Early and late evoked brain responsesdifferentially reflect feature encoding andperception in the flash-lag illusion

Abstract

In the flash-lag illusion (FLI), the position of a flash presented ahead of a moving bar is mislocalized, so the flash appears to lag the bar. Currently it is not clear whether this effect is due to early perceptual-related neural processes such as motion extrapolation or reentrant processing, or due to later feedback processing relating to postdiction, i.e. retroactively altered perception. We presented 17 participants with the FLI paradigm while recording EEG. A central flash occurred either 51ms ("early") or 16ms ("late") before the bar moving from left to right reached the screen center. Participants judged whether the flash appeared to the right ("no flash lag illusion") or to the left ("flash-lag illusion") of the bar. Using single-trial linear modelling, we examined the influence of timing ("early" vs. "late") and perception ("illusion" vs. "no illusion") on flash-evoked brain responses, and estimated the cortical sources underlying the FLI. Perception of the FLI was associated with a late window (368-452ms) in the ERP, with larger deflections for illusion than no illusion trials, localized to the left fusiform gyrus. An earlier frontal and occipital component (200-276ms) differentiated time-locked early vs. late stimulus presentation. Our results suggest a postdiction-related reconstruction of ambiguous sensory stimulation involving late processes in the occipito-temporal cortex, previously associated with temporal integration phenomena. This indicates that perception of the FLI relies on an interplay between ongoing stimulus encoding of the moving bar and feedback processing of the flash, which takes place at later integration stages. HighlightsFlash-lag illusion relates primarily to late evoked brain potentials (>300ms) Illusion vs. no-illusion trials showed difference in fusiform gyrus Flash-lag illusion could involve postdiction-driven integration of ongoing stimuli