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Yusuke Tani, Takao Sato; The spatial frequency characteristics of the Cafe wall illusion. Journal of Vision 2002;2(7):192. doi: 10.1167/2.7.192.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Effects of spatial frequency contents on cafe wall illusion have not been studied extensively except for an isolated report on its disappearance at very high spatial frequencies (Morgan & Moulden, 1986). The main objective of this study is to examine effects of spatial frequency of the square wave part on appearance of the cafe wall illusion in a more systematic fashion. Methods: An 8 (H) × 8.5 (V) deg cafe wall figure with 8 horizontal rows of tiles was presented on a CRT screen. Spatial frequency for the square wave part was varied in 10 steps between 0.25 and 7 c/d. Subjects were asked two questions (1) whether the mortar appeared to be slanted as a whole, and (2) whether tiles appeared as independent trapezoid (see results). Results: At very low (< 0.33 c/d) as well as high (> 3.5 c/d) spatial frequencies, the illusion disappeared and the upper and lower edge of each row appeared parallel. At lower middle spatial frequencies (0.5 – 1.0 c/d), regular cafe wall illusion was observed. The mortars (or the top and bottom edges of the tile rows) were connected and slanted, thus each row appeared as an elongated trapezoid. An intriguing phenomenon was found at moderately high frequencies (1.4 – 2.3) c/d. Each tile appeared as an independent trapezoid instead of connected elongated trapezoid seen with regular cafe wall, thus the top and bottom edges of each row appeared in a saw-tooth shape. Conclusions: It has been argued that the illusion occurs from interactions between the orientations of tile edges and local components of the twisted cords that appear in the mortar part as a result of band-pass filtering within the visual system. The present results support the hypothesis and extend it by defining the limits of such orientation integrations. When the twisted cords' local orientation is excessively slanted, it is impossible to integrate the two orientations into one and each tile appears trapezoidal.
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