Alpha and Theta Oscillations Differentiate Escalating Risk Levels During Reward Anticipation in Sequential Decision Making

Reward anticipation potentially guides sequential decision making, yet its underlying neural dynamics remain unclear. In this study, we investigated how within-trial escalating risk and contextual uncertainty modulate oscillatory brain activity during reward anticipation. EEG was recorded while participants (N = 44) performed a modified version of the Balloon Analogue Risk Task in which balloon burst probabilities changed across task phases, introducing contextual uncertainty. Spectral power was analyzed time-locked to three within-trial risk levels: early no-risk pumps, final successful pumps (preceding cash out), and unsuccessful pumps (preceding balloon burst). Time-frequency decomposition using Morlet wavelets revealed a parieto-occipital alpha power increase following early no-risk pumps, consistent with reduced deliberative engagement when anticipating certain rewards. In contrast, centroparietal alpha suppression emerged following final successful pumps, suggesting increased attentional engagement and reward expectancy at high within-trial risk levels. Frontocentral theta power also decreased following final successful pumps, with the strongest reduction observed when the burst probability function was shallower. Rather than reflecting simple reward encoding, this pattern may index reduced monitoring demands following commitment to the selected action. Overall, both alpha and theta dynamics tracked within-trial escalating risk, whereas theta activity was additionally modulated by contextual factors across task phases. These findings provide novel insights into the oscillatory mechanisms supporting reward anticipation in sequential decision environments.