In this study, we recruited 81 unmedicated patients with MDD and 123 demographically-matched healthy control subjects from the Mental Health Center of West China Hospital of Sichuan University that were matched for age and gender. Demographics of all subjects are shown in Table 1. All MDD patients were enrolled based on the Structured Clinical Interview for DSM-IV (SCID; First, 2002) for major depression, with Hamilton Depression Rating Scale (HDRS) score > 17 and age within 18–60 years. Exclusion criteria included patients with concurrent neurological illness, mental retardation, cardiovascular disease, bipolar disorder, schizophrenia, anxiety disorder, alcohol or drug abuse and history of loss of consciousness. To avoid the influence of medication, we ensured all patients enrolled in our study had stopped taking any antidepressants for at least 3 months before scanning (i.e. unmedicated MDD), while a subset of MDD patients (26/81, 32%) were previously treated with antidepressant medications. Detailed clinical information is presented in Table 1. All patients were assessed and scanned as soon as possible (usually within 3 days) to prevent treatment delay. Demographically, similar healthy volunteers were recruited by poster advertisements within the local community. The control subjects were also interviewed by professional psychiatrists using SCID non-patient edition (SCIDI/NP; APA, 2000) to ensure that none of them had a current or past history of depression, other major physical or neurological illnesses, or any substance abuse. All participants signed an informed consent form prior to participation in the study. This study was approved by the Ethics Committee of Sichuan University and conducted according to the Declaration of Helsinki. Tests from CANTAB (Robbins et al., 2010), imaging parameters and preprocessing details are presented in the Supplementary material.