| Summary: | Bipolar Disorder (BD) has a debilitating impact on psychosocial functioning and social decision-making. Recent evidence using the Ultimatum Game (UG) has shown increased rejection of moderately unfair offers in BD, suggesting impaired processing of ambiguous social information related to fairness. The present study builds upon this finding to investigate the neural substrates of fairness processing in BD. During functional magnetic resonance imaging scanning, euthymic BD patients (n = 41) and matched healthy controls (HC; n = 41) accepted or rejected very unfair, moderately unfair, or fair offers in the UG. Acceptance rates of moderately unfair offers were significantly lower in BD patients. This aberrant behavior co-occurred with abnormal brain responses to moderately unfair offers. Compared to HC, BD patients exhibited hypoactivation of right anterior insula in response to moderately unfair offers suggesting impaired integration of affective and contextual information. BD patients also displayed stronger deactivation of posterior and middle insula in response to moderately unfair offers reflecting impaired processing of the contextual aspects of fairness. The level of impulsivity of BD patients positively correlated with the abnormal deactivation of posterior and middle insula. A separate analysis revealed increased activation of dorsal ACC and left ventrolateral PFC in response to rejected compared to accepted offers in BD patients. Taken together, our findings suggest impaired processing of ambiguous social information in euthymic BD patients which is associated with increased rejection of moderately unfair offers. This impairment may reflect a failure to integrate contextual information and may be related to increased trait impulsivity.
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