Participants

A total of 56 healthy adults (all right-handed, Dutch-speaking men) were recruited to participate in this double-blind, randomized, placebo-controlled study with parallel design. All participants visited the laboratory once, during which electroencephalography and eye tracking recordings were performed before and after nasal spray administration (oxytocin or placebo).

However, due to technical difficulties, EEG and eye tracking recordings were not available for six participants (three oxytocin and three placebo). In addition, EEG data were not available from 10 additional participants (8 oxytocin and 2 placebo) due to excessive signal artefacts. Thus, EEG analyses were performed for a total of 40 participants (17 oxytocin, mean age 22.0 ± 3.0 years; 23 placebo, 22.65 ± 3.33 years). Only male participants were included to avoid the potential confound of gender-dependent effects of oxytocin (Zink and Meyer-Lindenberg, 2012). Participants abstained from alcohol and caffeine for 24 h before testing. Written informed consent was obtained from all participants prior to the study. Consent forms and study design were approved by the Ethics Committee Research UZ/KU Leuven in accordance to The Code of Ethics of the World Medical Association (Declaration of Helsinki). The study procedure was pre-registered at ClinicalTrials.gov (ClinicalTrials.gov Identifier: {"type":"clinical-trial","attrs":{"text":"NCT03272321","term_id":"NCT03272321"}}NCT03272321).

Note that participants were recruited to participate in a larger study, including additional physiological recordings of respiration, blood volume pulse and skin conductance. A detailed report of the effects of intranasal oxytocin on skin conductance is provided elsewhere (Daniels et al., 2020). Overall, the procedures outlined below for nasal spray administration and eye contact stimuli are the same as those described in Daniels et al. (2020).