Is subjective perceptual similarity metacognitive?

Perceptual similarity is a cornerstone for human learning and generalization. However, in assessing perceptual similarity between two stimuli, it is not well-defined which features or combinations of features one should focus on. The problem has accordingly been considered ill-posed, implying a lack of a clear ground-truth basis for similarity judgments. We hypothesized that the stimuli rated as subjectively similar are those that are, in fact, more challenging for oneself to discern in practice, in near-threshold settings (e.g., psychophysics experiments). Therefore, an individuals perceptual discrimination capacities can serve as a quasi-objective ground truth for determining whether two stimuli should be judged as similar. To test this idea, we asked participants to rank the similarity of face pairs and also separately measured their discrimination capacity for those pairs. We found a positive association between perceptual discrimination capacity and subjective perceptual dissimilarity, with this association being specific to each individual. The results indicate that perceptual similarity judgments reflect and predict ones own perceptual discrimination capacities. Overall, although subjective, similarity judgments are normative in that they are grounded in ones perceptual capacities, which can be measured through a psychophysical task with objective responses.