Integrating EEG and Pupillometry to Investigate Reward Learning With Neutral Feedback

While the brain's response to positive and negative feedback is well understood, neutral feedback, where outcomes are neither positive (such as a reward) nor negative (such as a loss), remains largely unexplored. How does the brain interpret and learn from neutral outcomes? This study aimed to answer that question by combining electroencephalography (EEG), a neuroimaging technique, and pupillometry, an eye-tracking technique, to investigate the brain's response to neutral feedback and how it differs from rewards and losses. Participants completed a computer-based task called "Beat the Dealer", where they selected one of two cards and received feedback on whether they beat the dealer (win), lost to the dealer (loss), or tied the dealer (neutral outcome). Unbeknownst to participants, the outcomes were pre-determined, so certain outcomes were more likely in different blocks. For example, in one block, ties were rare, while in another, they were the most common. This design allowed for measuring how the brain and pupils responded to neutral outcomes depending on their probability and context. Results suggest that neutral feedback is not inherently categorized as good or bad, but rather, its meaning depends on context and expectation. When neutral feedback was rare, it was processed more like a loss. However, when neutral feedback was expected, it was processed as an intermediate signal rather than a negative one. This study also revealed that pupil dilation, a measure linked to norepinephrine, which is a neuromodulator involved in attention and learning, played an important role in shaping how the brain responded to feedback. Larger pupil dilations were associated with better differentiation between expected and unexpected outcomes, highlighting the role of norepinephrine when learning from feedback. These findings challenge the traditional binary model of decision-making and suggest that neutral feedback can serve as a meaningful learning signal. Understanding how the brain processes neutral feedback could have important implications for psychology, behavioral economics, and clinical neuroscience, particularly in conditions like depression and anxiety where feedback learning is disrupted. By integrating EEG and pupillometry, this study provides new insights into how dopamine and norepinephrine interact to shape learning and decision-making.