ABSTRACT
Theorized vertical-horizontal illusion (VHI) mechanisms include differential eye movements, discord among retinal, visual, and gravitational orientations, and differentially excitatory retinal meridians. One causal VHI theory maintains that the VHI is the product of foreshortening of receding horizontals. Foreshortening applied to the VHI acknowledges that vertical retinal images can represent long horizontal surfaces extending away from the observer. Under this theory, the VHI increases the perceived length of vertical retinal images in order to represent environmental horizontal surfaces accurately. Alternatively, Gravity theory explanations of the VHI suggest that perceived distance corresponds with anticipated effort of traversing it. Gravity theory acknowledges that vertical surfaces require more energy to navigate than horizontal surfaces and suggests that this relationship produces over-perception of vertical surfaces. Evolved Fall Avoidance Theory suggests that some cognitive features reflect natural selection resulting from the risk of falling inherent in vertical surface navigation.
