Pupillary dynamics of optimal effort

While a substantial body of work has shown that cognitive effort is aversive and costly, a separate line of research on intrinsic motivation suggests that people spontaneously seek challenging tasks. According to one prominent account of intrinsic motivation, the Learning Progress Motivation theory, the preference for difficult tasks reflects the dynamic range that these tasks yield for minimization of performance accuracy prediction errors (Oudeyer, Kaplan & Hafner, 2007). Here we test this hypothesis, by asking whether greater engagement with intermediately difficult tasks, indexed by subjective ratings and objective pupil measurements, is a function of trial-wise changes in performance prediction error. In a novel paradigm, we determined each individuals capacity for task performance and used difficulty levels that are too low, intermediately challenging or high for that individual. We demonstrated that intermediately challenging tasks resulted in greater liking and engagement scores compared with easy tasks. Task-evoked and baseline pupil size tracked objective task difficulty, where challenging tasks were associated with smaller baseline and greater phasic pupil responses than easy tasks. Most importantly, pupil responses were predicted by trial-to-trial changes in expected accuracy, performance prediction error magnitude and changes in prediction errors (learning progress), whereas smaller baseline pupil responses also predicted greater subjective engagement scores. Together, these results suggest that what is underlying the link between task engagement and intermediate tasks might be the dynamic range that these tasks yield for minimization of performance accuracy prediction errors. ### Competing Interest Statement The authors have declared no competing interest.