Operant conditioning chambers (Med Associates Inc.; St. Albans, VT) were housed inside sound attenuating boxes and were equipped with a retractable active lever, an inactive lever, two 4W stimulus lights positioned 2cm above each lever, a 10W house light, a pellet dispenser and a pellet receptacle tray located between the two levers (Hansen and Mark, 2007). A syringe pump for cocaine delivery was located outside of the sound attenuating box. Presses of the active lever resulted in delivery of a reward (either a food pellet or cocaine infusion) and retraction of the lever. Pressing the inactive lever had no scheduled consequence.
