0.35 mg/kg methylphenidate will be administered after baseline testing and 45 min-1 hr before post-intervention testing.

We will compare acute pain dynamics-i.e., the time series of pain visual analog scale ratings (0-100, where 0 = no pain and 100 = worst pain imaginable) obtained while delivering a changing noxious stimulus-during the methylphenidate condition to that of the placebo condition.

We will regress the time course of nucleus accumbens BOLD activity onto the latent time series obtained from our valence competition model. This will be supported by regressing the nucleus accumbens time course onto the pain VAS time course, allowing our findings to be compared to standard analyses in the literature.