The Delboeuf Illusion is a visual illusion of assimilation where the size of two identical circles can be perceived differently when an additional circle is placed around one of the original circles and another circle is placed inside the other original circle. See Figure 1. The circle (A) with a larger circle (B) around it looks larger than circle (C) with a smaller circle (D) inscribed within it. This illusion is thought to be an example of the assimilation effect. The team of Tadas Surkys, Algis Bertulis and Aleksandr Bulatov (2006) discovered that if the test circle is filled in and has luminance contrast, the illusion will be stronger than the open test circles with isoluminant colors. They also measured the effect of the gap width on the illusion and they found that the magnitude of the illusion tended to decrease. It has also been found that this illusion appears only to be visual and does not work haptically (Gentaz 2004). This experiment continues to look at the gap width in the Delboeuf illusion. Tadas et al. (2006) found that the illusion decreases as the gap width increases but this experiment uses the Method of Adjustment to ask the question of whether or not continuing to increase the gap distance will reverse the illusion in favor of the contrast effect.
Figure 1. The Delboeuf Illusion In the figure above, circle A and circle C are actually the same size. Circle A appears to be larger because of the assimilation effect.
Methods
The 22 participants in this study were Alma College students from 18-22 years old. The Eye Lines computer software was used to present the stimuli and collect the data. The 6 stimuli consisted of circles such as shown in Appendix 1.
The inner circle on the left and the outer circle on the right remained the same size, with diameters of 70 mm, however each of the 6 stimuli pictures showed the left outer circle and right inner circle with varying sizes therefore varying gap widths. The exact circle diameter sizes are given in Table 1.
Results
After running the experiment we found that when the gaps were the smallest, there was the least amount of error but as they gap widths grew larger, there was more error until stimuli Image 5 (the second largest gap width). The average error decreased between Image 4 and Image 5 and continued to decrease between Image 5 and Image 6. Table 2 shows the average error for each stimuli.
Discussion
As the data in Figure 2 shows,the average error is highest in the mid ranges. One can argue that the illusion works best given these conditions. The assimilation effect still seems to be at work here. It is also shown that once the increasing gap widths reach a certain distance, the illusion begins to loose strength. In is also interesting to note that the stimulus with the smallest gap width (Image 1), had one of the lowest average errors. It may be concluded that the illusion is not as strong when the distance is so small. From our data, we can not conclude that the contrast effect is at work as the gap widths increase. We have instead found that the larger the gap width, the lower the strength of the illusion.
Appendix 1
Image 1
Image 2 
Image 3 
Image 4
Image 5
Image 6
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