Investigating the role of long-term perceptual priors in confidence

According to Bayesian models, both our perceptual decisions and our confidence about those decisions are based on the integration of incoming sensory information with our prior expectations. These models therefore assume that priors influence confidence and decisions in the same way, and to the same extent. While asymmetries have been found in the influence priors have on decisions versus confidence, challenging this assumption, those results were obtained with high level cognitive priors that were induced in the task context. It remains unclear whether this generalises to long-term, perceptual priors. Here, we investigated the influence of a low-level prior, namely the slow-motion prior, on confidence. Stimuli were parallel line segments in motion for which the slow-motion prior biases the perceived direction to be perpendicular to the line orientations. Observers had to decide whether the motion direction was clockwise or counterclockwise relative to a reference, and after two such decisions, judge which decision was more likely to be correct. We contrasted two conditions – one where the percept was dominated by the prior, and another where incoming sensory information was dominant. We then assessed which of these conditions participants were more likely to judge as their more confident decision. We found a confidence bias favouring the prior-dominant condition, even when accounting for differences in perceptual decision performance. This suggests that priors impact confidence more strongly than they do perceptual decisions, even in cases of low-level, perceptual priors. Further computational modelling indicates that this effect may be best explained by confidence using the degree of prior-congruent information as an additional cue, above and beyond the posterior evidence used in perceptual decisions. We propose that participants have a metacognitive bias to incorporate confirmatory evidence in favour of their own prior expectations, even when these priors are low-level and participants are unaware of them.