Sustained posterior negativity (SPN) elicited by brief (20 ms) symmetrical stimuli

These results demonstrate that brief visual evidence is sufficient to initiate symmetry-selective activity that unfolds over the established SPN time course in the absence of sustained viewing or response preparation. This pattern supports a hybrid account in which a rapid feedforward sweep flags global regularity while recurrent interactions sustain and consolidate the representation, thereby tightening temporal constraints on the neural mechanisms that extract structural order from transient input.