A new illusion demonstrates long-range processing

In the Fraser phase-illusion [Popple & Sagi. Vision Research, 40 (2000) 873-878] rows of Gabor patches appear tilted because successive elements are shifted in phase. We measured this bias in global orientation judgment, while varying the number of patches in each row, and their separation. We found that illusory tilt increases with the number of patches, at least up to ten patches for a separation of four carrier periods. This finding implies that the visual system is able to integrate information over large (>10 degrees ) strips of the central visual field. Our model shows that the tilt illusion might be the result of averaging the activity of oriented filters.