Reduced mu suppression and altered motor resonance in euthymic bipolar disorder: evidence for a dysfunctional mirror system?

Social cognitive difficulties are common in the acute phase of bipolar disorder and, to a lesser extent, during theeuthymic stage, and imaging studies of social cognition in euthymic bipolar disorder have implicated mirrorsystem brain regions. This study aimed to use a novel multimodal approach (i.e., including both transcranialmagnetic stimulation (TMS) and electroencephalogram (EEG)) to investigate mirror systems in bipolar disorder.Fifteen individuals with euthymic bipolar disorder and 16 healthy controls participated in this study. Single-pulseTMS was applied to the optimal site in the primary motor cortex (M1), which stimulates the muscle of interestduring the observation of hand movements (goal-directed or interacting) designed to elicit mirror system activity.Single EEG electrodes (C3, CZ, C4) recorded mu rhythm modulation concurrently. Results revealed that thepatient group showed significantly less mu suppression compared to healthy controls. Surprisingly, motorresonance was not significantly different overall between groups; however, bipolar disorder participants showeda pattern of reduced reactivity on some conditions. Although preliminary, this study indicates a potential mirrorsystem deficit in euthymic bipolar disorder, which may contribute to the pathophysiology of the disorder.