Psychology Department, University of Colorado Denver, Denver, CO 80217.
Psychology Department, University of Colorado Denver, Denver, CO 80217
Proc Natl Acad Sci U S A. 2017 Dec 26;114(52):E11303-E11312. doi: 10.1073/pnas.1706969114. Epub 2017 Nov 6.
The mesolimbic dopamine system is strongly implicated in motivational processes. Currently accepted theories suggest that transient mesolimbic dopamine release events energize reward seeking and encode reward value. During the pursuit of reward, critical associations are formed between the reward and cues that predict its availability. Conditioned by these experiences, dopamine neurons begin to fire upon the earliest presentation of a cue, and again at the receipt of reward. The resulting dopamine concentration scales proportionally to the value of the reward. In this study, we used a behavioral economics approach to quantify how transient dopamine release events scale with price and causally alter price sensitivity. We presented sucrose to rats across a range of prices and modeled the resulting demand curves to estimate price sensitivity. Using fast-scan cyclic voltammetry, we determined that the concentration of accumbal dopamine time-locked to cue presentation decreased with price. These data confirm and extend the notion that dopamine release events originating in the ventral tegmental area encode subjective value. Using optogenetics to augment dopamine concentration, we found that enhancing dopamine release at cue made demand more sensitive to price and decreased dopamine concentration at reward delivery. From these observations, we infer that value is decreased because of a negative reward prediction error (i.e., the animal receives less than expected). Conversely, enhancing dopamine at reward made demand less sensitive to price. We attribute this finding to a positive reward prediction error, whereby the animal perceives they received a better value than anticipated.
中脑边缘多巴胺系统强烈参与动机过程。目前公认的理论表明，短暂的中脑边缘多巴胺释放事件激发了对奖励的追求，并对奖励价值进行编码。在追求奖励的过程中，奖励和预测其可用性的线索之间形成了关键的关联。通过这些经验，多巴胺神经元开始在提示出现的最早时刻以及在收到奖励时开始发射。由此产生的多巴胺浓度与奖励的价值成正比。在这项研究中，我们使用行为经济学方法来量化短暂的多巴胺释放事件如何与价格成比例缩放，并在因果关系上改变价格敏感性。我们向大鼠提供一系列价格的蔗糖，并对由此产生的需求曲线进行建模，以估计价格敏感性。使用快速扫描循环伏安法，我们发现与提示呈现时间锁定的伏隔核多巴胺浓度随价格降低而降低。这些数据证实并扩展了这样一种观点，即起源于腹侧被盖区的多巴胺释放事件编码主观价值。我们使用光遗传学来增加多巴胺浓度，发现增强提示时的多巴胺释放会使需求对价格更加敏感，并降低奖励时的多巴胺浓度。从这些观察结果中，我们推断出价值降低是因为负奖励预测误差（即动物得到的奖励少于预期）。相反，增强奖励时的多巴胺会使需求对价格的敏感度降低。我们将这一发现归因于正奖励预测误差，即动物认为他们得到的价值比预期的要好。
