Changes in impulsive choice occur in numerous psychiatric disorders. For example,
individuals suffering from substance use disorders and problem gambling show increased
discounting of future rewards (Bickel et al., 2014; Ring et al., 2022). Maladaptive
alterations in dopaminergic circuits are assumed as functional correlates of such effects
(Volkow et al., 2004). In line with these findings, pharmacological manipulation of
dopaminergic neurotransmission modulates decision making. Enhancing dopamine
signalling has been reported to increase (Pine et al., 2010) and reduce impulsive choice
operationalised as temporal discounting (de Wit et al., 2002), while some studies reported
no overall effect of modulating dopamine signalling on impulsive choice (Petzold et al.,
2019). The variability suggests that manipulating dopamine neurotransmission may have
contrary effects in different individuals, possibly related to baseline differences in
dopamine function (Cools & D’Esposito, 2011).
We studied the effects of manipulating dopaminergic transmission on the discounting of
future rewards and assessed a possible interaction with putative proxies of dopamine
function. Following a randomised, double-blind placebo-controlled design (see
preregistration at https://osf.io/a4k9j/), 76 healthy men and women performed an
intertemporal choice task with two magnitude conditions under 150 mg L-DOPA and
placebo, respectively. To describe the devaluation of future rewards we took a model-
based approach of hyperbolic discounting and modelled the decision process with drift
diffusion models via linear and non-linear linkage functions. We further assessed
spontaneous eye blink rate, working memory capacity, and impulsivity as putative proxies
of baseline dopamine function.
Replicating previous findings (Smith & Peters, 2022), we found a magnitude effect,
meaning that higher rewards are discounted less. Model comparison showed that the
choices and response times were best accounted for by a non-linear drift rate mapping.
Crucially, L-DOPA reduced discounting, challenging the results from Pine et al. (2010) by
using a substantially larger sample. The results are more in line with the idea that
increases in dopamine transmission attenuate temporal discounting (Wagner et al. 2020).
However, we found no evidence that the drug effects were modulated by the putative
dopamine proxies. While these measures may reliably reflect interindividual differences,
using them as proxy measures for baseline dopamine function should be reconsidered.