Divergent effects of expectations on behavior and brain

Expectations have a significant impact on sensory experiences, as evidenced by placebo and nocebo effects. In both cases, positive or negative outcomes are products of expectations, where an individual assimilates sensory experience with expectations. However, assimilation effects may not always occur, particularly when expectations are highly discrepant from sensory experience. Using a factorial experiment with different levels of cues and stimulus intensities, we find that higher expectations lead to higher pain ratings when expectations match incoming stimulus, indicating an assimilation effect. In contrast, when expectations and stimulus intensities do not match, expectations have less influence on outcome ratings, implying greater weighting on the stimulus input itself. Our results suggest that expectations and noxious input are weighted differently depending on congruence between expectations and pain experience. A computational model simulation supports these findings. Interestingly, the behavioral and neural results show opposite patterns, where low cues were associated with higher pain-predictive brain activation. Using our computational framework, further investigation is required to identify the discrepancy between the behavioral and neural results and understand the divergent effects of expectations on sensory perception.