How Can Visual Illusions Help Us to Understand Lower and Higher Level Process in Visual Perception?

How can visual illusions help us to understand lower and higher level process in visual perception?
A visual illusion may refers to the case where what we perceive differs from what we suppose to be correct. As Nicholas et al. (2001) note that the idea of a visual illusion presupposes that the object or pattern concerned would be different under other conditions. One view of illusions is that they can be used as tools to probe the mechanisms of visual perception, because perceptual errors give us clues about the way in which normal perception takes place .
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