A brain imaging study of the choice procedure

We study the behavior of subjects facing choices between certain, risky, partially ambiguous, and ambiguous lotteries in an experimentally controlled environment. Our observations are subjects' choice behavior, response times, and brain activations. The choices of subjects are consistent with economic theories designed to predict these choices, namely theories modeling ambiguity aversion. The additional evidence we present supports a specific interpretation of the decision process that is implementing these choices. In particular it supports the conjecture that subjects face the choice task as an estimation of the value of the two lotteries; and that a measure of the difficulty of the choice provides an important explanatory variable (in addition to risk and ambiguity aversion) of the observed behavior. Further support for the interpretation of choice as cognitive task in our experiment comes from the observation that emotional factors seem to play a minor role. SpeciÞcally, the medial orbito frontal region and amygdala are not activated. Compared with the set of results organized in the Somatic Marker Hypothesis (Damasio [9], Bechara, Damasio and Damasio [1]), these results suggest that a static choice without learning and feedback on outcome is a task of a different nature than a choice with learning and feedback. The brain imaging data suggest that the estimation is of an approximate nature when the choices involve ambiguous and risky lotteries, and requires mental faculties that are shared by all mammals and in particular are independent of language. The regions in the brain that are activated are located typically in parietal lobes, which are known to be involved in approximate calculations. Choices involving partial ambiguous lotteries produce in addition an activation of the frontal region, which indicates a different, more sophisticated cognitive process. The time to decide is shorter for arguably harder choices, a finding that suggests the need for new models of the allocation of effort in the choice process.