In economic decision making, outcomes are described in terms of risk (uncertain outcomes with certain probabilities) and ambiguity
(uncertain outcomes with uncertain probabilities). Humans are more averse to ambiguity than to risk, with a distinct neural system
suggested as mediating this effect. However, there has been no clear disambiguation of activity related to decisions themselves from
perceptual processing of ambiguity. In a functional magnetic resonance imaging (fMRI) experiment, we contrasted ambiguity, defined as
a lack of information about outcome probabilities, to risk, where outcome probabilities are known, or ignorance, where outcomes are
completely unknown and unknowable.Wemodified previously learned pavlovian CSstimuli such that they became an ambiguous cue
and contrasted evoked brain activity both with an unmodified predictive CS(risky cue), and a cue that conveyed no information about
outcome probabilities (ignorance cue). Compared with risk, ambiguous cues elicited activity in posterior inferior frontal gyrus and
posterior parietal cortex during outcome anticipation. Furthermore, a similar set of regions was activated when ambiguous cues were
compared with ignorance cues. Thus, regions previously shown to be engaged by decisions about ambiguous rewarding outcomes are
also engaged by ambiguous outcome prediction in the context of aversive outcomes. Moreover, activation in these regions was seen even
when no actual decision is made. Our findings suggest that these regions subserve a general function of contextual analysis when search
for hidden information during outcome anticipation is both necessary and meaningful
