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Yusuke Tani, Kazushi Maruya, Takao Sato; Reversed Cafe wall illusion. Journal of Vision 2003;3(9):351. doi: https://doi.org/10.1167/3.9.351.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: The main objective of this study was to examine the contributions of luminance (1st order) and higher-order components on Cafe wall illusion by comparing the illusion induced by stimuli with regular square wave (SQ) and missing fundamental (MF) gratings. Although the two types of gratings look alike, the lowest frequency component contained in MF is the third harmonic (3f component) of fundamental frequency of original square-wave. Experiments: We used two types of Cafe wall figure. One was a normal SQ, consisted of three square-wave gratings and two gray mortars. In the other type, we replaced the square waves by MF gratings. Both stimuli subtended 10.2 (H) × 6.95 (V) deg, and presented at the center of a CRT screen. The spatial frequency for the gratings was 0.2 c/d. Subjects were asked to judge the direction of illusory mortar tilt (2AFC). In Exp. 1, we examined typical configuration where the phase difference (offset) between two adjacent gratings was 90 deg phase angle. In Exp. 2, we systematically varied the offset from 15 to 165 deg in small steps. Results: In Exp. 1, although both types of stimuli subjectively looked quite alike, they brought about tilts in opposite directions. In Exp. 2, the tilt direction was reversed twice for MF stimuli when offset was varied between 15 to 165 deg. No such reversal occurred for SQ stimuli. Conclusion: The tilt reversal found for MF stimuli can be related to 3f-wave components since a 90 deg shift for MF fundamental frequency corresponds to 270 deg (= −90 deg) for 3f components. The direction reversal in Exp. 2 can be also related to processing of 3f component. Although the MF stimulus subjectively appeared quite alike to regular square-wave pattern, the direction of the illusion was determined by invisible 3f component. These results, thus, clearly demonstrated that Cafe wall illusion is dominantly a 1st order, luminance phenomenon.
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