Neural and Cognitive Basis of Third-Party Altruistic Decision-Making and Its Modulators

Abstract

Human beings live in a world full of social connections. Favoring by the evolution, humans could survive the challenges of nature by not only maximizing their own interests (i.e., selfish motives) but also by considering the well fare of others even at a cost to their own resources (i.e., altruistic motives). Beyond the kindness between relatives and direct reciprocity between friends, humans, as third-party bystanders, will sometimes engage in a costly situation where social norms are vio-lated, to achieve justice via either punishing the unknown offender or compensating the anony-mous victim, even when such a violation does not directly affect their own interests and the costs incurred by them will not be paid back. Why do unaffected third parties intervene at a personal cost and what might be the underlying neural as well as cognitive mechanism? What factors might influence their decisions in such situations? To address these questions, the present dissertation used four studies by adopting a modified third-party economic paradigm to capture the third-party altruistic behaviors (i.e., third-party help and punishment) in response to an unfair situation, with the help of the technique of functional magnetic resonance imaging (fMRI; Studies 1-3) and eye-tracking (Study 4). By mainly investigating neural correlates during altruistic decision-making of third parties, Study 1 showed that signals in the bilateral striatum (esp. the ventral part) were stronger when third-party deciders chose to either help the victim or punish the selfish offender. Further analyses revealed an association between either choice of altruistic behavior, or its neural activation, and the empathic concern level, a personality trait closely related with altruism (esp. helping behavior). Studies 2-4 further tested whether, and how, other factors modulate third-party decision-making and the underlying neural or cognitive processes. In particular, Studies 2A and 2B focused on oxytocin, a so-called ―pro-social‖ hormone, and tested whether its effect on other altru-istic behaviors extended to the third-party context. As revealed by Study 2A, and replicated by Study 2B, we observed that intranasal oxytocin affects neither type of third-party altruistic deci-sions; rather, it modulated neural processing, especially via enhancing activity in the temporo-parietal junction (TPJ), a region shown to support mentalizing ability, during the perception of helping decision made by a computer (Study 2A). Study 3 manipulated the attention focus on different aspects of the norm violation (i.e., asking participants to consider either the unfairness of the offender or the feelings of the victim), and showed not only an effect on third-party altruistic choice behavior, but also confirmed the role of TPJ and control-related regions in such modula-tion. Replicating the effects of empathic concern (Study 1) and attention focus on choice behavior (Study 3), Study 4 provided the first empirical evidence that eye-movement pattern during third-party altruistic decision-making could also be biased by both factors and their interaction, shed-ding light on the cognitive mechanism underlying attention and information searching. Limitations of the studies and future research directions were also discussed.