Condition-related dynamics of EEG inter-subject synchrony during naturalistic threat viewing

Background Naturalistic threat viewing provides a useful context for examining how observers align their neural responses to shared, continuously unfolding stimulus structure. This study used electroencephalography inter-subject correlation (EEG-ISC) of band-limited power time courses to characterize condition-related neural alignment during rest, threat-related viewing, and neutral viewing. Methods Seventeen healthy adults viewed an eyes-open resting baseline, a high-arousal parkour clip labeled as the fear condition, and an emotionally neutral parkour clip while 64-channel EEG was recorded. Band-limited power was extracted using sliding-window fast Fourier transform, and ISC was estimated using leave-one-out correlation followed by Fisher z transformation. We examined condition-related ISC differences across delta, theta, alpha, and beta bands, as well as time-dynamics, window-length sensitivity, matched-duration controls, and exploratory associations with trait anxiety. Results Across the analyzed delta–beta frequency bands, ISC differed reliably across conditions, showing stronger synchrony during video viewing than rest and higher ISC during fear viewing than neutral viewing. This effect was most pronounced for alpha-band power ISC, although frequency-specific interpretation remains tentative because periodic and aperiodic spectral components were not separated. Time-dynamics analyses showed that ISC was largely stable within clips, with a selective attenuation of fear–alpha ISC over time. Window-length and matched-duration control analyses broadly preserved the qualitative condition pattern. Exploratory individual-differences analyses did not provide robust evidence for an association between fear-evoked ISC and trait anxiety in this small sample. Conclusions These findings support EEG-ISC as a marker of shared, stimulus-locked processing during naturalistic threat viewing. Threat-related events may promote more convergent tracking of unfolding stimulus structure, yielding stronger neural alignment at the group level. The exploratory anxiety-related findings require confirmation in larger samples.