The purpose of this study is to assess the mechanisms of the effect of transcranial direct current stimulation (tDCS) in alleviating subthreshold depressive symptoms in young individuals, and individualized outcome prediction is carried out.

After being informed about the study and potential risks, all participants providing written informed consent will undergo screening to determine eligibility for study entry. On day 0, participants who meet the eligibility requirements will be randomized in a double-blind manner (participant and investigator) in a 1:1:1 ratio to receive tDCS targeting the left dorsolateral prefrontal cortex (lDLPFC), tDCS targeting the right ventrolateral prefrontal cortex (rVLPFC), or Sham stimulation (10 sessions in 2 weeks). At baseline, after 2 weeks of stimulation, and at follow-up after 1 month and 3 months, participants will be assessed for brain network function, depressive symptoms, emotional status and social interactions.

Inclusion Criteria:

• 1) Be between the ages of 12-25;
• 2) is right-handed on the Edinburgh Handedness Inventory;
• 3) 2-4 of the 9 depressive symptoms defined by the American Diagnostic and Statistical Manual of Diseases, Fifth Edition (DSM-5), lasting more than 1 week; or more than 5 depressive symptoms lasting more than 1 week but less than 2 weeks (i.e., not meeting criteria for a major depressive episode) (by DSM-5 Clinical Semi-Definitive Interview (K-SADS-PL) (Gilbody et al., 2017, JAMA);
• 4) Hamilton Depression Scale (HAMD-24) 24 points≥ 8 points, < 24 points;
• 5) Gender is not limited;
• 6) Subjects and their families voluntarily participate and sign the informed consent form.

Exclusion Criteria:

• 1) Individuals with the following diagnoses according to DSM-5: psychiatric disorders due to organic diseases, substance-related and addictive disorders, neurodevelopmental disorders, schizophrenia spectrum and other psychotic disorders, bipolar disorder (assessed by K-SADS-PL);
• 2) Have two or more DSM-V-defined manic symptoms for 4 days but do not meet the criteria for hypomania or meet the DSM-V defined criteria for hypomanic symptoms for only 2-3 days;
• 3) Severe or unstable physical illness;
• 4) have a moderate or higher risk of suicide (as assessed by the Columbia Suicide Severity Scale);
• 5) Received transcranial direct current stimulation in the past three months (continuous intervention time: >2 weeks);
• 6) Contraindications to transcranial direct current stimulation, such as brain implants, personal or family history of epilepsy;
• 7) Magnetic resonance scanning is contraindicated, such as in vivo metal implants.