Figure 1. The mean and standard deviation values for the cafe wall illusion and its variations.
The cafe wall illusion consists of rows of tiles of alternating black and white within each row, and with alternate rows shifted by one half the width of a tile. These rows of tiles are separated by a thin line of mortar (Gregory, 1972). The illusory effect created by the cafe wall illusion is that of a tilt to the mortar lines with alternate mortar lines being tilted in the opposite direction and is increased with the peripheral rather than foveal vision (Gregory & Heard, 1979). This study looks at the effects created when either the black or white tiles are considerably wider than the others and determines whether these variations are stronger or weaker than the original version.
Participants consisted of 18 Alma College students. An original version of the cafe wall illusion was constructed and placed into the Eyelines program at a 20 degree angle along with a variation with wider white tiles and one with wider black tiles. An adjustable line was placed with each trial to be used to measure the strength of the illusion. Subjects were asked to adjust the line until it appeared parallel to the mortar lines of each trial.
The mean and standard deviations of the original illusion, the altered illusion with wider white tiles, and the altered illusion with wider black tiles were computed. As shown in the figure below, the standard deviation with the original Cafe Wall illusion where the black and white tiles were of equal size was 2.565 while the mean was -.878. In the trial with wider white tiles the standard deviation was 2.717 and the mean was -.916. With the wider black tiles the standard deviation was 2.144 and the mean was -.586.
The results given above indicate that the strength of the Cafe Wall illusion does not depend greatly on the width of the black or white tiles. In the first trial with the black tiles wider the illusion is slightly less than that of the second trial where the tiles are of equal size. In the third trial where the white tiles are wider there is a very slight increase in this illusion according to teh results of this experiment. If we relate this to the twisted cord theory given by Morgan and Moulden (1986) there is one possible conflict. Using the twisted cords there would be more of a twist in the original version of the Cafe Wall illusion, making this illusion stronger. The two variations would have less of a twist because there are longer periods of black or white before it shifts. Therefore the variations should be weaker than the original. However, in the present findings there is not a significant difference. Future studies should possibly look at the twisted cords revealed in the mortar of the variations of the illusioin in this study.
References
Earle, D. & S. Maskell, (1993) "Fraser cords and reversal of the cafe wall illusion," Perception, 22, 383-390.
Gregory, R.L., (1972) "Editorial," Perception, 1, 492.
Gregory, R.L. & P. Heard, (1979) "Border locking and the cafe wall illusion," Perception, 8, 365-380.
Morgan, M.J. & B. Moulden, (1986) "The Munsterberg figure and twisted cords," Vision Research, 26, 1793-1800.
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