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As I was working on a basic chess application for Android, I loaded some chess clip art into my imageviews. Then this happened. Look closely at the top two rows.

The Illusion

At first I was startled. My imageviews had reformatted themselves! But no. All the squares are still identically sized.

Question is this: How does this image fool our minds? What can illusions like this tell us about how the brain is wired?

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This is likely related to other illusions of relative size, such as the Ebbinghaus (A) and Delboeuf (B) illusions.

 Ebbinghaus (A) and Delboeuf (B) illusions (Mrucek et al. 2014)

These illusions show that perception is not a 1:1 representation of retinal input. Instead it is a mental (re-)construction.

As reviewed by Mrucek et al:

an object's size is not inherently represented in the size of its projected retinal image. Rather, the perceived size of an object is constructed by integrating multiple sources of information including, but not limited to, retinal image size, physical and perceived distance (Berryhill, Fendrich, & Olson, 2009; Boring, 1940; Emmert, 1881; Ponzo, 1911), an object's geometrical and textural properties (Giora & Gori, 2010; Kundt, 1863; Lotze, 1852; Oppel, 1855; Helmholtz, 1867; Westheimer, 2008), knowledge of an object's typical size (Konkle & Oliva, 2012), and the relative size of different objects in a scene (Coren & Girgus, 1978; Roberts, Harris, & Yates, 2005; Robinson, 1972).

A number of factors produce assimilation and contrast effects in size (as in your example), central variables seem to be the distance and the similarity of target and inducing stimuli (Roberts et al. 2005).

This is getting a little bit off topic, but Mruczek et al. (2014) show that dynamic visual aspects can feed into this illusion too. Also check out the StarTrek illusion which also has a relative size component. Isn't that cool? The name alone...

These illusions may seem to be cute effects. However, they have important real world consequences. For example, Wansink and van Ittersum (2012; Ittersum & Wansink, 2013) investigated the effect of plate size on consumption. Imagine that the white circle in picture B is the plate and that the black cirle is your dessert. If you have a big plate you eat more, which is (also) related to the Delboeuf illusion. These illusions seem difficult to counteract:

Even a 60-min, interactive, multimedia warning on the dangers of using large plates had seemingly no impact on 209 health conference attendees, who subsequently served nearly twice as much food when given a large buffet plate 2 hr later.

I could go on..., sorry.

References

Ittersum, K. V., & Wansink, B. (2012). Plate Size and Color Suggestibility: The Delboeuf Illusion’s Bias on Serving and Eating Behavior. Journal of Consumer Research, 39(2), 215–228. http://doi.org/10.1086/662615

Mruczek, R. E. B., Blair, C. D., & Caplovitz, G. P. (2014). Dynamic illusory size contrast: A relative-size illusion modulated by stimulus motion and eye movements. Journal of Vision, 14(3), 2. http://doi.org/10.1167/14.3.2

Roberts, B., Harris, M. G., & Yates, T. A. (2005). The roles of inducer size and distance in the Ebbinghaus illusion (Titchener circles). Perception, 34(7), 847–856.

Wansink, B., & van Ittersum, K. (2013). Portion size me: Plate-size induced consumption norms and win-win solutions for reducing food intake and waste. Journal of Experimental Psychology: Applied, 19(4), 320–332. http://doi.org/10.1037/a0035053

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    $\begingroup$ Interesting read and a great answer +1 $\endgroup$ – AliceD Mar 29 '15 at 23:52
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I can't tell you the exact underlying mechanism unfortunately. I can analyze the illusion so as to approximate an answer.

You can see that the effect is virtually absent when the white squares on the two rows are nearly similarly sized (rook - pawn). The effect is biggest when the white surfaces are most unequal (king - pawn).

Additionally, to me, the white fields seem to be bigger than the black ones, not so much the reverse. I think it is the illusion of the remainder of the black fields looking like they are offset and widening, which is most evident in pawn - bishop.

So in all, the white squares are central to the illusion, and the black fields are seemingly bordering the white fields. It now looks as if the black outlines are widened around the white squares. This may be related the fact that the white squares seem to be interpreted as squares and the black field (-remainders) as the borders of the white squares. In other words, the white fields are interpreted as squares, the black fields as borders.

Hence, I think this illusion is to be sought higher up in the cortex, where visual images are interpreted as opposed to 'simple' retinal illusions.

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