No abstract

Humans work to improve the situation of others through sympathetic concern. The empathic joy hypothesis proposes that the urge to help is stimulated by enhanced sensitivity to vicarious joy achieved through helping a recipient to meet their needs. We further hypothesized that the positive feeling-related brain activation that occurs in the striatum during sympathetic concern is enhanced by empathy in a familiarity-dependent manner. To test this, we conducted behavioral and functional magnetic resonance imaging experiments in which two participants simultaneously played a virtual ball-toss game where either the subjects' partner or an unfamiliar player was isolated by the other players. The subjects showed sympathetic behavior by tossing the ball to the isolated player and reported enhancement of self-positive feelings and anticipation of feeling improvements of the isolated player during sympathetic behaviors. Activation in the bilateral dorsal striatum became more prominent during the behaviors, supporting the empathic joy hypothesis. Dorsal striatal activity when helping a romantic partner positively correlated with affective empathic traits toward that partner, whereas the activity measured when helping a stranger positively correlated with perspective-taking traits. The correlation results further revealed a familiarity-dependent empathic enhancement of positive feelings. Therefore, sympathetic concern is invoked through empathic positive feeling.

Positive social interactions contribute to the sense that one’s life has meaning. Enjoyment of feelings associated through social interaction motivates humans to build social connections according to their personal preferences. Therefore, we hypothesized that social interaction itself activates the reward system in a manner that depends upon individual interaction preferences. To test this hypothesis, we conducted a functional magnetic resonance imaging (fMRI) study in which 38 participants played a virtual ball-toss game in which the number of ball tosses to the participant was either similar to (normal-frequency condition) or higher than (high-frequency condition) the number of tosses to the other players. Participants reported greater-than-anticipated enjoyment during the high-frequency condition, suggesting that receiving a social reward led to unexpected positive feelings. Consistent with this, the high-frequency condition produced stronger activation in the ventral striatum, which is part of the reward system, and the precuneus, representing positive self-image, which might be translated to social reward. Furthermore, ventral striatal activation covaried with individual participants’ preference for interactions with others. These findings suggest that an elevated frequency of social interaction is represented as a social reward, which might motivate individuals to promote social interaction in a manner that is modulated by personal preference.