Stimulus familiarity shapes hierarchical structure learning and metacognitive dynamics

Humans learn multiple probabilistic regularities at once, yet it remains unclear how learning at different structural levels co-evolves with metacognition when feedback is unavailable. Here, participants (N=63; N=50 completed both sessions) learned a two-level probabilistic environment in which lower-order stimulus co-occurrences were embedded within a higher-order sequence, using unfamiliar fractals or familiar line drawings. We measured learning using a cover task during exposure periods, and using forced-choice questions followed by confidence judgments in repeated test blocks. Results revealed stimulus-dependent learning and metacognitive signatures. Learning dynamics differed across stimuli with fractal sequence test accuracy outpacing co-occurrence accuracy and vice versa for line drawings. Confidence increased across blocks, and was higher for line drawings and first but not second sequence selections. Metacognitive sensitivity increased alongside test accuracy for fractals but remained stagnant for line drawings. These results highlight stimulus-dependent effects on hierarchical structure learning that can dissociate from metacognitive dynamics.