A comparison of length-matching and length-fractionation measures of Müller-lyer distortions

Two experiments were performed to evaluate the informational equivalence of length-matching (e.g., reproduction) and length-fractionation (e.g., bisection) procedures that are frequently used to quantify geometrical size distortions, such as the Müller-Lyer (M-L) illusion. In Experiment 1, the distortion in the apparent length of a horizontal test line was measured as a function of the angle between it and an abutting inducing line, and in Experiment 2 distortion was measured in the apparent length of the shaft of one-angle versions of the M-L illusion. Both procedures indexed the expansion of the obtuse-angle and the wings-out M-L illusion and the contraction of the acute-angle and the wings-in M-L illusion. However, whereas the reproduction measures indicated substantially greater expansion than contraction distortion, the bisection measures indicated greater contraction distortion. Some possible reasons for this difference, particularly the possibility that the reproduction and bisection procedures are unequally sensitive to the outputs of the mechanisms producing the M-L illusion, are discussed.