Neural representation of social, monetary and chocolate reinforcer processing
Little attention has been paid to social reinforcer processing compared with food and monetary reinforcers, in the reward-related functional magnetic resonance imaging (fMRI) literature. This is surprising as social reinforcers pervade our daily lives and are often experienced more frequently than food or monetary reinforcers. The question of whether social reinforcers are processed in the same or different brain regions as other reinforcer types remains poorly understood. In this thesis, three fMRI studies were employed to investigate this question, in healthy individuals. The experimental paradigms focused on two main aspects of reward processing: neural patterns of activation associated with different reward types and valance, and also correlations between neural activation to rewards and participants’ hedonic level. The studies reported in this thesis revealed that amygdala and a subregion of the OFC responded more sensitively to social reinforcers than monetary, or food reinforcers, indicating social reinforcers modulate the affective response more strongly in the brain reward network. The results also provide evidence for a medial-lateral functional dissociation in the OFC to rewards and punishment, so that medial OFC responded more strongly to rewards and lateral OFC to punishments. Moreover, fMRI study-1 revealed a crossover interaction between reinforcement valence and reward type in the lateral OFC, indicating this region may be involved in the functional integration of both reward type and valence. This is consistent with the theory of a common neural currency, for valuing different rewards in the OFC. As activation in the reward network may also be attributed to the hedonic experience of gaining rewards, fMRI study-2 and study-3 also explored the relationship between BOLD activity in response to rewards and participants’ hedonic scores. These two studies demonstrated highly significant correlations between BOLD activity in the OFC (positive correlation) and insula (negative correlation) and self-reported levels of hedonic response. The findings of the correlations between reward and hedonic level could have important implications for understanding how human hedonic levels affect responses to various reinforcements.