Abstract
Perceptual metamers occur when identical estimates are formed for physically distinct sensory signals. Multisensory integration represents one example of this phenomenon, as small misalignments in temporal or spatial properties of audiovisual stimuli may result in the same perceptual report as judgments for fully congruent audiovisual stimuli (DeRoy, Spence, Noppeney, TICS, 2016). This scenario raises two intriguing questions about perceptual metacognition for multisensory metamers: (1) are confidence judgments for metameric stimuli similar, or different? (2) Can subjects discriminate between conflicting and non-conflicting multisensory events, even when the perceptual report is the same? In this investigation, we investigated these questions using the sound-induced flash illusion. On each trial, observers were presented with 1-4 flashes and 1-4 beeps, and were asked to judge three things: (1) the number of flashes that were presented, (2) their confidence in the judgment about the number of flashes, (3) their confidence in whether the number of stimuli presented in each modality were the same, or different. We selected subsets of trials across conditions yielding perceptual metamers; specifically, we selected trials across pairs of conditions where the flash response was the same (“metamer trials”). Results showed that for metamer trials with smaller numbers of stimuli, participants were significantly more confident in their response about the number of flashes when audiovisual stimuli were congruent than when they were incongruent. For metamer trials with larger numbers of stimuli, confidence was similar across conditions. Interestingly, when making judgments about confidence in whether the number of beeps and flashes were the same or different on each trial, these “causal confidence” judgments did not significantly differ across any condition pairs. These results provide preliminary evidence that while some perceptual metamers may be causal metamers, metacognition for individual sensory signals may still index differences between metameric stimuli that yield the same perceptual reports.