The neurotransmitters dopamine and serotonin in the brain have a significant influence on our social behavior, as new research shows. For the first time, their interaction has been directly observed thanks to the participation of four Parkinson's patients in an experiment during brain surgery. The results reveal a neurochemical mechanism that explains our tendency to make decisions in a social context. Additionally, in the long term, this technique could help to better understand changes in brain chemistry associated with Parkinson's disease.