Two distinct neural representations of confidence in categorization of a natural image

Metacognition represents evaluation of our own knowledge and behaviours. Deficits or alterations in metacognition have been implicated in a range of cognitive functions and mental health issues, but the neural mechanisms of metacognition in naturalistic tasks remain poorly understood. Decision making and metacognition are typically studied in 2 alternative forced choice (2-AFC) tasks, with simple artificial stimuli varying on a few discrete levels of a one-dimensional feature (e.g. number of dots). Here, we apply Bubbles sampling to a naturalistic ambiguous image (Dali’s “Slave Market with a Bust of Voltaire”) that affords two perceptions in a 3-AFC task—i.e. perceiving the nuns, the bust of Voltaire, or neither. Bubbles selectively reveal parts of the image randomly on each trial, resulting in a high-dimensional stimulus evidence space while we concurrently recorded MEG. Each participant also rated their confidence in each response. To explore the neural mechanisms of metacognition, we use mutual information (MI) to quantify the trial-by-trial dependence between confidence ratings and MEG responses, and Conditional Mutual Information (CMI) to control for the stimulus features and response choice. We find two distinct neural responses related to confidence: a slow response, visible in the evoked response after 500ms, and an alpha/beta desynchronisation effect between 800-1500ms. Using source localisation, we show these responses originate from different locations: slow response from frontal sources, alpha desynchronisation from occipital-parietal. We also show dissociated patterns of multiple stimulus feature selectivity for these two responses. This suggests that rather than reflecting the same evidence processing, these two neural representations of confidence may reflect different stages of metacognitive evaluation.