Müller-Lyer: Illusion of Size or Position?

Abstract

Holding (1970) described a version of the Müller-Lyer illusion in which the arrowheads point in the same direction. This does not produce a change in the apparent length, but a shift in the apparent position of the shafts. Current models of the Müller-Lyer illusion based on hypothetical length-processing mechanisms cannot explain this position shift. An alternative explanation is that both versions of the illusion result from an additive displacement of the endpoints of the shafts. Measurement of both illusions using a group of 32 subjects indicated that a greater endpoint shift is needed to account for the strength of the standard length version of the illusion. Some possible reasons for this difference are suggested, and the implications for future theories of this class of illusions are discussed.