Abstract
We investigated the effect of varying figural proportions on perceived depth and on the horizontal vertical illusion in the frontal plane. Stimuli were rectangles 10cm in width and varying in length from 8cm to 20 cm. The rectangles were presented, one at a time, either in depth (lying on the surface of a table) or in the frontal plane (standing vertically on the table). Viewing was either monocular or binocular at a distance of 108cm. Observers verbally estimated the length of each rectangle assuming a width of 100 arbitrary units. Results were normalized by dividing estimated length by true length. For frontal plane rectangles, binocular and monocular results were very similar. When true length was 10 (equal to width), the normalized estimated length was 1.05. This was a significant horizontal vertical illusion (without a bisection component) of 5%. As the true length increased, the normalized estimated length increased significantly. A linear regression line fitted to the normalized results had a slope of 0.0165, indicating that for every 10% increase in length, relative to width, another 1.65% was added to the horizontal vertical illusion. For rectangles receding in depth, monocular and binocular results were quantitatively different although qualitatively similar. When true length was 1.0, the monocular normalized estimated length was 0.76 and the corresponding binocular length was 0.88, showing depth compression in both cases. As the true length increased, the normalized estimated ratios increased significantly, with regression line slopes of 0.005 monocularly and 0.010 binocularly. Thus, for all four conditions, the estimated length to width ratio, when normalized so that the correct value was always 1.0, increased significantly as length increased. This produced an increasing horizontal vertical illusion for stimuli in the frontal plane and a decreasing compression of perceived depth for receding stimuli.