Neural dynamics of trust-based decision-making in the context of social interaction and reward sensitivity

Abstract

This dissertation aims to elucidate the impact of the degree of social interaction and the sensitivity to the reward type on the trust-based decision-making and to understand dynamics of its neural mechanisms. We selected the Prisoner’s Dilemma Game and the Trust Game as task representing the trust-based decision-making. We simultaneously recorded the EEG activity of subjects participating in the game and analyzed the results as following: 1. Behavioral analysis: To investigate the relationship between the degree of social interaction and mutual cooperation rate, the motivation of mutual cooperation in the Prisoner’s Dilemma Game and to investigate the relationship between the reward sensitivity differences and the amount of investment and return ratio, the relationship between the social interaction level differences and the amount of investment and return ratio, and the motivation of investors and trustees in the Trust Game. 2. EEG time-frequency analysis: To investigate the time and frequency range and brain region of EEG activity related to the degree of the social interaction during the Prisoner’s Dilemma Game and to investigate the time and frequency range and brain region of EEG activity related to the reward sensitivity differences and the social interaction levels differences during the Trust Game. 3. Brain synchronization analysis: To quantify the inter-brain synchronization between two players participating in the Prisoner’s Dilemma Game and the Trust Game, to investigate its relationship to the social interaction, and to examine associated brain region. In the Prisoner’s Dilemma Game, the face-to-face interaction led to more frequent mutual cooperation. The power of the alpha activity in the right temporoparietal region immediately after seeing the outcome significantly differed between face-to-face and face-blocked conditions and predicted the strategy to cooperate or to defect. Moreover, inter-brain synchronization across the right temporoparietal area within this time and frequency range reflected the use of these strategies. In the Trust Game, the reward sensitivity was related to the motivation of investors and trustees. Monetary-sensitive investors were motivated by perceiving risk to lose money while social-sensitive investors were motivated by the betrayal aversion. The effect of the reward sensitivity on investors was affected by the degree of face-to-face contact. Monetary-sensitive trustees were motivated by financial gain whereas social-sensitive trustees were motivated by prosocial behavior. In particular, the effect of social interaction level differences was greater for monetary-sensitive trustees compared to others. EEG activities in parietal, left and right temporoparietal regions around 1 sec after the decision were related to the reward sensitivity differences and EEG activities in frontal, left and right temporoparietal regions around 1 sec after the decision were associated with the social interaction level differences. Processing of the reward sensitivity and the social interaction in temporoparietal region occurred simultaneously, providing the neural level evidence for the interaction effect of these two factors. Inter-brain synchronization between the left temporoparietal area of the investor and the frontal area of the trustee and between the left temporoparietal area and the right temporoparietal area indicated the degree of interactions with respect to the decision to invest, mainly synchronized across the superior frontal gyrus, inferior frontal gyrus, superior temporal gyrus, and angular gyrus. Inter-brain synchronization between the frontal area of the investor and the right temporoparietal area of the trustee indicated the extent of interactions with respect to the decision to return, mainly synchronized across the superior frontal gyrus and precuneus. In addition, inter-brain synchronies across the inferior parietal lobule was related to the interaction effect of the reward sensitivity and the social interaction level. This dissertation uncovered that the trust-based decision-making, where the conflict between monetary and social rewards exists, was affected by both the reward sensitivity and the social interaction and revealed the importance of temporoparietal regions on the processing of these two factors. Most of all, results of current dissertation emphasize that brain synchronization analysis enable us to quantify the degree of the interpersonal social interaction. Furthermore, our results are expected to contribute to the deeper understanding and treatment of impaired reward processing of depressed patients and impaired social interaction of autistic patients.