Abstract:

Cooperation is a prosocial behavior that develops along with human social development. Cooperation involves brain activation of the reward system and enables people to form cooperative relationships so to pursuit social rewards and self-affirmation. Previous studies have shown that depressed patients have severe social dysfunctions, e.g., they have reduced willingness to cooperate and exhibited increased negative emotions during cooperation.
This study employed the prisoner's dilemma game (PDG) to investigate the effect of depression on social cooperation using functional near-infrared spectroscopy (fNIRS) hyperscanning technique. A total of 156 participants were screened using Beck Depression Inventory Second Edition and allocated into three paired groups, i.e., low - low depressive tendency pairs (n = 26), low - high depressive tendency pairs (n = 26), and high - high depressive tendency pairs (n = 26). The fNIRS optrodes were placed at frontal and right temporoparietal junction of two participants, with 29 channels in each participant.
Behavioral and self-reported emotion ratings showed that compared to participants with low depressive tendency, the high depressive tendency group were less cooperative and less satisfied with their partner during the prisoner's dilemma task. The brain imaging results showed that, first, the orbitofrontal cortex (OFC) was activated most significantly in the reciprocal cooperation condition, followed by the condition with self defection but opponent cooperation. Furthermore, the significantly increased neural activation in these two conditions could only be observed in the low depressive tendency group. This finding suggests that people with high depressive tendency have deficits in reward processing, especially for social reward processing. Second, the neural activation of bilateral dorsolateral prefrontal cortex (dlPFC) in participants with high depressive tendency was significantly weaker than that in participants with low depressive tendency. Depressive tendency had a significant modulation effect on inter-brain synchronization of the right dlPFC, i.e., the enhanced inter-brain synchronization induced by reciprocal cooperation could not be observed in participants with high depressive tendency. Third, the right temporoparietal junction (TPJ) inter-brain synchronization in the low-low depressive tendency group was higher than that in the high-high and high-low depressive tendency groups. Furthermore, this effect was significant only if both participants in the PDG made the same choice (both cooperation or both defection).
The result of this study suggests that depressive population have dysfunctions in the brain regions involved in social reward processing (reflected by the OFC), conflict control (the dlPFC) and theory of mind (the right TPJ). Our findings provide experimental evidence to help understand the brain mechanism of decreased cooperation in depressed individuals, which further lays a foundation to improve social functions in depressed patients in clinical practice.

Key words: depression, inter-brain synchronization, cooperation, orbitofrontal cortex, dorsolateral prefrontal cortex, temporoparietal junction