A research team from the University of Cologne conducted one of the most comprehensive studies on dopamine and decision-making in humans so far, providing evidence for effects of the former on the latter. Dopamine is a neurotransmitter involved in several functions, including motivation and reward. The team at the Psychology Department led by Dr Elke Smith and Professor Jan Peters found that L-DOPA, a precursor of dopamine that increases dopamine levels in the brain, slightly increased the study participants' willingness to wait for larger delayed rewards, decreasing impulsivity by about a 20 percent compared to placebo. This modest effect challenges some earlier influential findings from much smaller studies, which had found that L-DOPA increased impulsive choices. The study "Dopamine and temporal discounting: revisiting pharmacology and individual differences" has appeared in the Journal of Neuroscience.
When making decisions, people often prefer smaller immediate rewards over larger delayed ones, a tendency known as temporal discounting. Strong discounting is linked to more impulsive choices and is common when the brain's dopamine system is altered, such as in substance use disorders and behavioural addictions. While it is well known that dopamine influences decision-making, previous studies have produced inconsistent effects, sometimes making people more impulsive, and other times more willing to wait. Many of these studies had small sample sizes, making it difficult to draw firm conclusions. To clarify these mixed findings, the research team conducted a comparatively large study, including additional covariates that may underlie individual differences in dopamine function and that may influence how people respond to dopamine-enhancing drugs.
In a double-blind randomised placebo-controlled within-subject study, 76 healthy male and female participants received either a placebo or L-DOPA, and chose between smaller immediate and larger delayed rewards. Using cognitive modelling, a method that uses computer-based mathematical and statistical models to understand mental processes, they further examined how dopamine influenced more nuanced aspects of decision-making, including the rate of evidence accumulation, response caution and processing speed.
Participants showed the well-known "magnitude effect", such that larger rewards lost their value less over time than smaller ones. L-DOPA made participants slightly more willing to wait for rewards overall, but it did not credibly change the magnitude effect. Also, it did not credibly influence how quickly participants gathered information, how cautiously they made decisions, or how long they took to respond. This suggests that dopamine's effect on waiting for rewards may not stem from changes in basic decision processes, but rather from how future rewards are valued over time.
The scientists also analysed measures that have long been assumed to reflect baseline dopamine levels, like working memory capacity, spontaneous eye-blink rate, and impulsivity, that would be expected to influence how individuals respond to L-DOPA. These measures have been linked to dopamine activity in different brain circuits, including prefrontal areas involved in cognitive control and subcortical regions that support reward processing. However, the team found no such interaction, suggesting that these measures may not be reliable direct indicators of baseline dopamine.
"Our findings show that L-DOPA increases humans' willingness to wait for rewards, providing new evidence that challenges some earlier influential studies conducted in relatively small samples," says Dr Elke Smith. "Interestingly, we did not find that commonly used proxies for baseline dopamine, such as working memory capacity or, spontaneous eye-blink rate, or impulsivity, influenced this effect. "In my view, while these measures may capture meaningful individual differences, they likely do not directly reflect baseline dopamine levels, and using them as such may not be valid."
These insights contribute to a better understanding of the dopaminergic brain mechanisms guiding choices and help explain impulsive choices in conditions where dopamine signalling is altered, such as in addictions. Future studies may address how dopamine influences decision-making in patient populations to help inform future interventions targeting dopaminergic function. "Looking forward, future studies should examine how dopamine affects decision-making in patient populations and investigate potential interactions with hormonal fluctuations", says Elke Smith.
