This study investigated the roles of dopamine and serotonin in human social interaction by measuring their sub-second fluctuations in the substantia nigra pars reticulata (SNr) during the ultimatum game. Parkinson's disease patients undergoing deep brain stimulation surgery participated, responding to offers from human and computer players. Results showed that participants rejected more offers from humans, an effect linked to higher dopamine levels but not serotonin. Dopamine changes tracked trial-by-trial offer value changes (reward prediction errors), while serotonin tracked current offer value. This indicates distinct roles for dopamine and serotonin in processing social context and value signals.

Seth R. Batten, Dan Bang, Brian H. Kopell, Arianna N. Davis, Matthew Heflin, Qixiu Fu, Ofer Perl, Kimia Ziafat, Alice Hashemi, Ignacio Saez, Leonardo S. Barbosa, Thomas Twomey, Terry Lohrenz, Jason P. White, Peter Dayan, Alexander W. Charney, Martijn Figee, Helen S. Mayberg, Kenneth T. Kishida, Xiaosi Gu, P. Read Montague