Oloriz, A.; Hassall, C. D.

For methodological reasons, reward processing is commonly studied using random feedback and unlearnable tasks. It remains unclear whether task learnability influences reward-related brain activity, and whether this effect depends on individual differences such as reward responsiveness. We addressed this question by administering a behavioural activation system (BAS) scale before recording electroencephalography (EEG) while participants completed learnable and unlearnable versions of the "doors" task, a standard two-choice paradigm. Despite matched outcome likelihoods across conditions, participants reported greater motivation, enjoyment, and perceived performance in the learnable task. Contrary to our predictions, the amplitude of the reward positivity (RewP) - a frontocentral ERP index of reward processing - did not depend on task learnability and reward responsiveness. However, learnability and reward responsiveness effects became apparent when the analysis was restricted to high performers. Within this subgroup, participants low in reward responsiveness showed an enhanced RewP when the task was learnable. These findings suggest that contextual factors such as task learnability can interact with individual differences, informing ongoing efforts to identify the RewP as a biomarker of disordered reward processing.