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Simultaneous contrast example. The green squares look darker on the left, where they are in yellow bands, compared to the right, where they are in blue bands. There is no actual difference between them. This is a demonstration of the effect known as simultaneous contrast - the phenomenon whereby the perception of different colours is altered by their surroundings. There has been some debate - most notably amongst the 19th century scientists Helmholtz and Hering - as to whether this results from neurological effects or from retinal pre-processing. It is likely that it is a combination of the two.
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