Abstract
Given that vernier acuity is lower for the oblique than the cardinal axes due to the oblique effect, illusory contour figures without adjacent line ends should be less subject to distortion when orthogonal squares are rotated 45 degrees. Experiment 1 tested this prediction using 4 types of inducers (pacmen, irregular shapes, lines, and corner-ends) with 8 different edge alignments: 0, 2, 4, 6, 8, 10, 12, and 14 degrees. Using a scale of “goodness-of-square” magnitude estimates, observers rated the stimuli that were presented randomly on a high-resolution computer screen. The results showed that the perception of “squareness” or “diamondness” was more stable in illusory contour figures for greater degrees of misalignment than in real line figures. While these findings indicate that the former benefits from the oblique effect, experiment 2 compared these results to vernier judgements of the same stimuli using only 1/2 of the figures. The results revealed that the “figure-superiority” effect found in experiment 1 appears to be due to vernier acuity but not global figural factors.