Brain Functional Connectivity Mechanism of Cognitive Flexibility Impairment and rTMS Intervention in Major Depressive Disorder

Major depressive disorder (MDD) is a highly prevalent and recurrent chronic psychiatric illness. Over 50% of patients in the acute phase show widespread and significant cognitive impairments affecting executive function, attention, processing speed, and memory. Among executive subdomains, cognitive flexibility - the capacity to adapt thoughts and behaviors to changing environmental demands - is particularly difficult to treat. Even after 6 months of antidepressant treatment, cognitive flexibility often fails to return to healthy levels, and its impairment worsens with recurrent episodes and longer illness duration. Functional MRI (fMRI) evidence points to the inferior parietal lobule (IPL) as a key node activated during cognitive flexibility tasks, while the dorsolateral prefrontal cortex (DLPFC) mediates top-down cognitive control. We therefore hypothesize that decreased functional connectivity between the left IPL and right DLPFC is the critical neural basis for cognitive flexibility impairment in MDD, and that targeted modulation of this connection can improve flexibility. Study Design This is a randomized, double-blind, sham-controlled, multi-center trial. A total of 105 MDD patients with cognitive flexibility impairment will be enrolled and allocated 1:1:1 to: Active dual-target group (n=35): active rTMS over left IPL + active rTMS over right DLPFC; Active single-target group (n=35): active rTMS over left IPL + sham over right DLPFC; Sham control group (n=35): sham rTMS over both targets. All participants will continue their stable SSRI or SNRI treatment as usual (TAU). rTMS will be delivered using MRI-guided neuronavigation for individualized target localization. The stimulation regimen consists of 5 paired sessions per day (with a 50-minute inter-session interval) for 10 consecutive days. Outcomes and Follow-up Assessments will occur at baseline, immediately post-treatment (Day 10), and at 2-week and 4-week post-treatment: Primary outcome: cognitive flexibility, measured by the Trail Making Test difference score (TMT-B minus TMT-A) and by task-switching behavioral performance. Secondary outcomes: depressive symptom severity (HAMD), anxiety (HAMA, ), functional disability (Sheehan Disability Scale, SDS), suicidal ideation (Beck Suicidal Ideation scale, BSI), and other clinical measures. Imaging: structural, resting-state acquired on a Siemens 3.0T MRI scanner to assess IPL-DLPFC functional connectivity changes. Safety: adverse events, vital signs, and pain rating (Visual Analogue Scale, VAS). Significance This study may identify the key functional connectivity basis of cognitive flexibility impairment in MDD and provide a novel, individually-targeted rTMS strategy. If effective, it will offer new evidence for treating cognitive dysfunction in depression and establish a clinically actionable brain network target.