Increased Baseline Pupil Size Linked to Uncertainty Avoidance in Decision Making

Uncertainty is a key contributor to decision making, and humans show inconsistent attitudes towards it. Although excessive uncertainty-avoidance or uncertainty-seeking are hallmark symptoms of several mental conditions, the neural mechanism underlying uncertainty seeking and avoidance remains unclear. Here, we probed whether changes in pupil-linked arousal are indicative of uncertainty avoidance in humans. Investigating baseline pupil size to capture endogenous fluctuations across two experiments (N₁ = 24, N₂ = 21), we found that pretrial pupillary responses (as early as 700 ms prior to the onset of a trial) were closely related to uncertainty attitudes during multiarmed bandit tasks. Although increased baseline pupil size signalled avoidance in uncertainty-related decisions, it did not foreshadow value processing per se. The specificity of our results suggests that uncertainty processing is dynamic and depends on (potentially noradrenergic) endogenous pupil fluctuations.