The evolutionarily highly conserved neuropeptide oxytocin seems to be involved in the regulation of complex forms of social behavior. It enhances the processing of positive social stimuli, reduces behavioral and neuroendocrine stress responses and modulates amygdala activity in humans. Moreover, it has been proposed that oxytocin dampens sympathetic nervous system activity. This hypothesis was tested in a double-blind, placebo-controlled study with 38 men either receiving 24 IU oxytocin intranasally or a placebo spray. While accomplishing an emotion classification task, electrodermal responses were measured as an index of sympathetic activity. Moreover, heart rate changes were recorded that are additionally mediated by the parasympathetic nervous system. Oxytocin enhanced differential heart rate responses to facial expressions as a function of the emotional valence, but had no effect on electrodermal activity or tonic measures of physiological arousal. These results indicate that oxytocin specifically modulates phasic activity of the parasympathetic nervous system which potentially reflects an increased motivational value of facial expressions following oxytocin treatment. Findings suggest that anxiolytic effects of oxytocin are not reflected in short-term sympathetic responses and may even be a consequence of rather than a prerequisite for improved social information processing.
