Is subjective perceptual similarity metacognitive?

Abstract

Perceptual similarity is a cornerstone for human learning and generalization. However, in assessing the similarity between two stimuli differing in multiple dimensions, it is not well-defined which feature(s) one should focus on. The problem has accordingly been considered ill-posed. We hypothesize that similarity judgments may be, in a sense, metacognitive: 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). This self-knowledge about one's own perceptual capacities provides a quasi-objective ground truth as to whether two stimuli should be judged as similar. To test this idea, we measure perceptual discrimination capacity between face pairs, and ask subjects to rank the similarity between them. Based on pilot data, we hypothesize a positive association between perceptual discrimination capacity and subjective dissimilarity, with this association being importantly specific to each individual.