The many illusions of the Müller-Lyer: comparisons of the wings-in and wings-out illusions and manipulations of standard and dot forms.

The assumed role of peripheral distortion mechanisms in both wings-in and wings-out Müller-Lyer illusions was investigated by requiring subjects to reproduce the central extent of standard Müller-Lyer figures and dot variations. Illusory magnitude of the line and dot variations was also examined with increasing wing length and wing angle. A reduction in the overestimation for the wings-out illusion occurred with the removal of intersecting lines; the dot variations evidenced a significant overestimation effect. In contrast, no reliable decrease in underestimation was found with the removal of intersecting lines in the wings-in illusion, and both standard and dot variations were significantly underestimated. These results support a conclusion that the wings-in and wings-out Müller-Lyer illusions are two distinct illusions, and may be differentially determined at the loci of distortion within the visual system. Parallel changes in illusory magnitude were noted with configural manipulations of standard and dot wings-out illusions. However, standard and dot forms of the wings-in illusion were not equivalently affected by equivalent configural manipulations, which suggests that they are different illusions. Thus, the use of the wings-in dot variation to separate empirically peripheral from non-peripheral distortion mechanisms may be ill-advised.