Dopamine and serotonin are hypothesized to guide social behaviours. In humans, however, we have not yet been able to study neuromodulator dynamics as social interaction unfolds. Here, we obtained subsecond estimates of dopamine and serotonin from human substantia nigra pars reticulata during the ultimatum game. Participants, who were patients with Parkinson’s disease undergoing awake brain surgery, had to accept or reject monetary offers of varying fairness from human and computer players. They rejected more offers in the human than the computer condition, an effect of social context associated with higher overall levels of dopamine but not serotonin. Regardless of the social context, relative changes in dopamine tracked trial-by-trial changes in offer value—akin to reward prediction errors—whereas serotonin tracked the current offer value. These results show that dopamine and serotonin fluctuations in one of the basal ganglia’s main output structures reflect distinct social context and value signals.

多巴胺和血清素被认为可以指导社会行为。然而，在人类中，随着社会互动的展开，我们还无法研究神经调节剂的动态变化。在这里，我们在最后通牒游戏期间从人类黑质网状部获得了多巴胺和血清素的亚秒估计值。参与者是正在接受清醒脑部手术的帕金森病患者，他们必须接受或拒绝来自人类和计算机玩家的不同公平性的金钱提议。他们拒绝的人类条件比计算机条件更多，这是社会环境的影响，与较高的多巴胺总体水平有关，但与血清素水平无关。无论社会背景如何，多巴胺的相对变化都会跟踪提供价值的逐次试验变化（类似于奖励预测错误），而血清素则跟踪当前的提供价值。这些结果表明，基底神经节主要输出结构之一的多巴胺和血清素波动反映了不同的社会背景和价值信号。