Additivity of cues in perception of verticality

Visual cues affecting perception of verticality in different situations were combined within a single percept and shown to be additive. When the independent effects of the Zollner illusion cues and the rod-and-frame test cues affected the perception of verticality in the same direction, the total illusion was enhanced; when these cues counteracted each other, the illusion was attenuated by an equal magnitude.