Delay discounting as a behavioral measure of impulsivity has been widely used in neuroeconomy, psychopathology, clinical neuroscience, and drug addiction studies. Previous psychological studies have suggested that: 1) a hyperbolic function best describes the decision behaviors of humans and other animals; 2) drug users tend to have higher delay discounting rates than controls; and 3) the associations between delay discounting rate and personality measures of delay discounting are inconsistent across studies. However, neuroimaging studies have often observed two neural systems involved in delay discounting, and a number of neural models have been proposed to describe delay discounting. The studies reported in this dissertation investigated delay discounting as a behavioral measure of impulsivity when considering the neural models.
In Chapter 1, delay discounting and its designs, task procedures, analysis models, reliability, and validity are reviewed. Based on previous studies, the delay discounting rate is influenced by the design of delay discounting tasks such as reward magnitude; the delay discounting rate reliably differentiates drug users from controls; and its reliability is high within a normal population but is relatively lower in clinical populations. In Chapter 2, the current studies are introduced.
In Chapter 3, three neural model fitting equations are compared with the standard exponential model and hyperbolic model. The studies suggest the saturating-hyperbolic model fits better than the standard models when the reward magnitude is low ($10). The superiority of the saturating-hyperbolic model is even more robust when clinical populations are involved. However, the saturating-hyperbolic model does not fit the empirical data better than the standard models when the reward magnitude is high ($1000).
In Chapter 4, cocaine users are compared with matched controls and with individuals with binge eating disorder on their delay discounting rates; the parameters are analyzed by the saturating-hyperbolic function. The results show that cocaine users do not have significantly higher delay discounting rates; rather, they have significantly higher saturation indices, indicating the observed decision making bias in cocaine users is associated with the decision factor related to reward utility instead of the decision factor related to time utility. Furthermore, the findings suggest the observed decision making bias in cocaine users is not associated with binge eating disorder, indicating the decision preference is likely to be specific to drug users.
In Chapter 5, a personality measure based on the construct of time preference (the Time Preference Scale) is introduced. Its psychometric properties and its association with delay discounting and with the Barrat Impulsiveness Scale are investigated. The Time Preference Scale appears to have high reliability and validity. The latent trait of time preference is significantly associated with delay discounting rate. In addition, time preference is better than the overall score on the Barrat Impulsiveness Scale, but not better than the score on the non-planning subscale of the Barrat Impulsiveness Scale, in predicting the delay discounting rate. In Chapter 6, overall conclusions are drawn based on results of the current studies, and practice implications and research recommendations are provided.