. Our results showed that the degrees of delay and probability discounting, assessed with standard discounting parameters (e.g., hyperbolic k and AUC), were stable across three months. By combining this result and the previous studies’ findings that the degrees of discounting in ex drug dependent individuals are smaller than those in current drug dependent individuals, it can be concluded that we may be able to use the measures of drug dependent patients’ degrees of discounting tendencies in order to estimate their degrees of recovery from drug dependence.
In Study 2, we examined the relation between discounting tendencies and nicotine dependence for elucidating the biological bases of discounting tendencies. In this experiment, participants were required to choose between immediate and delayed monetary rewards (or losses), or between certain and probabilistic rewards (losses). As a result, the degree of delay discounting of monetary gains was significantly correlated with both the number of cigarettes smoked per day and the estimated amount of nicotine intake per day. In contrast, the degrees of the other three types of discounting were correlated with neither the number of cigarettes smoked per day nor the estimated amount of nicotine intake per day. Chronic nicotine intake-induced neuroadaptaion in reward-processing neural circuitry may possibly increase impulsivity in delay discounting of monetary gains in smokers of cigarettes. This result will help us understand the common mechanism of personality changes and the onsets of mental disorders, such as chronic stress-induced depressive illnesses.