Table 1 Datasets used

High vs low BDI scores

MDD vs controls

23 participants during an episode of MDD and 25 controls matched for age, sex, education, ethnicity and marital status. MDD participants met DSM-IV criteria for MDD, had Hamilton Rating Scale for depression scores ≥ 17, and no other axis I comorbidities except for anxiety. Inclusion required a minimal drug-free period of 2 weeks. Payment: US$10/hour plus 5US$ average task earnings.

History of MDD vs no history

Currently healthy participants with and without a history of major depressive disorder (MDD). Participants received US$15/hr in compensation for their time, as well as their task “earnings” (on average, US$5).

BPD vs controls

Euthymic outpatients (matched to the same 25 controls as in dataset ‘MDD’). The outpatients had a long-standing diagnosis of Bipolar Disorder, currently satisfying criteria on the Affective Disorder Evaluation, which contains modified SCID mood and psychosis modules. Patients were classified as euthymic if they currently Young Mania Rating Scale [51] score <12, and if their Hamilton Rating Scale for Depression [52] score was below 8. Exclusion criteria included other axis I disorders, a past history of MDD, substance abuse and ECT in the past 6 months. Participants were paid US$25 for participation and task earnings.

Pramipexole vs placebo

Healthy volunteers received either placebo or a single dose of the D2/D3 agonist pramipexole hydrochloride (PPX) 0.5 mg 2 hours prior to the task. At this low dose, PPX is thought to reduce phasic DA release through autoreceptor stimulation. Payment: US$ 40 for the pharmacological session and US$24.60 for the task session.

Threat-of-shock acute vs no stressor

Healthy volunteers took part in the task twice (one missing session), once in a no-stress condition and once in a stress condition. Participants were told that poor performance on the task might lead to a shock being delivered through electrodes attached to the back of their neck. In the stress condition, they were told that this was quite likely, whereas they were told that no shock would be delived in the no-stress condition. No shocks were actually delivered. Notably, the version of the task used in this study was more difficult, with the difference in size between long and short mouth being smaller. This resulted in fewer correct discriminations (see Figure 1F). Payment: either course credit or US$10/hour as well as money “won” during the task (US$10.60 on average).