Subjects

Forty-five young adults (25 females) were recruited to participate in a study on L-tyrosine and cognition. One subject was excluded from all analyses due to them using marijuana in the days before the study; two subjects had technical problems while performing the reference-back task; four subjects had technical problems during the Stroop task; three subjects did not fill in an affect grid to measure self-reported mood at all time points. This left 42, 40, and 41 subjects for analysis of the reference-back task, the Stroop task, and the affect grid, respectively.

Subjects met the following criteria: (1) aged 18 to 30 years; (2) no use of drugs or psychoactive medication, nor smoking more than one cigarette per day; (3) no history of psychiatric or neurological conditions; (4) no colorblindness; and (5) women had to be using hormonal contraception, to limit confounding fluctuations in hormone and dopamine levels related to the menstrual cycle (Czoty et al., 2009; Jacobs & D’Esposito, 2011). Subjects were instructed to refrain from caffeine intake in the 3 hours before participation.