RELATIVE SPATIAL EXPANSION AND CONTRACTION WITHIN THE MULLER-LYER ANDJUDD ILLUSIONS

The shaft portions of Muller-Lyer (M-L) figures, one-ended M-L figures , Judd figures, and their respective control (tails-up) figures were d ivided by subjects into eight equal-appearing intervals by means of su ccessive bisections. For most of the control stimuli the length of the left half of the shaft tended to be overestimated relative to the len gth of the right side. For the tails-out version of the M-L figure, th ere was relative overestimation of segments of the shaft adjacent to t he tails, while for the tails-in version there was relative underestim ation of these segments. These results indicate that the distortion of perceived length in the M-L illusion is not distributed evenly along the shaft. For the one-ended M-L figures the apparent overestimations and underestimations extended further along the shaft than for the sta ndard figures. For the Judd figure perceived length varied systematica lly along the length of the shaft from underestimation near the rails- in end of the figure to overestimation near the tails-out end. These r esults are contradictory to the hypothesis that the M-L illusion resul ts from inappropriate size scaling produced through the operation of s ize-constancy mechanisms, since this conjecture would predict uniform expansion or contraction. The results are compared with findings that localization responses are accurate for M-L figures but biased for one -ended M-L figures and Judd figures.