The Effect of Transcranial Direct Current Stimulation (tDCS) in Improving Emotion Health

The goal of this clinical trial is to investigate how transcranial direct current stimulation (tDCS) affects emotion functions in young adults aged 18-35 from the local community, including both male and female participants. The main question\[s\] it aims to answer are: * Does tDCS improve emotional functions, such as mood regulation and motivation, in individuals with subthreshold depression (StD)? * Can tDCS enhance emotional regulation compared to a sham stimulation (placebo)? Researchers will compare participants receiving tDCS on either the left dorsolateral prefrontal cortex (lDLPFC) or right ventrolateral prefrontal cortex (rVLPFC) with those receiving sham stimulation to see if tDCS has a stronger effect on emotional functions. Participants will: * Complete online and in-person screening to assess depressive symptoms using the Chinese version of the Beck Depression Inventory II (C-BDI-II) and be selected based on their depressive symptoms (C-BDI-II score ≥ 13). * Be randomly assigned to one of three groups: lDLPFC tDCS, rVLPFC tDCS, or Sham control group (1:1:1 ratio). * Receive 10 sessions of tDCS or Sham tDCS over 2 weeks, with each session lasting 20 minutes. * Complete assessments at baseline, post-intervention, 1-month follow-up, and 3-month follow-up, with each assessment lasting 2-2.5 hours. This includes questionnaires and perform emotional and cognitive tasks.