Investigating the Relationship Between Delay and Probability Discounting: Evidence from Healthy Aging and Focal Lesion Patients

Each day, we engage in complex decisions that involve weighing different reward outcomes by considering the prolonged impact (delay discounting) or risk (probability discounting) associated with our choices. Surprisingly, the nature of the relationship between delay and probability discounting remains unclear despite their interdisciplinary relevance to trait impulsivity and future-oriented thinking. The current dissertation aims to understand whether a common mechanism underlies delay and probability discounting through a neurocognitive lens. In Study 1, I examined the neural substrates affecting delay and probability discounting by assessing both decision-making tasks in individuals with vmPFC or MTL damage and in matched controls. Overall, only vmPFC patients discounted delayed rewards more steeply, but discounted probabilistic rewards more shallowly, than controls, representing a significant negative correlation in performance between the two discounting tasks. In Study 2, the relationship between delay and probability discounting was evaluated by determining if personal event cues, which are known to modulate delay discounting, also modulate probability discounting. Similar to previous studies, event cues led to shallower discounting of delayed rewards; however, this effect was significantly less pronounced in older adults compared to young adults. In contrast, event cues had little to no effect on the discounting of probabilistic rewards for both age groups. Study 3 extended the findings from the first two studies by examining whether personal event cues modulate probability discounting in vmPFC patients, who were recently shown to reduce delay discounting when presented with event cues. When presented with cues, vmPFC patients discounted probabilistic rewards more steeply than controls. Importantly, their rates of discounting with cues were aligned with the typical discounting observed in controls without cues, suggesting a modulatory effect that also reduced their risk-taking. MTL patients, like controls, showed no differences in probability discounting with and without cues. Collectively, my dissertation provides evidence against the theory that a unitary trait of discounting underlies delay and probability discounting. Nevertheless, at a neurocognitive level, this set of studies shows that the vmPFC is central to reward discounting, and within its contributions to reward valuation and episodic future thinking, may instantiate self-schematic representations to modulate delay and probability discounting.