Vernier line stimuli consisted of two short abutting vertical dark line segments, each 0.540 in width and 300 in length presented on a mean luminance background. All stimuli were shown with suprathreshold Maxwell contrast of mc ¼ 0:18, which was far above detection threshold. In all threshold measurements the lower segment of the Vernier stimulus was displaced at one of five possible offsets, representing four equal horizontal offsets to the right of alignment, including a no- offset condition. The step size of these offsets was chosen on the basis of pilot experiments, to be close to the observers threshold and appropriate for all stimulus and mask conditions used. Simultaneous presentation of oblique Gabor mask patterns and Vernier line segments was accomplished simply by local superposition of mask and Vernier stimulus (see example in the lower right of Fig. 1). Figural representations of Vernier/mask configurations were provided at the beginning of the experiment.