The temporal precision of reward prediction in dopamine neurons.

Midbrain dopamine neurons are activated when reward is greater than predicted, and this error signal could teach target neurons both the value of reward and when it will occur. We used the dopamine error signal to measure how the expectation of reward was distributed over time. Animals were trained with fixed-duration intervals of 1-16 s between conditioned stimulus onset and reward. In contrast to the weak responses that have been observed after short intervals (1-2 s), activations to reward increased steeply and linearly with the logarithm of the interval. Results with varied stimulus-reward intervals suggest that the neural expectation was substantial after just half an interval had elapsed. Thus, the neural expectation of reward in these experiments was not highly precise and the precision declined sharply with interval duration. The neural precision of expectation appeared to be at least qualitatively similar to the precision of anticipatory licking behavior.