In our studies of motivation, we have found that monkeys work faster and with fewer errors when a cue indicates that a juice reward will be delivered immediately after the next correct response than when the cue indicates that additional trials will be needed. Single neurons in the ventral striatum signal the rewarded trial when it follows one or more unrewarded trials, thus providing a neural signal that could reinforce complex behavior. The neuronal responses are thus directly related to the associative learning of the meaning of the cue in a complex behavioral task. Changing the meaning of the cue is accompanied by a parallel change in neural activity. Specifically, the neurons keep track of whether the animal is at the beginning, end, or in the course of a behavioral sequence that ultimately leads to reward. The neurons that responded when intravenous cocaine was used as the reward were different from those that responded to juice, and they responded equally strongly whether the trial was rewarded or not, indicating that neuronal responses related to cocaine are not simply enhanced normal signals, but are qualitatively different.
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