Pupil dilation and P3 reflect subjective surprise about decision outcome

Central to human and animal cognition is the ability to learn from feedback in order to optimize future rewards. Such a learning signal might be encoded and broadcasted by the brain9s arousal systems, including the noradrenergic locus coeruleus. Pupil responses and the P3 component of event-related potentials reflect rapid changes in the arousal level of the brain. Here we ask whether and how these variables may reflect 9subjective surprise9: the mismatch between one9s expectation about being correct and the outcome of a decision, when expectations fluctuate due to internal factors (e.g., engagement). We show that during an elementary decision-task in the face of uncertainty both physiological markers of phasic arousal reflect subjective surprise. We further show that pupil responses and P3 are unrelated to each other, and that subjective prediction error computations depend on feedback awareness. These results further advance our understanding of the role of central arousal systems in decision-making under uncertainty.