Discussion of nunuphar's comment: This paper seeks to validate the hypothesis that DA cell bursting in response to a positive reward prediction error is causally linked with the learning to associate the predictive cue to the reward, and that the pause in DA cell firing in response to a negative reward prediction error is causally linked to the forgetting of a cue-reward association. The authors spend some time discussing why the methods previously used to probe this question have been insufficient because the time course of drugs and genetic manipulations is too long for testing of the effect of DA cell firing in response to specific cues. They also mention that optogenetics has been used before to stimulate DA neurons which has been found to have a reinforcing effect, but has not been used in a behavioral task specifically to link cue-reward learning. The authors accomplish this by using a blocking procedure in which a second cue is introduced after an initial cue-reward association has been made, which has been shown to prevent the formation of an association between the second redundant cue with the reward. They use TH-Cre rats with a Cre dependent ChR2 virus to achieve ChR2 expression in DA cells in the VTA. In their first experiment, they show that when they stimulate DA cells (or ChR2 expressing cells whatever they may be) during the presentation of the second blocked cue they find that the animals that received this stimulation approach the reward well after just the second cue is presented much more frequently than the control animals who received the stimulations at random times. In their second experiment, they show that when they stimulate DA neurons during the assumed pause that occurs when they omit the reward after cue presentation, the animals are more likely to continue approaching the reward well then the control animals who underwent the same extinction training.