Schultz and colleagues have demonstrated that dopamine cells display consistent tonic firing patterns during maintenance of associative learning tasks [33]. However, phasic burst activity of dopaminergic cells occurs when discrepancies between predicted and actual reinforcement contingencies transpire [14]. This robust increase of dopaminergic cell activation in response to unpredicted outcomes has been referred to as an "error" signal [14]. To date, the molecular basis of this error signal has not been identified. The inability of the D2R-/- mice to desist responding to a previously reinforced stimulus suggests that the dopamine D2R might in fact be the focal point of this error-signaling cascade.