A 2-odor discrimination paradigm was chosen because it permits a measure of attentional focus in goal-directed behavior. Odor discrimination was chosen as the particular task because it is a highly robust behavior in rodents [18]. To minimize experimental error, the experimenter was blind to genotype during testing. Mice were mildly food deprived to approximately 85% ad libitum weight. Dopamine D2R-/- and D2R+/+ mice (n = 8 per group) were initially trained to ambulate the length of a polyvinyl mouse cage (30-cm) to retrieve ~30-mg of Pico® fat supplemented rodent food located in a small plastic cup (3-cm) attached to the floor of the testing apparatus with Velcro. Each subject received five trials, and the latency to locate and start to consume the food pellet was recorded manually with a standard hand-held stopwatch. Twenty-four hours later subjects were trained to ambulate towards and dig through the same 3-cm dish filled with sterile sand to locate a food pellet buried underneath the sand. Subjects received 10 trials. If a mouse failed to locate a food pellet within 10-min of the first trial, an additional food pellet was dropped on top of the dish. This manipulation usually ensured that a subject would dig through the sand to locate the original pellet and learn this task.