Abstract
How the brain values uncertain prospects and translates them into choices is a central question in decision neuroscience. Current accounts propose that decisions under risk and ambiguity share a valuation process that integrates outcome with probability information, with ambiguity additionally requiring evaluation of the uncertainty about the probabilities themselves. Here we used computational modeling to estimate trial-wise subjective value and tested whether it was tracked by shared or distinct neural dynamics under risk and ambiguity. Participants accepted or rejected risky and ambiguous monetary gambles while we recorded neural activity using electroencephalography (Experiment 1) and magnetoencephalography (Experiment 2). Across both experiments, alpha-band (9–14 Hz) activity localized to the precuneus and superior parietal cortex tracked subjective value under both risk and ambiguity. By contrast, delta-band (1–3 Hz) activity localized to the ventromedial prefrontal and anterior cingulate cortices tracked subjective value only under ambiguity. These findings reveal dissociable neural signatures for the two forms of uncertainty and suggest that the brain resolves ambiguity by combining a shared valuation process indexed by alpha activity with an additional process indexed by delta activity that may reflect the motivational salience of options with ambiguous outcome probabilities.