Neural Mechanisms of Reward Processing

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DESCRIPTION (provided by candidate): When rewards are expected only after some delay, their present value is reasonably determined by discounting their worth relative to the length of the delay. Experiments aimed at determining the nature of such time discount functions in people and animals have found that time is valued significantly more in the near-term than in the long-term: a phenomenon known as hyperbolic time discounting. The exact nature of hyperbolic discounting remains hotly debated, with no empirical data able to clearly distinguish between alternative hypotheses. In this proposal, a series of functional MRI experiments are developed to distinguish between two of the most popular theories for how hyperbolic discounting arises from brain function. We test several hypotheses as subjects make choices between rewarding outcomes of differing magnitudes at different delays. Our experiments employ both natural rewards (fruit juice) and monetary rewards to determine whether the associated brain structures generalize across reward modalities. In addition to this experimental work, we will conduct computational research using reinforcement learning methods to determine whether there are normative reasons for why people discount hyperbolically through time.