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Much is known about how people make decisions under varying levels of probability (risk). Less is known about the neural basis of decision-making when probabilities are uncertain because of missing information (ambiguity). In decision theory, ambiguity about probabilities should not affect choices. Using functional brain imaging, we show that the level of ambiguity in choices correlates positively with activation in the amygdala and orbitofrontal cortex, and negatively with a striatal system. Moreover, striatal activity correlates positively with expected reward. Neurological subjects with orbitofrontal lesions were insensitive to the level of ambiguity and risk in behavioral choices. These data suggest a general neural circuit responding to degrees of uncertainty, contrary to decision theory.
In theories of choice under uncertainty used in social sciences and behavioral ecology, the only variables that should influence an uncertain choice are the judged probabilities of possible outcomes and the evaluation of those outcomes. But confidence in judged probability can vary widely. In some choices, such as gambling on a roulette wheel, probability can be confidently judged from relative frequencies, event histories, or an accepted theory. At the other extreme, such as the chance of a terrorist attack, probabilities are based on meager or conflicting evidence, where important information is clearly missing. The two types of uncertain events are often called risky and ambiguous, respectively. In subjective expected utility theory, the probabilities of outcomes should influence choices, whereas confidence about those probabilities should not. But experiments show that many people are more willing to bet on risky outcomes than on ambiguous ones, holding judged probability of outcomes constant (1). This empirical aversion to ambiguity motivates a search for neural distinctions between risk and ambiguity. Here, we extend the study of the neural basis of decision under risk to encompass ambiguity.
The difference between risky and ambiguous uncertainty is illustrated by the Ellsberg paradox (2). Imagine one deck of 20 cards composed of 10 red and 10 blue cards (the risky deck). Another deck has 20 red or blue cards, but the composition of red and blue cards is completely unknown (the ambiguous deck). A bet on a color pays a fixed sum (e.g., $10) if a card with the chosen color is drawn, and zero otherwise (Fig. 1A).
In experiments with...