Title: the Neural Representation of Subjective Value under Risk and Ambiguity 1 2

28 Risk and ambiguity are two conditions in which the consequences of possible 29 outcomes are not certain. Under risk the probabilities of different outcomes can 30 be estimated, while under ambiguity even these probabilities are not known. 31 Although most people exhibit at least some aversion to both risk and ambiguity, 32 the degree of these aversions is largely uncorrelated across subjects, suggesting 33 that risk aversion and ambiguity aversion are distinct phenomena. Previous 34 studies have shown differences in brain activations for risky and ambiguous 35 choices, and have identified neural mechanisms that may mediate transitions 36 from conditions of ambiguity to conditions of risk. Unknown, however, is whether 37 the value of risky and ambiguous options is necessarily represented by two 38 distinct systems or whether a common mechanism can be identified. To answer 39 this question we compared the neural representation of subjective value under 40 risk and ambiguity. fMRI was used to track brain activation while subjects made 41 choices regarding options that varied systematically in the amount of money 42 offered and in either the probability of obtaining that amount or the level of 43 ambiguity around that probability. A common system, consisting of at least the 44 striatum and the medial prefrontal cortex, was found to represent subjective 45 value under both conditions. 46