Spatial range of illusory effects in Müller-Lyer figures

The spatial range of the illusory effects in Müller-Lyer (M-L) figures was examined in three experiments. Experiments 1 and 2 assessed the pattern of bisection errors along the shaft of the standard or double-angle (experiment 1) and the single-angle (experiment 2) M-L figures: Subjects bisected the shaft and the resulting two half-segments of the shaft to produce apparently equal quarters, and then each of the quarters to produce eight equal-appearing segments. The bisection judgments of each segment were referenced to the segment's physical midpoints. The expansion or wings-out and the contraction or wings-in figures yielded similar patterns of bisection errors. For the standard M-L figures, there were significant errors in bisecting each half, and each end-quarter, but not the two central quarters of the shaft. For the single-angle M-L figures, there were significant errors in bisecting the length of the shaft, the half-segment, and the quarter, of the shaft adjacent to the vertex but not the second quarter from the vertex nor in dividing the half of the shaft at the open end of the figure into four equal intervals. Experiment 3 assessed the apparent length of the half-segment of the shaft at the open end of the single-angle figures. Length judgments were unaffected by the vertex at the opposite end of the shaft. Taken together, the results indicate that the length distortions in both the standard and single-angle M-L figures are not uniformly distributed along the shaft but rather are confined mainly to the quarters adjacent to the vertices. The present findings imply that theories of the M-L illusion which assume uniform expansion or contraction of the shafts are incomplete.