Euthymic bipolar disorder (euBD) patients exhibit deficits in neurocognitive and social cognitive functioning compared to healthy controls (HCs). Our prior research has shown that the excitatory/inhibitory (E/I) imbalance in the default mode network (DMN) is linked to executive function in euBD. Neurocognitive impairments are associated with social cognition deficits in individuals with mental disorders. Given this connection, this study posits E/I imbalance within the DMN is associated with social cognition, with executive function as a mediator.

Seventy-five HCs and 49 euBD individuals were recruited. Using the emotion recognition task, Diagnostic Analysis of Nonverbal Accuracy 2-Taiwan version (DANVA-2-TW) and cognitive flexibility task, Wisconsin Card Sorting Test (WCST), we assessed emotion recognition and prefrontal function. Proton magnetic resonance spectroscopy (1H-MRS) measured metabolites in the posterior cingulate cortex (PCC) and medial prefrontal cortex/anterior cingulate cortex (mPFC/ACC), quantifying excitatory glutamate+glutamine (Glx) and inhibitory GABA to calculate the E/I ratio.

euBD patients showed poorer emotion recognition (p = 0.020) and poorer cognitive flexibility (fewer WCST categories completed, p = 0.002). A negative association was found between emotion recognition and the E/I ratio in the mPFC/ACC of the BD patients (r = −0.30, p = 0.034), which was significantly mediated by cognitive flexibility (Z = −2.657, p = 0.007).

The BD patients demonstrate deficits in emotion recognition, linked to an altered E/I balance in the prefrontal cortex, and the cognitive flexibility, a key aspect of executive function, mediates the impact of the E/I ratio on emotion recognition accuracy in euBD patients.