Prior Expectations Bias Confidence Judgments Through Parietal Alpha-Band Modulation

: Humans possess the metacognitive ability to estimate the likely accuracy of their own decisions through confidence judgments. Yet, whether prior information shapes confidence and the neural mechanisms mediating such influence, remain to be determined. Here, two complementary studies identify parietal alpha modulation as an oscillatory mechanism linking prior expectations to selective shifts in metacognitive bias. In Study 1, 75 participants performed a visual detection task with symbolic cues signaling target probability. Within the Type II Signal Detection Theory framework, probabilistic cues induce a congruence-dependent metacognitive bias: confidence increases when perceptual responses align with expectations and decreases when they diverge. Importantly, metacognitive sensitivity is unaffected. Critically, electroencephalography shows that cue-induced right-parietal alpha modulation predicts interindividual differences in the magnitude of this bias. In Study 2 (N = 88), right-parietal continuous Theta-Burst Stimulation (cTBS) disrupts cue-induced alpha modulation, relative to sham stimulation. This oscillatory change, in turn, drives the selective reduction in metacognitive bias. Together, these findings demonstrate that parietal alpha modulation causally and selectively shapes metacognitive bias, revealing an oscillatory mechanism for predictive metacognition.