Participant recruitment
substance-use, schizophrenia-spectrum, first-degree, cardiovascular disorder, word of mouth, ADHD, schizophrenia, psychotic disorders, bipolar affective disorder, neurological illness
CARDIOVASCULAR DISORDER, SEPARATION, SYMPATHOMIMETIC
All forty participants were recruited by word of mouth, lecture announcements, advertisements on campus notice boards, social media, or university-group emails in 2017 and 2018. Participants were informed to abstain from any psychoactive substance (including caffeine) at least 24 hours before the test.Inclusion criteria were being >17 and <60 years of age, and if female, not be pregnant, and be using contraceptives if sexually active and fertile to avoid the potential of harming a fetus. Exclusion criteria included, inability to provide valid consent, sensitivity to d-amphetamine or sympathomimetic amines for the d-amphetamine study or to caffeine in the caffeine study, history of psychotic disorders (schizophrenia, schizophrenia-spectrum, or bipolar affective disorder) in themselves or their first-degree relatives, substance-use dependence, neurological illness, or language difficulties that could interfere with assessment, currently taking any (prescription) medicine that affect cognitive functions, childhood diagnosis of ADHD, women who are pregnant or lactating and cardiovascular disorder risks.The testing sessions were held between 9:00 am to 5:00 pm for a total of two days (with one-week separation) for each participant. As mentioned in the general procedures, the times of the experiments at each session and test were well-controlled to avoid inconsistencies. Transport and lunch were provided with no additional financial incentive. Informed consent and medical assessments by psychiatrists were conducted before the experiment on the first day. The basic demographic information (age, years of education, sex, and weight) was then taken.Both experiments were randomized, double-blind, counter-balanced, placebo controlled cross-over studies with permuted block randomization for drug order. Each participant received both the active drug and placebo in identical gelatin capsules. Half of the participants (n = 10) in each experiment received placebo on the first day and active drug on the second day which followed the first day by at least a week, while the remaining half received active drug first and placebo second (Figs 