Size Illusions Caused by Contour Lines

Summary

There is a group of visual illusions which give the impression that straight border lines of a rectangle are bent or that rectangles look wedge shaped. In a simple form, it may be described in the following way: On a white surface area, e.g., an upright rectangle is surrounded by a black border line. At half height, the interior of the rectangle is black, while on moving towards the bottom and the upper end, the black gradually turns into grey and finally fades away into white. When the width of the contour line is chosen such that it approaches the resolution limit of the eye, the rectangle seems to change its shape, giving the impression of being wider at the top and the bottom and narrower in between. Consequently this effect may be considered to be a size illusion. Arrays of such rectangles produce a variety of effects. We have investigated this illusion experimentally on 13 different examples. Numerical results are given. Possible explanations are discussed:

1. Blurring of the border line. Calculations have been performed in order to check to what amount this may contribute to the illusion.

2. Perceived contrast of the borderline depending on the hue of the adjacent area.

3. Structure effect. Variation of the conspicuity range is related to the existence of fine structural details. An explanation is given based on the fundamental law of data transmission and processing. Examples are presented which seem to support this hypothesis.

4. The irradiation illusion.

Though several effects seem to contribute to the illusion it is concluded that the main cause of the illusion is the structure effect.