Posterior parietal cortex mediates rarity-induced decision bias

Optimal decision-making under uncertainty is critical for survival, yet real-world decisions often deviate from optimality. Here, we report a rarity-induced bias in humans and mice, where rare events exert a stronger and more persistent impact on subsequent decisions than common events. Optogenetic manipulations reveal distinct learning stage-specific roles of the posterior parietal cortex (PPC): in the naïve stage, PPC drives the rarity-induced bias and interferes with optimal decisions; in the expert stage, PPC becomes essential for stimulus-based decisions. In vivo recordings support these dynamic roles, showing that rare reward history bidirectionally modulates choice-selective PPC neurons to induce bias, while learning enhances stimulus-encoding of PPC neurons to promote optimal decisions. A data-inspired dual-agent model recapitulated the bias and learning process, and predicted PPC's contribution to learning as confirmed by optogenetics. Our study uncovers a rarity-induced bias, and reveals PPC's essential roles in mediating this bias and stimulus-based learning under uncertainty.