An experimental approach to examine behavior and brain activity during decision-making under risk as compared to ambiguity

Neural correlates of decisions under risk and under ambiguity have been examined for several decades. In the current study, we focus on a neglected aspect that is a potential confounding of the average risk of alternatives in a trial, the average magnitude of the outcomes, and the difference in risk between the two alternatives. We present an experimental approach to solve this problem here and explicitly separate risk and ambiguity. In the present fMRI-study with 20 participants, we created different levels of risk and ambiguity to investigate their effects on behavior and brain activation. In a first experimental block, decisions with mixed-risk options (one high-risk option paired with one low-risk option) were compared to decisions with two high-risk or two low-risk options in terms of the effects on decision-making and neural activity. The second block consisted of the same risk levels crossed with high and low ambiguity by withholding information about the probabilities of the outcomes. During mixed-risk trials participants made cautious decision significantly more often. This effect was strongest during mixed trials with high ambiguity in the second block. In addition, risk behavior of subjects was correlated with the subjective importance of the amount of potential monetary losses or wins and the related probabilities. The fMRI results revealed activation of the dorsal anterior cingulate cortex (dACC), the insula, and the orbitofrontal cortex (OFC) during mixed risk trials without ambiguity. In contrast, activation of the amygdala was specifically present during mixed-risk trials with high ambiguity.