Singer Rebecca A, Zentall Thomas R
Department of Psychology, Georgetown College, Georgetown, KY 40324, ,
Learn Motiv. 2011 Aug 1;42(3):255-271. doi: 10.1016/j.lmot.2011.06.001.
Pigeons prefer a positive discriminative (S+) stimulus that follows a less preferred event (a large number of required responses, a longer delay, or the absence of food) over a different S+ with a similar history of reinforcement that follows a more preferred event (a single required response, no delay, or food). We proposed that this phenomenon results from contrast (referred to as within-trial contrast) between the less preferred initial event and the signal for reinforcement. Delay reduction theory (Fantino, 1969) can account for these results by proposing that the less preferred initial event lengthens the duration of the trial, thereby allowing the S+ stimulus to occur later in the trial and thus become a better predictor of reinforcement. In the present experiments, we further explored this effect. In Experiment 1, we controlled for trial duration by using a fixed ratio response (30 pecks) as one initial event and the absence of pecking for the same duration as the other initial event (0 pecks). The pigeons showed a reliable preference for the positive stimulus that followed the least preferred initial event. In Experiment 2, we controlled for trial duration by using 30 pecks as one initial event and 1 peck followed by a delay that matched the duration of the preceding 30-peck trial. (Group Time Same). For Group Time Different, there was no delay following the 1-peck initial event. For Group Time Same, preference for the initial event negatively predicted the pigeons' preference for the S+ stimulus that followed, supporting the contrast account. A somewhat greater preference for the discriminative stimulus that followed the least preferred initial event was found for Group Time Different suggesting that in addition to contrast, delay reduction also may play a small role. However, the greater initial-event preference found for Group Time Different suggests that contrast can account for the group difference as well.
相较于另一个具有相似强化历史、跟随更偏好事件（单次所需反应、无延迟或有食物）的正性辨别性（S+）刺激，鸽子更喜欢跟随不太偏好事件（大量所需反应、更长延迟或无食物）的S+刺激。我们提出，这种现象是由不太偏好的初始事件与强化信号之间的对比（称为试验内对比）导致的。延迟减少理论（Fantino，1969）可以通过提出不太偏好的初始事件延长了试验持续时间，从而使S+刺激在试验中出现得更晚，进而成为更好的强化预测指标，来解释这些结果。在本实验中，我们进一步探究了这种效应。在实验1中，我们通过使用固定比率反应（30次啄击）作为一个初始事件，以及在相同持续时间内不进行啄击（0次啄击）作为另一个初始事件，来控制试验持续时间。鸽子对跟随最不偏好初始事件的正性刺激表现出可靠的偏好。在实验2中，我们通过使用30次啄击作为一个初始事件，以及1次啄击后跟随与前一个30次啄击试验持续时间匹配的延迟（时间相同组）来控制试验持续时间。对于时间不同组，1次啄击初始事件后没有延迟。对于时间相同组，对初始事件的偏好对鸽子对随后S+刺激的偏好具有负向预测作用，支持了对比解释。在时间不同组中，发现对跟随最不偏好初始事件的辨别性刺激有稍大的偏好，这表明除了对比之外，延迟减少也可能起了较小的作用。然而，在时间不同组中发现的对初始事件更大的偏好表明，对比也可以解释组间差异。
