Investigating the Neural Mechanisms of Social Values via Hormones and Neuroimaging

Recent research on the social functions of the neuropeptide hormones oxytocin and vasopressin has increasingly shown them to be dependent on context, but the nature of the contexts in which their behavioral and neural effects are elicited remains unclear. Despite the rise in interest, questions remain regarding whether oxytocin increases perceptions of trust and whether its influence on prosocial behavior extends to egalitarian actions or is limited to parochial concern. Exploration into the effects of vasopressin on social behavior in humans has only recently begun. This study examined the influence of these hormones on behavioral and blood oxygen level-dependent (BOLD) responses in two social tasks. Participants were assigned to a pseudo-randomized order in which to receive nasal sprays containing oxytocin, vasopressin and placebo and performed these tasks during three separate functional Magnetic Resonance Imaging sessions conducted within a double-blinded procedure. In one task, faces were judged on how trustworthy and how dominant they appeared. In the other task, single sentence descriptions of real-world activities were evaluated on how worthwhile they were and how likely a participant was to participate in them. Each activity was categorized according to four motivational contexts conceptualized in the Basic Human Values Theory, a model of core values. Two of these contexts capture the distinction between parochial and egalitarian prosocial behavior as relating, respectively, to the values of Benevolence and Universalism. Compared to placebo, vasopressin did not affect either facial judgment. Oxytocin did not affect trustworthiness ratings but did increase ratings of facial dominance compared to placebo. Oxytocin did not influence activity ratings when compared to placebo, but vasopressin increased how worthwhile Universalism activities were perceived to be. The hormones affected BOLD signal more in the activities task than the facial judgments. Rating Benevolence stimuli during the activities task revealed different effects of the hormones on a wide network stretching from the junction of the parietal and temporal cortices to the inferior frontal gyrus when compared to placebo. Specifically, oxytocin increased activation in this network while vasopressin decreased it. When directly contrasting oxytocin and vasopressin across evaluations of Benevolent activities, oxytocin enhanced BOLD in the left anterior insula and posterior inferior frontal gyrus, potentially indicating a role for oxytocin in emotional awareness during relevant social contexts. This study presented the first evidence of oxytocin increasing the perception of facial dominance and of vasopressin promoting egalitarian prosocial behavior. I also provide evidence of oxytocin enhancing, and vasopressin attenuating, BOLD signal in brain regions associate with socio-affective information processing.