The primary objective of this study is to identify any interaction between oxytocin and serotonin on neural and behavioral activity related to social reward and decision making by a double-blind, placebo-controlled, between-subject, pharmacological fMRI design

Serotonin and oxytocin have been proved to play an important role in social reward processing and decision making in human. The aim of this study is to determine whether they interact in those essential human behaviors by a double-blind, placebo-controlled, between-subject, pharmacological fMRI design, in which 4 groups of healthy males will be administered with a random combination of nasal oxytocin (24 international units, IU) or placebo and acute tryptophan depletion (ATD, 75.2g) or placebo (a tryptophan-balanced mixture, 78.2g). The social-reward related neural activity and behavior will be assessed by a series of social reward task in and out of fMRI scanner and the decision making will be assessed by reversal learning task, ultimatum game (UG) and loss aversion task. The interaction between serotonin and oxytocin on brain networks will also be tested by resting state.

Inclusion Criteria:

• Healthy subjects without past or current psychiatric or neurological disorders
• Right-handedness

Exclusion Criteria:

• History of head injury;
• Medical or psychiatric illness.
• High blood pressure, general cardio-vascular alterations
• History of drug or alcohol abuse or addiction.
• Allergy against medications or general strong allergies
• Sleep disorders.
• Visual or motor impairments