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Making sense of ourselves
Making sense of ourselves

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3.2 Adelson’s checkerboard illusion

A further, classic example of the impact of context and surrounding is Adelson’s checkerboard

Activity 5 Adelson’s checkerboard illusion

In this example, you are asked which square is darker: A or B?

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Square A seems to be darker than square B. Why might this be the case? Note down what you think might cause this illusion, or any thoughts you have on your experience of it.

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Discussion

Now click on the arrow to reveal the answer. This illusion makes use of two effects: context/surroundings and shadow. As with the previous example, we assume that square A is darker as it is surrounded by lighter squares. Similarly, square B is surrounded by darker squares, making it appear lighter. However, the shadow cast by the green cylinder makes colour judgements based on luminance (the light coming from a surface) more difficult. Our visual system accounts for this, and we assume that square B may appear slightly darker than it is, because it is cast in shadow. However, equally, our visual system is more alert to more obvious, hard-line changes in colour (as demonstrated by the checkerboard, where individual squares are easily picked out) and less so to gradual colour changes (as demonstrated by shadow). For these combined reasons, participants generally claim that square A is darker than square B, whereas in fact they are the same shade of grey.