An experimental and theorectical appraisal of the inappropriate size-depth theories of illusions.

Abstract

Many explanations have been proposed to account for the distortions evident in illusions. Important examples of these suggest that the apparent size of a stimulus is determined by its apparent distance. The Müller‐Lyer figure is frequently used to illustrate this principle. Problems are raised which make it difficult to explain the illusion in this way. Figures embodying depth features similar to those in the Müller‐Lyer illusion are illustrated. These may be seen in depth and they will reverse in aspect also. If the ‘inappropriate size—depth’ theories are valid, illusory distortions should be seen in them. The apparent lengths of lines bounded by arrowheads are measured: they are inconsistent with the outcome expected if their sizes were determined from their apparent locations in depth. Detailed consideration is given to the implications of the findings for the inappropriate size—depth theories. Particular reference is made to the ‘perspective’, ‘carpentered environment’ and ‘size‐constancy’ theories. It is concluded that these theories fail to explain why illusions appear in two‐dimensional figures.