Our social activity is heavily influenced by the process of introspection, with emerging research suggesting a role for the Default Mode Network (DMN) in social cognition. We hypothesize that oxytocin, a neuropeptide with an important role in social behaviour, can effectively alter the connectivity of the DMN. We test this hypothesis using a randomized, double-blind, crossover, placebo-controlled trial where 15 healthy male participants received 24 IU oxytocin or placebo prior to a resting-state functional MRI scan. We used Granger Causality Analysis for the first time to probe the role of oxytocin on brain networks and found that oxytocin reverses the pattern of effective connectivity between the bilateral precuneus and the left dorsolateral prefrontal cortex (dlPFC), a key central executive network (CEN) region. Under placebo, the bilateral precuneus exerted a significant negative causal influence on the left dlPFC and the left dlPFC exerted a significant positive causal influence on the bilateral precuneus. However, under oxytocin, these patterns were reversed, i.e. positive causal influence from the bilateral precuneus to the left dlPFC and negative causal influence from the left dlPFC to the bilateral precuneus (with statistically significant effects for the right precuneus). We propose that these oxytocin-induced effects could be a mechanistic process by which it modulates social cognition. These results provide a measurable target for the physiological effects of oxytocin in the brain and offer oxytocin as a potential agent to enhance the cooperative role of the predominantly 'task-inactive' 'default mode' brain regions in both healthy and patient populations.
Keywords: Default Mode Network; Effective connectivity; Granger Causality; Oxytocin; Resting fMRI.