JHIBRG:Abstract May 03 2007

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Self-control in Macaque Monkeys Studied with an Intertemporal Choice Task
Efficient decision making requires consideration of both benefit and cost before choosing an option. The influence of benefit on decision-making has been studied extensively, but the influence of cost on decision-making is less understood. Many factors can contribute to behavioral cost. In this study, we concentrated on delay cost, i.e. the waiting time until a reward can be consumed, using macaque monkeys working in an intertemporal choice task. Behaviorally, the influence of delay cost on decision-making manifests itself in a decreasing probability of choosing a certain reward, if the reward is delivered with an increasing delay time. Previous studies, as well as ours, show a reversal of preference from a larger delayed reward to a smaller immediate reward as the larger reward is delivered later. Three types of neuronal activity are necessary to select between options with different costs and benefits. First, before the decision is made, expectations of benefit and cost associated with each option should be represented. Expected cost and benefit might be combined into an expected value representation. Second, decision-related activity should indicate the choice while the decision is made. Third, after the decision, outcome-related activity should exist so that the expectation of cost and benefit associated with the particular choice can be updated. While it is important to be sensitive to the costs of actions, expected costs might also create a temptation to choose the less costly, but ultimately less beneficial option (e.g. the immediate, smaller gratification). To overcome this temptation, it is necessary to exert effort to bias one's decision to the more costly, but ultimately more beneficial option. Thus, effort -related activity should show more modulation when an option is chosen in spite of its associated cost (longer delay time). The medial frontal cortex (MFC) is likely to be involved in regulating the influence of cost on decision making. MFC is thought to receive a feedback signal evaluating a particular action and to use that information to select an action leading to a desired outcome. Therefore, we expect MFC neurons to carry signals related to the outcome of particular actions, including the costs and benefits. We also expect MFC neurons to carry internal self-control signals, such as the effort-related activity. To investigate the cortical representation of delay cost and effort, we did single cell recording in MFC while monkeys worked in an intertemporal choice task.