Effects of methamphetamine on human effort task performance are unrelated to its subjective effects

Stimulant drugs increase objective indices of reward-related behavior, including willingness to expend effort for reward, and also produce feelings of well-being and positive mood. However, it is not known to what extent these different measures are related to each other.

The present study was designed to assess the relationship between the behavioral measure of effort expenditure and positive subjective responses to methamphetamine (MA).

96 healthy adults completed the Effort Expenditure for Rewards Task (EEfRT) during two laboratory sessions after receiving 20 mg MA or placebo (PL) under double blind conditions. They also self-reported their mood states and drug effects.

MA (vs. PL) increased willingness to complete a high effort/high reward option vs. a low effort/low reward option during the EEfRT (N = 96), and this effect was greater in participants with low effort at baseline. A subjective value modeling analysis (N = 91) showed that MA decreased sensitivity to the perceived cost of effort for the low baseline performance group only. MA also increased self-reported positive affect (euphoria; N = 94, liking the drug; N = 92) in the full sample, but this increase was unrelated to either baseline EEfRT performance or MA-induced EEfRT performance changes (N = 91).

As reported previously, MA increased choice of the high effort/high reward option, particularly in participants with low effort at baseline, who also showed drug-induced changes in effort sensitivity. These behavioral effects were not related to drug liking and drug-induced euphoria. These findings suggest that the effects of stimulants on reward-related behavior and mood are dissociable.