EFFECT OF BACKGROUND DENSITY ON ZOLLNER ILLUSION

The Zöllner illusion consists in the distortion of two parallel lines when superimposed on a background of oblique intersecting lines. The size of the illusion was measured for intersect angles 2 to 45° with pattern density constant. The angle distortion is maximum at 20° and declines to either side of this value. Different explanations for this are considered and predictions made as to how different pattern densities should affect the function. The illusion was measured for 2 pattern densities over the range 2-20°. The results show that increasing the pattern simply multiplies the entire illusion function by a constant. These measurements are an important first step before forming any theory of the illusion. © 1969.