Brain stimulation over dorsomedial prefrontal cortex causally affects metacognitive bias but not mentalising

Despite the importance of metacognition for everyday decision-making, its neural substrates are far from understood. Recent neuroimaging studies linked metacognitive processes to dorsomedial prefrontal cortex (dmPFC), a region known to be involved in monitoring task difficulty. dmPFC is also thought to be involved in mentalising, consistent with theoretical accounts of metacognition as a self-directed subform of mentalising. However, it is unclear whether, and if so how, dmPFC causally affects metacognitive judgements, and whether this can be explained by a more general role of dmPFC for mentalising. To test this, participants performed two tasks targeting metacognition in perceptual decisions and mentalising whilst undergoing excitatory anodal versus sham dmPFC tDCS. dmPFC tDCS significantly decreased subjective confidence reports while leaving first-level performance in accuracy and reaction times unaffected, suggesting a causal contribution of dmPFC to representing metacognitive bias. Furthermore, we found no effect of dmPFC tDCS on neither metacognitive sensitivity and efficiency nor on mentalising, providing no evidence for an overlap between perceptual metacognition and mentalising in the dmPFC. Together, our findings highlight the dmPFC's causal role in metacognition by representing subjective confidence during evaluations of cognitive performance.