Postreward delays and systematic biases in measures of animal temporal discounting
Postreward delays and systematic biases in measures of animal temporal discounting
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DOI：
10.1073/pnas.1310446110
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发表时间：
2013-09-17
影响因子：
11.1
通讯作者：
Hayden, Benjamin Y.
中科院分区：
文献类型：
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作者：
Blanchard, Tommy C.;Pearson, John M.;Hayden, Benjamin Y.
跨期选择任务（Intertemporal choice task），即用较小/较快的奖励对抗较大/较晚的奖励，经常被用来研究时间偏好，进而研究冲动和自我控制。当在动物身上使用时，许多试验按顺序串联在一起，并在较小/较快选项后添加一个调整缓冲，以保持每次试验的总持续时间不变。选择更小/更快的选项并不是奖励最大化，因此这表明动物在对未来的奖励打折扣。然而，如果动物没有正确地考虑到后向缓冲的持续时间，假定的折扣行为可能反而反映了受限的奖励最大化。在这里，我们报告了与这个无贴现假设一致的三个结果。我们发现(i)猴子对后向延迟的持续时间和它们的选择之间的关系不敏感；（ii）他们对事后延迟的长度很敏感，尽管他们大大低估了延迟的长度；(3)增加了猴子对较大/较晚选项的延迟偏好的显著性，降低了测量的贴现率。这些结果与标准的基于贴现的帐户不兼容，但与另一种启发式模型兼容。我们的数据表明，测量动物的跨期偏好可能不能反映冲动，甚至不能反映对未来选择的心理折扣，而标准的人类和动物跨期选择任务测量的是不相关的心理过程。
Intertemporal choice tasks, which pit smaller/sooner rewards against larger/later ones, are frequently used to study time preferences and, by extension, impulsivity and self-control. When used in animals, many trials are strung together in sequence and an adjusting buffer is added after the smaller/sooner option to hold the total duration of each trial constant. Choices of the smaller/sooner option are not reward maximizing and so are taken to indicate that the animal is discounting future rewards. However, if animals fail to correctly factor in the duration of the postreward buffers, putative discounting behavior may instead reflect constrained reward maximization. Here, we report three results consistent with this discounting-free hypothesis. We find that (i) monkeys are insensitive to the association between the duration of postreward delays and their choices; (ii) they are sensitive to the length of postreward delays, although they greatly underestimate them; and (iii) increasing the salience of the postreward delay biases monkeys toward the larger/later option, reducing measured discounting rates. These results are incompatible with standard discounting-based accounts but are compatible with an alternative heuristic model. Our data suggest that measured intertemporal preferences in animals may not reflect impulsivity, or even mental discounting of future options, and that standard human and animal intertemporal choice tasks measure unrelated mental processes.