The next day, mice were presented to the testing apparatus that now contained 2 dishes, each filled with different scented sand. One odor signaled that a food reinforcer was buried in that particular dish (S+) while the other odor signaled that no reinforcer was contained within that particular dish (S-). The sand was scented by adding 0.80 grams of either cinnamon or dill weed to 98.8 grams of autoclaved playground sand purchased from a local farm and garden store. Finally, 0.40 grams of finely ground food was mixed with the scented sand in order to mask any odor emitted by the food reinforcer in the S+ dish. This final mixture did not appear to be palatable to the subjects. A trial terminated when the mouse: dug in the correct dish and consumed the hidden pellet, or began to dig in the incorrect dish. Digging was considered the actual physical displacement of sand with the forepaws and/or snout. Placing forelimbs on either of the dishes and sniffing was not considered digging and was not penalized or scored. Each completed trial was separated by a 20-sec inter-trial-interval (ITI). Preliminary experiments demonstrated that an extended time-out period following an incorrect response was not required to ensure goal-directed behavior in the wild-type or mutant mice for this particular task (data not shown). Therefore, the 20-sec ITI was used to separate all trials (successful and unsuccessful). The S+ and S- scents were counterbalanced across subjects in order to control for potential scent bias (pilot data suggest there are no biases, data not shown). Following accurate discrimination, defined as 8 correct retrievals across 10 consecutive trials, reversal trials began.