Fig. 2.

Crowding effect on performance. In the baseline condition observers chose between two stimuli with the same distractor orientation and the same target contrast (i.e., they were identical with the exception of target orientation, which was random). We varied the contrast of the stimulus and the orientation of the distractors across trials. (A) Data for observer AF. The blue and red circles represent measured frequency correct with horizontal and vertical distractors. Psychometric functions were fit to the data to summarize the effect of contrast on performance. (B) Psychometric functions for all six observers: Performance was systematically lower with vertical distractors than with horizontal ones (at equal contrast levels), showing that vertical distractors reliably induce a crowding effect.