Frequency-tagging SSVEP reveals late-stage sustained attention to sad faces in subthreshold depression

Attentional bias toward negative information has been regarded as a central cognitive mechanism underlying depression. However, the temporal dynamics of emotional attention in individuals with subthreshold depression (StD) remain insufficiently understood. Conventional measures of attentional bias are generally unable to continuously track the dynamic allocation of attentional resources between competing stimuli. In the present study, a frequency-tagged steady-state visual evoked potential (SSVEP) combined with a dot-probe task was employed to continuously measure visuocortical responses to emotional faces in individuals with StD (n = 25) and healthy controls (HC; n = 28). Participants viewed concurrently presented happy-neutral, sad-neutral, and neutral-neutral stimulus pairs while SSVEP amplitudes were recorded across different stages of attentional processing. The results showed no significant differences in SSVEP amplitudes among happy, sad, and neutral faces in the StD group during early processing (180-500 ms), potentially suggesting reduced early sensitivity to emotional information. During late processing (2000-3000 ms), the StD group exhibited greater sad-neutral difference SSVEP amplitudes than the HC group, whereas no corresponding group difference was detected during earlier stages. Larger late-window sad-neutral difference amplitudes were also associated with greater depressive symptom severity across the full sample. These findings suggest altered late processing of negative emotional information in StD, which may be consistent with, but does not directly establish, impaired attentional disengagement. Frequency-tagged SSVEPs may therefore provide a useful neural measure for characterizing the temporal dynamics of emotional processing in individuals with subthreshold depression.