Uncertainty can be fractioned into risk and ambiguity psychologically and neurobiologically. However, whether and how risk and ambiguity are dissociated in terms of neural dynamics during value‐based decision making remain elusive. The present event‐related potential (ERP) study addressed these issues by asking participants to perform a wheel‐of‐fortune task either during a risky context (Experiment 1; N = 30) where outcome probability was known or during an ambiguous context (Experiment 2; N = 30) where outcome probability was unknown. Results revealed that the cue‐P3 was more enhanced for risk versus ambiguity during the anticipatory phase, whereas the RewP was more increased for ambiguity than risk during the consummatory phase. Moreover, the SPN and the fb‐P3 components were further modulated by the levels of risk and ambiguity, respectively. These findings demonstrate a neural dissociation between risk and ambiguity, which unfolds from the anticipatory phase to the consummatory phase.

Contextual valence is an important dimension during value‐based decision‐making. Previous research has revealed behavioral and neural asymmetries between the gain context and the loss context. The present event‐related potential study investigated the effects of contextual valence on neural dynamics underlying magnitude and time, two important reward dimensions, during feedback evaluation. Forty‐two participants performed a simple guessing task in which they experienced both a gain context wherein high or low rewards were delivered immediately or six months later, and a loss context wherein high or low losses were delivered in the same way. Results showed that in the gain context, time and magnitude information were processed in a parallel way during the time windows of the reward positivity (RewP) and the P3. In the loss context, however, time and magnitude information were processed in a serial way such that time information was encoded during the RewP and P3 periods, whereas magnitude information was not tracked until the time window of the late positive potential. Our findings suggest that the neural dynamics underlying time and magnitude information are distinct between the gain and loss contexts, thus providing a novel perspective for the well‐known gain–loss asymmetry.