While major depression causes substantial distress and impairment for affected individuals and society, the effectiveness of cognitive behavioural therapy (CBT) in treating the condition has been established. However, the therapeutic mechanism underlying the efficacy of CBT remains unknown. This study aimed to describe a protocol for a randomised controlled trial that will measure the CBT-induced clinical and neural changes in patients with non-psychotic major depression. Methods and analysis The current study is a 16-week assessor-blinded, randomised, parallel-group trial with a 12-month follow-up as part of usual depression care at an outpatient clinic. Patients aged 20-69 years with major depressive disorder will be randomly assigned to receive either CBT in addition to their usual treatment or talking control in addition to their usual treatment for 16 weeks. The primary outcome is the functional changes in the brain areas that have been associated with future-oriented thinking at 16 weeks; secondary outcomes include changes in functional brain connectivity, severity and changes in the scores of objective and subjective clinical depression symptoms, proportion of responders and remitters and quality of life. The intention-to-treat analysis will be used. Ethics and dissemination All protocols and the informed consent form are compliant with the Ethics Guideline for Clinical Research (Japanese Ministry of Health, Labour and Welfare). Ethical Review Committees at the Keio University School of Medicine have approved the study protocol (version 3, 11 September 2017). We will disseminate research findings to scientific and general audiences through national and international conference presentations as well as lay summaries to the general public, including mental health consumer and publications in international peer-reviewed psychiatry and brain imaging journals. Trial registration number UMIN Clinical Trials Registry (UMIN000018155); Pre-results.
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