Fairness has long been a topic of human society, and numerous studies confirm that people have fairness preference. Previous neurobiology researches have found that some brain regions and serotonin neurotransmitter systems play significant roles in fairness-related social behavior, and these brain regions and serotonin neurotransmitter systems have been considered to play important roles in the pathophysiology of major deoressive disorder (MDD). Therefore, the trait of fairness preference may be an important behavioral marker of maladaptation of MDD. The main objective of this study was to investigate whether fairness preference is a potential behavioral marker of MDD. To answer this question, the present thesis utilized a game theory paradigm which is often used to examine fairness preference and carried out three studies.
In Study 1, by using cognitive-behavioral research methods, we explored the characteristics of fairness-related social decision-making behavior of MDD patients who played as responders in the Ultimatum Game (UG). This study included two substudies which have different clinical samples. Both substudies found that depressed patients show abnormal decision-making behavior in a social interaction context. Specifically, (1) the acceptance rates of the patients were lower than those of the normal controls; (2) unfair proposals were accepted at similar rates from computer partners and human partners in the MDD patients, unlike the acceptance rates in the normal controls, who were able to discriminatively treat unfair proposals from computer partners and human partners. These findings light up direction and provide the basis for further investigation of MDD patients’ potiential behavioral markers from the point of acceptance rate and human-computer indiscrimination.
In Study 2, by using functional magnetic resonance imaging (fMRI), we explored the neural basis of abnormal fairness preference behaviors in MDD and the potential biomarkers. This study found that the brain activation pattern and the brain functional connectivity pattern of MDD patients were different from the normal controls when playing the UG. Specifically, (1) there are neural basis for the abnormal behavior of decreased acceptance rate in MDD. Abnormal brain activation patterns and brain functional connectivity patterns, such as decreased activation of the right MFG, brain activation-behavior uncoupling of the left insula and ACC, functional network uncoupling of the right insula-ACC, and enhanced compensatory connectivities, may be the potiential neural mechanisms; (2) there are neural basis for the abnormal behavior of human-computer indiscrimination in MDD. Brain activation-behavior uncoupling of the right insula may be the potiential neural mechanisms. These results showed that there are neural basis for the two abnormal behavioral patterns found in Study 1, i.e., decreased acceptance rate and human-computer indiscrimination. These abnormal brain activation patterns and brain functional connectivity patterns may be the potiential biomarkers of MDD patients.
However, whether a characteristic can be a biomarker, the core standard is that the characteristic must be heritable. Thus, in Study 3, by using twin studies, we explored whether there are genetic bases for the behavior performance and brain activity in the UG. This study found that (1) the acceptance rate of the responder in the UG were moderately heritable (34.0%), which provides a basis for acceptance rate in the UG as a potential marker of MDD; (2) the genetic basis of the decision differences between human and computer proposers was not found, thus whether this behavior can be a potential marker of MDD has yet to be determined; (3) ROI-based and voxel-based analysis of the fMRI data both showed that the activations of the bilateral insula which is crucial in the fairness-related decision-making also have moderately genetic basis, which indicated that brain activity of the bilateral insula may act as potential biomarkers of MDD. According to the proposed standard that biomarkers or endophenotype must be heritable, the findings in this study has provided direct evidence for the fairness preference as a potential marker of MDD.
Taken together, this study indicated that fairness preference is a potiential marker of major depressive disorder. Decreased acceptance rate and abnormal brain activation in bilateral insula are significant indeces of altered fairness preference behavior in MDD. These new potential behavioral markers may be used for early diagnosis and prognosis for MDD, and also for the psychosocial intervention and rehabilitation programs of MDD. This can further help to improve their treatment, thereby improve the social cognition and social adaptability of MDD, ultimately promote their return to society.