Long-term perceptual priors drive confidence bias which favors prior-congruent evidence

Within a Bayesian framework, both our perceptual decisions and confidence about those decisions are based on the precision-weighted integration of prior expectations and incoming sensory information. This assumes priors to influence both decisions and confidence in the same way. Against this assumption, asymmetries have been found in the influence that priors have on confidence compared to discrimination decisions. However, these patterns were found for high-level probabilistic expectations that are flexibly induced in the task context. It remains unclear whether this generalizes to low-level perceptual priors that are naturally formed through long term exposure. Here we investigated human participants’ confidence in decisions made under the influence of a long-term perceptual prior: the slow-motion prior. Participants viewed moving line stimuli for which the slow-motion prior biases the perceived motion direction. On each trial, they made two consecutive motion direction decisions followed by a confidence decision. We contrasted two conditions – one in which the prior biased perceptual decisions and one in which decisions were driven by the sensory information alone. We found a confidence bias favoring the condition in which the prior influenced decisions, even when accounting for performance differences. Computational modeling revealed this effect to best be explained by confidence using the prior-congruent evidence as an additional cue, beyond the posterior evidence used in the perceptual decision. This suggests a confirmatory confidence bias favoring evidence congruent with low-level perceptual priors, revealing that, in line with high-level expectations, even long-term priors have a particularly strong influence at the metacognitive level.