Ready, set, explore! Event-related potentials reveal the time-course of exploratory decisions

The decision trade-off between exploiting the known and exploring the unknown has been studied using a variety of approaches and techniques. Surprisingly, electroencephalography (EEG) has been underused in this area of study, even though its high temporal resolution has the potential to reveal the time-course of exploratory decisions. We addressed this issue by recording EEG data while participants tried to win as many points as possible in a two-choice gambling task called a two-armed bandit. After using a computational model to classify responses as either exploitations or explorations, we examined event-related potentials locked to two events preceding decisions to exploit/explore: the arrival of feedback, and the subsequent appearance of the next trial's choice stimuli. In particular, we examined the feedback-locked P300 component, thought to index a phasic release of norepinephrine (a neural interrupt signal), and the reward positivity, thought to index a phasic release of dopamine (a neural prediction error signal). We observed an exploration-dependent enhancement of the P300 only, suggesting a critical role of norepinephrine (but not dopamine) in triggering decisions to explore. Similarly, we examined the N200/P300 components evoked by the appearance of the choice stimuli. In this case, exploration was characterized by an enhancement of the N200, but not P300, a result we attribute to increased response conflict. These results demonstrate the usefulness of combining computational and EEG methodologies, and suggest that exploratory decisions are preceded by two characterizing events: a feedback-locked neural interrupt signal (enhanced P300), and a choice-locked increase in response conflict (enhanced N200).