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The Journal of Neuroscience, July 30, 2008, 28(31):7837-7846; doi:10.1523/JNEUROSCI.1600-08.2008
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Behavioral/Systems/Cognitive
Influence of Reward Delays on Responses of Dopamine Neurons
Shunsuke Kobayashi andWolfram Schultz Department of Physiology, Development, and Neuroscience, University of Cambridge, Cambridge CB2 3DY, United Kingdom
Correspondence should be addressed to Shunsuke Kobayashi, Department of Physiology, Development, and Neuroscience, University of Cambridge, Downing Street, Cambridge CB2 3DY, UK. Email: skoba-tky{at}umin.ac.jp
Psychological and microeconomic studies have shown that outcome values are discounted by imposed delays. The effect, called temporal discounting, is demonstrated typically by choice preferences for sooner smaller rewards over later larger rewards. However, it is unclear whether temporal discounting occurs during the decision process when differently delayed reward outcomes are compared or during predictions of reward delays by pavlovian conditioned stimuli without choice. To address this issue, we investigated the temporal discounting behavior in a choice situation and studied the effects of reward delay on the value signals of dopamine neurons. The choice behavior confirmed hyperbolic discounting of reward value by delays on the order of seconds. Reward delay reduced the responses of dopamine neurons to pavlovian conditioned stimuli according to a hyperbolic decay function similar to that observed in choice behavior. Moreover, the stimulus responses increased with larger reward magnitudes, suggesting that both delay and magnitude constituted viable components of dopamine value signals. In contrast, dopamine responses to the reward itself increased with longer delays, possibly reflecting temporal uncertainty and partial learning. These dopamine reward value signals might serve as useful inputs for brain mechanisms involved in economic choices between delayed rewards.
Key words: single-unit recording; dopamine; neuroeconomics; temporal discounting; preference reversal; impulsivity
Received Oct. 25, 2007; revised June 14, 2008; accepted June 23, 2008.
Correspondence should be addressed to Shunsuke Kobayashi, Department of Physiology, Development, and Neuroscience, University of Cambridge, Downing Street, Cambridge CB2 3DY, UK. Email: skoba-tky{at}umin.ac.jp
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