Audiovisual congruency drives confidence in presence and absence

The formation of a subjective sense of confidence often requires the integration of signals from multiple sources of evidence. This is of particular relevance when one needs to determine with a high degree of certainty whether a multisensory stimulus is present or absent. Understanding the mechanisms underlying this ability to map evidence strength from multiple modalities into a single confidence value is therefore central to the study of metacognition. To this end, we asked healthy adults to detect the presence or absence of near-threshold stimuli that could be visual, auditory, or both, and then rate their confidence. In two pre-registered experiments (N = 48 and N = 54), audiovisual stimuli were better detected than unimodal ones, but were not associated with better metacognitive performance. Surprisingly, participants were more confident in their absence than presence judgments. To explain these results, we fitted a Bayesian evidence accumulation model in which sensory evidence is available for presence only, rendering decisions about absence dependent on counterfactual inference. The model reproduced decision patterns by assuming that a stimulus was perceived if sensory evidence from either modality exceeded a threshold (a disjunctive integration rule). In contrast, it reproduced confidence judgments by assuming that high confidence requires that the two modalities align (conjunctive for presence, disjunctive for absence). Together, these findings reveal that distinct computational mechanisms drive perception and confidence when detecting near-threshold multisensory signals.