 Subjects are assigned to one of 5 groups: Paired, high reward PairHi; n24; Paired, low reward PairLo; n24; Paired no reward PairNone; n23; Unpaired, high reward UnpHi; n7; and Unpaired, low-reward UnpLo; n9. Each participant attends an orientation session, followed by four 4h conditioning sessions separated by at least 48h, and a 2h test session. During the initial orientation session, subjects practice the computer tasks together with the environmental stimuli to be paired during the conditioning sessions, and carry out a series of choices for the tasks and images see below. All subjects receive methamphetamine 20 mg on two conditioning sessions and placebo on the other two sessions, administered under double-blind conditions. Subjects differ in whether environmental stimuli are consistently paired with the drug administration. The three paired groups consistently receive drug and placebo in combination with one of the two sets of visual and auditory background stimuli see below. The two unpaired groups receive each treatment in combination with one of the stimuli sets on one session, and the other on the other. 