Vertical-horizontal illusion present for sighted but not early blind humans using auditory substitution of vision

This experiment was undertaken to investigate the effect of sensory modality (vision vs. audition) and if visual status (early blind vs. sighted) on sustainability to the vertical-horizontal illusion. Early blind and blindfolded sighted subjects explored variants of the vertical-horizontal illusion using a device that substituted audition for vision, whereas sighted subjects from an independant group inspected the same stimuli visually. Sensitivity to the vertical-horizontal illusion, including an illusion of moderate strenght when using the sensory substitution device, was observed only in the two sighted groups. The existence of an illusion effect when using such a device supports the idea of a visual perception provided by sensory substitution, whereas the attenuation of the vertical-horizontal illusion strenght is consistent with the visual field shape theory (Kunnapas, 1955a). The absence of the illusion effect in early blind subjects suggests that the sensory experience influences the nature of perception and that the visual experience plays a crucial role in the vertical-horizontal illusion, in accordance with the size-contancy scaling theory (Gregory, 1963).