Neural correlates of acceptance and commitment therapy in major depressive disorder: a task-based fMRI study.

Acceptance and Commitment Therapy (ACT) has demonstrated effectiveness in treating Major Depressive Disorder (MDD); however, its underlying neural mechanisms remain poorly understood. Therefore, this study aims to investigate neural changes following an 8-week, group-based ACT intervention using task-based functional magnetic resonance imaging (fMRI) in patients diagnosed with MDD. Forty-two patients (21 in the ACT group and 21 in a waitlist control group) underwent fMRI scans before and after the intervention, using stimuli designed to evoke cognitive fusion alongside psychological assessments. The ACT group exhibited significant improvements in depressive symptoms and ACT-related psychological processes. fMRI analyses revealed increased posttreatment activation in brain regions central to emotion regulation, particularly the medial prefrontal cortex (mPFC) and anterior cingulate cortex. These neural changes were associated with improvements in depressive symptoms and reductions in cognitive fusion and experiential avoidance. Additionally, enhanced functional connectivity between the mPFC and posterior cingulate cortex/precuneus was observed, which was negatively correlated with rumination. These findings suggest that midline brain structures may serve as critical neural substrates for reducing ruminative thinking and enhancing cognitive defusion and acceptance, offering insights into the neurobiological mechanisms through which ACT exerts its antidepressant effects.