''Frozen'' motion and ''melted'' stereopsis: Linking the double nail and Ternus illusions

Purpose. Two nails appropriately placed behind one another may appear side by side instead (the Double Nail illusion). Casual observation suggests no connection between this and the well known Ternus illusion in apparent motion. Method. Correspondence matching patterns in the two illusions were compared, and stimulus onset asynchrony (SOA) was varied in a dichoptic Ternus display with "nails" (lines) as elements. Results. Correspondence matching in the two illusions is lawfully related. Either illusion can be obtained from the other by the manipulation of one variable (SOA). Several earlier claims concerning these illusions were falsified (e.g. that "element" motion cannot be obtained with long SOAs, and that the Double Nail illusion cannot be disrupted by differences in nail length). Conclusion. Results reveal a lawful relationship between correspondence matching in motion and depth perception.