Scaling of Retinal Image Induced by Vertical Magnification Disparity

A surface lying in a frontal plane appears slanted about a vertical axis when the image in one eye is horizontally magnified relative to the image in the other eye. This is predicted from the geometry of the situation. The same surface appears to slant in the opposite direction when the image in the first eye is vertically magnified relative to the image in the other eye (induced effect). One explanation of the induced effect is that the retinal image is isotropically scaled to the same height and the horizontal magnification disparity as a by-product elicits the slant impression. In the current study, scaling of retinal image induced by the stereogram with vertical magnification disparity is investigated. Two horizontal lines were superimposed on the stereogram to measure the scaling. One line presented to one eye was parallel to the other line presented to the other eye, to avoid problems of rivalry or fusion. Three subjects responded apparent lengths of the lines. The result showed that the line superimposed on the vertically magnified image appeared longer than the other line. This supports that scaling of retinal image induced by vertical magnification exists.