Sustained Posterior Negativity Elicited by Brief (20ms) Symmetrical Stimuli: An ERP Study on the Temporal Limits of Visual Regularity Processing

Sustained Posterior Negativity (SPN) is a strong event-related potential (ERP) component associated with the visual processing of regularity. This study investigated whether SPN can be elicited by extremely brief visual stimuli, pushing the temporal limits of symmetry processing. We recorded brain activity using electroencephalography (EEG) while participants performed a symmetry detection task in which symmetrical and asymmetrical octagonal stimuli were presented, each for a duration of only 20 milliseconds. Notably, SPN is typically observed between 200–650 milliseconds post-stimulus and is thought to reflect feedback-related visual processing. Prior studies have relied on stimulus durations of a few hundred milliseconds, as this is ample time for recurrent processing, leaving it unclear whether such response can be triggered with near-threshold stimuli. In the present experiment, conditions for symmetry detection were brought further suboptimal by varying the salience of the symmetry axis on a trial-by-trial basis. An SPN with reduced amplitude compared to longer-exposure paradigms, but reliable nonetheless, emerged between 250 and 512 milliseconds post-stimulus, indicating that a single transient input is sufficient to initiate symmetry-selective processing in ventral visual cortex, likely via magnocellular pathways. These findings support a hybrid framework in which early feedforward signals can initiate global form processing, with later feedback refining the representation. Ultimately, the efficiency of the visual system in extracting structural regularity from minimal input is highlighted.