Discussion
hypotensive, tiredness
DILATION, HYPOTENSIVE, DISORDERS
Adaptive decision-making requires an optimal balance between choosing known options and trying new paths when the environment changes or new information is required. Given the ubiquity of exploration-exploitation tradeoffs in everyday life and their potential relevance for psychopathology, understanding the mechanisms involved in this tradeoff is important. Here we investigated the specific roles of dopamine and noradrenaline in the exploration-exploitation tradeoff by pharmacological blockade of either system using propranolol and amisulpride and systematically examining the effects of reward values, depleting returns, and opportunity costs on choice behavior. The action of the administered drugs was confirmed by specific changes in blood pressure, heart rate, pupil dilation, and blink rate. As expected, (systolic) blood pressure and heart rate decreased most prominently in the propranolol group, consistent with its action as a hypotensive agent [Previous studies suggested a role of dopamine in exploration [In sharp contrast to the amisulpride group, none of these choice aspects had a significant effect on choice behavior in the propranolol group. Interestingly, participants in the propranolol group tended to switch even more after higher rewards, compared to the placebo group. Specifically, they still switched less after higher than lower rewards, but this was less pronounced than in the placebo group, while this effect was significantly more pronounced in the amisulpride group than in the placebo group. This pattern points to a reduced usage of decision-relevant information for choice behavior, in line with evidence suggesting a role of noradrenaline in random, but not directed exploration [In addition to differences between tonic and phasic noradrenergic activity, a possible inhibitory mechanism of β-adrenergic receptors may explain why we found a tendency towards an increase of stochasticity. Specifically, β-adrenergic receptors enhanced inhibitory synaptic mechanisms in rats by a noradrenaline-mediated enhancement of GABA efficacy [Overall, however, the influence of propranolol on the exploration-exploitation tradeoff was less pronounced than for amisulpride. A potential explanation for this could be that noradrenaline does not drive specific components of decision-making, but rather exerts higher-order control signals, such as an urgency signal that stops ongoing information gathering, presumably by inducing decision noise.At this point, it should be noted that other factors such as tiredness or boredom might have affected switching behavior. Although these factors may also contribute to more random exploration and we do not think that these could explain the influence of the drugs on the dependency of switch behavior on relevant decision parameters, future studies should measure these additional variables to explicitly control for their influence. Moreover, future studies should consider including baseline measures of task performance to rule out performance differences between groups before drug administration or use a within-subject design instead of a between-subjects design.Taken together, our findings suggest functionally dissociable roles of dopamine and noradrenaline in the exploration-exploitation tradeoff during human decision-making. Compared to placebo, participants in the amisulpride group switched less when the prospects in the current environment were still advantageous (i.e., high rewards and low depletion rates) and the costs associated with exploration were high (i.e., long travel time). After propranolol intake, participants tended to switch even more, compared to the placebo group, when the rewards in the current environment were still high. Thus, these data show that dopamine modulates the sensitivity to choice relevant aspects, while noradrenaline regulates when to disengage from the current information paths to randomly explore new options. Our results are thus generally in line with previously hypothesized roles of dopamine and noradrenaline in directed and random exploration, respectively. The present findings enhance our understanding of the differential roles of dopamine and noradrenaline in decision-making and might have relevant implications for mental disorders characterized by biases in the exploration-exploitation tradeoff.