Prefrontal EEG spectral and nonlinear signatures of subthreshold depression during resting state and affectively valenced picture/video viewing: a participant-level analysis

Subthreshold depression (StD) is clinically important because it is associated with functional impairment and increased risk of major depressive disorder, yet low-burden objective markers for early identification remain limited. We investigated whether a lightweight three-channel prefrontal EEG protocol could differentiate university students with StD from healthy controls (HC) during eyes-open rest and standardized positive- and negative-valence picture and video viewing. Eighty-four participants (42 StD, 42 HC) were recruited, and EEG was recorded from Fp1/Fpz/Fp2. Nine participant-level spectral and nonlinear features were analyzed using linear mixed-effects models with fixed effects of group, condition, and their interaction, followed by Benjamini-Hochberg false discovery rate correction; sensitivity analyses additionally adjusted for education level. The surviving effects were concentrated in the nonlinear domain. Power spectral entropy showed a significant condition effect, whereas C0 complexity and Kolmogorov entropy showed significant group-by-condition interactions. In condition-specific contrasts, both measures were higher in StD than HC during sad-video and happy-video viewing, and after education adjustment, Kolmogorov entropy remained significantly higher in StD during both video conditions. These findings support nonlinear prefrontal EEG features as exploratory candidate markers of subthreshold depression in standardized viewing contexts.