The horizontal-vertical illusion: Binocular and dichoptic investigations of bisection and verticality components

Abstract

Two experiments were conducted to examine the relative contributions of bisection and verticality to the inverted T variant of the horizontal-vertical illusion. The first study which utilized binocular presentation of stimuli supported the hypothesis that the magnitude of the horizontal-vertical illusion could be accounted for by simple additive combination of the magnitude of the vertical and bisection sub-illusions. The second study replicated the findings of the first study using dichoptic presentation (horizontal line to one eye, vertical to the other). The results of both studies taken together indicate that the amount of the horizontal-vertical illusion is due to the separate factors of bisection and verticality and that these processes are central (cortical) rather than peripheral in origin.