Humans must often estimate reward values in the presence of ambiguous information to determine the best course of action. The degree of ambiguity during decision-making has been related to levels of activity in the parietal cortex, however, its specific computations and causal role remain unknown. We tested the hypothesis that the parietal cortex is causally related in the computation of ambiguous probabilities during decision-making, studied via fMRI and concurrent TMS-EEG recordings. We found that the parietal cortex computes the degree of ambiguity assigned to subjective probability estimates. Disruption of these parietal signals selectively increased the assignment of unknown probabilities to choice outcomes, and these effects were accompanied by a modulation of frontal theta oscillations related to prediction error signals emerging from ambiguous choices. These results contribute to evidence supporting a fundamental causal role for the parietal cortex in the computations of ambiguous information during risky decisions and learning in humans.

Humans often face the challenge of making decisions between ambiguous options. The level of ambiguity in decision-making has been linked to activity in the parietal cortex, but its exact computational role remains elusive. To test the hypothesis that the parietal cortex plays a causal role in computing ambiguous probabilities, we conducted consecutive fMRI and TMS-EEG studies. We found that participants assigned unknown probabilities to objective probabilities, elevating the uncertainty of their decisions. Parietal cortex activity correlated with both the objective degree of ambiguity and a process that underestimates the uncertainty during decision-making. Conversely, the midcingulate cortex encodes prediction errors and increases its connectivity with the parietal cortex during outcome processing. Disruption of the parietal activity increased the uncertainty evaluation of the options, decreasing cingulate cortex oscillations during outcome evaluation. These results provide evidence for a causal role of the parietal cortex in computing uncertainty during ambiguous decisions made by humans.