Participants completed both sessions twice on two separate days, with different stimuli, once under placebo and once after the administration of the drug. For each event, an update term was calculated as the absolute difference between the participant's first and second estimations. ![]() The second session was the same as the first session. For each event, an estimation error term was calculated as the difference between the participant's estimation and the information provided. They were then presented with the average probability of that event occurring to a person like themselves, living in the same sociocultural environment. (A) On each trial, participants were presented with a short description of one of 40 adverse life events and asked to estimate how likely this event was to occur to them. ![]() They also completed a memory test for the information presented and rated all stimuli on different subjective scales (for a full description, see Supplemental Experimental Procedures). We then assessed whether participants used this information to update their predictions by subsequently asking them to again estimate their likelihoods for the same 40 events in a second session, taking place ∼15 min after the first session. After each trial, they were presented with an actuarial average probability of that event occurring to a person from the same sociocultural environment. At each session, participants provided estimates of their likelihood of experiencing 40 different types of adverse life events (e.g., Alzheimer's disease, robbery see the List of Stimuli in the Supplemental Experimental Procedures available online) adapted from a previous study. The task was identical on both days except for the fact that different stimuli were used on each day (lists were counterbalanced). On one of the days, participants received placebo and on the other they received L-DOPA (150 mg), in a counterbalanced order (n = 21). To test whether an optimism bias is modulated by dopamine, participants completed a belief update task on two separate days, one week apart ( Figure 1), in a double-blind placebo-controlled pharmacological intervention study. Given these set of findings, we hypothesize that enhancing dopamine function will influence how healthy individuals incorporate information about probabilities of future life events in an asymmetric manner, increasing an optimism bias. However, it also impacts on domains as diverse as working memory, episodic memory, and reversal learning. Dopamine effects on learning have been extensively studied in the context of model-free reinforcement learning. In Parkinson's disease, drugs enhancing dopaminergic function (e.g., dihydroxy-L-phenylalanine L-DOPA) influence learning of positive and negative outcomes in an asymmetric manner, enhancing the former and impairing the latter. A prominent candidate is the monoamine dopamine, a neuromodulator suggested to provide a teaching signal that indexes when predictions fail to align with outcomes. Īn unresolved question is whether neuromodulators associated with generating expectations of future outcomes influence this process. Selective updating is mediated by regions of the frontal cortex that track errors in estimation when these call for positive update but show a relative failure to code for errors that might induce a negative update. This bias is related to a striking asymmetry whereby people update their beliefs more in response to information that is better than expected compared to information that is worse than expected. Humans are optimistically biased when making predictions about the future, habitually underestimating the likelihood of negative events. These findings provide the first evidence that the neuromodulator dopamine impacts on belief formation by reducing negative expectations regarding the future. This effect is due to L-DOPA impairing the ability to update belief in response to undesirable information about the future. ![]() Here, we show that administration of a drug that enhances dopaminergic function (dihydroxy-L-phenylalanine L-DOPA) increases an optimism bias. This question assumes great importance in light of evidence that common neuropsychiatric disorders, such as depression, are characterized by pessimism. However, it is unknown how neuromodulatory systems impact on the generation of optimistically biased beliefs. This well-known bias, termed unrealistic optimism, is observed across age, culture, and species and has a significant societal impact on domains ranging from financial markets to health and well being. When predicting financial profits, relationship outcomes, longevity, or professional success, people habitually underestimate the likelihood of future negative events (for review see ).
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