The components of the standard and reverse Müller-Lyer illusions.

Abstract

The absolute contribution of each type of oblique figure in the standard and reverse Müller-Lyer illusions is determined by placing one oblique figure at different distances from the end of a test line (either horizontal or vertical) and measuring shift in the line's apparent midpoint as a function of proximity of the oblique figure. The absolute effect of both types of figure, apex-in > < or apex-out < > is one of contraction, and superimposed on this, again for both types of figure, is a smaller expansion effect which is phasic and varies with proximity of the figure to the line. The major factor in the reverse illusion appears to be the apex-in > < and not the apex-out < > figure as previously supposed. For both standard and reverse illusions, and again for both types of figures, there is a visual field effect, since contraction is greater when the obliques lie in the right or in the lower hemifield. The fundamental similarities in mode of action of each type of oblique are further demonstrated by showing that geometrical figures in general, regardless of shape or orientation, give rise to similar patterns of absolute contraction with a phasic expansion component superimposed. The Müller-Lyer oblique figures therefore operate as two of many possible examples of a single underlying mechanism, and recent arguments that the Müller-Lyer is really two separate constituent illusions are not supported.