The goal of this clinical trial is to determine whether modulating default mode network (DMN) alpha connectivity using transcranial alternating current stimulation (tACS) can reduce rumination and, in turn, mitigate feelings of entrapment and suicidal ideation in individuals with active suicidal ideation and depression. The main questions it aims to answer are: 1. Is personalized tACS stimulation of the DMN associated with reduced rumination 24 hours after stimulation? 2. Does a reduction in rumination result in lower feelings of entrapment and suicidal ideation? 3. Does personalized tACS stimulation of the DMN lead to a reduction of DMN alpha connectivity? Researchers will compare active tACS stimulation to sham stimulation to assess whether modulating alpha connectivity has a specific effect on rumination, entrapment, suicidal ideation, and DMN alpha connectivity. Participants will: * Receive either active or sham tACS stimulation during stimulation sessions, but all participants will receive active tACS at least once. * Complete self-report measures of rumination, entrapment, and suicidal ideation before and after stimulation. * Undergo EEG recordings to assess changes in DMN alpha connectivity. This clinical trial will be preceded by a pilot study in healthy participants with an anticipated completion of data collection in August 2025.
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EEG alpha functional connectivity (phase synchronization)
Timeframe: Pre-stimulation to post-stimulation at visit 2 (up to 1 hour)
Response style questionnaire (RSQ-10D)
Timeframe: Visit 2 to visit 3 (24 hours)