To quantify motivational state in monkeys, we have developed a behavioral paradigm in which monkeys are faced with the same task demands from trial to trial but with different expectancies regarding the amount of work needed to earn a juice reward. On each trial, the monkey is required to respond manually to a change in the color of a visual stimulus in the center of a video monitor. A modified variable-ratio reinforcement schedule is used in which progress toward earning a reward is indicated by a second visual cue located above the color stimulus. The monkey's accuracy and latency improve dramatically as the time of reward gets closer. Since the behavioral demands remain constant, the improvement in performance appears to reflect increased motivation. When a monkey was given intravenous cocaine for reward instead of juice, the effect of cueing the animal's progress toward earning a reward was disrupted. This paradigm will be used in studies directed at relating the activity of neurons in the ventral striatum and associated structures to levels of motivation for juice or cocaine reward.
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