Alpha and theta activity during reward anticipation are modulated by implicit expectations about sequential risk

Reward anticipation potentially guides sequential decision making, yet its underlying neural dynamics remain unclear. In this study, we investigated how risk levels and 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 where balloon burst probabilities changed throughout the task, promoting uncertainty. We analyzed induced spectral power time-locked to three risk levels: early no-risk pumps, final successful pumps (preceding a cash out), and unsuccessful pumps (preceding a balloon burst). Time- frequency decomposition using Morlet wavelets revealed a parieto-occipital alpha power increase following early no-risk pumps, interpreted as disengagement from deliberative processing when anticipating sure rewards. In contrast, centroparietal alpha and frontocentral theta power decreased most prominently following final successful pumps, suggesting heightened attentional and expectancy-related processes in response to high reward potential. The results indicate that alpha and theta dynamics are sensitive to risk levels and reward expectations. These findings provide novel insights into the oscillatory mechanisms of reward anticipation in uncertain decision environments.