Past events

The intensity or vigor of goal-directed behavior is a correlate of the motivation underlying it. Motivation is related to the subjective value of rewards and is moderated, or even completely dissipated, if the perceived effort or discomfort seems too great. Under what circumstances do we seek a goal or a reward? To study motivated behavior in monkeys, we use several variants of a task in which monkeys must perform some work, in this case detecting when a target spot turns from red-to-green, to obtain a drop of juice. We use another visual stimulus, a cue, to indicate how much discomfort must be endured, e.g., the number of trials to be worked, to obtain the reward. The monkeys learn about the cues quickly, often after just a few trials. The number of errors becomes proportional to amount of work remaining before reward, achieving our goal of manipulating motivation. This is a behavior in which the monkeys decrease their performance in response to an increased predicted workload. Temporal difference models have provided an important framework for interpreting goal directed-behavior, and in economics, game theory has been used to model choice behavior. A key concept in these models is to determine how the value of the reward is modulated by some parameter of the experiment, such as changing the reward size, or the amount of time needed to obtain the reward. In learning or adaptation the TD algorithm predicts that behavior should be (and in artificial systems is) adapted to maximize long-term reward. By examining the influence of reward size, waiting time, and amount of work, we can examine in what ways different model succeed and fail. Our data show that performances depend on work completed since preceding reward (sunk cost effect), and accumulated reward (over whole sessions) and work. In addition this behavior can be used to learn about categorization and rule learning. Using single neuronal recording, regional ablation, and molecular ablation of the D2 receptor we show that dopamine-rich brain regions have signals related to the balance between reward and work.