Repeated presentation of a stimulus leads to reductions in measures of neural responses. This phenomenon, termed repetition suppression (RS), has recently been conceptualized using models based on predictive coding, which describe RS as due to expectations that are weighted toward recently-seen stimuli. To evaluate these models, researchers have manipulated the likelihood of stimulus repetition within experiments. They have reported findings that are inconsistent across hemodynamic and electrophysiological measures, and difficult to interpret as clear support or refutation of predictive coding models. We instead investigated a different type of expectation effect that is apparent in stimulus repetition experiments: the difference in one's ability to predict the identity of repeated, compared to unrepeated, stimuli. In previous experiments that presented pairs of repeated or alternating images, once participants had seen the first stimulus image in a pair, they could form specific expectations about the repeated stimulus image but not the alternating image, which was often randomly chosen from a large stimulus set. To assess the contribution of stimulus predictability effects to previously observed RS, we measured BOLD signals while presenting pairs of repeated and alternating faces. This was done in contexts whereby stimuli in alternating trials were either i.) predictable through statistically learned associations between pairs of stimuli or ii.) chosen randomly and therefore unpredictable. We found that RS in the right fusiform face area (FFA) was much larger in trials with unpredictable compared to predictable alternating faces. This was primarily due to unpredictable alternating stimuli evoking larger BOLD signals than predictable alternating stimuli. We show that imbalances in stimulus predictability across repeated and alternating trials can greatly inflate measures of RS, and potentially even mimic RS effects. Our findings indicate that stimulus-specific expectations as described by predictive coding models may account for a sizeable portion of observed RS effects.

重复出现刺激会导致神经反应测量值降低。最近，已使用基于预测编码的模型对这种现象（称为重复抑制（RS））进行了概念化，该模型将RS描述为归因于对最近出现的刺激加权的期望。为了评估这些模型，研究人员在实验中操纵了重复刺激的可能性。他们报告的发现在血液动力学和电生理学措施之间不一致，并且难以解释为对预测编码模型的明确支持或反驳。相反，我们研究了在刺激重复实验中显而易见的另一种类型的预期效果：与未重复的刺激相比，个人预测重复的身份的能力有所不同。在先前的呈现成对重复或交替图像的实验中，一旦参与者看到一对中的第一个刺激图像，他们就可以形成对重复刺激图像的特定期望，而不是对交替图像的期望，后者通常是从大型刺激集中随机选择的。为了评估刺激可预测性效应对先前观察到的RS的贡献，我们测量了BOLD信号，同时呈现了成对的重复和交替面孔。这是在这样的情况下完成的，即交替试验中的刺激要么是（i）通过对刺激之间的统计学习关联来预测的，要么是ii。）随机选择的，因此是不可预测的。我们发现，在无法预测的试验中，右侧梭形面部区域（FFA）的RS比可预测的交替面部大得多。这主要是由于不可预测的交替刺激引起比可预测的交替刺激更大的BOLD信号。我们表明，在重复和交替试验中，刺激可预测性的不平衡会大大提高RS的测量效果，甚至可能模仿RS的影响。我们的发现表明，如预测编码模型所描述的，刺激特定的期望可能占观察到的RS效应的很大一部分。