Posterior parietal cortex mediates rarity-induced decision bias and learning under uncertainty

Making decisions when outcomes are uncertain requires judgment of outcome probability, yet such judgments are often inaccurate, owing to reliance on heuristics that introduce systematic errors like overweighting of low probabilities. Here, using a decision-making task in which the participants were unaware of outcome probabilities, we discovered that both humans and mice exhibit a rarity-induced decision bias (RIDB), i . e ., a preference towards rare rewards, which persists throughout task performance. Optogenetics experiments demonstrated that activity in the posterior parietal cortex (PPC) is required for the RIDB. Using in vivo electrophysiology, we found that rare rewards bidirectionally modulate choice-encoding PPC neurons to bias subsequent decisions towards rare rewards. Learning dynamically recruits PPC neurons and enhances their stimulus encoding, and the activity of these neurons is essential for stimulus-guided decisions. We then developed a dual-agent model that successfully recapitulates the decision-making and learning behaviours, and corroborates the specific functions of PPC neurons in mediating decision-making and learning. This study not only extends our understanding of rare probability overweighting to a context where the outcome probability is unknown and characterizes the neural basis for RIDB in the PPC, but moreover reveals an evolutionarily conserved heuristic that persistently impacts decision-making and learning under uncertainty.