Dopamine and Temporal Discounting: Revisiting Pharmacology and Individual Differences.

Disorders characterized by changes in dopamine (DA) neurotransmission are often linked to changes in the temporal discounting of future rewards. Likewise, pharmacological manipulations of DA neurotransmission in healthy individuals modulate temporal discounting, but there is considerable variability in the directionality of reported pharmacological effects, as enhancements and reductions of DA signaling have been linked to both increases and reductions of temporal discounting. This may be due to meaningful individual differences in drug effects and/or false-positive findings in small samples. To resolve these inconsistencies, we (1) revisited pharmacological effects of the DA precursor l-DOPA on temporal discounting in a large sample of N = 76 healthy participants (n = 44 male) and (2) examined several putative proxy measures for DA to revisit the role of individual differences in a randomized, double-blind placebo-controlled preregistered study (https://osf.io/a4k9j/). Replicating previous findings, higher rewards were discounted less (magnitude effect). Computational modeling using hierarchical Bayesian parameter estimation confirmed that the data in both drug conditions were best accounted for by a nonlinear temporal discounting drift diffusion model. In line with recent animal and human work, l-DOPA reliably reduced the discount rate with a small effect size, challenging earlier findings in substantially smaller samples. We found no credible evidence for effects of putative DA proxy measures on model parameters, calling into question the role of these measures in accounting for individual differences in DA drug effects.