Visual size analysers and Mueller-Lyer illusions.

The characteristics of the structural components inducing a length distortion were studied in Mueller-Lyer configuration and in other similar or different configurations. When placed in a more central position both ingoing and outgoing fins induced an apparent shrinkage of the shaft. It was found that a comparable effect is obtained as well with perpendicular lines and various other forms placed slightly centrally from the ends of the shaft (when placed away from the ends, an elongation is obtained). The shrinkage effect depended primarily on the ratio of the inner extent defined by the inducing forms to the extent of the shaft. With perpendicular inducing lines the most pronounced effects were observed at ratios of about 3/4. Configurations containing instead of perpendicular lines angled fins in equivalent positions showed a slightly more pronounced effect for the ingoing fins and less pronounced for the outgoing fins with a shift of optimal positions towards the ends for the ingoing and towards the center for the outgoing fins. If the ends of the shaft are marked by vertical lines the effect of the inducing forms is drastically reduced; it becomes significant as they exceed in vertical extent the end vertical lines. The findings support the idea of an interaction between size detectors as a basis of the length distorsion associated to Mueller-Lyer and other related configurations.