The neural basis of social decision making in patients with major depressive disorder

Abstract

Social decision making is a complex process of selecting an optimal option with the most desirable outcome in the interpersonal context. Because of the involvement of human interactions, social decision making usually demands heavily on the affective neural system. Within the system mood plays a vital role in the social interaction. In this connection, given the fact that depression is characterized as a stable state of low mood, researches have begun focusing on exploring the relationship between depression and social decision making. As yet, research on how the depression influence social decision has been scarce. Moreover, the neural basis underpinning the relationships between depressed mood and altered social interactions is undertermined. This thesis contains two studies conducted to extend the understanding of the behavioural presentation and the neural underpinnings of people with depression when they were involved in a social decision making process.

Study one examined altered ability of decision making in the context of social interaction among patients with major depressive disorders (MDD). A modified trust game (social interactive context) was adopted to measure the behavioural differences between 50 female patients with MDD and 49 healthy matched controls. Relatively to the controls, the MDD patients made less frequently and smaller ratio of deceptive decisions when the repayment proportion was high and when the risk was low. They also made less frequently benevolent responses than healthy participants when the repayment proportion was low and medium. These findings indicate that the MDD patients tended to be risk avoidant and fail to adjust their responses even when the risk was low.

Study two examined the neural correlates associated with social decision making in people with MDD. They performed on a modified trust game while their brain activities during risky decision making (high vs. low) and choices of behavior (benevolence vs. deception) were monitored by a 3T MRI scanner. Fifteen MDD patients and 15 healthy controls participated in this study. The behavioural patterns of both groups were very comparable to that of study one. Findings revealed that, compared with low risk condition, MDD patients exhibited hyperresponsivity to high risk in the insula, a key brain in decision making. Attenuated differentiated activity in the caudate nucleus, and exaggerated differentiated activity in the dorsolateral prefrontal cortex when displaying deception was observed in the clinical subjects. Healthy participants, on the other hand, exhibited increased activity in the interaction between risk and choice in the middle frontal lobe. These findings indicate that the impaired reward processing, as well as the inflexible adaptation of behaviors to external conditions, is affected by the depressed mood. These neural dysfunctions subserve the altered social decision making in MDD patients. From the observations that MDD patient’s social decision making is altered both in the behavioural pattern and in neural activity, mood is demonstrated to affect social decision making processes and risk is associated with depression. Our findings provide neural evidence of social decision making in MDD patients, which will shed light on the interaction between mood and social cognition, and further extend our insight about possible mechanisms explaining the relationship of depressed moods and the presented social deficits in people with MDD. This knowledge will facilitate future development of cost-effective interventions for people with affective disorders.