The effects of neuromodulation on human-robot interaction in games of conflict and cooperation

Game theory has been useful for understanding risk-taking, cooperation, and social behavior. However, in studies of the neural basis of decision-making during games of conflict, subjects typically play against an opponent with a predetermined strategy [1–3]. In the present study, human subjects played Hawk-Dove games against a neural agent, both simulated and robotic, with the ability to assess the potential costs and rewards of its actions and adapt its behavior accordingly. The neural agent's model was based on the assumption that the dopaminergic and serotonergic systems track expected rewards and costs, respectively [4]. The study consisted of two experimental days, one in which subjects' serotonin levels were lowered through acute tryptophan depletion (ATD), where human subjects played against neural agents whose simulated serotonin systems were altered as well. When the neural agent's serotonergic system was compromised, by turning off neural activity in its raphe nucleus, the neural agent tended towards aggressive behavior, due to its inability to assess the cost of its actions [4]. When subjects played against an aggressive neural agent, there was a significant shift in their strategy from Win-Stay-Lose-Shift (WSLS) to Tit-For-Tat (T4T). This shift to a T4T strategy may be similar to the rejection of unfair offers in the Ultimatum Game [2]. A T4T strategy, which is strategically less advantageous than WSLS, could send a message to another player that the subject believes he is being treated unfairly. In other studies, ATD led to increased defections in the Prisoner's Dilemma [3] and more rejections of offers in the Ultimatum Game [1]. In contrast, we did not observe a decrease of cooperativeness in our subjects due to ATD, but rather the emergence of a strongly significant shift in strategies based on opponent type. It may be that iterative interactions with a responsive, adaptive agent outweighed the effects of ATD in our human subjects. Additionally, the physical instantiation of the neural agent did not evoke stronger responses from subjects than did the simulated neural agent. We suggest that both the simulated and embodied versions of the neural agent evoked strong responses in subjects because of the neural agent's adaptive behavior. These results highlight the important interactions between human subjects and an agent that can adapt its behavior. Moreover, they reveal neuromodulatory mechanisms that give rise to cooperative and competitive behaviors.