Multiple systems support the neural representation of value over time [electronic resource]

Time, when prolonged to engender delays, has a dramatic impact on the way we value events and experiences in the world. Long delays make future rewards less attractive and make memories more difficult to retain. Thus, in the following series of studies, I investigate how time can be used as a manipulation to study the underlying neural processes that support valuation, learning, and decision making. Experiment 1 investigated how delays impact neural representations of value during intertemporal choice, where subjects chose between smaller, immediate rewards and larger, delayed rewards. Using FMRI, we dissociated neural representations of subjective value for delayed rewards according to reward magnitude and delay time. We found that mesolimbic dopamine regions correlated with the magnitude of future rewards, while dissociable activation in lateral cortical regions correlated with the delay of future rewards. The findings suggest that considering rewards far into the future recruits deliberative cognitive control processes in addition to the reward valuation processes of the mesolimbic system. Even when the timing of outcomes is not made explicit, the experience of delays during learning or decision making may affect how values are calculated and updated. Thus, Experiment 2 explored how the introduction of delays into an instrumental reward learning task affects which neural structures and learning strategies are recruited to support successful learning. We found that both mesolimbic (MPFC and NAcc) and declarative (MTL and lateral PFC) systems were recruited to support prediction error and value signals for learning over delay periods. Additionally, we provided evidence that value signals in the MTL were dependent on attention and working memory processes, as a secondary distraction task disrupted value signals in the MTL but not in the MPFC. The results demonstrate that reinforcement learning over delay periods requires shared activation between mesolimbic dopamine regions and declarative memory systems. Finally, Experiment 3 examined more directly how experiencing delays during learning diminishes explicit assessments of value. Participants learned the values associated with different visual cues by playing a repeated gambles game, experiencing reward outcomes that varied in reward magnitude, probability, and delay until delivery. We found that cues associated with longer delays were perceived as less valuable than cues with shorter delays, as indicated by subjects' explicit recall of reward probabilities. Delays also biased subjects' choices towards more risk-averse preferences. These finding suggest that the experience of delays diminishes assessments of value and biases choice behavior, even when people are not explicitly aware of delay times. Taken together, the present studies provide evidence that delays influence the neural processes underlying learning, valuation, and choice, and they suggest new applications for leveraging the timing of events to shift learning strategies and bias decision making heuristics.

Keywords: Psychology
Advisor: Knutson Brian primary advisor. Thesis advisor
McClure Samuel M. advisor. Thesis advisor
Wagner Anthony David advisor. Thesis advisor