Total rewards delivered, total rewards delivered as % vehicle, total responses for stimulation, total responses for stimulation as % vehicle, respectively. No effect of Morphine Treatment or interactions with Methamphetamine Dose for any of these measures. There was a significant effect of Methamphetamine Dose on total rewards (A: ***p = 0.001) and total responses (C: **p = 0.007), indicating methamphetamine-induced potentiation of brain stimulation reward. Data are plotted as the mean ± S.E.M. Sample sizes are shown in panel A next to the figure legend. Sample size by Sex: SAL: n = 10 (6 females, 4 males); MOR: n = 10 (7 females, 3 males).