Experimental procedure
The experimental session took place in a quiet room of the CAML’s Clinical Research Centre (Centro para Investigação Clínica) at the Hospital de Santa Maria, Lisbon, Portugal. We used a double-blind (throughout data collection up to statistical analysis, inclusive), randomized placebo-controlled, cross-over design, whereby each participant took part in two sessions: one for IN-OT and another for placebo administration, in a counterbalanced order, and at the same time each day (by 2 pm). The IN-OT administration of 24 IU was via three puffs of 0.1 ml each, in each nostril, from a 40 IU mlParticipants spent seven time windows of 5-min eyes-closed and 5-min eyes-open (−10–0 min before administration and 15–27 min, 30–42 min, 45–57 min, 1 h to 1 h 12 min, 1 h 15 min to 1 h 27 min, 1 h 30 min to 1 h 42 min after administration) (see The resting-state task. Neurophysiological data were recorded in eyes-closed (HRV) and eyes-open (HRV and pupil size) conditions in seven time windows, one prior to drug administration (baseline) and six post administration. Each time window was preceded by a 30-s countdown. Then followed 5 min of eyes-closed, a beep as an instruction to open eyes, a 15-s countdown, and finally five more minutes of recording. Afterwards, the participants filled in on-screen Likert-type scales for alertness, excitement and sociability. Between time windows, participants were allowed to rest until the start of the following recording period. OT: oxytocin; PL: placebo; HRV: heart rate variability.